Ch2 Subsoil Exploration (15-71)

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CHAPTER

2

SUBSOIL EXPLORATION

2.1 SOIL EXPLORATION All office, laboratory and field works are done in order to explore the subsurface of soil or rock conditions at any given site to obtain the necessary information required in design and construction. Subsoil exploration is the first step in the design of a foundation system. Soil exploration consists essentially of boring, sampling and testing.

(a) Office, laboratory and field works:

Planning for exploration program and phases of soil investigation

Boring

Sampling

Testing

Report

Usually, the proper program of soil investigation for a given project depends on the type and importance of structure; nature of the subsoil involved; type of equipment available; ground water condition; and the budget allocated for the exploration. At present, site investigation drilling and testing are carried out in a routine way, with the absence of any significant plan. This may result in a significant loss of money and time, since the work is carried out without reference to the special requirements of the project. If previous site investigation reports exist for construction in the same soil, this allows the geotechnical engineer to judge the likely performance of the ground under and around the proposed development. In any case, geological maps coupled with experience will give a considerable amount of information of great value in the initial stages of design. At this stage, there should also be interaction between the client and all his design professionals. Thus, the design should be modified to reduce possible geotechnical problems. For example, if a large site is to be developed as a business park, the buildings might be re-aligned with their long sides parallel to the contours; this will reduce the amount of cut and fill, thus keeping the cost of foundations and retaining structures to a minimum, while also reducing the risks of slope instability.

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

From the knowledge of probable ground, groundwater conditions at and around the site, and the required structural form(s), the geotechnical engineer can predict the types of foundations and earth-retaining structures required for the project, and any possible problems (such as slope instability, chemical attack on foundation concrete and construction difficulties) which may therefore require further investigation. (b) To explore subsurface conditions: Drilling and trial pitting are normally carried out for a number of reasons, such as: 1. To establish the general nature of the strata below a site. This will decide whether the site is good or not for the proposed project; 2. To determine depth (below the ground surface); thickness; extent of all soil layers in both vertical and lateral directions. Also if rock is existing, the depth and the extent of its surface, its thickness and nature should be defined and determined; 3. To determine for clay layers (if possible) whether it is a recent deposit or an old one that has been compressed by pervious overburden which may be removed by erosion and whether the clay has been subjected to cycles of wetting and drying; 4. To obtain disturbed and undisturbed soil samples for laboratory testing; 5. To allow field tests to be carried out; 6. To locate the level of water table, its seasonal variation and the methods of controlling it during construction; 7. To install instruments such as piezometers, or extensometers, and 8. To verify the interpretation of geophysical surveys. (c) To obtain information required in the design and construction (i) For new structures:  Selection of the type and depth of foundation;  Determination of bearing capacity and settlement of the selected foundation;  Evaluation of the earth pressure against walls and abutments;  Suitability of soil and degree of compaction of fill material under slabs and earth works;  Establishment of the ground water level;  Provisions against constructional difficulties;  Swelling potential; and  Chemical contents. (ii) For existing structures:  Investigation of the safety of the structure;  Prediction of settlement; and  Determination of remedial steps against water table problems, serious settlement and cracks. (iii) For highways and airfields:  The location of the road and runways both vertically and horizontally;  The location and selection of borrow materials for fills and subgrade treatments;  The design of ditches, culverts and drains;  The design of roadway section;

16

Foundation Engineering for Civil Engineers

 

Chapter 2: Subsoil Exploration

The type of subgrade treatments; and The location of local sources of construction materials for base, subbase, stabilizer, binder and wearing courses and other structural works.

Mainly, subsoil exploration involves three phases; reconnaissance phase, preliminary site investigation phase, and detailed site investigation phase.

2.1.1 RECONNAISSANCE PHASE This phase consists of: (a) Collection of all available information, and (b) Reconnaissance of the site. So that, it will indicate any settlement limitations and help to estimate foundation loads.

2.1.2 PRELIMINARY SITE INVESTIGATION PHASE This phase consists of: (a) Preliminary design data that satisfy building code requirements, and (b) Number and depth of boreholes. So, it involves knowing the distribution of structural loads which is required in the design of foundations. Also, a few borings or tests pits are to be opened to establish the stratification types of soil and location of water table. In addition, one or more borings should be taken to rock when the initial boreholes indicate that the upper soil is loose or highly compressible.

2.1.3 DETAILED SITE INVESTIGATION PHASE In this phase, additional boreholes, samples will be required for zones of poor soil at smaller spacing and locations which can influence the design and construction of the foundation.

2.2 DRILLING OR BORING 2.2.1 TEST PITS A pit is dug either by hand or by a backhoe. Probably in a test pit, the engineer can examine in detail the subsoil strata and take disturbed or undisturbed samples at the desired location (see Fig.(2.1)): Advantages:  Inexpensive.  Provide detailed information of stratigraphy.  Large quantities of disturbed soils can be obtained for testing.  Large blocks of undisturbed samples can be carved out from the pits, and  Field tests can be conducted at the bottom of the pit.

17

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Disadvantages:  Depth limited to about 6m.  Deep pits are uneconomical as in the case of investigation that involves basement construction.  Excavation below groundwater (high water table) and into rock is difficult and costly.  Too many pits may scar site and require backfill soils.  When the soil is unstable and has a tendency to collapse, this prevents the engineer from entering the pit and accompanied by certain risks, and  Unsuitable in granular soils below water level or when the standard penetration resistance test (N-value) is required.

Walls of test pit indicate four layers (1) Clayey silt (2) Sandy silt (3) Clean sand (4) Sandy gravel

Fig.(2.1): Test pits.

2.2.2 DRILLING METHODS Several methods can be used for drilling a hole into ground, these are: (1)

Auger Drilling (a) Hand - auger drilling. (b) Power - auger drilling.

(2) Wash Boring

(a) Jetting. (b) Sludging (reverse drilling). (3)

Rotary Drilling (a) Rotary drilling with flush. (b) Rotary - percussion drilling.

18

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

(4) Percussion Drilling

Each of these methods has its merits and its demerits. However, Table (2.1) gives a guide for selecting the most appropriate drilling method. Table (2.1): Drilling method selection (after Budhu, 2007). Wash boring

Rotary drilling Rotary Rotary drilling percussion with drilling flush ? X  ?

Hand auger drilling

Jetting

Sludging

Sand

X 

X 

X 

Silt









?

?



?





 slow

 slow

X

X

X

X

?

?

Low to medium strength formations

X

X

X



 slow



X

X

X







Limestone Medium to high Igneous (granite, strength basalt) formations Metamorphic (slate, gneiss) Rock with fractures or voids

X

X

X

 slow



 slow

X

X

X

X



 slow

X

X

X

X



 V slow

X 

X

X





!

? 

X 













Type of soil Gravel

Clay Sand with pebbles or boulders shale Sandstone

Unconsolidated formations

Above water-table Below water-table  = Suitable drilling method ? = Possible problems

?  ? = Danger of hole collapsing

Percussion drilling ? ?

 ! = Flush must be maintained to continue drilling

x = Inappropriate method of drilling

(5) Rock Core Drilling

o o

Used for obtaining rock cores. A core barrel which is fitted with a drill bit, is attached to hollow drill rods. Fig.(2.2) shows one box of bedrock drill core. The core can be drilled in various diameters. The core in the picture is about 2.0 inches in diameter and is the most common size drilled for mineral exploration.

Fig.(2.2): Core extracted from a drill hole placed in a core box. 19

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.2.2.1 Auger Drilling (a) Hand - Augers The auger of (10−20) cm in diameter is rotated by turning and pushing down on the handlebar. Then withdrawing and emptying the soil-laden auger to remove the excavated soil. Several new auger sections are added until the required depth is reached. These augers can be available in different types such as (see Fig.(2.3)):  Helical Auger.  Short flight Auger, and  Iwan Auger. Advantages:  Inexpensive.  Simple to operate and maintain.  Not dependent on terrain.  Portable.  Used in uncased holes, and  Groundwater location can easily be identified and measured. Disadvantages:  Slow compared with other methods.  Depth limited to about 6m.  Labor intensive.  Undisturbed samples can be taken only for soft clay deposit, and  Cannot be used in rock, stiff clays, dry sand, or caliches soils.

a.

Helical (worm types) Augers

b. Short flight Auger

c. Iwan (posthole) Auger

Fig.(2.3): Hand−augers (after Allen, 1993).

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

(b) Power - Augers Truck or tractor mounted type rig and equipped with continuous flight augers that bore a hole 100 to 250 mm in diameter. These augers can have a solid or hollow stem of (20 -75) cm in diameter (see Fig.(2.4)).

Advantages:  Used in clay or sand or silt soils.  Quick.  Used in uncased holes, therefore no need for using drilling mud.  Undisturbed samples can be obtained quite easily, and  Groundwater location can easily be identified and measured.

Disadvantages:  Depth limited to about 15m. At greater depth, drilling becomes expensive, and  Site must be accessible to motorized vehicle.

a. Continuous flight augers.

b. Solid-stem auger

Fig.(2.4): Power or mechanical-augers.

21

c. Hollow-steam auger

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.2.2.2 Wash Boring Water is pumped to bottom of borehole and soil washings are returned to surface. A drill bit is rotated and dropped to produce a chopping action (see Fig.(2.5)).

Fig.(2.5): Wash boring rig.

(a) Jetting Method

Method: Water is pumped down the center of the drill-rods, emerging as a jet. It then returns up the borehole or drill-pipe bringing with it cuttings and debris. The washing and cutting of the formation is helped by rotation, and by the up-and-down motion of the drillstring. A foot-powered treadle pump or a small internal-combustion pump is equally suitable. (b) Sludging (Reverse Jetting)

Method: A hollow pipe of steel is moved up and down in the borehole while a one-way valve can be used to improvise successfully and provide a pumping action. Water flows down the borehole annulus (ring) and back up the drill pipe, bringing debris with it. A small reservoir is needed at the top of the borehole for recirculation. Simple teeth at the bottom of the drill-pipe, preferably made of metal, help cutting efficiency.

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Advantages:    

The equipment can be made from local, low-cost materials, and it is simple to use. Possible above and below the water-table. Suitable for clay to silt clay, silt soils and unconsolidated rocks, and Used in uncased holes.

Disadvantages:      

Slow drilling through stiff clays and gravels. Undisturbed soil samples cannot be obtained. Water is required for pumping. Difficulty in obtaining accurate location of groundwater level. Boulders can prevent further drilling, and Depth is limited to about 30m.

2.2.2.3 Rotary Drilling (a) Rotary Drilling with Flush Method: A drill-pipe and bit are rotated to cut the rock. Air, water, or drilling mud is pumped down the drill-pipe to flush out the debris. The velocity of the flush in the borehole annulus must be sufficient to lift the cuttings (see Fig.(2.6)).

Advantages:       

Quick. Can drill any type of soil or rock. Possible to drill to depths of over 40 meters. Operation is possible above and below the water-table. Undisturbed soil samples or rock cores can easily be recovered. Water and mud support unstable formations, and Possible to use compressed air flush.

Disadvantages:      

Expensive equipment. Terrain must be accessible to motorized vehicle. Water is required for pumping. Difficulty in obtaining accurate location of groundwater level. There can be problems with boulders, and Rig requires careful operation and maintenance (additional time required for setup and cleanup).

23

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

(b) Rotary - Percussion Drilling Method: In very hard rocks, such as granite, the only way to drill a hole is to pulverize the rock, using a rapid-action pneumatic hammer, often known as a 'down-the-hole hammer' (DTH). Compressed air is needed to drive this tool. The air also flushes the cuttings and dust from the borehole. Rotation of 10−30 rpm ensures that the borehole is straight and circular in cross section (see Fig.(2.6)).

Advantages:    

Drills hard rocks. Possible to penetrate gravel. Fast, and Operation is possible above and below the water-table.

Disadvantages:   

Higher tool cost than other tools illustrated here. Air compressor required, and Experience needed to operate and maintain.

Fig.(2.6): Rotary drilling.

24

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.2.2.4 Percussion Drilling Method: The lifting and dropping of a heavy (+ 50 kg) cutting tool will chip and excavate material from a hole. The tool may be fixed to rigid drill-rods, a rope or a cable. With a mechanical winch, depths of hundreds of meters can be reached. Advantages:     

Simple to operate and maintain. Suitable for a wide variety of rocks. Operation is possible above and below the water-table. It is possible to drill to considerable depths, and Can be used for boring observation wells.

Disadvantages:    

Slow, compared with other methods. Problems can occur with unstable rock formations. Water is needed for dry holes to help remove cuttings, and Due to high disturbance of soil, the obtained samples cannot be used for testing.

2.2.2.5 Rock Core Drilling 

Steps in the Process: (after IDEA Drilling, 1997)

1.

Drilling holes begins with installing a pipe called casing from the surface through soils and sealed into bedrock. Diamond core drilling uses a diamond bit, which rotates at the end of drill rod (or pipe) inside the casing. The opening at the end of the diamond bit allows a solid column of rock to move up into the drill pipe and be recovered at the surface. Most drill rods are 3m long. After the first 3m drilling, a new section of pipe is screwed into the top end, so the combination of pipes can be drilled deeper into the ground. The diamond bit is rotated with gentle pressure while being lubricated with water and drilling fluid to prevent overheating. The driller adjusts rotation speed, pressure and water circulation for different rock types and drilling conditions. Inside the drill pipe is a core tube, which has a latching mechanism attached to a cable. At the end of each 3m run, the cable is lowered to winch the core tube containing the new rock core to the surface where it can be recovered. The drill core is stored in specially designed core boxes containing compartments to hold sections of the core, and The drill core is then logged and analyzed by a geologist.

2. 3. 4.

5. 6. 7.

8. 9.

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.3 UNDERGROUND WATER IN THE TEST HOLE The depth of the water table (W.T.) as measured during drilling and sampling should be carefully evaluated. It is always necessary to wait for at least 24 hours to check on the stabilized water table for the final measurement. The technician should plug the top of the drill holes and flag them for identification. Care is required to ensure that the water level in the drill hole is always maintained. Any sudden drop or rise of the water table should be carefully recorded in the field logs of borings.

2.4 GEOPHYSICAL METHODS These methods represent indirect methods of subsoil exploration and mainly consist of: (1) (2) (3) (4)

Ground Penetration Radar (GPR). Electrical Resistivity Method (ERM) Electromagnetic Method (EM), and Seismic Methods.

In subsoil investigation, the seismic methods are most frequently used. These methods are based on the variation of the wave velocity in different earth materials. They involve in generating a sound wave in the rock or soil, using a sledgehammer, a falling weight, or a small explosive charge, and then recording its reception at a series of geophones located at various distances from the shot point, as shown in Fig.(2.7). The time of the refracted sound arrival at each geophone is noted from a continuous reader. Typical P−wave velocity in various soils and rocks in (m/sec) are shown in Table (2.2).   

Requirements of seismic exploration: Equipment to produce an elastic wave, such as a sledgehammer used to strike a plate on the surface. A series of detectors, spaced at intervals along a line from wave origin point, and A time-recording mechanism to record the time of origin of the wave and the time of its arrival at each detector.

Advantages of seismic exploration: 1. 2. 3.

Permits a rapid coverage of large areas at a relatively small cost. Not hampered by boulders and cobbles which obstruct borings, and Used in regions not accessible to boring equipment, such as the middle of a rapid river.

Disadvantages of seismic exploration: 1. 2. 3. 4.

Lack of unique interpretation. It is particularly serious when the strata are not uniform in thickness nor horizontal, Irregular contacts often are not identified, and The strata of similar geophysical properties sometimes have greatly different properties. Note: Whenever possible, seismic data should be verified by one or two borings before definite conclusions can be reached.

26

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

(b) Ground penetration radar.

(a) Electrical resistivity method.

(c) Seismic survey method.

Fig.(2.7): Geophysical methods.

27

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Table (2.2): Typical P− wave velocity in various soils and rocks (after Das ,2007). Type of soil or rock Soil: Sand, dry silt, and fine – grained topsoil Alluvium Compacted clays, clayey gravel, and dense clayey sand Loess Rock: Slate and shale Sandstone Granite Sound limestone

P−wave Velocity (m/ sec) 200 -1000 500 - 2000 1000 - 2500 250 - 750 2500 - 5000 1500 - 5000 4000 - 6000 5000 -10000

2.5 SAMPLING During the boring, three types of representative soil samples should be collected which are valuable to geotechnical engineers; these are as follows: (a) The disturbed samples (D): which were collected from auger cuttings at specified

depths? (b) The undisturbed samples (U): which were obtained using a thin Shelby tubes of

100mm in diameter and (400-450)mm in length, and (c) The (SS) samples: which were taken from standard split spoon sampler used in a

standard penetration test (S.P.T.) that performed at different intervals depending on soil stratification. All these samples are then sealed tightly in plastic bags to retain their in situ moisture content, labeled and transported to the soil mechanics laboratory, to perform the required tests.

2.5.1 DISTURBED SAMPLES Disturbed samples can be collected during the drilling process from the auger cuttings at certain intervals and/or each different stratum. In test pit excavation, large samples will sometimes be required in order to fulfill the laboratory testing requirements. Such samples should be at least (30 cm x 30cm) in size, wrapped in wax paper, and carefully transported to the laboratory. Representative samples can also be obtained by driving into the ground an open-ended cylinder known as “Split Spoon.” Spoons with an inside diameter of about 5 cm consists of four parts: a cutting shoe at the bottom; a barrel consisting of a length of pipe split into one half; and a coupling at the top for connection to the drill rod.

28

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2.5.2 UNDISTURBED SAMPLES These samples can be obtained by pushing a thin-walled “Shelby” or seamless steel tube which is attached to a sampler into the soil at the desired depth. The operation must be performed carefully so as to experience minimum deformation. The sampler head contains a check valve and vents for the escape of air or water. The principal advantages of the Shelby tube sampler are its simplicity and the minimal disturbance of soil. Fig.(2.8) shows some details of standard split-spoon and thin-wall tube samplers that are commonly used in in-situ testing and sample recovery equipment. A modification in the design of the split spoon sampler allows the insertion of brass thin-wall liners into the barrel. Four sections of brass liners (each 4 inch long) can be used. Such a device allows the sampling and penetration test at the same time. This method was initiated in California and known as the “California” sampler.

2.5.3 ROCK CORE SAMPLES Samples of rock are generally obtained by rotary core drilling. Diamond core drilling is primarily used in medium-hard to hard rocks. Special diamond core barrels up to 20 cms in diameter are occasionally used and larger ones can be used. Such large samples enable the geologist to study the formation and texture of the foundation rock in detail. A summary of different sampler types which can be used to obtain disturbed or undisturbed samples of each type of soil are listed in Table (2.3).

Fig.(2.8): Details of commonly used samplers for in−situ testing (after Moore, 1980). 29

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Table (2.3): Types of samplers used for taking soil and rock samples from test holes. Type of sampler 1. Highly disturbed sampler

2. Split spoon sampler

3. Thin wall Shelby tube

Procedure Auger boring, wash boring, and percussion drilling.

Standard Penetration Test.

5. Piston samplers 6. Hard carved samples: (a) Spring core catcher, and (b) Scraper bucket.

7. Hand-cut samples

 All types of soils,  Due to high disturbance it is unsuitable for foundation exploration.  Cohesive, cohesionless soils and soft rocks,  For taking disturbed samples which are required for physical and geotechnical analysis of soil as well as chemical tests.  In cohesionless soils, the penetration number (N) is used for making both strength and settlement estimates.

16 gauge seamless steel tube (7.5- 15) cm dia.; preferably pushed by static force instead of driven by hammer.

 For taking undisturbed samples from cohesive soil,  Unsuitable for granular soils and hard materials.

Rotary drilling

 For taking undisturbed continuous rock samples.

Rotary drilling

 For taking undisturbed samples in soft and slightly stiff cohesive soils.

Cut by hand from side of test pit.

 For taking disturbed samples in cohesive or cohesionless soils.

Cut by hand from side of test pit.

 For taking disturbed samples in cohesionless soil or disturbed and undisturbed block samples in cohesive soil.

4. Core barrel sampler: (a) Single tube, and (b) Double tube core barrel.

Type of soil and Remarks

2.6 SAMPLE DISTURBANCE Certain amounts of disturbance during sampling must be regarded as inevitable:1. Effect of stress relief: Due to boring, the stress state in soil will be changed as a result of a stress relief. 2. Effect of area ratio (Ar %): It is the ratio of the volume of soil displacement to the volume of the collected sample.

Ar 

Do 2  Di 2

x100 ………………………….……………………………(2.1) Di 2 where, Di and Do are inner and outer diameters of the used sampler. For stiff clay < 20%, for soft clay  10% and samples with A r > 20% considered as disturbed samples. 3. Effect of friction and adhesion: If the length of sampler is large with respect to diameter, a bearing capacity failure may occur due to disturbance of sample.

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Do  Di x100 ………………………………………..…………………(2.2) Di where, Ci  inside clearance = (0.3−0.4)% and not more than 1%. Ci 

4. Effect of the way in which the force is applied to the spoon: that means by pushing or driving or by constant rate of penetration.

2.7 TESTING The tests performed on each type of the three different soil samples are as follows: As a rule, undisturbed samples (U) can be tested for strength and compressibility to determine the stress strain characteristics of the material, in addition to classification and chemical tests. Whereas, disturbed (D) or (SS) samples as available were mainly used for physical and geotechnical analysis of soil as well as chemical tests.

2.7.1 LABORATORY TESTS The obtained samples should be tested according to the procedure of the American Society for Testing and Materials (ASTM) or the British Standards (BS) whichever is appropriate. The test program of the samples includes the followings: 1. Classification Tests: Sieve and hydrometer analysis, natural water content, Atterberge limits, specific gravity, and wet and dry unit weights. 2. Compaction Test: Modified Procter compaction test must be carried out on some soil samples to obtain the maximum dry density (  dmax . ) and the relevant optimum moisture content (OMC). 3. Shear Strength and Compressibility Tests: Unconfined or Triaxial compressive strength test, one-dimensional consolidation, and swelling tests. 4. Chemical Tests: Sulphate Content (SO3-2)%, Total Soluble Salts (T.S.S.), Organic Matter Content (ORG.)%, PH- value, Carbonate Content (CO3-2), Chlorides Content (Cl-1)% , and Gypsum content %.

2.7.2 FIELD TESTS During the subsoil exploration, several field tests for soils or rocks as given in Tables (2.4) and (2.5), respectively, can be performed depending on the available testing equipment, required parameters for design of foundations, and the economic point of view.

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Foundation Engineering for Civil Engineers

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Table (2.4): Types of field tests for soils. Purpose of test

Type of test

1. SPT N-value (for granular soil)

 Standard or Dynamic Penetration Test (SPT).  Static Penetration Test (CPT)  Vane shear test (for soft to medium fine grained soil, clay and silt clay; up to Cu =1.0 kg/cm2),  Tor vane shear test (for soft soil; up to Cu =5.0 kg/cm 2),  Pocket penetrometer,  Pressuremeter test; it is of three types: a- Menard (to obtain; R D ,.,.Su ,.Es ,.G,.mv ,.Cc ),

2. Undrained shear strength (for cohesive soil)

b- Self boring (to obtain; R D ,.,.Su ,.u,.Es ,.G,.mv ,.Cc ,.Cv ), c- Screw boring (to obtain; E s ...and...G  ).

    

3. Bearing capacity

4. Elastic and shear modulus

5. Permeability

6. Compaction control

Pavements: plate bearing ;CBR test, Footings: plate bearing test, Piles subjected to vertical loads: load test, Batter piles: lateral load test. Seismic Tests: a- Cross-hole, b- Down-hole, and c- Surface refraction (to measure R D , Es , G , liquefaction resistance and thickness of soil layers).  Pumping Test: a- Constant head test, b- Variable head test, c- Piezometers test (or ground water observation).  Field or In-place Density: For Sand: a- Sand cone method, b- Rubber balloon method, For Clay: a- Penetration needle, b- Core cutter method.

Table (2.5): Types of field tests for rocks. Purpose of test

Type of test

1. Strength & Rock Quality Designation (RQD %)

Field Vane Direct Shear, Point Load, Pressuremeter2, Uniaxial Compressive2, Borehole Jacking2.

2. Bearing capacity

Plate Bearing1, Standard Penetration1.

3. Stress Conditions 4. Mass Deformability 5. Anchor Capacity 6. Rock Mass Permeability Notes:

Shear1,

Hydraulic Fracturing, Pressuremeter, Overcoring, Flat Jack, Uniaxial (Tunnel) Jacking2, Chamber (Gallery) Pressure2. Geophysical (Refraction)3, Pressuremeter or Dilatometer, Plate Bearing, Uniaxial (Tunnel) Jacking2, Borehole Jacking2, Chamber (Gallery) Pressure2. Anchor / Rockbolt Loading. Constant Head, Rising or Falling Head, Well Slug Pumping, Pressure Injection.

1. Primarily for clay shales, badly decomposed, or moderately soft rocks, and rock with soft seams. 2. Less frequently used. 3. Dynamic deformability.

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2.8 LOGS OF BORINGS AND RECORDS OF TESTING RESULTS At the beginning, a map giving specific locations of all borings should be available. Each boring should be identified (by number) and its location documented by measurement to permanent features. Such a map is shown in Fig.(2.9). For each boring, all pertinent data should be recorded in the field on a boring log sheet. These sheets are preprinted forms containing blanks for filling in appropriate data. Fig.(2.10) shows an example of a boring log sheet. Soil data obtained from a series of test borings can best be presented by preparing a geologic profile, which shows the arrangement of various layers of soil, the groundwater table, existing and proposed structures, and soil properties data. An example of a geologic profile is shown in Fig.(2.11). Depending on the results of the laboratory tests and the field observations, the actual subsoil profiles or logs of borings can more accurately be sketched (see Fig.(2.12)). In addition to, the actual description of soil strata in each borehole is summarized within records of tests results. 90m

B-1

B-2

7.5m

15m

30m

30m

15m

45m

B-3

15m

B-4

B-5

7.5m

Fig.(2.9): Example map showing boring locations on site plan. DRILLING COMPANY, INC. PROJECT:

BORE HOLE NO.: ------------LOCATION: --------------------

Name ---------------------------------------------------Date Time Depth Casing at Address ------------------------------------------------ ------------- -------------- ---------------- -----------------------------------CASING (SIZE AND TYPE) --------------------------------SAMPLE SPOON (SIZE AND TYPE) --------------------HAMMER (CSG): WT. ------------, DROP -----------------(SPOON): WT. ------------, DROP -----------------DATE: STARTED --------------------------------------------, COMPLETED ----------------------, DRILLER ----------------------

Field Samples No. Type 1 2 3 4 5 6

D U S.S D U S.S

Depth of Sampling (m) From To 0.0 2.0 4.5 5.0 7.0 9.5

2.0 4.0 5.0 7.0 9.0 10.0

'N'- Value

6

6

6

11

14

6

4

8

3

Visual Description of Soil Black and grey moist fill, Black peat. Sandy clay and silt mixture. Sandy silt and clay mixture. Silt with fine gravel and traces of fine sand. Sandy clay and silt mixture.

Fig.(2.10): Boring log sheet. 33

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Fig.(2.11): Example of geologic profile. Natural ground surface

(N.G.S.)

BH.no.2

BH.no.1

BH.no.3

0

2

Sandy clay and silt mixture

Sandy clay and silt mixture

Depth (m)

4

6

8

10

Sandy clay and silt mixture

E.O.B.

Sandy silt and clay mixture

Sandy clay and silt mixture

12 E.O.B.

E.O.B.

Fig.(2.12): Log of borings for 1st. stage of garden city housing project Tanahi District / Duhok city.

34

Chapter 2: Subsoil Exploration

A RECORD OF TESTS RESULTS

Foundation Engineering for Civil Engineers

35

Chapter 2: Subsoil Exploration

A RECORD OF TESTS RESULTS

Foundation Engineering for Civil Engineers

36

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.9 NUMBER OF BOREHOLES can be drawn with reasonable accuracy and then the preliminary program can be adjusted to suit subsoil conditions. Obviously, the more boreholes and the closer they are spaced, the more accurate the resulting geologic profile will be. Boreholes number and layout may need to be changed as more information emerges, so that, additional boreholes may be required during the survey. The layout of the borings should aim not only to provide soil profiles and samples at positions related to the proposed structures and their foundations, but should also be arranged to check the hypotheses formed during the survey, the geological succession, the presence of drift deposits and the extent of the various materials on site in order to allow cross-sections to be drawn. Also, when a structure is to be found on slope, the overall stability of the structure and the slope obviously must be investigated, and a deep borehole near the top of the slope is very useful. For example, a typical investigation for a motorway in the UK might use 5 to l0m deep borings every 150m along the proposed road line, with four 25 to 30m deep borings at the proposed position of each bridge structure. However, for rough guidelines, if soil conditions are relatively uniform or the geological data are limited, Tables (2.6, 2.7 and 2.8) can be used as a guide in planning of the preliminary program: Table (2.6): Number and spacing of boreholes according to the type of project (after Hvorslev 1949, and Road Research Laboratory 1952). Distance between borings (m) Project

Horizontal stratification of soil uniform average erratic

Multi-story building 1 or 2 story building Bridge, pier, abutment, Tv. Tower Highways Borrow pits Isolated small structures: such as small houses.

45 60 ---300 150−300

30 30 30 150 60−150

15 15 7.5 30 15−30

Minimum number of boreholes 4 3 1-2 ------1

Compact projects: such as buildings, dams, bridges or small landslips

4 deeper and closely spaced

Extended projects: such as motorways, railways, reservoirs and land reclamation schemes.

shallower and widely spaced

Table (2.7): Number of borings for medium to heavy weight buildings, tanks, and other similar structures on shallow foundations (after Sowers, 1979). Subsurface Conditions

Structure Footprint Area for Each Exploratory Boring (m2) 100 – 300 200 – 400

Poor quality and / or erratic Average High quality and uniform

300 – 1000

37

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Table (2.8): Minimum borings guidelines (after MDT Manual, 2008). Geotechnical feature

Minimum number of borings

Shallow Foundations

 Provide at least 1 boring per substructure unit ≤ 30 m width,  Provide at least 2 borings per substructure unit > 30 m width,  Provide additional borings in areas where erratic subsurface conditions are encountered.

Deep Foundations

 Provide at least 1 boring per bridge pier location for driven piles,  Provide at least 1 boring per drilled shaft location, unless the shaft is part of a group in which case a single boring per group is sufficient,  Provide additional borings in areas where erratic subsurface conditions are encountered, especially for shafts socketed into bedrock.

Retaining Structures

 Provide at least 1 boring for each retaining wall,  For walls of length > 30 m, space borings 30 m to 60 m with locations alternating from in front of the wall to behind the wall,  For anchored walls, provide additional borings in the anchorage zone space at 30 to 60 m,  For soil-nailed walls, provide additional borings at a distance of 1.0 −1.5 times the height of the wall behind the wall spaced at 30 to 60 m.

Bridge Approach Embankments over Soft Ground

 When approach embankments are to be placed over soft ground, provide at least 1 boring at each embankment to determine if any problems associated with stability and settlement of the embankment are present,  Typically, test borings taken for the approach embankments are located at the proposed abutment locations and serves a dual function.

Roadways

 Provide borings at a minimum spacing of approximately 300 m,  Provide additional borings in areas where erratic subsurface conditions are encountered.

Roadway Cuts Embankments

 Typically, space borings every 60 m for erratic conditions, to 120 m for uniform conditions with at least 1 boring taken in each separate landform,  For high cuts and fills and other critical locations, provide a minimum of 3 borings along a line perpendicular to centerline or planned slope face to establish geologic cross-section for analysis.

and

Landslides

 Provide a minimum of 3 borings along a line perpendicular to centerline or planned slope face to establish geologic cross-section for analysis,  The actual number of borings depends on the extent of stability problem.

Ground Improvement Techniques

 Varies widely depending on ground improvement technique(s) being employed, for more information, see FHWA Ground Improvement Technical Summaries, FHWA SA−98−086R.

Material Sites (Borrow Sources, Quarries)

 Space boring every 30 m to 60 m.

38

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.10 DEPTH OF BORINGS It is good practice on any site to sink at least one deep borehole to establish the site geology. On extended projects several of these may be necessary, partly in order to establish the depth of weathering. Hvorslev (1949) and MDT Manual (2008) suggested a number of general rules which remain applicable:  The borings should be extended to strata of adequate bearing capacity and should penetrate all deposits which are unsuitable for foundation purposes such as unconsolidated fill, peat, organic silt and very soft and compressible clay;  The soft strata should be penetrated even when they are covered with a surface layer of high bearing capacity;  When structures are to be founded on clay and other materials with adequate strength to support the structure but subject to consolidation by an increase in the load, the borings should penetrate the compressible strata or be extended to such a depth that the stress increase for still deeper strata is reduced to values so small that the corresponding consolidation of these strata will not materially influence the settlement of the proposed structure;  In case of very heavy loads or when seepage or other considerations are governing, the borings may be stopped when rock is encountered or after a short penetration into strata of exceptional bearing capacity and stiffness, provided it is known from explorations in the vicinity of the area that these strata have adequate thickness or are underlaid by still stronger formations. But, if these conditions are not satisfied, some of the borings must be extended until it has been established that the strong strata have adequate thickness irrespective of the character of the underlying material;  When the structure is to be founded on rock, it must be verified that bedrock and not boulders have been encountered, and it is advisable to extend one or more borings from 3 to 6m into solid rock in order to determine the extent and character of the weathered zone of the rock; For rough guidelines, the following criteria can be used for minimum depths, from considerations of stress distribution or seepage,:

1. Foundations:  All borings should extend below all deposits such as top soils, organic silts, peat, artificial fills, very soft and compressible clay layers;  Boring should be sufficiently deep for checking the possibility of a weaker soil at greater depth which may settle under the applied load;  Deeper than any strong layer at the surface checking for a weaker layer of soil under it which may cause a failure (see Fig.(2.13a));  The depth at which the net increase in stress due to the foundation or building load is less than 5% of the effective overburden pressure;  The depth at which the net vertical total stress increases because the foundation or building load is less than 10% of the stress applied at foundation level (contact pressure);  For isolated spread footings or raft foundations, explore to a depth equal 1.5B (B = least width of the footing or the raft) (see Fig.(2.13b));

39

Foundation Engineering for Civil Engineers

 

Chapter 2: Subsoil Exploration

For group of interfering footings, explore to a depth equal 1.5B′ (where, B′ = width of interfering footings) (see Fig.(2.14)); For heavy structures (pressure ≥ 200 kPa), the depth of borings should be extended to 2B (width of footing); L  10 . B



For strip footings, explore to not less than 3B (width of footing) for B  6m and



For multistory buildings, explore to: (i) D  Df  3.S0.7 (in meter)………. for light steel or narrow concrete buildings, (ii) D  Df  6.S0.7 (in meter) .…….. for heavy steel or wide concrete buildings. where, D = Depth of boring, D f = Depth of footing, and S = Number of stories.



2

If piled foundation is expected, the borehole depth D =(𝐷𝑓 + 3 L+ 1.5B) or D = (L + 3m) into the bearing stratum (see Fig.(2.15a));

2. Reservoirs: Explore soil to: (i) The depth of the base of the impermeable stratum, or (ii) Not less than 2 x maximum hydraulic head expected. 3. Dams: Because the critical factor is the safety against seepage and foundation failure, boreholes should penetrate not only soft or unstable soils, but also permeable soils to such a depth that seepage patterns can be predicted. Thus, it is recommended: For earth structures, explore soil to 1.5 times the base width of the dam, and  For concrete structures, explore to a depth 1.5−2.0 times the height of the dam, and for any case, the depth of boring must be checked with (1.5−2.0) of the water head. 4. Roads, highways, and air fields: the minimum depth is 5m below the finished road level, provided that vertical alignment is fixed but should extend below artificial fill. In practice, some realignment often occurs in cuttings, and side drains may be dug up to 6m deep or to bore to at least 1.5 times the embankment height in fill areas, and to at least 5m below finished road level in cut. 5. Retaining walls, slopes stability problems: Explore to:  1.5B (wall base width) or 1.5H (wall height) whichever is greater below the bottom of the wall or its supporting piles (see Fig.(2.15b)), Moreover,  It must be below an artificial fills, and deeper than possible surface of sliding; 6. Canals, deep cut and fill sections on side hills: Explore at least to: (i) 3m below the finished level in cut, or (ii) B when B  H , or (iii) H when B  H (see Figs.(2.16a and 2.16b)). 7. Embankments: The depth of exploration should be at least equal to the height of the embankment and should ideally penetrate all soft soils if stability is to be investigated. If settlements are critical then soil may be significantly stressed to depths below the bottom of the embankment equal to the embankment width (see Fig.(2.16c)).

40

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

P G.S.

Borehole Strong layer Weak layer

(a) Existence of rock layer

B

S

B

LB Column Footing

Plan P1

G.S.

P2

when

P

S  4B

G.S. Df

Borehole depth

 (Df  1.5B)

D = 1.5B

Borehole depth

B

 (Df  1.5B)

Section

(b) Isolated spread footing

(c) Raft or mat foundation.

Fig.(2.13): Depth of borings for spread and raft foundations.

41

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

W B

S

B

S

B

B

S

L=W

B

S

B

P2

P1

P3 G.S.

S

S

Df

Borehole depth

 (D f  1.5B)

B

B

B

B (a) Single row of adjacent spread footings when S  4B P2

P1

P3

G.S. S Borehole depth

 (Df  D)

S B

B

Df B

D = 1.5B when S  4B = 3.0B when S  4B..and..S  2B = 4.5B when S  2B (b) Multiple rows of adjacent spread footings. Fig.(2.14): Depth of borings for adjacent spread footings.

42

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

P

G.S. B

Df

L

Pile cap

2 L 3

Borehole depth

 (D f 

Individual pressure bulbs Combined pressure bulb

Rock or hard layer (a) Piles

G.S.

Backfill Soil H

G.S. B Base Soil

Depth of B.H. = 1.5B or 1.5H whichever is greater

(b) Retaining walls Fig.(2.15): Depth of borings for piles, and retaining walls.

43

2 L  1.5B) 3

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Fill H

Cut B Depth of borehole = 3m minimum or

= B when B  H or = H when B > H

(a) Deep cut and fills sections on side hills. Fill Side hill

Fill Cut Depth of borehole = 3m mimm.

(b) Normal section of a canal.

H

Depth of borehole = H minm.

(c) High embankment Fig.(2.16): Depth of borings for cuts and fills, canals, and embankments. 44

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.11 FIELD LOAD TEST It is a method to investigate the stress-strain (or load−settlement) relationship of soils. Then, the results are used in estimating the bearing capacity. In this test, the load is applied to a model footing and the amount of load necessary to induce a given amount of settlement is measured. Round plates from (150−750)mm in diameter by 150mm increment (i.e., 150, 300, 450, 600, 750)mm are available as well as square plates of (1.0 ft2) area. The minimum thickness of plate is (1 inch or 25.4mm).

1.0 ft2

25.4 mm

square plate

Round plate

Procedure of load test as given by ASTM D110−72: (1) Excavate a pit to width at least 6 times as wide as the used plate, and to the depth that the foundation is to be placed. P G.S. B

Df 2.5 B

2.5 B

If it is specified that three sizes of plates are to be used for the test, the pit should be large enough so that, there is an available spacing between tests of 3 times the diameter (D) of the largest plate. This is useful for studying the size effect of footings. G.S.

300mm

D = 750mm

450mm

Df 2.5 B

(2)

(3)

3D

3D

2.5 B

A square loading plate 2.5cm thick and (30 x 30)cm is placed on the surface of the soil at the bottom of the pit. There should not be any surcharge load placed on the soil within a distance of (60cm) from around the plate. A vertical load is placed on the plate in increments and settlements are recorded as an average from at least three dial gauges accurate to (0.025mm) attracted to an independent suspension system. Load increment should be approximately 1/10 of the estimated allowable soil pressure. For each load increment, settlement readings should be taken at regular intervals of not less than (1 hr.) until there is no further settlement. The same time duration should be used for all the loading increments.

45

Foundation Engineering for Civil Engineers (4) (5) (6)

Chapter 2: Subsoil Exploration

The test is continued until a settlement of 25mm is observed or until the load increments reached 1.5 times the estimated allowable soil pressure. If the load is released, the elastic rebound of the soil should be recorded for periods of time equal to the same time durations of each applied load increment. The result of each test can be represented graphically as follows (see Fig.2.17):(a) Settlement versus log time curve (for each load increment), (b) Load-settlement curve (for all increments) from which qult . is obtained. Pressure (kN/m2)

q ult .

q ult .

Settlement (mm)

Settlement (mm)

q ult .

Log Time (min.)

b

a

settl.

c

(a) Load - settlement curve

(b) Log time- Settlement curve

Fig.(2.17): Typical load test results.



For cohesive soil (bearing capacity is independent of footing size):

 qf  qp  s  s B f ................................................................................(2.3) p  f Bp  

For cohesionless soil (bearing capacity increases with size of footing):

Bf   qf  qp B p    s  s  2B f p  f  Bp  Bf 



   

2 .................................................................(2.4)

Settlement for both cohesive and cohesionless soils:

sf / Bf Bf n ( ) .....................................................................(2.5) sp / Bp Bp

46

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

where, s f and s p are settlements of footing and plate, B f and B p are their respective widths; provided that B p = 1.0 ft for

Bf Bp

 5 as well as the footing and plate carry the same

intensity of load, and (n) is an exponent that depends on soil type; with some of its values are:



Type of soil

n

Clay Sandy clay Dense sand Medium sand Loose sand

0.03 - 0.50 0.08 - 0.10 0.40 - 0.50 0.25 - 0.35 0.20 - 0.25

For c   soils (bearing capacity from two-plate load tests; after Housel, 1929):

V  A.q  P.s .................................................................................(2.6) where, V = total load on a bearing area, A = contact area of footing or plate, q = bearing pressure beneath A, P = perimeter of footing or plate, and s = shear perimeter. This method needs data from two-plate load tests so that Eq.(2.6) can be solved for q and s (for given settlement). After the values of q and s are known, the size of a footing required to carry a given load can be calculated.

2.12 FIELD PENETRATION TESTS 2.12.1 DYNAMIC OR STANDARD PENETRATION TEST (SPT) This test is preferred for very hard deposits, particularly of cohesionless soils for which undisturbed samples cannot be easily obtained. It utilizes a split-spoon sampler shown previously in Fig.(2.8a) that is driven into the soil. The test consists of driving the standard split-barrel sampler of dimensions (680mm length, 30mm inside diameter and 50mm outside diameter) a distance of 460mm (18'') into the soil at the bottom of the boring. This was done by using a 63.5kg (140Ib) driving mass (or hammer) falling "free" from a height of 760mm (30"). Then, counting the number of blows required for driving the sampler the last 305mm (12") to obtain the (N) number (neglecting the no. of blows for the upper first 150mm).

Note: The SPT- value is rejected or halted in any one of the following cases: (a) if 50 blows are required for any 150mm increment, or (b) if 100 blows are obtained, or (c) if 10 successive blows produce no advancement.

47

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

The number of blows (N) can be correlated with the relative density ( D r ) of cohesionless soil (sand) and with the consistency of cohesive soil (clay) as shown in Tables (2.9, 2.10 and 2.11). Table (2.9): Relative density of sands according to results of standard penetration test. Relative density SPT- value N/30cm

Dr 

emax  einsitu x100 emax  emin



0-4

0 -15

Very loose

28

4 - 10

15 - 35

Loose

28 - 30

10 - 30

35 - 65

Medium

30 - 36

30 - 50

65 - 85

Dense

36 - 41

> 50

85 - 100

Very dense

> 41

Table (2.10): Relation of consistency of clay, SPT N-value, and unconfined compressive strength ( q u ). SPT- value N/30cm

consistency

q u (ksf )

Below

Very soft

0 - 0.5

0 - 0.25

2-4

Soft

0.5 - 1

0.25 - 0.5

4-8

Medium

1-2

0.5 - 1

8 - 15

Stiff

2-4

1-2

15 - 30

Very stiff

4-8

2-4

> 30

Hard

>8

>4

q u (kg / cm 2 )

Table (2.11): Empirical values for ,..Dr , and  of granular soils based on the SPT at about 6 m depth and normally consolidated. Description

Very loose

Loose 0.15 – 0.35

Medium

Dense

Very dense

0.35 – 0.65

0.65 - 0.85

> 0.85

Relative density D r

0 - 0.15

SPT N70 : Fine Medium Coarse

1-2 2-3 3-6

3-6 4-7 5-9

7-15 8-20 10-25

16-30 21-40 26-45

26-28 27-28 28-30

28-30 30-32 30-34

30-34 32-36 33-40

33-38 36-42 40-50

70-100 (11-16)

90-115 (14-18)

110-130 (17-20)

110-140 (17-22)

 :  wet :

Fine Medium Coarse Pcf (kN/m3)

48

-----> 40 > 45 -----< 50 -----130-150 (20-23)

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.12.1.1 Corrections for N−value (1)

and

W.T. Correction (in case of presence of W.T.): For N > 15:

N corr .  15  0.5( N field  15) .…….…………………………..…(2.7)

For N  15:

N corr .  N field …….……………..…......……..……………..….(2.8)

 If N-value is measured above water table, no need for this correction. (2)

Overburden pressure, C N ; Energy ratio, 1 ; Rod length,  2 ; Sampler;  3 ; and Borehole dia.,  4 Corrections:

N70  Nfield .CN ..1..2..3..4 …………….……………..…….(2.9) where,

N70  corrected (N) using the subscript for the energy ratio E rb and ( ' ) to indicate it has been adjusted or corrected, C N = adjustment for overburden pressure for p  25.(kPa) and can be calculated from the following formula:

C N  0.77 log

2000 ………..………………....……………….(2.10) Po

 If p  25.(kPa) , no need for overburden pressure correction. where,

p o  overburden pressure in ( kPa ), i : factors obtained from (Table 2.12) as: 1  hammer correction = (average energy ratio)/(drill rig energy) = E r / E rb ; 2  rod length correction; 3  sampling method correction; and 4  borehole diameter correction.

49

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Table (2.12): Hammer, borehole, sampler, and rod  i correction factors. Hammer correction 1

Country

Average energy ratio Er Donut Safety R-P Trip R-P Trip

USA North America

45

----

70-80

80-100

Japan

67

78

------

-------

UK

----

----

50

60

China

50

60

------

-------

R-P = Rope -Pulley: 1  E r / E rb For USA trip/auto w / E r = 80

1  80/70 = 1.14

Rod length correction 2

Length

2 = 1.00

> 10m 6 -10 4-6 0-4

= 0.95 = 0.85 = 0.75

N is too high for L < 10 m

Sampling method correction 3

3 = 1.00

Without liner: With liner: Dense sand, Clay Loose sand.

= 0.80 = 0.90

N is too high with liner

Borehole diameter correction 4

Hole diameter

60 -120 mm 150 mm 200 mm

4 = 1.00 = 1.05 = 1.15

N is too small for oversize hole

Notes: 1. It is evident that all i =1.0 for the case of a small borehole, no sampler liner, length of

drill rod > 10 m and the given drill rig has E r  70 . In this case, the only adjustment is for overburden pressure (i.e., Ncorr .  Nfield .CN ). E 2. Large values of E r decrease the blow count (N) linearly (i.e., N 2  r1 ..N1 ). This E r2 equation is used to convert any energy ratio to any other base.  30  3. If N field  10...blows / 10cm , then Ncorr .  10.   30...blows / 30cm.  10 

50

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.12.3 STATIC OR CONE PENETRATION TEST (CPT) This is a simple static test used for soft clays and fine to medium coarse sands. The test is not applicable in gravels and stiff hard clays. It is performed by pushing the standard cone (according to ASTM D3441 with a 60o point and base diameter = 35.7mm with cross-section area of 10 cm2) into the ground at a rate of (10 – 20) mm/sec. Several cone configurations can be used such as: 1. Mechanical or the earliest "Dutch Cone Type", 2. Electric friction with strain gauges, 3. Electric piezo for pore water measurement, 4. Electric piezo/friction to measure q c , q s and u or (pwp), and 5. Seismic cone to compute dynamic shear modulus. Fig.(2.18b) shows the operations sequence of a mechanical cone as: in position (1) the cone

is seated; position (2) advances the cone tip to measure q c ; position (3) advances the friction sleeve to measure q s ; and position (4) advances both tip and sleeve to measure q t = q c + q s . Therefore, at any required depth, the tip and sleeve friction resistances q c and q s are measured and then used to compute a friction ratio f R as: q f R (%)  s x100 ; f R < 1% for sands; f R > 5 or 6% for clays and peat. qc The data collected from the CPT can be correlated to establish the undrained shear strength S u of cohesive soils, allowable bearing capacity of piles, to classify soils; and to estimate ,..Dr for sands. A typical data set is shown in Fig.(2.19b).

 CPT Correlations 1. The cone resistance q c is related to the undrained shear strength S u of cohesive soils as:

q  po ……………………………………………….(2.11) Su  c Nk where, p o  .z = overburden pressure at point where q c is measured., and N k = cone factor computed from Fig.(2.20). 2. The q c and f R results are used to classify soils from Fig.(2.21) for standard electronic cone.

The cone resistance q c can be related to the pore pressure as: q  po ….…………………………………………….(2.12) Su  T N kT where, q T  q c  u.(1  a ) , a = area ratio depends on the cone type, u = measured pore water pressure, and NkT  13  (5.5.Ip / 50) . 3.

For normally consolidated clays of low sensitivity (S t  4) and I p  30 a value of N k about 18 and N kT of 14 may be satisfactory (see Figs. (2.18) to (2.25)).

51

Foundation Engineering for Civil Engineers

(a) Dutch cone modified to measure both point resistance q c and skin friction q f .

Chapter 2: Subsoil Exploration

(b) Positions of the Dutch cone during a pressure record.

(c) Typical output.

Fig.(2.18): Mechanical (or Dutch) cone, operations sequence, and tip resistance data.

(a) Piezocone.

(b) Cone Penetration record for clay soil.

Fig.(2.19): Electric cone and CPT data.

52

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Fig.(2.20): Cone factor N k vs. I P (after Anagnostopoulos etal., 2003 ).

Fig.(2.22): Approximate relationship between q c and D r (after Schmertmann, 1978).

53

Fig.(2.21): Soil classification chart for standard electric cone. (after Anagnostopoulos etal., 2003 ).

Fig.(2.23):  vs. D r (after Schmertmann, 1978).

Foundation Engineering for Civil Engineers

(a) Cone bearing vs.  relationship.

Chapter 2: Subsoil Exploration

(b) Correlation between peak friction angle  and q c for uncemented, quartz sands (after Robertson and Campanella, 1983).

Fig.(2.24): Correlations between cone data and angle of internal friction (  ).

Fig.(2.25): Relationship between mean grain size ( D50 ) and q c / N ratio. (after Robertson et al., 1983 and Ismael and Jeragh, 1986)

54

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2.13 VANE SHEAR TEST It is a field test used to determine the in-situ shearing resistance (undrained shear strength) of soft to medium clay and silt clay having U.C.S.< 1.0 ( kg / cm 2 ), then to be used for design of foundations and slopes.



Apparatus (see Fig.(2.26):

1. 2. 3. 4.

Van shear test equipment; Drilling rig; Casing (as required); and Other necessary tools and supplies such as stop watch, pipe,… etc..

Fig.(2.26): Vane shear apparatus.

55

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

 Procedure: 1. The equipment is installed in place properly either at the ground surface without a hole (case 1) or at the bottom of a borehole (case 2) and then the vane is pushed into the soil layer to the required depth; (see Fig.(2.27)). 2. A torque is applied at a uniform rate of 0.1o per sec. or (1o-6o per minute). 3. Readings are taken every minute interval until failure happens. T T T G.S.

t H

S

H

S S S

S

a S

4t  11% .D

S D

Case 2

Standard dimensions of vane B.S. 1377 Rate of test (6 -12)deg./ min.

dr r

D

S

Case 1

where, t = thickness of plate, and D = diameter of vane.

D

D

H

Soil strength (kPa) < 50 50-75 > 75

H (mm) D (mm) 150 75 100 50 Not suitable

Fig.(2.27): Vane shear standards.

 Calculation: (i) Case (1): In this case, the vane is not embedded in soil, so that only the bottom end takes pant in shearing. If the soil is isotropic and homogenous, then: (a) Total shear resistance at failure developed along cylindrical surface = .D.H.S (b) Total resistance of bottom ends, considering a ring of radius r and thickness dr D/2

=  (2.r.dr ).S 0

56

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

(c) The torque T at failure will then equal: T  (.D.H.S)

or

T

D D/2   (2.r.dr ).S.r 2 0

.D2Su D (H  ) ……………………….………..………..……(2.13) 2 6

(ii) Case (2): If the top end of the vane is also embedded in soil, so shearing takes place on top and bottom ends: or

T

.D2Su D (H  ) ………………………….……..………..……(2.14) 2 3

Notes:  Use consistent units, such as: T in (kg-cm); S u in (kg/cm2); and H and D in (cm).  It is found that the S u values obtained by vane shear test are too large for design.

Therefore, Bjerrum's (1972) proposed a reduction factor using the following formula: S u , design  ..S u , field ……………………….………..………..……(2.15)

where,  is a correction factor depends on plasticity index I p and obtained from Fig.(2.28a); Also, Aas et al. (1986) proposed another charts (see Fig.(2.28b)) taking into account the effects of aging and OCR (Overconsolidation ratio).

Ip , %





Ip , %

Vane strength ratio Su, v / P′o

(a) Bjerrum correction factor for vane-shear test. [(Bjerrum, 1972) and Ladd et al., 1977)].

(b) Reinterpretation of the Bjerrum chart of part a by (Aas et al. (1986) to include effects of aging and OCR ).

Fig.(2.28): Vane shear correction factor  .

57

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

SOLVED PROBLEMS Problem (2.1): A thin-walled tube (OD = 76.2mm, ID = 73mm) was pushed into a soft clay at the bottom of a borehole a distance of 600mm. When the sampler was recovered a measurement done inside the tube indicated a recovered sample length of 575mm. Calculate the recovery and area ratios. Solution: Recovery ratio: L r  Area ratio:

Ar 

575  0.958 600 (76.2) 2  (73) 2 (73) 2

x100  8.96%

Problem (2.2): A three story steel frame office building will be built on a site where the soils are expected to be of average quality and uniformity. The building will have a (30m x 40m) footprint and is expected to be supported on spread footing foundations located about (1m) below the ground surface. The site appears to be in its natural condition, with no evidence of previous grading. Bedrock is several hundred feet below the ground surface. Determine the required number and depth of the borings. Solution:  Number of borings: From Table (2.7), one boring will be needed for every 200 to 400 m2 of footprint area. Since the total footprint area is 30 x 40 =1200 m2, use (4) four borings.  Depth of borings: For subsurface condition of average quality, the minimum depth is: 4.

4.5.S0.7  Df  4.5(3) 0.7  1  11m. However, it would be good to drill at least one of the borings to a slightly greater depth to check lower strata. In summary, the exploration plan will be 4 borings with, 3 borings to 11 m, and 1 boring to 15 m.

Problem (2.3): Structure:

Given: Available information about: Multistory building with 3 stories and basement No. of columns = 16, Column load = 1000 kN Raft dimensions: 16m x 16m x 1m, Foundation at 3m below G.S.

Soil profile:  d = 16 kN/m3 ,  sat = 20 kN/m3 , W.T. at 6m below G.S. Required: Number, layout, and depth of B.Hs.?

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Solution:  Number and layout of borings: From Table (2.7), for poor quality and/or erratic subsurface conditions, one boring is needed for every (100 to 300) m2 of footprint area. Since the total footprint area is 16x16 = 256m2 > 200m2 (average value), use one or two borings.  Depth of borings: (a)

d  1.5(16)  24 m

(b)

10% of contact pressure:

q contact ( net )  0.1.(38.5)  (c)

16.(1000)  24(16)(16)(1)  (3)(16)  38.5..kPa (16)(16)

38.5(16)(16) (16  d) 2

, . ………………..….………..……d = 34.6 m

5% of overburden pressure:

0.05.[16(6)  (d  3)(20  10)] 

38.5(16)(16) (16  d) 2

, …………...d = 15.5 m

From (b and c) take the lower d = 15.5 m (d)

d  6.S0.7  6.(4)0.7  15.83m From (24m, 15.5m, and 15.38m) take the highest d = 24 m

 Use D = 24 + 3 = 27 m from G.S. Problem (2.4): A wide strip footing applying net pressure of 35 kPa is to be constructed 1.0 m below the surface of uniform soil having unit weight of 19 kN/m 3. The footing is 5.0 m wide and the water table is at ground surface. Is 12 m depth of boring (measured from ground surface) sufficient for subsoil exploration program. Solution: (a) d  3( B)  3(5)  15 m

(35)(5)(1) ,....................... d = 4.3 m (5  d)(1  d) (35)(5)(1) ,................ d = 5.2 m (c) 5% of overburden pressure: 0.05(9  9d)  (5  d)(1  d) From (b and c) take the lower d = 4.3 m (b) 10% of contact pressure:

0.1.(35) 

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

From (15 m, and 4.3 m) take the highest d = 15 m, and so the depth from ground surface D  15  1  16 m,

 12 m is not sufficient.

Problem (2.5): A standard penetration test was performed on dense sand soil. If Nfield = 20, rod length = 12m, hole diameter = 150mm, po  205...kPa , determine N60 and N70 for each of the following conditions: a. Use safety hammer with E r  80, and no liner.

Use safety hammer with E r  60, and with sample liner. c. po  100.kPa , 205 mm hollow stem auger, hole depth = 6m, use safety hammer with b.

E r  55, and no liner. Solution: (a)

Since po  205...kPa  25 kPa  C N  0.77. log10

2000 2000  0.77. log10  0.76 po (kPa) 205

From (Table 2.12):

1  E r / E rb  80 / 70  1.14  2 = 1.00 (for L = 12m (rod length  10m)),  3 = 1.00 (for no liner),  4 = 1.05 (for B.H. diameter = 150mm), N70  Nfield ..CN ..1..2..3..4 = 20 (0.76)(1.14)(1.00)(1.00)(1.05) = 18 and

N60..Er 60  N70.. Er 70 , (b)

or

N60 

70 (18)  21 60

C N  0.76 From (Table 2.12):

1  Er / Erb  60 / 70  0.86  2 = 1.00 (for L = 12m (rod length)  10m),  3 = 0.80 (dense sand with liner),  4 = 1.05 (for B.H. diameter = 150mm), N70  Nfield ..CN ..1..2..3..4 = 20 (0.76)(0.86)(1.00)(0.80)(1.05) = 10 and

N60..Er 60  N70.. Er 70 ,

or

N60 

70 (10)  11 60

60

Foundation Engineering for Civil Engineers

(c)

Chapter 2: Subsoil Exploration

Since po  100...kPa  25 kPa  C N  0.77. log10

2000  1.00 100

From (Table 2.12):

1  Er / Erb  55 / 70  0.79  2 = 0.95 (for L  6m (rod length)  10m),  3 = 1.00 (for no liner),  4 = 1.00 (for 205mm hollow stem auger), N70  Nfield ..CN ..1..2..3..4 = 20 (1.00)(0.79)(0.95)(1.00)(1.00) = 15 and

N60..Er 60  N70.. Er 70 ,

or

N60 

70 (15)  17 60

Problem (2.6): A standard penetration test SPT has been conducted in a coarse sand to a depth of 4.8 m below the ground surface. The blow counts obtained in the field were as follows: 0 – 6 in: 4 blows; 6 -12 in: 6 blows; 12 -18 in: 8 blows. The test was conducted using a USA-style donut hammer in a 150mm diameter boring with a standard sampler and liner. If the vertical effective stress at the test depth was 70 kN/m2, determine N60 ? Solution: The raw SPT value is N = 6 + 8 = 14 Since po  70...kPa  25 kPa  C N  0.77. log10

2000  1.12 70

From (Table 2.12): 1  Er / Erb  45/60 = 0.75

 2 = 0.85 (for L  4.8m (rod length)  6m),  3 = 0.90 (for loose sand with liner),  4 = 1.05 (for B.H. diameter = 150mm),

N60  Nfield ..CN ..1..2..3..4 = 14(1.12)(0.75)(0.85)(0.90)(1.05) = 10 blows Problem (2.7): A standard penetration test was carried out in sand at 5m depth below the ground surface gave (N = 28) as shown in the figure below. Find the corrected N-value? G.S.

Solution:  Water table correction:

2m

For N  15 ..…. N  15  0.5.( Nfield  15)

N = 15 + 0.5 (28 −15) = 21

W.T.

 = 18 kN/m3 Fine sand

3m

 sat = 20 kN/m3 Nfield  28..blows / 30cm

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

 Overburden correction:

Po = 2(18) + 3(20 – 9.81) = 66.57 kPa  25 kPa 2000 2000  1.14 = 0.77 log  C N  0.77 log Po 66.57  Ncorr .  N..CN = 21(1.14) = 23 blows Problem (2.8): A standard penetration test was carried out at a depth of (10m) in a saturated fine sand yielded a blow count of (N = 41). If the saturated unit weight of the sand equals (19 kN/m3), find the corrected value of (N).

Solution:  Water table correction: For N  15 ………. N  15  0.5.( Nfield  15)

N = 15 + 0.5 (41 −15) = 28 blows

 Overburden correction:

Po = 10 (19 – 9.81) = 91.9 kPa  25 kPa 2000 2000  1.03 = 0.77 log  C N  0.77 log Po 91.9  Ncorr .  N..CN = 28(1.03) = 28.8, say 28 blows

Problem (2.9): It is proposed to construct a spread wall footing of (3m width) in sand at (1.5m) below the ground surface to support a load of 12 Ton/m. The SPT results from a soil boring are as shown below. If the water table is located at 0.9m from G.S. and  soil(sat .)  17.6 kN/m3, determine the average corrected N-value required for design? SPT sample depth (m)

1.5

2.25

3.0

3.75

4.5

5.25

6

Nfield

31

25

22

20

28

33

31

Solution: Find Po at each depth and correct Nfield values up to at least a depth B below the base of foundation according to the magnitude of overburden pressure in comparison with 25 kPa. Overburden pressure correction: C N  0.77 log

62

2000 Po

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

For 1.5m depth:

Po = 0.9(17.6) + (0.6)(17.6 – 9.81) = 20.5 kPa  25 kPa, therefore, C N =1.00

For 4.5m depth:

Po = 0.9(17.6) + (3.6)(17.6 – 9.81) = 43.9 kPa  25 kPa, therefore, C N =1.28

Find the average corrected N-value as a cumulative average down to the depth indicated, and then, choose the N-value for design as the lowest average N-value. SPT sample depth (m) 1.5 2.25 3.0 3.75 4.5

Po

Nfield

CN

N  CN .Nfield

N  15  0.5( N  15)

 . N avg

1.00 1.45 1.38 1.32 1.28

31 36 30 26 35

23 25 22 20 25

23 24 23 22 23

2

(kN/m )

31 25 22 20 28

20.5 26.3 32.2 38.0 43.9

For 1.5m depth:

Navg.  23

23  25  24 2 23  25  22  23 For 3.0m depth: N avg.  3 23  25  22  20  22 For 3.75m depth: N avg.  4 23  25  22  20  25  23 For 4.5m depth: N avg.  5 N-value for design = Navg. (lowest)  22 blows For 2.25m depth: N avg. 

qc  300.(kg / cm 2 ) at depth z = 8m in sand with    11.15 (kN/ m3). Required: estimate angle of internal friction (  ).

Problem (2.10): Given:

Solution:

po  8(11.15)  89.2...kPa

q 300(100)  336 qc  Vb .po , Vb (bearing capacity factor)  c  po 89.2 2 1. Using Fig.(2.22) with qc  300.(kg / cm ) and p o  89.2.kPa , the intersection is above D r  100% ; with D r  100% and using Fig.(2.23) obtain  = 42 to 46, say  = 44o,

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Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

2. From Fig.(2.24a) at Vb =336 project to curves and down to obtain:  = 34.5 to 39.5 , say  =37o,

300.(98.07)  29.4.MPa , obtain  = 46o. 1000 (37  44  46) A better estimate might be obtained by taking an average value:    42 o 3 but this value is still, high, because it is recommended to use  not over (40o). 3. From Fig.(2.24b) and q c =

Problem (2.11): Classify the soil of Fig.(2.19b) at (10-12)m depth. Also, estimate the undrained shear strength S u if the average  =19.65 kN / m 3 for the entire depth of CPT. It is known that the profile is entirely in cohesive soil. Solution: From Fig.(2.19b), at average depth of 11.0m: q c = 11 MPa and f R =4%  p o =19.65(11) = 216 kPa, from Fig.(2.19) estimate N k  18 . 

Using Fig.(2.21), with q c = 11000 kPa and f R = 4%, the soil is stiff, sandy silt and (the expected I p = 10 or less),

 Su =

11000  216  600 kPa. 18

Problem (2.12): Load-settlement data obtained from load test of square plate of size (1.0 ft) are as shown below. If a square footing of size (7.0 ft) settles (0.75 inch), what is the allowable soil pressure of the footing? Consider sandy soil. Load (Tsf) Settlement (inch)

2 0.1

5 0.2

8 0.3

10 0.4

14 0.6

16 0.8

19 1.0

Solution:

Bf   qf  qp B p  For cohesionless soil:    s  s  2B f f p   Bp  Bf 

   

2 ,

   0.75  sp   2   2x 7   1 7   

Now by drawing the given data and for s p  0.25 , q p  6.5 T/ft2, and

Pressure (Tsf)

s p  0.25 Settlement (inch)

B 7 q f  q p f  6.5  45.5 T/ft2 . Bp 1

64

   0.75  0.25   3.05  

q p  6.5

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

Problem (2.13): Use Housel’s method to determine the size of square footing required to carry a column load P = 45 tons if the two plate loading tests results are as given below: plate size (1) = 35 x 35 cms, corresponding load = 5.6 tons; relative to 1.0 cm settlement.  plate size (2) = 50 x 50 cms, corresponding load = 10 tons; relative to 1.0 cm settlement. Solution: From Housel's method (Eq. 2.6):

V = A. q + P. s 5.6 = 0.123 q + 1.4 s 10 = 0.25 q + 2 s

Solving the two equations, gives: q = 26.9 and s = 1.63. Again from Eq.(2.6) shown above, the footing area required to carry 45 tons load is calculated as: 45 = B2 q + 4B s 45 = B2 (26.9) + 4B (1.63) 26.9 B2 + 6.52 B – 45 = 0 B2 + 0.24 B – 1.67 = 0

B

 0.24  (0.24) 2  4(1)(1)(1.67)  0.24  2.59   1.18 m (2)(1) 2

Take the footing size as 1.20 m x 1.20 m.

Problem (2.14): A vane tester with a diameter d = 9.1cms and a height h = 18.2 cms requires a torque of 110 N-m to shear a clay soil sample, with a plasticity index of 48%. Find the soil un-drained cohesion Su ? Solution: For CASE (2) with top and bottom vane ends embedded in soil, the torque is given by:

.D2.Su ,field D (H  ) 2 3 T 0.110 or S u , field    40 kN/m2 2 2 .D D .(0.091)  0.091 (H  ) 0.182   2 3 2 3   From Fig.(2.28a) for a plasticity index of 48%, Bjerrum's correction factor  = 0.80, and T

Therefore,

Su ,design  ..Su ,field  0.8(40)  32 kPa

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PROBLEMS P2.1

Name several sources for information and preliminary evaluation of a building site.

P2.2

Explain briefly the difference between preliminary exploration and detailed exploration.

P2.3

Give useful information obtained from a typical subsurface investigation.

P2.4

Give factors that are relevant to planning for a well-balanced exploration program.

P2.5

How does someone select the depth of boring, boring layout, and type of samples?

P2.6

What is a test pit? Give few negative and positive aspects of it.

P2.7

What is a test boring? How does it differ from a test pit?.

P2.8

What are disturbed and undisturbed soil samples? How do you obtain each of them?.

P2.9

Describe Shelby-tube sampler features. How does it differ from split-spoon sampler?

P2.10 What is a core sample? How is it obtained? What sort of information can be obtained after evaluation. P2.11 What is a vane shear test? Describe the apparatus. P2.12 What are boring logs? Define them. P2.13 Can a split-spoon sampler penetrate a typical rock formation? Can a fight auger penetrate a rock formation? Explain briefly. P2.14 Shelby tubes are usually pushed into a strata. However, some practitioners regard driving the tube an acceptable approach. How might the disturbance be affected by the two methods for each type listed below? (a) Very soft clay, and (b) Hard clay. P2.15 a- List general rules of estimating the number and depth of soil investigation boreholes. b- What are the different types of samplers you may use to obtain undisturbed samples of cohesive soil? P2.16 a- The load-settlement data obtained from load test of a square plate of size (1ft X1ft) are as shown below. If a square footing of size (7ft X 7ft) settles 0.75 inch, what is the allowable soil pressure (consider the soil is a cohesionless soil)? Load (Tsf) Settlement (inch)

2 0.1

5 0.2

8 0.3

10 0.4

14 0.6

16 0.8

19 1.0

b- Compute the area ratio (Ar) of the standard split spoon dimensions (OD = 51 mm and ID = 38mm). Then, what ID would be required to give Ar = 10%?

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P2.17 For cone penetration records shown in the table below, plot the CPT data including f R and then estimate Su and  at a depth of 5.6m if I P = 30. Take  av = 16.5 kN/m3 up to ground water table (GWT) at depth of 3m and  = 19.81 kN/m3 for soil below the GWT. Depth (m) 0.50 1.50 1.60 2.10 2.50 3.10 3.50 4.10 4.50 5.60

q s (kPa)

q c (MPa)

22.02 27.77 28.72 32.55 24.89 22.02 12.44 15.32 21.06 28.72

1.86 1.83 1.16 1.15 2.28 0.71 0.29 0.38 1.09 1.57

Soil Classification sandy silt silt and clayey silt silty clay silty clay silty sand silty clay Clay Clay silty clay silt and clayey silt

P2.18 For the cone penetration records of clay soil shown in the figure below, if LL = 45 and PL = 20% estimate Su and  at the 7−8 m depth. Assume  avg. = 16.5 kN/m3 for soil above the GWT and  = 19.81 kN/m3 below it. Friction ratio fR %

Depth z , meters

Point resistance qc kPa

Depth z , meters

Sleeve friction qs kPa

67

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

P2.19 In the soil profile shown in the figure below, if the cone penetration resistance ( q c ) at point (x) as determined by an electric friction cone penetrometer is 0.80 MN/m2 find: (a) the over-consolidation ratio, OCR , (b) the un-drained cohesion C u . 1.01

 q  o   Use: OCR  0.37 c  o 

C ; and OCR  . u.field  o

  where,   22.( PI) 0.48 

and o , o are total and effective vertical stresses in (MN/m2). G.S. 2m

W.T.

Clay

 = 18 KN/m3 4m

 sat = 20 KN/m3 x

P2.20 For a vane shear data shown in the figure below, estimate Su ,.v and Su,.remolded if 100 mm diameter vane is used with H/ D =2 (rectangular). Also estimate  if I p = 40 and Po =125 kPa. 200

100 50 -

Friction

150 -

T for Su,v

Total torque, N-m

200 -

Tremolded

16.7

0Rotation, 

Typical vane shear data. P2.21 Vane shear tests were conducted in a layer of clay. The vane dimensions were 63.5 mm (D) x 127mm (H). At certain depth, the torque required to cause failure was 0.051 N-m. The liquid limit of the clay was 46 and the plastic limit was 21. Estimate the undrained cohesion of the clay to be used in design by applying Bjerrum's  relationship:

  1.7  0.54 log10 (PI(%)) .

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Chapter 2: Subsoil Exploration

P2.22 Given: A two-thin walled sample tubes of dimensions are as follows: OD (inch)

ID (inch)

Length (inch)

3.0 3.5

2.875 3.375

24 24

Required: What is the area ratio of each of these two sample tubes? and what kind of sample disturbance is obtained? P2.23 For the soil profile shown in the figure below, along with the standard penetration numbers in the clay layer, determine the variation of C u ..and..OCR with depth.

N  Use: Cu.(kN/m )  and OCR  0.193 60   o  where, o = effective vertical stresses in (MN/m2). 2

0.689

29N060.72 ;

G.S. W.T.

2.0m

N 60 5

2.0m 1.5m

Dry sand  = 17 KN/m3 Sand

 sat = 19 KN/m3

8

1.5m

Clay

6

1.5m

 sat = 17 KN/m3

4

1.5m

6

Sand P2.24 For the soil profile shown in the figure below, along with the standard penetration numbers in sand layers, calculate the corrected N-values and then estimate an average peak soil friction angle  using:

1. .(deg .)  27.1  0.3.N 60  0.00054.( N 60 )2  2. .(deg .)  tan1 ( N 60 / 12.2  20.3( o ))0.34 100

N 60

G.S.

6.0m

Dry sand  = 18 KN/m3 W.T.

Sand

 sat = 20.2 KN/m3

69

1.5m

6

3.0m

8

4.5m

9

6.0m

8

7.5m

13

9.0m

14

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

P2.25 For the log of boring shown the in figure below, make reasonable estimates of the relative density D r ..and.. for the sand both above and below the GWT. Assume that

E r  60 for the N-values shown, the unit weight of sand increases linearly from (15 to 18.1) kN/m3 close to the ground surface up to water table level and  sat  19.75 kN/m3 below GWT. Also, estimate the N-value you would use for a square footing of (2m x 2m) located at -2m depth? N−values

G.S.

0.0 Top soil

- 0.3 -6

Sand

Depth (m)

-10 - 4.0

-8

W.T.

-7 - 6.0

 sat  19.75 kN/m3

-9 -11

 q u  50.kPa - 8.0

-15

 q u  60.kPa -14

Sandy clay

70

Foundation Engineering for Civil Engineers

Chapter 2: Subsoil Exploration

REFERENCES Aas, G., et al., (1986),“Use of in situ tests for foundation design on clay”, 14th. PSC, ASCE, pp. 1-30. Allen, D.V., (1993),“Low-cost hand drilling”, Consallen Group Sales Ltd., Loughton. Anagnostopoulos, A., Koukis, G., Sabatakakis, N., and Tsiambaos, G. (2003),“Empirical correlations of soil parameters based on cone penetration tests (CPT) for Greek soils,” Geotechnical and Geological Engineering, Vol. 21, No. 4, 377–387. Bjerrum, L. (1972),“Embankments on soft ground”, In Proc. ASCE Spec. Conf. Performance Earth Earth-Supported Structures, Purdue University, 2:1–54, Geotechnical Journal, 20(4): 718-745. Budhu, M. (2007),”Soil mechanics and foundations”, 2nd. edition, Wiley, section 2.6, Soils exploration Program, Pgs.21. Das, Braja M. (2007),” Principles of foundation engineering”, 6th Edition, Nelson a division of Thomson Canada Limited. Housel, W.S. (1929),“Discussion of: ’The science of foundations”, Trans. ASCE, Vol. 93, pp.322330. Hvorslev, M.J. (1949),“Subsurface exploration and sampling of soils for civil engineering purposes”, US Waterways Experimental Station, Vicksburg, Mississippi. IDEA Drilling,(1997),”Bedrock core drilling: Mineral exploration”, In Minnesota, http:// Idea drilling.com/ 1997 9th Avenue North Virginia. Ismael, N.F. and Jeragh, A.M. (1986).”Static cone tests and settlement of calcareous desert sands”, CGJ, Vol. 23, No. 3, Aug.,pp. 297-303. Ismael, Nabil F. and Vesic, Aleksandar S. (1981), "Compressibility and bearing capacity," ASCE Journal of the Geotechnical Engineering Division. Vol. 107. No. GTI2, pp. 1677-1691. Ladd, C.C., Foote, R., Ishihara, K., Schlosser, F. and Poulos, H.G. (1977), "Stress deformation and strength characteristics”, State-of-the-art report, Proceedings. Ninth International Conference on Soil Mechanics and Foundation Engineering. Vol. 2, p. 421-494, Tokyo. MDT Geotechnical Manual, (2008),”Subsurface Investigations / Field Tests”, Section 8.2.4, Pgs.12-14. Moore, R. (1980), “Reasoning about knowledge and action”, PhD. Dissertation, Cambridge MA, MIT, Published as TN-191, SRT International, Menlo Park, CA. Road Research Laboratory (1952),“Soil mechanics for road engineers”, Department of scientific and industrial research, HMSO, London. Robertson, P.K. and Campanella, R.E. (1983),“Interpretation of cone penetration tests, Part I: sand”, CGJ, Vol.20, No.4, Nov., pp. 718-733. Robertson, P.K., Campanella, R.G. and Wightman, A. (1983),“SPT-CPT correlation”, JGED, ASCE, Vol. 109, No. 1, pp. 1449-1459. Schmertmann, J.H. (1978),“Guidelines for cone penetration test: performance and design”, FHWA-TS-78-209(report), U. S. Dept. of Transportation, 145 pp. Sowers, G.F. (1979), “Introductory soil mechanics and foundations: Geotechnical engineering”, 4th ed., MacMillan Publishing Co., New York, NY, p. 621. Terzaghi, K. (1943),” Theoretical soil mechanics”, John Wiley & Sons, New York. Terzaghi, K., Peck, R. and Mesri, G. (1996),“Soil mechanics in engineering practice”, John Wiley-Interscience Publication, John Wiley & Sons, New York.

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