Deep Foundation_final Rev1.docx

1 1.1

General Principles

INTRODUCTION

In some cases, shallow foundations are inadequate to support the structural loads, so deep foundations or pile foundations are required. In this chapter, you will study about the basic principles of pile foundations, it’s function, type, load transfer mechanisms, and standard procedures in design. Learning Outcomes:   

Appreciate and understand the complexity of the stress and strain states imposed by pile installation and structural loads on the soil. Understand the function and type of pile foundations Understand the load transfer mechanism from pile to surrounding soils.

Function of piles Piles are columnar elements in a foundation which have the function of transferring the structural loads include axial loads, lateral loads, and moments from the superstructure through weak compressible strata or through water, onto stiffer or more compact and less compressible soils or onto rock. They may be required to carry uplift loads when used to support tall structures subjected to overturning forces from winds or waves. Piles used in marine structures are subjected to lateral loads from the impact of berthing ships and from waves. Combinations of vertical and horizontal loads are carried where piles are used to support retaining walls, bridge piers and abutments, and machinery foundations. Pile foundations are used when: • • • • •

The soil near the surface does not have sufficient bearing capacity to support the structural loads. The estimated settlement of the soil exceeds tolerable limits (i.e., settlement greater than the serviceability limit state). Differential settlement due to soil variability or non-uniform structural loads is excessive. The structural loads consist of lateral loads, moments, and uplift forces, singly or in combination. Excavations to construct a shallow foundation on a firm soil layer are difficult or expensive.

1

1.2

CLASSIFICATION OF PILES

Piles may be classified as long or short in accordance with the L/D ratio of the pile (where L = length, D = diameter of pile). A short pile behaves as a rigid body and rotates as a unit under lateral loads. The load transferred to the tip of the pile bears a significant proportion of the total vertical load on the top. In the case of a long pile, the length beyond a particular depth loses its significance under lateral loads, but when subjected to vertical load, the frictional load on the sides of the pile bears a significant part to the total load. Piles may further be classified as vertical piles or inclined piles. Vertical piles are normally used to carry mainly vertical loads and very little lateral load. When piles are inclined at an angle to the vertical, they are called batter piles. Batter piles are quite effective for taking lateral loads, but when used in groups, they also can take vertical loads. Piles may be classified in a number of ways based on different criteria: (a) (b) (c) (d) (e) (f) 1.2.1

Material and composition Installation method Method of load transfer Amount of ground displacement during pile installation Function or action Method of pile fabrication

Classification Based on Material and Composition

Piles may be classified as follows based on material and composition: Timber piles These are made of timber of sound quality. Length may be up to about 8 m; splicing is adopted for greater lengths. Diameter may be from 30 to 40 cm, with maximum design load is about 250kN. Timber piles perform well either in fully dry condition or submerged condition. Alternate wet and dry conditions reduce the life of a timber pile; to overcome this, creosoting is adopted. Steel piles These are usually H-piles (rolled H-shape), pipe piles with closed or open ended, or sheet piles (rolled sections of regular shapes). They may carry loads up to 1000kN or more. Concrete piles These may be ‘precast’ or ‘cast-in-situ’. Precast piles are reinforced and or prestressed to withstand handling stresses. They require space for casting and storage, more time to cure and heavy equipment for handling and driving. Cast-in-situ piles are installed by pre-excavation or boring, thus eliminating vibration due to driving and handling.

2

Composite piles These may be made of either concrete and timber or concrete and steel. These are considered suitable when the upper part of the pile is to project above the water table. Lower portion may be of untreated timber and the upper portion of concrete. Otherwise, the lower portion may be of steel and the upper one of concrete. 1.2.2

Classification Based on Method of Installation

Piles may also be classified as follows based on the method of installation: Driven piles Timber, steel, or precast concrete piles may be driven into position either vertically or at an inclination. If inclined they are termed ‘batter’ or ‘raking’ piles. Pile hammers and pile-driving equipment are used for driving piles. Cast-in-situ piles Only concrete piles can be cast-in-situ. Holes are drilled and these are filled with concrete. These may be straight-bored piles or may be ‘under-reamed’ with one or more bulbs at intervals. Reinforcements may be used according to the design requirements. Driven and cast-in-situ piles This is a combination of both types. Casing or shell may be used. The Franki pile falls in this category. Screw piles Two types of screw piles are available in construction: (1) steel screw piles and (2) concrete screw piles. Screw piles made of steel are circular hollow sections of shaft with one or more tapered steel plates (helices) welded to the outside of the tube at the base. In the case of concrete screw piles, the hollow tube with an auger head is screwed into the ground until it reaches the base depth. Then, the hollow cavity is filled with reinforced concrete while the tube and the auger are screwed back. Jack-in piles, Jetting piles, Vibrating piles 1.2.3

Classification of piles based on the method of load transfer

Pile types based on the method of load transfer can be placed into the following four categories: Classification of piles based on the method of load transfer from the pile to the surrounding soil consists of end-bearing piles, friction piles, combining endbearing and friction piles, and laterally loaded piles. End-bearing piles are driven through soft and loose material and their tips rest on the underlying stiff stratum, such as dense sand and gravel, clay shale, or hard rock. Friction piles primarily transfer the load to various soil layers along its shaft. Figure 1.1 gives an 3

illustration of end-bearing and friction piles. Combined end-bearing and friction piles support the load partly through skin friction to the soil around them and the remaining load is transferred to the underlying denser or stiffer stratum.

Soft highly compressible soils

Soil progressively increasing in stiffness or relative density with increasing depth Rock or hard relatively incompressible soil

Figure 1.1 Types of bearing piles (a) Friction or Adhesion piles (b) Point or End-bearing piles

1.2.4 Classification based on amount of ground displacement during pile installation Pile types based on the amount of ground disturbance during pile installation can be placed into the following four categories: Large-displacement piles Piles displace soil during their installation, such as driving, jacking, or vibration, into the ground. Examples of these types of piles are timber, precast concrete, prestressed concrete, close-ended steel pipe, and fluted and tapered steel tube piles. Small-displacement Piles displace a relatively small amount of soil during installation. These piles include steel H-sections, open-ended pipe piles, steel box sections, and screw piles. These categories are based on the amount of soil disturbed during pile installation. The terms “large” or “small displacement” used are for qualitative description only, since no quantitative values of displacement have been assigned. Non-displacement Piles do not displace soil during their installation. These piles are formed by first removing the soil by boring and then placing prefabricated or cast-in-place pile into the hole from which an equal volume of soil was removed. Their placement causes little or no change in lateral ground stress, and, consequently, such piles develop less shaft friction than displacement piles of the same size and shape. Piling operation is done by such methods, as augering (drilling, rotary boring) or

4

by grabbing (percussion boring). Most common types of non-displacement piles are bored and cast-in-place concrete piles. Composite Piles can be formed by combining units in above categories. An example for a displacement and non-displacement type composite pile is by first driving an open-ended tube, then drilling out the soil and extending the drill hole to form a bored and cast-in-place pile. Numerous other combinations may be formed by combining units in each of the above categories. 1.2.5

Classification of piles based on function or action

Piles may be classified as follows based on the function or action: End-bearing piles It is used to transfer load through the pile tip to a suitable bearing stratum, passing soft soil layer or water. Friction / Adhesion piles It is used to transfer loads to a depth in a frictional material by means of skin friction along the surface area of the pile. Tension or uplift piles It is used to anchor structures subjected to uplift due to hydrostatic pressure or to overturning moment due to horizontal forces. Compaction piles It is used to compact loose granular soils in order to increase the bearing capacity. Since they are not required to carry any load, the material may not be required to be strong; in fact, sand may be used to form the pile. The pile tube, driven to compact the soil, is gradually taken out and sand is filled in its place thus forming a ‘sand pile’. Anchor piles It is used to provide anchorage against horizontal pull from sheet piling or water. Fender piles It is used to protect water-front structures against impact from ships or other floating objects. Sheet piles Commonly used as bulkheads, or cut-offs to reduce seepage and uplift in hydraulic structures. Batter piles It is used to resist horizontal and inclined forces, especially in water front structures. Laterally-loaded piles 5

Commonly used to support retaining walls, bridges, dams, and wharves and as fenders for harbour construction. 1.3 FACTORS GOVERNING CHOICE OF TYPE OF PILE The advantages and disadvantages of the various forms of pile described in previous sections affect the choice of pile for any particular foundation project and these are summarized in the following subsections: 1.3.1

Driven displacement piles

Advantages (1) Material forming pile can be inspected for quality and soundness before driving (2) Not liable to ‘squeezing’ or ‘necking’ (3) Construction operations not affected by groundwater (4) Projection above ground level advantageous to marine structures (5) Can be driven in very long lengths (6) Can be designed to withstand high bending and tensile stresses (7) Can be re-driven if affected by ground heave (8) Pile lengths in excess of 25 m are common and pile loads over 10000kN are feasible for large diameter piles. Disadvantages (1) Un-jointed types cannot readily be varied in length to suit varying levels of bearing stratum (2) May break during driving, necessitating replacement piles (3) May suffer unseen damage which reduces carrying capacity (4) Uneconomical if cross-section is governed by stresses due to handling and driving rather than by compressive, tensile or bending stresses caused by working conditions (5) Noise, and vibration, and pollution due to driving may be unacceptable (6) Displacement of soil during driving may lift adjacent piles or damage adjacent structures (7) End enlargements, if provided, destroy or reduce shaft friction over shaft length (8) Cannot be driven in conditions of low headroom. 1.3.2

Driven and cast-in-place displacement piles

Advantages (1) Length can easily be adjusted to suit varying levels of bearing stratum (2) Driving tube driven with closed end to exclude groundwater (3) Enlarged base possible (4) No spoil to remove; important on contaminated sites (5) Formation of enlarged base does not destroy or reduce shaft friction (6) Material in pile not governed by handling or driving stresses (7) Noise and vibration can be reduced in some types by driving with internal drop-hammer (8) Reinforcement determined by compressive, tensile or bending stresses caused by working conditions 6

(9) Concreting can be carried out independently of the pile driving (10) Pile lengths up to 25 m and pile loads to around 1500kN are common. Disadvantages (1) Concrete in shaft liable to be defective in soft squeezing soils or in conditions of artesian water flow where with draw-able tube types are used (2) Concrete cannot be inspected after installation (3) Concrete may be weakened if artesian groundwater causes piping up shaft of pile as tube is withdrawn (4) Length of some types limited by capacity of piling rig to pull out driving tube (5) Displacement may damage fresh concrete in adjacent piles, or lift these piles or damage adjacent structures (6) Noise and vibration due to driving may be unacceptable (7) Cannot be used in river or marine structures without special adaptation (8) Cannot be driven with very large diameters (9) End enlargements are of limited size in dense or very stiff soils (10) When light steel sleeves are used in conjunction with with-drawable driving tube, shaft friction on shaft will be destroyed or reduced. 1.3.3

Bored and cast-in-place replacement piles

Advantages (1) Length can readily be varied to suit variation in levels of bearing stratum (2) Soil or rock removed during boring can be inspected for comparison with site investigation data (3) In-situ loading tests can be made in large-diameter pile boreholes, or penetration tests made in small boreholes (4) Very large (up to 7.3 m diameter) bases can be formed in favourable ground (5) Drilling tools can break up boulders or other obstructions which cannot be penetrated by any form of displacement pile (6) Material forming pile is not governed by handling or driving stresses (7) Can be installed in very long lengths (8) Can be installed without appreciable noise or vibration (9) No ground heave (10) Can be installed in conditions of low headroom (11) Pile lengths up to 50 m over 3 m in diameter with working loads over 30000kN are feasible. Disadvantages (1) Concrete in shaft liable to squeezing or necking in soft soils where conventional types are used (2) Special techniques needed for concreting in water-bearing soils (3) Concrete cannot be inspected after installation (4) Enlarged bases cannot be formed in coarse-grained soils (5) Cannot be extended above ground level without special adaptation (6) Low end-bearing resistance in coarse-grained soils due to loosening by conventional drilling operations 7

(7) Drilling a number of piles in a group can cause loss of ground and settlement of adjacent structures. 1.4 LOAD TRANSFER MECHANISM The load transfer mechanism from a pile to the soil is complicated. To understand it, consider a pile of length L, as shown in Figure 1.2a. The load on the pile is gradually increased from zero to Q(z=0) at the ground surface. Part of this load will be resisted by the side friction developed along the shaft, Q1 and part by the soil below the tip of the pile, Q2. Now, how are Q1 and Q2 related to the total load? If measurements are made to obtain the load carried by the pile shaft, Q(z) at any depth z, the nature of the variation found will be like that shown in curve 1 of Figure 1.2b. The frictional resistance per unit area at any depth z may be determined as (1.1) where p = perimeter of the cross section of the pile. If the load Q at the ground surface is gradually increased, maximum frictional resistance along the pile shaft will be fully mobilized when the relative displacement between the soil and the pile is about 5 to 10 mm, irrespective of the pile size and length L. However, the maximum point resistance Q2 = Qp will not be mobilized until tip the pile has moved about 10 to 25% of the pile width (or diameter). (The lower limit applies to driven piles and the upper limit to bored piles). At ultimate load (curve 2 in Figure 1.2b), Q(z=0) = Qu. Hence, Q1 = Qs, and Q2 = Qp

(1.2)

The preceding explanation indicates that Qs (or the unit skin friction, f, along the pile shaft) is developed at a much smaller pile displacement compared with the point resistance, Qp.

Figure 1.2 Load transfer mechanism for piles

8

This means if piles are designed to carry a working load equal to 1/3 to 1/2 the total failure load, there is every likelihood of the shaft resistance being fully mobilized at the working load. This has an important bearing on the design. The type of load-settlement curve for a pile depends on the relative strength values of the funding and underlying soil. Fig. 1.3 gives the types of failure (Kezdi, 1975). They are as follows:

Figure1.3 Types of failure of piles. (a) buckling in very weak surrounding soil; (b) general shear failure in the stro

9

1.5

DESIGN OF PILE FOUNDATIONS

The design of pile foundations can be performed by the following steps: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Calculate the total load acting on the pile. The loads to be used for bearing capacity analysis and the loads for the settlement analysis have to be identified. Sketch the soil profile and the soil properties up to a depth beyond the expected maximum length of piles. Locate the ground water level in the sketch. Determine the type and length of the pile with alternatives. Evaluate the single pile capacity. Establish the number and spacing of piles based on the pile capacity and loads to be supported. Establish the pile group and the number of piles in each group and size of the pile cap. Check the stresses transmitted to lower strata, particularly if there is a weak layer of soil below the bottom of the piles. Carry out the structural design of piles and pile cap. Carry out settlement analysis of the pile group. Check for uplift pressure and lateral load capacity of each pile group. Plan for proper pile load tests for verifying the computed values.

The details of vertical load on piles and design are presented in detail in Chapter 2. The input for the pile design such as length, diameter and pile capacity determination, and settlement analysis etc. are discussed in the following sections.

10

2

AXIAL CAPACITY OF A SINGLE VERTICAL PILE

Learning Outcomes: 

Estimate the allowable axial load capacity of single piles based on; static method, semi-empirical correlation with in-situ testing, field static loading test, and driving formulas.

2.1

INTRODUCTION

The load carrying capacity of piles is governed both by its structural strength and the supporting soil properties. Obviously, the smaller of the two values should be used for the design. Usually, the pile capacity based on soil properties governs the design except probably in timber piles. However, the methods for determination of these values are similar in all these types of piles. The capacity of piles based on structural strength can be obtained by multiplying the area of pile cross section with the allowable compressive strength of the material of the pile. The bearing capacity of groups of piles subjected to vertical or vertical and lateral loads depends upon the behavior of a single pile. The bearing capacity of a single pile depends upon: (1) Type, size and length of pile, (2) Type of soil, and (3) The method of installation. In order to be able to design a safe and economical pile foundation, we have to analyze the interactions between the pile and the soil, establish the modes of failure and estimate the settlements from soil deformation under dead load, service load etc. The design should comply with the following requirements. 1. It should ensure adequate safety against failure; the factor of safety used depends on the importance of the structure and on the reliability of the soil parameters and the loading systems used in the design. 2. The settlements should be compatible with adequate behavior of the superstructure to avoid impairing its efficiency. The pile capacity determination of supporting soil can be divided into three categories:    

Static (Analytic) methods, which are base on soil properties obtained from laboratory test. Semi-empirical correlation with in-situ test results (CPT and SPT). Full-scale static load test on prototype foundations Dynamic methods, which are based on the dynamic of pile or wave propagation

11

2.2

STATIC (ANALYTIC) METHODS

These methods are developed for piles and deep foundations using the soil properties in which they are founded. They assume equilibrium of the pile under the applied loads and resistance offered by the soil in terms of point bearing capacity and the friction and adhesion of the shaft. A single pile subjected to a vertical load and the mechanism of load transfer to the soil is shown in Figure 2.1. Thus the load is transferred to the soil partly as point bearing pressure at its base and partly as friction and or adhesion along the surface of the shaft. 2.2.1 Equation of Estimation Single Pile Capacity The ultimate load-carrying capacity Qu of a pile is given by the equation (2.1) where Qu Qp Qs

= ultimate load capacity of the pile = ultimate point/end bearing capacity = ultimate resistance due to adhesion and or friction along the shaft of pile

Numerous published studies cover the determination of the values of Qp and Qs, for example Meyerhof (1976), Berezantsev (1961), and Coyle and Castello (1981). Qu Steel Soil plug

Qs

(b) Open-Ended Pipe Pile Section

L = Lb

Steel Soil plug

Qp L = length of embankment Lb = length of embankment in bearing stratum

(c) H-Pile Section (Note: Ap = area of steel + soil plug)

Figure 2.1 Ultimate load-carrying capacity of pile

12

Point Bearing Capacity, Qp The ultimate bearing capacity of shallow foundations based on Terzaghi’s equations, given as: qu = 1.3cNc + qNq + 0.3DN qu = 1.3cNc + qNq + 0.4BN

(circular footing of diameter D) (square footing of width B=D)

(2.2a) (2.2b)

Similarly, the general bearing capacity equation for shallow foundations was given in (for vertical loading) as: (2.2c) Hence in general, the unit ultimate bearing capacity maybe express as (2.3) where Nc*, Nq*, and N* are the bearing capacity factors that include the necessary shape and depth factors. The ultimate resistance per unit area developed at the pile tip qp, may be expressed by an equation similar in form to Eq. 2.3, although the bearing capacity values will change. Hence, Eq. (2.3) can be rewrite as: (2.4) Because the width B of a pile is relatively small, the term BN* may be dropped from the right side of the preceding equation without introducing a serious error; thus, we have (2.5) Note that the term q has been replaced by σ 'V 0 in Eq. (2.4), to signify effective vertical stress. Thus, the point bearing of piles is (2.6) Ap = qp = c = σ 'V 0 =  = L = o =

area of the pile tip unit point bearing capacity of the pile cohesion of the soil supporting the pile tip = effective vertical stress at the level of pile tip L+ o unit weight of soil length of the pile embedded in soil surcharge load on the surface (if any) 13

Nc*, Nq*= bearing capacity factors = f ()  = angle of internal friction of soil Shaft Resistance, Qs Shaft or skin resistance due to the frictional or adhesion resistance of a pile may be written as (2.7) where As p L fs

= = = =

surface area of the shaft of embedded length of pile in soil perimeter of the pile section incremental pile length over which p and fs are taken to be constant unit skin resistance at any depth z

Ultimate Load, Qu The ultimate load-carrying capacity Qu of a pile is given in equation (2.1) can be rewritten as: (2.8a) (2.8b) (2.8c) The various methods for estimating Qp and Qs are discussed in the next several sections. It needs to be reemphasized that in the field, for full mobilization of the point resistance the pile tip must go through a displacement of 10 to 25% of the pile width (or diameter). Allowable Load, Qall After the total ultimate load-carrying capacity of a pile has been determined by summing the point bearing capacity and the frictional (or skin) resistance, a reasonable factor of safety should be used to obtain the total allowable load for each pile, or (2.9) Where: Qall = allowable load-carrying capacity for each pile Qu = ultimate load-carrying capacity for each pile FS = factor of safety The factor of safety generally used ranges from 2.5 to 4, depending on the uncertainties surrounding the calculation of ultimate load.

14

15

2.2.2 Capacity of Pile in Sands (-soils) When a pile is driven into loose sand its density is increased (Meyerhof, 1959), and the horizontal extent of the compacted zone has a width of about 6 to 8 times the pile diameter. However, in dense sand, pile driving decreases the relative density because of the dilatancy of the sand and the loosened sand along the shaft has a width of about 5 times the pile diameter (Kerisel, 1961). However, for design practice it is wise to use the value of  that should represent the in situ condition that existed before driving. With regard to bored and cast-in-situ piles, the soil gets loosened during boring. Tomlinson (1986) suggests that the  value for calculating both the base and skin resistance should represent the loose state. However, Poulos et al., (1980) suggests that for bored piles, the value of  be taken as  = 1 - 3, where 1 is the angle of internal angle prior to installation of the pile. A. Methods of estimating ultimate point bearing pile capacity, Qp Meyerhof Methods (1976) The point bearing capacity qp of a pile in sand generally increases with the depth of embedment in the bearing stratum and reaches a maximum value at an embedment ratio of Lb/B = (Lb/B)cr. Note that in a homogeneous soil Lb is equal to the actual embedment length of the pile L. However, where a pile has penetrated into a bearing stratum, Lb < L. Beyond the critical embedment ratio, (Lb/B)cr, the value of qp remains constant (qp= ql). That is, as shown in Figure 2.2 for the case of a homogeneous soil, Lb = L. Unit point resistance, qp (Lb/B)cr

qp=ql L/B=Lb/B Figure 2.2 Nature of variation of point unit resistance in homogeneous sand

For piles in sand, c = 0, and Eq. (2.6) simplifies to (2.10)

16

The variation of Nq* with soil friction angle  is shown in Figure 2.3. The interpolated values of Nq* for various friction angles are also given in Table 2.1. However, Qp should not exceed the limiting value Ap ql that is, (2.11) The limiting point resistance is (2.12)

Bearing capacity factors, Nc & Nq Angle of internal friction in degree

Figure 2.3 Bearing capacity factors and critical depth ratios L/d for driven piles (after Meyerhof, 1976)

Table 2.1 Interpolated Values of Nq*based on Meyerhof’s Theory Soil friction angle, 0 20 21 22 23 24 25 26 27 28

Nq*

0

Nq*

0

Nq*

12.4 13.8 15.5 17.9 21.4 26.0 29.5 34.0 39.7

29 30 31 32 33 34 35 36 37

46.5 56.7 68.2 81.0 96.0 115.0 143.0 168.0 194.0

38 39 40 41 42 43 44 45

231.0 276.0 346.0 420.0 525.0 650.0 780.0 930.0

17

Coyle and Castello’s Methods (1981) Coyle and Castello (1981) analyzed 24 large-scale field load tests of driven piles in sand. On the basis of the test results, they suggested that, in sand: (2.13) where q’ = effective vertical stress at the pile tip Nq* = bearing capacity factor The variation of Nq* with L/D and the soil friction angle are shown in Figure 2.4 Bearing capacity factor Nq* Bearing capacity factor Nq*

Relative depth L/D Figure 2.4 Nq* versus L/D (after Coyle and Castello, 1981

Berezantsev’s Methods (1961) & Brinch Hansen (1961) Berezantsev (1961) and Brinch Hansen (1961) in their individual research projects, take into account the critical depth ratio L/D for estimating the value of Qp. Reduction in the rate of increase in base resistance with increase in penetration depths is also shown by Berezantsev et al. The values of Nq related to  and depth/width ratios are shown in Figure 2.5. It will also be seen that the Berezantsev Nq values gave lower base resistance than those of other researchers.

18

In Figure 2.5 the Berezantsev factors are compared with those of Brinch Hansen. The latter have been adopted by the American Petroleum Institute.

Bearing capacity factor, Nq

L/D

Brinch Hansen

Berezantsev

Angle of shearing resistance,

Figure 2.5 Bearing capacity factors of Berezantsev et al. & Brinch Hansen

19

B. Methods of estimating pile ultimate shaft resistance, Qs The unit frictional resistance, fs, is hard to estimate. In making an estimation of fs, several important factors must be kept in mind: 1. 2.

For driven piles in sand, the vibration caused during pile driving helps densify the soil around the pile. The zone of sand densification may be as much as 2.5 times the pile diameter, in the sand surrounding the pile. It has been observed that the nature of variation of fs in the field is approximately as shown in Figure 2.6. The unit skin friction increases with depth more or less linearly to a depth of L’ and remains constant thereafter. The magnitude of the critical depth may be 15 to 20 pile diameters. A conservative estimate would be L’  15D Unit frictional resistance, fs

fs

L

L

(a)

Depth

(b)

Figure2.6 resistance for in sand sand Figure 11.11 Unit Unit frictional frictional resistance for piles piles in

3. 4.

At similar depths, the unit skin friction in loose sand is higher for a highdisplacement pile, compared with a low-displacement pile. At similar depths, bored, or jetted, piles will have a lower unit skin friction compared with driven piles.

According to Eq. (2.7), the frictional resistance (2.14) Taking into account the preceding factors, we can give the following approximate relationship for fs (see Figure 2.6): For z = 0 to L’

(2.15a)

And for z = L’ to L

(2.15b) 20

In these equations, K = earth pressure coefficient effective vertical stress at the depth under consideration σ´ 'o =  = soil-pile friction angle The value of K that will use in Eq. (2.15a) can be obtained as follows: Pile type Bored or jetted Low-displacement High-displacement

K  K0 = 1 - sin  K0 to 1.4 K0  K0 to 1.8 K0

Based on load test results in the field, Mansur and Hunter (1970) reported the following average value of K. Pile type H – piles Steel pipe piles Precast concrete piles

K 1.65 1.26 1.5

The value of ’ from various investigations appears to be in the range from 0.5 ’ to 0.8’. Coyle and Castello’s Methods (1981) In conjunction with the material previously presented, Coyle and Castello (1981) proposed that, (2.16) where average effective overburden pressure σ´ 'v0 =  = soil-pile friction angle = 0.8 The lateral earth pressure K, can be obtained from Figure 2.7, so (2.17)

21

22

Earth pressure coefficient K

Relative depth L/D Figure 2.7 Variation of Ks with L/D (after Coyle & Casttelo, 1981)

2.2.3 Capacity of Pile in Clay (c-soils) A. Methods of estimating point bearing pile ultimate capacity, Qp The long term (drained) point-bearing capacity of a pile in clay will be considerably larger than the un-drained capacity. However, the settlement required to mobilize the drained capacity would be far too large to be tolerated by most structures. In addition, the pile must have sufficient immediate load carrying capacity to prevent short term failure. For these reasons, it is customary to calculate point bearing capacity of piles in clay in terms of the un-drained shear strength conditions ( = 0) of the clay cu and a bearing factor Nc*. For depths relevant to piles, the appropriate value of Nc* is 9, so (2.18) Therefore the ultimate point bearing resistance can be given as: (2.19)

23

B. Methods of estimating pile ultimate shaft resistance, Qs Estimating the frictional (or skin) resistance of piles in clay is almost as difficult a task as estimating that in sand (see Section 2.2.2-B), due to the presence of several variables that cannot easily be quantified. Several methods for obtaining the unit frictional resistance of piles are described in the literature. We examine some of them next.  Method This method, proposed by Vijayvergiya and Focht (1972), is based on the assumption that the displacement of soil caused by pile driving results in a passive lateral pressure at any depth and that the average unit skin resistance is (2.20) where σ´ 'v0 = c´ u =

mean effective vertical stress for entire embedment length mean undrained shear strength ( = 0)

The value of  changes with the depth of penetration of the pile can be obtained from Figure 2.8 or Table 2.2. Figure 2.8 & Table 2.2 Variation of  with pile embedment length, L

Depth of penetration, m

Value of 

fs =(0’+2cu)

Embedment length, L (m) 0 5 10 15 20 25 30 35 40 50 60 70 80 90

 0.5 0.336 0.245 0.200 0.173 0.150 0.136 0.132 0.127 0.118 0.113 0.110 0.110 0.110

The total frictional or skin Qs therefore may be calculated as: (2.21) 24

Figure 2.9 gives an illustration how to calculate the mean value of effective vertical stress and un-drained shear strength for entire embedment length by weighted area method.

L1

cu1 cu2

cu3

Figure 2.9 Application of , method in layered soil

Hence, the mean value of undrained shear strength cu = (cu1L1 + cu2L2 + cu3L3)/L and the mean effective stress is 0’ = (A1 + A2 +A3)/L. It should be noted that  method has been found very useful for the design of heavily loaded pipe piles for offshore structures.  Method According to the  method, the unit skin resistance in clayey soils can be represented by the equation (2.22) where  = empirical adhesion factor. The approximate variation of the value of  is shown in Table 2.3. It is important to realize that the values of  may vary somewhat, since  is actually a function of vertical effective stress and the undrained cohesion. Sladen (1992) has shown that (2.23) where average vertical effective stress σ´ 'v0 = C = 0.4 to 0.5 for bored piles, and  0.5 for driven piles The ultimate side resistance can thus be given as (2.24) 25

Table 2.3 Variation of  (interpolated values based on Terzaghi, Peck and Mesri, 1996) cu (kN/m2) ≤10 20 30 40 60 80 100 120 140 160 180 200 240 280

 1.00 0.92 0.82 0.74 0.62 0.54 0.48 0.42 0.40 0.38 0.36 0.35 0.34 0.34

 Method When piles are driven into saturated clays, the pore water pressure will be buildup in the soil around the piles. The excess pore water pressure in normally consolidated clays may be four to six times. However, within a month or so, this pressure gradually dissipates. Hence, the unit frictional resistance for the pile can be determined on the basis of the effective stress parameters of the clay in a remolded state. Thus, at any depth, (2.25) where

σ´ 'v0  = R = K =

= vertical effective stress K tan R drained friction angle of remolded clay earth pressure coefficient

(2.26)

Conservatively, the magnitude of K is the pressure coefficient at rest, or For normally consolidated clays

:

(2.27)

For over-consolidated clays

:

(2.28)

where, OCR is over-consolidated ratio of clay. Combining equations (2.25), (2.26), (2.27), and (2.27), yields

For normally consolidated clays For over-consolidated clays :

:

(2.29) (2.30) 26

With the value of fs determined, the ultimate shaft or skin resistance may be evaluated as (2.31) 2.2.4 Capacity of pile in sandy– clayey soils (c- -soils) Where piles are installed in sandy clays or clayey sands which are sufficiently permeable to allow dissipation of excess pore pressure caused by application of load to the pile, the base and shaft resistance can be calculated for the case of drained loading using Eq. 2.6. The angle of shearing resistance used for obtaining the bearing capacity factor Nq should be the effective angle ’, obtained from unconsolidated drained triaxial compression tests. In a uniform soil deposit, Eq. 2.6 gives a linear relationship for the increase of base resistance with depth. Therefore, the base resistance should not exceed the peak value of 11 MN/m2 unless pile loading tests show that higher ultimate values can be obtained. The effective overburden pressure, σ 'v0 , in Eq. 2.6 is the total overburden pressure minus the pore water pressure at the pile toe level. It is important to distinguish between uniform c- soils and layered c and  soils, as sometimes the layering is not detected in a poorly executed soil investigation. (2.32)

27

2.3

SEMI-EMPIRICAL CORRELATION WITH IN-SITU TEST RESULTS

Today, due to its simplicity, many engineers were interested to estimate pile load capacity based on the results of in-situ tests. In-situ methods are based on cone penetration tests (CPT), standard penetration tests (SPT), or other in-situ tests. In principle, these methods are applicable to all soil types, but have been most often applied to sandy soils because they are difficult to sample and thus are not well suited to laboratory testing. 2.3.1

Bearing capacity of piles based on Static Cone Penetration (CPT)

The Cone Penetration Test is the measurement of the resistance at tip and friction along sides when an instrumented cone (electrical or mechanical) is pushed into the ground. The cone penetration test may be considered as a small scale pile load test. As such the results of this test yield the necessary parameters for the design of piles subjected to vertical load. Various methods for using CPT results to predict vertical pile capacity have been proposed. The following methods will be discussed: 1. Vander Veen's method. 2. Schmertmann's method. Vander Veen's Method for Piles in Cohesionless Soils In the Vander Veen et al., (1957) method, the ultimate end-bearing resistance of a pile is taken equal to the point resistance of the cone. To allow for the variation of cone resistance which normally occurs, the method considers average cone resistance over a depth equal to three times the diameter of the pile above the pile point level and one pile diameter below point level. Experience has shown that if a safety factor of 2.5 is applied to the ultimate end resistance as determined from cone resistance, the pile is unlikely to settle more than 15 mm under the working load (Tomlinson, 1986). The equations for ultimate bearing capacity and allowable load may be written as, pile point resistance,

qp = q´

ultimate point capacity,

Qp = Ap q´

where, q´ pile toe.

c

c

(cone) c

(2.33) (2.34)

= average cone resistance over a depth of D below and 3D above

The skin friction on the pile shaft in cohesionless soils is obtained from the relationships established by Meyerhof (1956) as follows. For displacement piles, the ultimate skin friction, fs, is given by (2.35)

28

and for H-section piles, the ultimate limiting skin friction is given by (2.36) where, q´ c = average cone resistance in kg/cm2 over the length of the pile shaft under consideration. Meyerhof states that for straight sided displacement piles, the ultimate unit skin friction, fs, has a maximum value of 107 kPa and for H-sections, a maximum of 54 kPa (calculated on all faces of flanges and web). The ultimate skin load is Qs = As fs

(2.37)

The ultimate load capacity of a pile is Qu = Qp + Qs

(2.38)

The allowable load is (2.39)

Schmertmann's Method for Cohesionless and Cohesive Soils Nottingham and Schmertmann (1975), and Schmertmann (1978) recommend one procedure for all types of soil for computing the point bearing capacity of piles. However, for computing side friction, Schmertmann gives two different approaches, one for sand and one for clay soils. Point Bearing Capacity Qp in All Types of Soil The procedure used in this case involves determining a representative cone point penetration value, q´ c1, within a depth between 0.7 to 4D below the tip level of the pile and 8D above the tip level as shown in Fig. 2.10 (a) and (b). The value of unit end-bearing, qp may be expressed as (2.40) where q´ c1 = average cone resistance below the tip of the pile over a depth which may vary between 0.7D and 4D, where D = diameter of pile, q´ c2 = average of the envelope of minimum cone resistance recorded above the pile tip to a height of 8D. = correction factors for gravel content or consolidation (using Table 2.4)

29

Table 2.4 Correction factor  for use in Eq. 2.40



Soil Condition Sand with OCR = 1 Very gravelly course sand; and with OCR = 2 to 4 Fine gravel; sand with OCR = 6 to 10

1.00 0.67 0.50

The method of computing q´ c1 and q´ c2 with respect to a typical qc-plot shown in Fig. 2.10 (a) and (b) is explained below. Case 1: When the cone point resistance qc below the tip of a pile is lower or decrease in resistance between 0.7D and 4D, the lowest value in this range should be selected for qc1 calculation. Case 2: When the cone resistance qc below the pile tip is greater or increases continuously to a depth of 4D, the average value of qc1 is obtained only over a depth of 0.7D. To obtain qc2 the diagram is followed in an upward direction and the envelope is drawn only over those values which are decreasing or remain constant at the value at the pile toe. Minor peak depressions are ignored provided that they do not represent clay bands; values of qc higher than 30 MN/m2 are disregarded over the 4D – 8D range. Schmertmann suggested an upper limit of 15 MPa for the unit point bearing capacity, qp.

D

D

Depth

z =8D

8D

4D

0.7D 4D

below pile tip greater than that at pile tip within depth 4D qc below pile tip lower than that at pileqctip within depth 4D

Figure 2.10 Pile capacity by use of CPT value - Schmertmann’s method

30

Ultimate skin friction load Qs in cohesionless (sandy and gravelly) soils For pile in sand the shaft resistance Qs is obtained by: (2.41) Coduto (1994) rewrite the Equation 2.41 in two parts, as For z < 8D:

(2.42)

For z  8D:

(2.43)

where fs fsc z s D L

= = = = = =

unit skin friction resistance local side (sleeve) friction from CPT test depth from ground surface to midpoint of segment friction correction factor for sand (from Table 2.5) pile width or diameter penetration of pile below ground surface

Note that s is based on the overall L/D ratio of the pile. Do not assign a difference value to each pile segment. Schmertmann suggested a limit of 120 kPa on fs. Table 2.5 Frictional resistance modification factor, s (for Mechanical CPT) L/D 5 10 15 20  25

Timber 1.49 0.85 0.58 0.49 0.48

Concrete 0.78 0.59 0.50 0.44 0.44

Steel 1.19 0.68 0.46 0.39 0.38

Note: - Back calculated from Schmertmann graph - For Electrical CPT = 2 x Mech. CPT

Ultimate skin friction load Qs in cohesion (clayey) soils According to Schmertmann’s method, the unit skin friction of the pile fs is given by: fs =c fsc

(2.44)

31

where: c is a reduction factor, which varies from 0.2 to 1.25 for clayey soil, and fsc is the sleeve friction. Figure 2.11 depicts the variation of c with fsc for different pile types in clay.

Penetrometer to pile friction ratio - c

Concrete & timber piles

Steel piles

Penetrometer sleeve friction fsc (kg/cm2) Figure 2.11 Penetration design curves for pile side friction in clay

Example 2.1 The CPT (mechanical cone) data shown on Table 2.5 represent the soil conditions at a proposed construction site. Based on Figure 2.12, the test results indicate that the upper 4.5 m is sand, and it is underlain by 3.4 m of clay, then additional sand. Compute the allowable load capacity of on 457 mm square, 10.36 m long prestressed concrete pile is to be driven into the soil. Solution Evaluation of CPT data shows that the lower qc value in a range of 4D below pile tip was found at the level 11.00 meter below the surface, so use Case 1. Calculate qc1 and qc2 use a minimum path rule as follows: qc1 = 1/8 {115 + 105 + 108 + 5(99)} = 102.9 kg/cm2 The upper limit of qc2 averaging located at the level of L-8D = 10.36 – (8 x 0.457) = 6.70 meter qc2 = 1/18[5(99) + 2(93) + 85 + 3(70) + 50 + 0.6{8 + 2(7) + 3(6)}] = 58.3 kg/cm2 (Note: the factor 0.6 in this equation reflects the use of the mechanical cone in a cohesive soil)

32

Table 2.6 Cone penetration numerical data in Example 2.1

33

Depth (m)

qc (kg/cm2)

fsc

FR

Depth (m)

qc (kg/cm2)

fsc

FR

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.60 3.80 4.00 4.20 4.40 4.60 4.80 5.00 5.20 5.40 5.60 5.80 6.00 6.20 6.40 6.60 6.80

0.00 15.00 23.00 25.00 24.00 29.00 27.00 20.00 23.00 33.00 30.00 25.00 30.00 30.00 25.00 26.00 32.00 25.00 22.00 30.00 35.00 37.00 34.00 15.00 9.00 5.00 4.00 3.50 4.00 4.50 6.50 5.50 5.50 5.00 7.00

0.00 0.32 0.44 0.40 0.41 0.52 0.43 0.30 0.37 0.56 0.40 0.43 0.46 0.21 0.32 0.43 0.45 0.50 0.52 0.53 0.54 0.52 0.54 0.52 0.49 0.26 0.25 0.21 0.23 0.28 0.39 0.30 0.26 0.30 0.43

0.0 2.1 1.9 1.6 1.7 1.8 1.6 1.5 1.6 1.7 1.3 1.7 1.5 0.7 1.3 1.7 1.4 2.0 2.4 1.8 1.6 1.4 1.6 3.5 5.4 5.2 6.3 6.0 5.8 6.2 6.0 5.5 4.7 6.0 6.1

7.00 7.20 7.40 7.60 7.80 8.00 8.20 8.40 8.60 8.80 9.00 9.20 9.40 9.60 9.80 10.00 10.20 10.40 10.60 10.80 11.00 11.20 11.40 11.60 11.80 12.00 12.20 12.40 12.60 12.80 13.00 13.20 13.40 13.60 13.80

7.00 6.00 8.50 7.00 8.00 50.00 75.00 72.00 70.00 85.00 98.00 93.00 103.00 110.00 107.00 102.00 109.00 115.00 105.00 108.00 99.00 108.00 114.00 117.00 111.00 105.00 96.00 86.00 89.00 85.00 115.00 165.00 180.00 173.00 200.00

0.44 0.35 0.51 0.38 0.50 1.20 0.90 0.72 0.91 1.27 0.88 1.02 1.03 1.32 1.17 1.33 1.20 1.15 1.57 1.19 1.39 1.84 1.60 1.52 1.22 1.36 0.96 1.20 1.16 1.36 1.95 1.98 1.62 1.73 -

6.3 5.8 6.0 5.4 6.3 2.4 1.2 1.0 1.3 1.5 0.9 1.1 1.0 1.2 1.1 1.3 1.1 1.0 1.5 1.1 1.4 1.7 1.4 1.3 1.1 1.3 1.0 1.4 1.3 1.6 1.7 1.2 0.9 1.0 -

No data are available to define the degree of over-consolidation. However, in a natural soil deposit at a depth of 35 ft (11.55 m), the ORC might be in the range of 2 to 3. Therefore from Table 2.4,  = 0.67 seems to be a reasonable value for design.

The area of pile tip Ap = 45.7 x 45.7 = 2088.5 cm2

34

Cone resistance qc kg/cm2

Friction ratio FR

D

Sand

Clay 6.7

8D

10.3

Sand 4D

12.1

Figure 2.12 CPT test curve in Example 2.1

Skin Friction Qs Layer depth (m)

z (m)

fsc (kg/cm2)

0.00 - 3.66 3.66 - 4.50 4.50 - 7.90 7.90 - 10.36

1.83 4.08 6.20 9.13

0.42 0.53 0.36 1.08

αs

αc

0.44 0.44 0.85 0.44

fs (kg/cm2) 0.092 0.233 0.306 0.475

As (cm2)

Qs (kg)

66,905 15,355 226,672 44,969

6,182 3,581 69,362 21,369

Total

100,494

Note: The depth of the first layer is equal to 8D = 8 x 0.457m = 3.66 m Hence,

35

2.3.2

Bearing capacity of piles based on Standard Penetration test (SPT)

The Standard Penetration Test (SPT) is a measure of resistance to penetration when a drive split spoon sampler is driven in with a hammer. The number of hammer blows to drive the split-spoon sampler to penetrate 300mm is counted, as “N” SPT value. When SPT values are used for pile design, the field blow counts need to be corrected to reflect the effect of testing procedure and the influence of overburden pressure to the sample depth on actual soil properties. Meyerhof’s method(1976) On the basis of field observations, Meyerhof (1976) also suggested that the ultimate point resistance qp in a homogeneous granular soil (L = Lb) may be obtained from standard penetration numbers as (2.45) where N60 = the average value of the standard penetration number measured 10D above and 4D below the pile point The average unit frictional resistance, ´f s , for high-displacement driven piles may be obtained from average standard penetration resistance value as ´f s=2 ( N ´ 60 ) kN /m2

(2.46)

However for low-displacement driven piles ´f s=( N´ 60) kN / m2 Where:

( N´ 60 )

(2.47)

= average value of standard penetration resistance

Briaud et al. (1985) Briaud et al. (1985) suggested the following correlation for qp in granular soil with the standard penetration resistance N60, as qp = 1970 (N60)0.36

kN/m2

(2.48)

For the average unit frictional resistance, he suggested that ´f =22.4 ( N ´ 60) 0.29 s

kN/m2

(2.49)

36

Example 2.2 Consider a concrete pile that is 300 mm x 300 mm in cross section in sand. The pile is 15.00 meter long. The following are the variation of N60 with depth. Calculate the allowable load-carrying capacity of the pile based on Meyerhof’s and Briaud’s method. Use a factor of safety, FS=3.

Depth below ground surface (m)

1.0 3.0 4.0 6.0 7.5 9.0 10.5 12.0 13.5 15.0 16.5 18.0 19.0 21.0

N60

6 10 11 12 14 18 11 17 20 28 29 32 30 27

Solution Based on Meyerhof’s method The tip of the pile is 15.0 meter below the ground surface, and the pile diameter, D = 300 mm. The average of N60, measured 10D = 10 x 0.3 = 3.0 meter above and 4D = 4 x 0.3 = 1.2 meter below the tip of the pile is

From equation (2.11)

Thus, select:

Qp = 864 kN 37

From equation (2.11) ´ ) =( 2 )( 15 ) =30 kN / m2 f´s=2 ( N 60

Qs= pL ´f s=( 4 x 0.300 )( 15.0 ) ( 30 )=540 kN

Meyerhof’s method: Based on Briaud’s method From equation (2.11) Qp = Ap qp = Ap{1970 (N60)0.36}=(0.300 x0.300){(1970)(24)0.36} = 575.4 kN From equation (2.11) ´f s=22.4 ( N ´ 60) 0.29=22.4 ( 15 )0.29 =49.13 kN / m2 Qs= pL ´f s=( 4 x 0.300 )( 15.0 ) ( 49.13 ) =88 4 kN

Briaud’s method:

So the allowable pile capacity may be taken to be about 470 kN.

38

2.4

FULL-SCALE STATIC LOAD TEST

The most precise way to determine the ultimate downward and upward capacities for pile foundation is to build a full-size prototype foundation at the proposed site, and slowly load it to a certain load or until failure. This method is known as a (conventional) static load test. However, static load test are much more expensive and time-consuming, and thus must be used more selective. The objective of a static load test is to develop a load-settlement curve or, in the case of uplift tests, a load-leave curve. This curve is then used to determine the ultimate load capacity of that pile. The purposes of a pile load test are:     

To determine the load capacity of a single pile or a pile group, especially when the design requires methods that are outside of accepted practice. To determine the settlement of a single pile at working loads. To verify estimated load capacity. To obtain information on load transfer in skin friction and in end bearing (for research requirement). To satisfy regulatory agencies.

Load tests may be made either on a single pile or a group of piles, but tests on a pile group are very costly and may be undertaken only in very important projects. Pile load tests on a single pile or a group of piles are conducted for the determination of, (1) vertical load bearing capacity, (2) uplift load capacity, and (3) lateral load capacity. Figure 2.13(a) and (b), shows a schematic diagram of the pile load arrangement for testing axial compression in the field, known as kentledge system and tension pile system respectively. Kentledge is still commonly used in Indonesia; this involves the use of dead weights supported by a deck of steel beams sitting on crib pads. The area of the crib should be sufficient to avoid bearing failure or excessive settlement of the ground. It is recommended that the crib pads are placed at least 1.3 m from the edge of the test pile to minimize interaction effects (ICE, 1988). If the separation distance is less than 1.3 m, the surcharge effect from the kentledge should be determined and allowed for in the interpretation of the loading test results. Tension piles used to provide reaction for the applied load should be located as far as practicable from the test pile to minimize interaction effects. A minimum centre-to-centre spacing of 2.0 m or three times test pile diameters, whichever is greater, between the test pile and tension piles is recommended. If the centre spacing between piles is less than three pile diameters, there may be significant pile interaction and the observed settlement of the test pile will be less than what should have been. A minimum of three reactions piles should be used to prevent instability of the set up during pile loading tests.

39

Figure 2.13a – Typical Arrangement of a Compression Test using Kentledge

Stiffener s Dial gauge

Load cel

Tension member

Hydraulic jack

Test pile

Reaction pile

Figure 2.13b – Typical Arrangement of a Compression Test using Tension Piles 40

Load tests may be carried out either on; (1) a working pile or (2) a test pile. A working pile (known as un-failed test) is a pile driven or cast-in-situ along with the other piles to carry the loads from the superstructure. The maximum test load on such piles should not exceed one and a half times the design load. A test pile (known as failed test) is a pile which does not carry the loads coming from the structure. The maximum load that can be put on such piles may be about 21/2 times the design load or the load imposed must be such as to give a total settlement not less than one-tenth the pile diameter. Procedure The load is applied to the pile by a hydraulic jack. Step loads are applied to the pile, and sufficient time is allowed to elapse after each load so that a small amount of settlement occurs. The settlement of the pile is measured by dial gauges. Most building codes require that each step load be about one-fourth of the proposed working load. The load test should be carried out to at least a total load of two times the proposed working load. After the desired pile load is reached, the pile is gradually unloaded. There are two categories of static load test: (1) Controlled stress tests (also known as Maintained Load or ML test), and (2) Controlled strain tests ML tests uses predetermined loads (the independent variable) and measured the settlement (the dependent variable), while the other one uses an opposite approach. The disturbance of surrounding soils can generate excess pore water pressures during driving process will temporarily change the ultimate load capacity. Therefore, it is best to allow time for these excess pore water pressures to dissipate before conducting the test. This typically requires a delay of at least 2 days in sands and at least 30 to 60 days in clays. Interpretation of Pile Load Test result Figure 2.14 shows a typical load–settlement diagram obtained from field loading and unloading. Once we have obtained the load settlement curve, it is necessary to determine the ultimate load capacity, which means we must define where “failure” occurs. For piles in soft clay this is relatively straightforward, because its load-settlement curve has a distinct plunge, as shown by curve 1 in Figure 2.14b, and the ultimate capacity is the load that corresponds to this plunge. However piles in sands, intermediate soils, and stiff clays have slope with no clear point of failure, as shown by curve 2.

41

(a) (a)

(b) (b)

Figure 2.14 (a) Plot of load against total settlement; (b) plot of load against net settlement

For any load Q, the net pile settlement can be calculated as follows: When Q = Q1, ---------- Net settlement, Snet(1) = St(1) – Se(1) When Q = Q2, ---------- Net settlement, Snet(2) = St(2) – Se(2) Where: Snet St Se

= net settlement = total settlement = elastic settlement of the pile it self

These values of Q can be plotted in a graph against the corresponding net settlement Snet, as shown in Figure 2.14. The ultimate load of the pile can then be determined from the graph. Pile settlement may increase with load to a certain point, beyond which the load-settlement curve becomes vertical. The load corresponding to the point where the curve of Q versus becomes vertical is the ultimate load, for the pile; it is shown by curve 1 in Figure 2.14. In many cases, the latter stage of the load–settlement curve is almost linear, showing a large degree of settlement for a small increment of load; this is shown by curve 2 in the figure. The ultimate load, for such a case is determined from the point of the curve of Q versus where this steep linear portion starts. One of the methods to obtain the ultimate load Qu from the load-settlement plot is that proposed by Davisson (1973). Davisson’s method is used more often in the field and is described here. Referring to Figure 2.15, the ultimate load occurs at a settlement level (su) of (2.50) where 42

Qu D Dr L Ap Ep

= = = = = =

ultimate load capacity (kN) pile diameter (mm) reference pile diameter or width (=300mm) pile length (mm) area of pile cross section (mm2) Young’s modulus of pile material (kN/mm2) Qu Load, Q (kN)

0.12Dr + 0.1(D/Dr)

QuL/AE

Eq. (11.22)

Settlement, s (mm)

Figure 2.15 Davisson’s method for determination of Qu

Example 2.3 A 305-mm steel pipe pile with a length of 16.00 meter was subjected to a pile load test. The results of the test were plotted and the load-settlement curve is shown in Figure 2.16 The local building code states that the allowable pile load is taken as one-half of that load which produces a net settlement of not more than 0.025 mm/kN, but in no case more than 20 mm. Required: Allowable pile load Solution: Net settlement = Total settlement – Net (rebound) settlement

43

0

500

1000

1500

0.00

10.00

20.00

30.00

Settlement (mm)

40.00

50.00

60.00

70.00

80.00

Load (kN)

Test Load (kN) 0 500 1000 1500 2000 2500

Settlement (mm) Loading Unloading 0 55.90 5,10 60.70 11,40 64.50 19,30 67.10 31,80 69.30 71,10 71.10

Rebound (Se) (mm 55.90-55.90 = 0 60.70–55.90 = 4.80 64.50–55.90 = 8.60 67.10–55.90 = 11.20 69.30–55.90 = 13.40 71.10–55.90 = 15.20

Net Settlement (mm)

Buliding Code

0 5.10-4.80 = 0.30 11.40-8.60= 2.80 19.30-11.20= 8.10 31.80-13.40=18.40 71.10-15.20 =55.90

12.5 25.0/20.0 37.5/20.0 50.0/20.0 62,5/20.0

St Since a test load of 2000 kN produces a net settlement of 18.40 mm and the maximum allowable settlement is 20 mm. Allowable load = 2000/2 = 1000 kN

44

2000

Example 2.4 The load-settlement data shown in Figure 2.15 were obtained from a full-scale static load test on a 406-mm square, 20.0-m long concrete pile (Ep = 30 x 106 kN/m2) embedded in sand. Use Davisson’s method to compute the ultimate load Qu optimum downward load capacity. Solution Dr = 300 mm, D =406 mm, L = 20 m = 20,000 mm, Ap = 406 mm x 406 mm = 164,836 mm2, and Ep = 30 kN/mm2. Hence,

= 3.6 + 0.135 + 0.004Qu = 3.735 + 0.004 Qu Plotting the line su (mm) = 3.375 + 0.004 Qu on the load-displacement curve produce Qu = 1640 kN

Figure 2.16 Static load test data for Example 2.2

45

2.5

DYNAMIC PILE DRIVING FORMULAE AND WAVE EQUATION

These formulae have been developed for driven piles (precast type) using dynamic principles. A drop/falling hammer is used to drive the pile to the desired depth or until refusal. It is assumed that the kinetic energy of the hammer falling from a height is used partly to drive the pile into the soil and partly as a loss due to damping and so on. Using the dynamic penetration of the pile, several empirical formulae have been developed by various professional bodies and manufacturers (Teng, 1964; Bowles, 1996; Das, 2007). Since most of these formulae are empirical and involve several parameters which are difficult to quantify, the evaluation of pile capacity may have a large range of variation, thus their utility may be limited. These dynamic equations are widely used in the field to determine whether a pile has reached a satisfactory bearing value at the predetermined depth. One of the earliest such equations commonly referred to as the Engineering News Record (ENR) formula is derived from the work-energy theory. That is,

(2.51) where WR h S C

= = = =

weight of the ram height of fall of the ram, in mm pile penetration per hammer blow, in mm empirical constant = 2.54 for steam hammers and 25.4 for drop hammers

The above formula is obtained using a factor of safety = 6. The pile penetration, S, is usually based on the average value obtained from the last few driving blows. In the equation’s original form, the following values of C were recommended: Note that, for single and double acting hammers, the term WRh can be replaced by EHE where E is the efficiency of the hammer and HE is the rated energy of the hammer. Thus, (2.52) Similarly there are other formulae that are also used for driven piles such as Pacific Coast Uniform Building Code (PCUBC) formula, Janbu’s formula, Danish formula, AASHTO formula, Hiley’s formula, and many others, some of them are tabulated in Table 2.11.

46

Table 2.7 Pile driving Formulas Name

Formula

47

The maximum stress developed on a pile during the driving operation can be estimated from the pile-driving formulas presented in Table 2.16. To illustrate, we use the modified EN formula: (2.53) In this equation, S is the average penetration per hammer blow, which can also be expressed as, (2.54)

(S is in mm, and N = number of hammer blows per 25.4 mm of penetration) Hence, (2.55) Different values of N may be assumed for a given hammer and pile, and Qu may be calculated. The driving stress Qu/Ap can then be calculated for each value of N. This procedure can be demonstrated with a set of numerical values. Suppose that a prestressed concrete pile 24.4 m in length has to be driven by a hammer. The pile sides measure 254 mm. From Table 2.3a, for this pile, Ap = 654 x 10-4 m2 The weight of the pile is ApLc = (654 x 10-4)(24.4m)(23.58 kN/m2) = 37.1 kN If the weight of the pile cap is 2.98 kN, then Wp = 37.1 + 2.98 = 40.08 kN For the hammer, let Rated energy = 26.03 kN-m = HE =WRh Weight of ram = 22.24 kN Assume the the hammer efficiency is 0.85 and that n = 0.35. Subtituting these values into Equation (2.55) yields

48

Now the following table can be prepared,

N

Qu (kN)

Ap (m2)

0 2 4 6 8 10 12 20

0 632 1084 1423 1687 1898 2070 2530

654 x 10-4 654 x 10-4 654 x 10-4 654 x 10-4 654 x 10-4 654 x 10-4 654 x 10-4 654 x 10-4

Qu/Ap (MN/m2) 0 9.79 16.80 22.06 26.16 29.43 32.12 39.22

Both the number of hammer blows N and the stress can be plotted in a graph, as shown in Figure 2.3.

Qu /Ap (MN/m2) Number of blows/25.4 mm

Figure 2.17 Plot of stress versus blows/25.4 mm

If such a curve is prepared, the number of blows per inch (25.4mm) of pile penetration corresponding to the allowable pile-driving stress can easily be determined. Actual driving stresses in wooden piles are limited to about 0.7 fu. Similarly, for concrete and steel piles, driving stresses are limited to about 0.6 f 'c and 0.85fy respectively. In most cases, wooden piles are driven with a hammer energy of less than 60 kN-m. Driving resistances are limited mostly to 4 to 5 blows per inch of pile penetration. For concrete and steel piles, the usual values of N are 6 to 8 and 12 to 14, respectively. 49

3

AXIAL VERTICAL CAPACITY OF A PILE GROUP

Learning Outcomes: 

Estimate the axial load capacity of pile groups by using of individual and block action concept.

3.1 INTRODUCTION In most cases, piles are used in groups, as shown in Figure 3.1, to transmit the structural load to the soil. A pile cap where constructed over group piles can be in contact with the ground, as in most cases (Figure 3.1a), or well above the ground, as in the case of offshore platforms (Figure 3.1b). PILE CAP

PILE CAP GWL

L L

s

s Lb

Lg Bg

(a)

Number of piles in group = n = n1 x n2 (Note: Lg  Bg Lg = (n1 - 1) s + 2 (D/2) Bg = (n2 - 1) s + 2 (D/2) (b) Figure 3.1 Group Piles

Piles in group are uses instead of single piles because:  A single pile usually does not have enough capacity.  Low degree of precession of pile installation will resulting eccentricities would generate unwanted moment and deflection in pile.

50



Multiple piles provide redundancy, and thus can continue to support the structure even if one pile is defective  The lateral soil compression during pile driving is greater, so the side friction capacity is greater than for a single pile. Each group of piles is connected with a pile cap, which is a reinforced concrete member that similar to a spread footing. It functions are to distribute the structural loads to the piles, and to tie the piles together so they act as a unit. The design of pile caps varies with the number of piles and the structural loads. Figure 3.2 shows typical pile cap layouts.

Figure 3.2 Typical pile arrangements in groups

Determining the load-bearing capacity of group piles is extremely complicated and has not yet been fully resolved. When the piles are placed close to each other, a reasonable assumption is that the stresses transmitted by the piles to the soil will overlap, as shown in Figure 3.3(b), and (d) reducing the load-bearing capacity of the piles. Ideally, the piles in a group should be spaced so that the load-bearing capacity of the group is not less than the sum of the bearing capacity of the individual piles.

51

If the overlap is large, the soil may fail in shear or settlement will be very large. Though the overlapping zone of stresses obviously decreases with increased pile spacing, it may not be feasible since the pile cap size becomes too large and hence expensive. In practice, the minimum center to center pile spacing, s, is 2.5D and, in ordinary situations, is actually about 3 to 3.5D.

Qg

Q1

Group of three piles

Single pile Q1

Qg

(c) Single pile

Group of two piles

Figure 3.3 Stress in group pile

In fine-grained soils, the outer piles tend to carry more loads than the piles in the center of the group. In coarse-grained soils, the piles in the center take more loads than the outer piles. The ratio of the load capacity of a pile group Qg(u) to the total load capacity of the piles acting as individual piles (n Qu), is called the efficiency factor  that is, (3.1)

52

Where  = group Qg(u) = Qu = n =

effisiency factor ultimate load capacity of the pile group ultimate load capacity of a single pile number of piles in the group

53

3.2

LOAD CAPACITY OF GROUP PILES IN SANDS

Depending on their spacing within the group, the piles may act in one of two ways: (1) as a block, with dimensions Lg x Bg x L, or (2) as individual piles. If the piles as a block, the frictional capacity is fav pgL  Qg(u). Where pg = perimeter of the cross section of block = 2 (n1 + n2 – 2)s + 4D, and fav = average unit frictional resistance. The key assumption in single pile failure mode is that each pile mobilizes its full load capacity. Thus, the group load capacity is Qg(u)= nQu, hence,

(3.2a)

(3.2b)

Hence,

(3.3)

From Eq. (3.11), if the center-to-center spacing d is large enough  > 1, In that case, the piles will behave as individual piles. Thus, in practice, if  < 1, then

Qg(u) =  n Qu

(3.4)

and if  ≥ 1, then

Qg(u) = n Qu

(3.5)

There are several other equations like Eq. (3.12) for calculating the group efficiency of friction piles. It is important, however, to recognize that relationships such as Eq. (3.13) are simplistic and should not be used. In fact, in a group pile, the magnitude of depends on the location of the pile in the group (ex., Figure 3.8). Converse-Labare equation (3.6)

Los Angeles Group Action equation (3.7) 54

3.3

LOAD CAPACITY OF GROUP PILES IN SATURATED CLAY

As shown in Figure 3.4, the ultimate load bearing capacity of group piles in saturated clay can estimate in the following manner: Qg(u) Pile cap 2(Lg +Bg)cu(s)L cu(s)1 L

cu(s)2

cu(s)3

Lg Bg cu(p) LNc* Bg Lg Figure 3.4 Load capacities of group piles in clay

Step 1. Determine

Qg(u) = n1n2(Qp+Qs) Qp = Ap (9cu(p))

(3.8) (3.9)

cu(p) = undrained cohesion of the clay at the pile tip Qs =  p cu(s) L So,

Qg(u)

= n1n2Ap (9cu(p))+  p cu(s) L

(3.10) (3.11)

Step 2. Determining the ultimate capacity by assuming that the piles in the group act as block with dimension Lg x Bg x L. The skin resistance of the block is, Qsg =  pg cu(s) L = 2(Lg + Bg)cu(s)L

(3.12)

55

Calculate the point bearing capacity Qpg=Apgqp = Apgcu(p)Nc* = (LgBg)cu(p) Nc*

(3.13)

Obtain the value of the bearing capacity factor Nc* from Figure 11.11. Thus, the ultimate load is Qg(u) = LgBgcu(p) Nc*+ 2(Lg + Bg)cu(s)L

(3.14)

Step 3. Compare the values obtained from Eqs. (3.711) and (3.814), the lower of the two values is Qg(u). 3.4

LOAD DISTRIBUTION BETWEEN PILES

Knowledge of the load distribution in a pile group is necessary in assessing the file of movement and the forces in the pile cap. In the previous sections, the behavior of a single vertical or groups of vertical piles subjected to central (or axial) vertical loads or lateral loads were discussed. In many situations such as under bridges and offshore structures, the pile groups may be subjected to simultaneous central vertical loads, lateral loads and moments. Such loads may either be resisted by a group of vertical piles or a pile group containing both the vertical and batter piles. Combination of such loads on the pile group may result into a system that is subjected to an eccentric and inclined load. In general, the following four methods are available to analyze this problem 1. Statically or traditional methods: This consists of analyzing the pile group as a simple, statically determinate system but ignoring the effect of the soil. 2. Considering pile group as a structural system utilizing the theory of subgrade reaction for soil support. 3. Consider interaction between piles and the soil by assuming soil to be an elastic continuum. 4. Interaction relationships between soil and pile by determining bearing capacity of piles under eccentric inclined loads. In the following paragraphs, only the first methods will be briefly outlined, (see (Prakash & Sharma, 1990) for the last three methods. This simple method considers pile group as a simple statically determinate system. It neglects the contribution of soil to support the load. Due to its simplicity, this method is widely used in design but should only be limited to small projects because little is known of the reliability of this method. In the following paragraphs, two general cases-first, the inclined load on vertical and

56

batter piles and second, eccentric vertical load on vertical piles-are analyzed by this method. 1.

Inclined Load on Vertical and Batter Piles. The simplified analysis of batter and vertical piles assumes that all piles are subjected to axial loads. The method of analysis described below is based on Culman’s method as described by Chellis (1961) and consists of the following steps: (a) As shown in Figure 3.5, case (A) represents the resultant force by R. (b) Replace each group of similar piles by an imaginary pile at the center of the group. For example, in Figure 3.5, case (A) item (a), it is assumed that group A, group B, and group C offer the axial forces RA, RB, and RC, respectively. Values of RA, RB, and RC, can then be obtained by following procedure: (i)

As shown in (b), draw pile cap and lines parallel to RA, RB, and RC (ii) Extend R to intersect RA at point a. (iii) Extend RB, and RC, to intersect at point b. Join point a and b. (iv) As shown in (c), first draw line ac parallel to and equal to R by selecting an appropriate scale. From a draw ab parallel to ab shown in item (b). Then from point c draw cb parallel to RA to intersect ab at point b. From b draw a line parallel to RB and from point a draw a line parallel to RC to obtain point d. Then RA, will equal cb, RB will equal bd and RC will equal ad. Figure 3.5, case (A), item (c), shows these forces drawn to scale: The force direction (e.g., tension and compression) are also shown on this force diagram. Similarly, when the piles are subjected to a resultant pullout force (Qpull), then the force polygon can be drawn as shown in Figure 3.5, case (B). 2.

Eccentric Vertical Load on Vertical Piles. Load on an individual vertical pile (R,) from an eccentric vertical load can be obtained from the following relationship (Figure 3.6):

The load on any particular pile within a pile group may be computed using the elastic equation:

(3.15) where Qm = axial load on any pile m Qg = total vertical load at the centroid of the pile group 57

n = number of piles Mx, My = moment with respect to x and y axes, respectively x, y = distance from pile to y and x axes, respectively It should be noted that shears and bending moment can be determined for any section of pile cap by using elastic and static equations.

kN

If R = 300 kN/m of structure then from above scale RA = 80 kN, RB = 290 kN kN Qpull

kN kN

kN

Figure 3.5 Analysis of load distribution for vertical and batter piles

Example 3.1 The section of a 3x4 group pile in a layered saturated clay is shown in Figure 2.5. The piles are square in cross section (350 mm x 350 mm). The center-to-center spacing, s, of the piles is 875 mm. Determine the allowable load-bearing capacity

58

of the pile group. Use FS = 4. Note that the groundwater table coincides with the ground surface. GWT

4.50 m

Clay-1 cu(s) = 52.0 kN/m2 sat = 17.6 kN/m3

14..00 m

Clay-2 cu(s) = 85.0 kN/m2 sat = 19.0 kN/m3

875 mm

Figure 3.6 Group pile of layered saturated clay

Solution From equation,

Qug = n1n2Ap (9cu(p))+  p cu(s) L Qug = n1n2Ap (9cu(p)) + 1 p cu(s)1 L1 + 2 p cu(s)2 L2

From Figure (3.5): For the top layer with cu(s)1 = 52.0 kN/m2  from table 1  0.68 For the bottom layer with cu(s)2 = 85.0 kN/m2  from table 2  0.51 Qug = 3x4(0.350)2 (9)(85.0) + (0.68)(4x0.350)(52)(4.50) + (0.51)(4x0.350) (85.0)(14.00) = 13994 kN (revisi) Calculating group pile capacity assumed as block action, given Lg = 3(0.875)+0.350= 2.98

,Bg = 2(0.875)+0.350= 2.10,

pg = 2 (Lg+ Bg) = 2(2.98+2.10.) = 10.15 ¿ ¿ Qug = Lg Bg cu(p) N C + pg cu L= Lg Bg cu(p) N C

+ 2(Lg + Bg)cu L 59

Qug = (2.98)(2.10)(85.0)(9) + 10.15{(52 x 4.50) + (85 x 14.00) = 19241 kN Hence,

Qug = 13994 kN,

4

Qall(g) = Qu /FS =13994/4 = 3499 kN

SETTLEMENT OF SINGLE PILE AND GROUP PILE

Learning Outcomes: 

4.1

Estimate the elastic and consolidation settlement of single piles and pile groups. Elastic Settlement of Group Piles

In general, the settlement of a group pile under a similar working load per pile increases with the width of the group (Bg) and the center-to-center spacing of the piles (s). Several investigations relating to the settlement of group piles have been reported in the literature, with widely varying results. The simplest relation for the settlement of group piles was given by Vesic (1969), namely, (4.1) where Sg(e) = elastic settlement of group piles Bg =width of group pile section D = width or diameter of each pile in the group Se = elastic settlement of each pile at comparable working load For group piles in sand and gravel, for elastic settlement, Meyerhof (1976) suggested the empirical relation (4.2) and, q = Qg/(LgBg) kN/m2

(4.3)

where Lg and Bg = length and width of the group pile section, respectively (m) N60 = average standard penetration number within seat of settlement ( deep below the tip of the piles) I = influence factor = 1 – L/8Bg  0.5 L = length of embedment of piles (m)

60

Similarly, the group pile settlement is related to the cone penetration resistance by the formula. (4.4)

Where, qc = average cone penetration resistance within the seat of settlement. Note that, in Eq. (11.126), all quantities are expressed in consistent units.) 4.2

Consolidation Settlement of Group Piles

The consolidation settlement of a group pile in clay can be estimated by using the 2:1 stress distribution method. The calculation involves the following steps (see Figure 4.1): Qg

Clay layer 1

GWL

L

L a

b

Clay layer 2

z 2V:1H

Clay layer 3

L2

L3

Clay layer 4 b’

a’

Rock

Figure 4.1 Consolidation settlement of group piles

Step 1.

Let the depth of embedment of the piles be L. The group is subjected to a total load of Qg. If the pile cap is below the original ground surface, Qg equals the total load of the superstructure on the piles, minus the effective weight of soil above the group piles removed by excavation.

Step 2.

Assume that the load Qg is transmitted to the soil beginning at a depth of (2/3)L from the top of the pile, as shown in the figure. The load Qg spreads out along two vertical to one horizontal line from this depth. Lines aa’ and bb’ are the two 2:1 lines. 61

Step 3.

Calculate the increase in effective stress caused at the middle of each soil layer by the load Qg. The formula is Qg. (4.5) Where ∆ σ 'i Lg, Bg zi

Step 4.

= = =

increase in effective stress at the middle of layer i length and width, respectively of the planned group piles distance from z = 0 to the middle of the clay layer i

Calculate the consolidation settlement of each layer by the increased stress. The formula is or

(4.6a)

(4.6b) where

Sc(i) e(i) eo(i) Hi Cc(i) σ´ 'o (i) σ´ '1 (i ) Step 5.

= = = = =

consolidation settlement of layer i change of void ratio caused by the increase in stress in layer i initial void ratio of layer i (before construction) thickness of layer i (before construction) compression index of layer i = initial overburden pressure in layer i = pressure increment in layer i due to external loads

The total consolidation settlement of the group piles is then

Sc(g) = Sc(i)

(4.7)

Note that the consolidation settlement of piles may be initiated by fills places nearby, adjacent floor loads, or the lowering of water table.

62

Example 4.1 A group pile in clay is shown in Figure 4.2. Determine the consolidation settlement of the piles. All clays are normally consolidated. Pile group dimension: Lg = 3.3 m, and Bg = 2.2 m. Solution Because the lengths of the piles are 15 m each, the stress distribution starts at a depth of 10 m below the top of the pile. We are given that Qg = 2000 kN . Calculation of Settlement of Clay Layer 1

Qg = 2000 kN

GWL

Sand 15m

2m

Sand: = 16.2 kN/m2

10m

16m

Clay 1: sat = 18.0 kN/m2 eo = 0.82 Cc = 0.3

Clay 2

4m

Clay 2: sat = 18.9 kN/m2 eo = 0.7 Cc = 0.2

Clay 3

2m

Clay 1

z

2V:1H

Rock

Clay 3: sat = 19 kN/m2 eo = 0.75 Cc = 0.25

Figure 4.2 Soil profile and loading condition in Example 4.1

For normally consolidated clays,

= 2(16.2) + 12.5 (18.0 – 9.81) = 134.8 kN/m2 63

Similarly, settlement of layer 2

= 2(16.2) + 16 (18.0 – 9.81) + (2(18.9 – 9.81) = 181.62 kN/m2

Hence,

Similarly, settlement of layer 3

= 181.62 + 2(18.9 – 9.81) + 1(19 – 9.81) = 208.99 kN/m2

Hence,

Hence, the total consolidation settlement is: Sc = 162.4 +15.7 + 5.4 = 183.5 mm

64

5

INSTALATION OF PILE

Learning Outcomes: 

5.1

Understand the basic principles of piling installation using impact hammer energy INSTALLATION OF DISPLACEMENT PILES

Displacement piles are installed by means of a driving hammer or a vibratory driver. There are a range of hammer types including drop hammer, steam or air hammer, diesel hammer and hydraulic hammer. The use of noisy diesel, pneumatic and steam hammers for percussive piling is generally banned in builtup areas surrounded by noise sensitive receivers. It is important to exercise directional control and maintain the pile in alignment during initial pitching and driving. Leaders held in position by a crane are suitable for support of both the pile and the hammer during driving, and may be used for vertical and raking piles. Alternatively, vertical piles may be supported in a trestle or staging and driven with a hammer fitted with guides and suspended from a crane. Where a hammer is used to produce impacts on a precast concrete pile, the head should be protected by an assembly of dolly, helmet and packing or pile cushion. The purpose of the assembly is to cushion the pile from the hammer blows and distribute the dynamic stresses evenly without allowing excessive lateral movements during driving. In addition, the life of the hammer would be prolonged by reducing the impact stresses. Pile cushion (or packing) is generally not necessary for driving steel piles. The length of the follower should be limited as far as possible because the longer the follower, the more difficult it will be to control the workmanship on site. Furthermore, limited site measurements indicated that for follower longer than 4 m, reduction in energy transferred to the pile may occur, even if it is of the same material as the pile section. Impact Hammers The impact energy for driving piles may be obtained by any one of the following types of hammers. They are 1. Drop hammers, 2. Single-acting steam hammers, 3. Double-acting steam hammers, 4. Diesel hammer, 5. Vibratory hammer. 65

5.2

CHARACTERISTICS OF HAMMERS AND VIBRATORY DRIVERS

The rating of a piling hammer is based on the gross energy per blow. However, different types of hammers have differing efficiencies in terms of the actual energy transmitted through the pile being driven. The range of typical efficiencies of different types of hammers is shown in Table 5.1.

Type of Hammer Drop hammers Hydraulic hammers

Typical Energy Transfer Ratio 0.45 – 0.6 0.7 -1

The operational principles and characteristics of the various types of driving equipment are briefly summarized in the following section. Figure 5.1 are shown in the types of driving equipment. Drop hammers A drop hammer (typically in the range of 80 to 160 kN) is lifted on a rope by a winch and allowed to fall by releasing the clutch on the drum. The stroke is generally limited to about 1.2 m except for the case of 'hard driving' into bedrock where drops up to 3 m can be used. The maximum permissible drop should be related to the type of pile material. The drawback to the use of this type of hammer is the slow blow rate, the difficulty in effectively controlling the drop height, the relatively large influence of the skill of the operator on energy transfer, and the limit on the weight that can be used from safety considerations. Steam or compressed air hammers Steam or compressed air hammers are classified as single-acting or double-acting types depending on whether the hammer falls under gravity or is being pushed down by a second injection of propellant. A chiselling action is produced during driving as a result of the high blow rate. Some single-acting steam hammers are very heavy, with rams weighing 1000 kN or more. A double-acting air hammer is generally not suitable for driving precast concrete piles unless the pile is prestressed. For maximum efficiency, these hammers should be operated at their designed pressure. The efficiency decreases markedly at lower pressures; excessive pressure may cause the hammer to 'bounce' off the pile (a process known as 'racking') which could damage the equipment.

66

Diesel hammers In a diesel hammer, the weight is lifted by fuel combustion. The hammer can be either single-acting or double-acting. Usually, only a small crane base unit is required to support the hammer. Due to the high noise level and pollutant exhaust gases associated with diesel hammers, the use of diesel hammers has been phased out in populated areas. The driving characteristics of a diesel hammer differ appreciably from those of a drop or steam hammer in that the pressure of the burning gases also acts on the anvil (i.e. driving cap) for a significant period of time. As a result, the duration of the driving forces is increased. The length of the stroke varies with the driving resistance, and is largest for hard driving. In soft soils, the resistance to pile penetration may be inadequate to cause sufficient compression in the ram cylinder of a 'heavy' hammer to produce an explosion, leading to stalling of hammer. In this case, a smaller hammer may be necessary in the early stages of driving. The ram weight of a diesel hammer is generally less than a drop hammer but the blow rate is higher. The actual efficiency is comparatively low (Table 5.1) because the pressure of the burning gas renders the ram to strike at a lower velocity than if it were to fall freely under gravity. The efficiency is dependent upon the maintenance of the hammer. Furthermore, as the hammer needs to exhaust gas and dissipate heat, shrouding to reduce noise can be relatively difficult. Where a diesel hammer is used to check the final set on re-strike at the beginning of a working day, results from the first few 'cold' blows may be misleading in that the hammer is not heated up properly and the efficiency may be very low. This source of error may be avoided by warming the hammer up through driving on an adjacent pile. Hydraulic hammers A hydraulic hammer is less noisy and does not produce polluting exhaust. Modern hydraulic hammers, e.g. double-acting hydraulic hammers, are more efficient and have high-energy transfer ratios. The ram of the hammer is connected to a piston, which is pushed upward and downwards by hydraulic power. Some complex models have nitrogen charged accumulator system, which stores significant energy allowing a shortened stroke and increased blow rate. As such, the kinetic energy of the hammer depends not only on the height of the stroke but also the acceleration due to the injection of hydraulic pressure. Most new hydraulic hammers are equipped with electronic sensors that directly measure the velocity of the ram and calculate the kinetic energy just before impact. An “equivalent stroke height” is computed by dividing the measured kinetic energy by the weight of the ram and is used in the pile driving formulae. HKCA (2004) reported that the energy transfer ratio of hydraulic hammers ranges between 0.8 and 0.9. Vibratory drivers 67

A vibratory driver consists of a static weight together with a pair of contrarotating eccentric weights such that the vertical force components are additive. The vibratory part is attached rigidly to the pile head and the pulsating force facilitates pile penetration under the sustained downward force. The vibratory driver may be operated at low frequencies, typically in the range of 20 to 40 Hz, or at high frequencies around 100 Hz (i.e. 'resonance pile driving'). Vibratory drivers are not recommended for precast or prestressed concrete piles because of the high tensile stresses that can be generated. Jetting Piles Water jetting may be used to aid the penetration of a pile into dense sand or dense sandy gravel. Jetting is ineffective in firm to stiff clays or any soil containing much coarse to stiff cobbles or boulders. Where jetting is required for pile penetration a stream of water is discharged near the pile point or along the sides of the pile through a pipe 5 to 7.5 cm in diameter. An adequate quantity of water is essential for jetting. Suitable quantities of water for jetting a 250 to 350 mm pile for fine sand, coarse sand, sandy gravels are, 1525 liters/second, 25-40 liters/second, and 45-600 liters/second respectively. A pressure of at least 5 kg/cm2 or more is required.

68

COMPR. AIR (OPTIONAL)

Static weight

RAM

RAM

RAM

Figure 5.1 Principles of operation of pile-driving hammers (Vesic, 1977). (a) Drop hammer, (b) single-acting hammer

69

References BerezanTsev, V. G. et al. (1961). Load bearing capacity and deformation of piled foundations, Proceedings of the 5th International Conference, ISSMFE, Paris, Vol. 2, 1961, pp. 11–12. Bowles, J. E. (1996). Foundation Analysis and Design, 5th. ed, McGraw-Hill, New York. Briaud,J. L. et al. (1985). Behavior of Piles and Pile Groups, Report No. Federal Highway Administration,Washington, DC. Budhu, M. (2011). Soil Mechanics and Foundations, 3rd ed., JohnWiley & Sons, Ltd., USA. Chellis, R.D. (1961). Pile Foundations, McGraw Hill, New York. Coduto, D. P. (2001). Foundation Design, Principles and Practices, 2 nd ed. PrenticeHall, Englewood Cliffs, NJ. Coyle, H. M., and Castello, R. R. (1981). “New Design Correlations for Piles in Sand,” Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, Vol. 107, No. GT7, pp. 965–986. Craig, R.F. (2004). Craig’s Soil Mechanics 7th Ed., Spon Press – Taylor & Francis, London & New York Das, B.M. (2009). Shallow Foundation Bearing Capacity and Settlement, 2nd ed., Thomson, CRC Press Taylor & Francis Group, Boca Raton, FL. Davisson, M. T. (1973). “High Capacity Piles” in Innovations in Foundation Construction, Proceedings of a Lecture Series, Illinois Section, American Society of Civil Engineers, Chicago. Hansen, J. B. (1968). A general formula for bearing capacity, Danish Geotechnical Institute, Bulletin No. 11, 1961; also, A revised and extended formula for bearing capacity, Danish Geotechnical Institute, Bulletin No. 28. Kameswara Rao, N. S. V. (2011). Pile Foundation Design Theory and Practice, John Wiley & Sons. Kerisel, J. (1961). Fondations Profondes en Milieu Sableux, Proc. 5th Int. Conf. SM and FE, Vol. 2. Meyerhof, G.G. (1959). Compaction of Sands and Bearing Capacity of Piles, JSMFD, ASCE, Vol. 85, SM6.

70

Meyerhof, G. G. (1976). “Bearing Capacity and Settlement of Pile Foundations,” Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, Vol. 102, No. GT3, pp. 197–228. Murthy, V.N.S. (2011). Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering, Marcell Dekker Inc, New York, USA. Nottingham, L. C., and Schertmann, J. H. (1975). An Investigation of Pile Capacity Design Procedures, Research Report No. D629, Department of Civil Engineering, University of Florida, Gainesville, FL. Prakash, S. and Sharma, H. D., (1990). Pile Foundation in Engineering Practice, John Wiley & Sons, Singapore. Poulos, H.G. and Davis, E.H. (1980). Pile Foundation Design Theory and Practice, John Wiley & Sons, Ltd.,Chichester, UK. Sladen, J. A. (1992). The Adhesion Factor: Applications and Limitations, Canadian Geotechnical Journal, Vol. 29, No. 2, pp. 323–326. Teng, W.C. (1969). Foundation Design, Prentice Hall, Englewood Cliffs, NJ. Tomlinson, M. and Woodward, J. (2008). Pile Design and Construction Practices 5th Ed., by Taylor & Francis, USA. Vesic, A.S. (1969). Effect of Scale and Compressibility on Bearing Capacity of Surface Foundations, Discussions, Proceedings, 7th Int. Conf. SM and FE. Mexico City, Volume III. Vesic, A.S. (1977). Design of Pile Foundations, Synthesis of Highway Practice 42, Res. Bd., Washington D.C. Vijayvergiya,V. N., and FOCHT, J. A., JR. (1972). A New Way to Predict Capacity of Piles in Clay, Offshore Technology Conference Paper 1718, Fourth Offshore Technology Conference, Houston.

71

LIST OF NOTATIONS Ap As Bg Cc c cu D Db DR Dr E Ec Ei Ep Er Es eo FS fs fsc Ix, Iy Ko Kp Ks L Lb Lc Lg Mx, My N Nc, Nq, Nγ n Q Qall Qg Qs Qu Sc Se Snet St WR p

cross-sectional area of pile surface area of the shaft of embedded length of pile in soil width of group pile section compression index of soil cohesion of soil undrained shear strength of soil pile width or width of pile foundation in the direction of rotation foundation base width or base diameter relative density of sand reference pile diameter or width pile driving energy young's modulus of concrete modulus of soil layer i young's modulus of pile young's modulus of rock young's modulus of soil initial void ratio factor of safety unit skin friction resistance sleeve friction of CPT-test moment of inertia of pile group with respect to x and y axes respectively coefficient of earth pressure at rest coefficient of passive pressure coefficient of earth pressure total length of pile embedded length of pile critical pile length length of group pile section moment about centroid of pile group with respect to the x and y axes respectively uncorrected SPT blow count bearing capacity factors number of piles in pile groups applied vertical load allowable load-carrying capacity for each pile load carried by pile group ultimate skin resistance ultimate load on single pile consolidation settlement elastic settlement of the pile it self net settlement total settlement weight of the ram perimeter of the cross section of the pile

72

qc qp xi, yi  c β δ φ γ γ' γw  ηh λ s ' σv

cone resistance of CPT - test unit of ultimate end-bearing resistance distance of pile i from y and x axes respectively adhesion factor reduction factor for clayey soils shaft friction coefficient soil-pile friction angle internal friction angle of founding material bulk unit weight of soil effective unit weight of soil unit weight of water group reduction or efficiency factor efficiency of hammer (allowing for energy loss on impact) pile stiffness ratio poisson's ratio of soil vertical effective stress

73

GLOSSARY OF TERMS End-bearing resistance. Load-carrying capacity of pile due to bearing capacity of the soil below pile tip. Composite piles. Special piles of various combinations of materials in driven piles or combinations of bored piles with driven piles. Downdrag. The downward movement of a pile due to negative skin friction and is expressed in terms of settlement. Dragload. The load transferred to a pile due to negative skin friction. Driven cast-in-place piles. Piles formed by driving a steel tube into the ground to the required set or depth and withdrawing the tube after concrete placement. Hand-dug caisson. A bored pile in which the bore is formed manually by using hand tools in stages. Kentledge. The use dead load as counter weight in pile load test Large-diameter bored piles. Bored piles of diameter greater than about 750 mm, e.g. machine bored piles. Large-displacement piles. All solid driven piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug. Mini-piles. Small diameter piles which are formed by small drilling rigs with the use of down-the-hole hammers, rotary or rotary percussive drills and are subsequently grouted. Negative skin friction. Soil traction act downward along the pile shaft as a result of a downdrag and induce compression in pile. Neutral plane. The depth where there is no relative movement between the pile and the surrounding soil. Precast concrete piles. Reinforced concrete piles, with or without prestress, cast and then driven into ground. Replacement pile. Pile formed by machine boring, grabbing or hand digging. Shaft resistance. Load-carrying capacity of pile due to soil resistance developed at pile/soil interface in response to applied load. Small-diameter bored piles. Bored piles of small diameter less than about 750 mm.

74

Small-displacement piles. Driven rolled steel sections such as H-piles and open-ende tubular piles. Special piles. Particular pile types or variants of existing pile types introduced to improve efficiency or overcome problems related to special ground conditions. Steel H-piles. Piles of rolled steel section of H-shape in cross-section. Steel tubular piles. Preformed hollow steel piles of circular section.

75