Shear Strength Feb2010

SHEAR STRENGTH 1.0 Introduction Strength is a measure of the maximum stress that can be induced in a material without it failing. Strength may be expressed in terms of compressive or tensile stress but essentially it is the material‟s ability to resist shear stress. The shear strength of a soil is the maximum value of shear stress that the soil can resist before failure occurs. Essentially, shear strength within a soil mass is due to the frictional resistance between adjacent soil particles. The force transmitted between two bodies in static contact can be resolved into two components: normal component N (kN), perpendicular to the interface and shear (tangential) component T (kN), parallel to the interface.

T = N where :

= N tan ‟

= coefficient of friction ‟ = angle of friction

Also the stress/strain(or displacement) curve is of interest. The curve below shows a typical stress/strain relationship for soils shearing under constant normal stress, n.

Y

yield point

P

Peak shear strength U Ultimate strength C Critical state strength* R Residual strength

If soil is stressed beyond this point strain is not 100% recoverable. Maximum shear stress that can be sustained, rapid loss of strength beyond this point. For loose sands and soft clays work hardening can increase the shear stress so that a maximum stress is not achieved. After an amount of strain a soil will achieve a constant volume and will continue to strain at this volume After a considerable amount of strain, platy clay particles on either side of the failure surface are re-arranged to become more parallel to produce a constant strength.

*Critical State soil mechanics is not covered in this module. Principal test methods DIRECT SHEAR TESTS Shear box test Vane test INDIRECT SHEAR TESTS Uniaxial compression test Triaxial compression test

2.0

Failure criterion

The Mohr-Coulomb relationship is adopted in soil mechanics. It relates the shear stress at failure (i.e. shear strength, f) on a failure plane to the normal effective stress n‟ acting on that plane: f

Where:

= c‟ +

= c‟ = n‟ = ‟ = f

Shear Stress,

n‟

tan ‟

shear strength cohesion normal stress angle of friction

Mohr-coulomb line f

= c‟ +

n‟

tan ‟

‟ c‟

Normal stress,

n‟

Any combination of shear stress, , and normal stress, n located; below the Mohr–Coulomb line is a safe state of stress. on the Mohr–Coulomb line is in a state of incipient failure. above the Mohr–Coulomb line has failed (impossible to plot). 2 Shear Strength (Dr. P McMahon)

All analyses of soil shear strength are dependent on three factors: 1. What are the appropriate values of c‟ and ‟? - depends on soil history, type, loading, drainage. 2. What combination of n and exists within the soil mass? - found using Mohr‟s circle. 3. Knowing shear strength f, from 1; knowing shear stress , from 2; then the Factor of Safety against shear failure can be found: F of S=

Strength, Stress,

f

[F of S = 3.0 for bearing capacity : 1.5 for slope stability] NOTE: BS EN ISO 1997 is changing this approach (known as EC7 !!) 3.0

Stress analysis - Mohr’s circle of stress

Mohr‟s circle of stress provides a convenient method of analysing two dimensional stress states. The shear strength of a soil may be expressed in terms of the effective major principal stress, 1‟, and effective minor principal stress, 3‟, at failure at the point in question. At failure the straight line represented by the Mohr - Coulomb equation will be tangential to the Mohr circle representing the state of stress, the coordinates of the tangent point being f and f‟. Also if is the angle between the major principal plane and the failure plane, then it can be shown that: = 45 +

‟ 2

3 Shear Strength (Dr. P McMahon)

The Mohr – Coulomb failure line is often called the failure envelope and Mohr‟s circles at different values of normal stress will all be tangential to this failure envelope. Shear Stress, Failure envelope, f = c‟ +

n‟

tan ‟

‟ c‟ θ = 45 + ‟/2 Effective normal stress,

4.0

n‟

Shear strength tests

The purpose of shear strength testing is to determine values for the shear strength parameters c‟ and ‟. The drainage conditions during the test influence the measured values considerably. Shear strength tests are carried out in two main stages: 1. Consolidation stage. After a sample has been prepared to size and its mass and water content determined, it is consolidated to a required state. This is done either; or

one dimensionally in direct shear tests, three dimensionally in triaxial tests.

The objective is to produce an initial stress state and to ensure full saturation in triaxial tests. The consolidation of coarse soils in direct shear tests is virtually immediate upon load application, but may require up to 24 hours for fine soils. 2. Shearing or axial loading stage. The consolidated sample is subjected to either direct shearing (e.g. shear box test) or increased in axial loading (triaxial test) until failure occurs. Readings of vertical strain, axial load, pore water pressure (not shear box test) and volume change are taken against time.

4 Shear Strength (Dr. P McMahon)

Types of laboratory shear test:

4.1

Vane test Shear box test Unconfined compression test Triaxial compression test

Vane Test

This is a direct shear test used both in the laboratory and on site to determine the undrained shear strength, cu, of firm to stiff silts and clays.

The vane consists of four rectangular blades in a cruciform at the end of a steel rod. After the vane has been pushed into the soil (in a laboratory sample or at the bottom of a borehole), it is rotated by applying a torque to the rod. The applied torque is measured and the undrained shear strength of the soil is calculated by equating the applied torque to the shearing resistance of the shear surface (perimeter and ends of a cylinder of soil).

T=c

[

(

DH

x

From which; c=

D 2

)+

D

2

Or where, k = vane constant k=

(

(

2

T H + 2

D2 4

x

D 6

)

D 6

)

2 3

x

D 2

)

]

c = kT

D2

(

1 H + 2

After the initial test, the vane is rotated rapidly, causing the clay to become remoulded and the shear strength in this condition is recorded as the remoulded shear strength, cr. The sensitivity of a cohesive soil is defined as the peak strength divided by the remoulded strength i.e. cu Sensitivity = cr 5 Shear Strength (Dr. P McMahon)

The higher the sensitivity value the more the soil will lose strength as a result of disturbance, e.g. remoulding of soil during earthworks. Most ordinary clays have values of sensitivity up to about 4. However some „quick‟ clays have sensitivities as high as 100. Class example 1 A laboratory vane test was conducted on a sample of saturated clay soil. The vane measured 38.0mm high by 19.0mm diameter. The vane was inserted into the soil to a depth of 76.0mm, rotated and a torque of 2.5Nm recorded. The vane was quickly rotated a further three revolutions and after a period of settling the torque was measured at 1.1 Nm Determine:

4.2

i) ii) iii)

Peak shear strength Remoulded shear strength Sensitivity of the clay Ans: 99.5kN/m2 ; 43.8 kN/m2 ; 2.3

Shear Box Test

This is a direct shear test, i.e. the normal and shear stresses on the failure surface are measured directly. It is usually used for testing granular soils and stiff clays. A rectangular prism of soil is cut and fitted into a square metal box that is split into two halves horizontally. The standard box is 100mm x 100mm in plan area and a large model, 300mm x 300mm. Normal load is applied to the sample by dead weights via a hanger and lever mechanism. Shear load is applied at a constant rate of strain by a screw jack driven by electric motor.

6 Shear Strength (Dr. P McMahon)

Measurements of vertical and horizontal movement are recorded from dial gauges and readings of the shear force are read from a proving ring or load cell. The test is repeated using different values of normal load, e.g. In the diagram 2 2 2 below normal stresses might be 1 = 50kN/m , 2 = 100kN/m , 3 = 200kN/m .

Values of normal stress n and shear stress are calculated and plotted and the value of the peak stress determined.

Shear stress at failure, 2 kN/m



f

 

, angle of friction (or angle of internal shearing resistance)

Cohesion, c 50

100 2 Normal stress, n, kN/m

200

Plotting shear stress at failure f (i.e. peak stress) against normal stress the angle of friction and the cohesion (if clay soil) to be found:

n

enables

Advantages: 1. Both shear stress and normal stress can be measured directly. 2. A constant normal stress can be maintained throughout the test. 3. Easy test for cohesionless soils (sands and gravels) and drained tests can be carried out in a reasonable time. 7 Shear Strength (Dr. P McMahon)

4. Volume changes can be easily measured. 5. Residual strengths of clays (ie. at large values of strain) can be found by reversing the shear direction (reversible shear box). Disadvantages: 1. Poor, uncertain control of drainage conditions and the inability to measure pore water pressure. 2. The distribution of shear stress over the failure plane is assumed to be uniform – not true, due to influence of sides and corners of box. 3. The soil is forced to fail along a predetermined failure plane – may not be the weakest zone. 4. The normal stress cannot be easily changed during testing.

8 Shear Strength (Dr. P McMahon)

Class example 2 A drained shear box test was carried out on a sandy clay soil with the following results: Normal load (N) 108 Shear load at failure (N) 172

202 227

295 266

390 323

484 374

576 425

The shear box measured 60mm x 60mm. Determine the cohesion, c, and the angle of friction, , of the soil. Ans: c = 32kN/m2 :

4.3

= 28o

Unconfined Compression Test

This test involves axially loading a cylindrical soil sample and recording the failure load. The samples are usually 38mm diameter and 76mm long and are placed in a test rig which is portable, self contained and hand operated – useful for on site determination of undrained strengths of clays. The test rig incorporates a pencil tracing load against deflection (shortening) of the sample, as shown below;

9 Shear Strength (Dr. P McMahon)

For brittle failure, the maximum compressive load defines failure.

For plastic failure (no distinct maximum), the failure load is taken as that at 20% strain (ie. 15.2mm shortening on a 76mm long sample);

Compressive strength calculations must take account of the increase in cross sectional area of the sample as the test proceeds:Uncorrected axial stress at failure

=

compressive failure Load F = original cross sectional area Ao

To allow for the increase in cross sectional area, Corrected axial stress at failure,

1

=

Where; Axial strain,

=

F (1 - ) Ao

change in sample height (shortening) = original sample height

h ho

When analysed using Mohr‟s circle construction, since the lateral stress the following is drawn:

3

is zero,

10 Shear Strength (Dr. P McMahon)

cu

1

3

Corrected compressive strength, 1

From the above diagram, it can be seen that for saturated clay: Undrained shear strength, cu = radius of Mohr‟s circle at failure Thus cu = Advantages:

1

2

The test is fast, simple, compact and inexpensive.

Limitations: 1. The sample must be fully saturated. 2. The sample must must not contain fissures, gravel particles, air voids etc, as these will be areas of weakness. 3. The test must be carried out quickly to ensure that undrained conditions apply. 4. It is generally used as a comparative test (not for design purposes) – to indicate if the soil is stiff, firm etc, see designations below;

Class example 3 A sample of clay, 38.0mm diameter and 76.0mm long is tested in an unconfined compression test. The sample fails at a load of 205N after being reduced in height by 8.6mm. Determine; i) The corrected compressive strength of the sample. ii) The shear strength of the clay soil. iii) State the strength designation of this soil Ans: 160.9kN/m2 ; cu = 80.5kN/m2 : “firm to stiff” clay 11 Shear Strength (Dr. P McMahon)

ASSESSMENT OF IN-SITU STRENGTH Soil Type Sands, gravels

Term Loose

Slightly cemented

Field test Can be excavated with a spade; 50 mm wooden peg can be easily driven Requires a pick for excavation; 50 mm wooden peg is hard to drive Visual examination; A pick removes soil in lumps which can be abraded

Silts

Soft or loose Firm or dense

Easily moulded or crushed in the fingers Can be moulded or crushed by strong pressure in the fingers

Clays

Very soft Soft Firm Stiff Very stiff

Exudes between the fingers when squeezed in the hand Moulded by light finger pressure Can be moulded by stronger finger pressure Cannot be moulded by the fingers; Can be indented by the thumb Can be indented by the thumb nail

Firm Spongy Plastic

Fibres already compressed together Highly compressible and open structure Can be moulded in the hand and smears the fingers.

Dense

Organic, peats

STRENGTH SCALES (NOTE: EC7 is changing the way that soil strength is assessed) A scale (from BS 5930:1999), historically has been described in terms of undrained shear strength, as follows: Term

Undrained shear strength, cu kN/m2

Very soft Soft Firm Stiff Very stiff Hard

< 20 20 to 40 40 to 75 75 to 150 150 to 300 >300

Subdivisions of these strength ranges are widely (unofficially) used, as follows: Soft to firm Firm Firm to stiff Stiff

40 to 50 50 to 75 75 to 100 100 to 150

NOTE: Level 1 BSc Lecture Material covered in BLT1013 and 2 nd Year HNC Material covered in BLT1114 provides up to date guidance for awareness of students. Refer to Lecture Material on website for detailed exaplantion.

12 Shear Strength (Dr. P McMahon)

4.4

Triaxial Compression Test

This is the most widely used shear strength test to provide data for design purposes. It is suitable for most cohesive soil types except for clays sensitive to disturbance. It is just possible to test granular soils, e.g. sand, but the sample is difficult to setup. The test is normally carried out in two stages; Stage 1.

Apply a constant value of all round (or cell) pressure, i.e. minor principal stress, 3.

Stage 2

Increase the vertical or axial stress, i.e. major principal stress, 1, until failure occurs.

Advantages;

drainage conditions can be controlled, enables saturated soils of low permeability to be consolidated, if required, as part of the test procedure, and pore water pressure measurements can be made.

13 Shear Strength (Dr. P McMahon)

The sample, sealed in a rubber membrane, is positioned in the cell which, in turn, is filled with water. The water is pressurised to the required confining, or cell, pressure, 3. The sample is compressed using a constant rate of drive motor until the force on the sample stops increasing, or the strain in the sample reaches 20%. The increase in cross sectional area of the sample must be taken into account when the compressive stress is calculated The compressive strength is equal to the maximum applied compressive stress (or deviator stress).

Three or more identical samples (Test 1, 2 & 3 below) are axially loaded to failure under different values of cell pressure, 3 (e.g. 3-1 = 50, 3-2 = 100, 3-3 = 200 kN/m2). Proving ring and axial deformation dial gauges are recorded. The data is converted to axial stress (with area correction) and graphed vs % strain. From each plot, the stress at failure, 1, value is obtained, see 1-1, 1-2 and 1-3 below.

Test 1

c

3-1

3-2

Test 2

Test 3

n

3-3

1-2 be drawn, hence 1-3 Mohr‟s circle constructions enable1-1a failure envelope to the shear strength parameters c and to be determined.

Types of Triaxial Test Engineering works will change the stresses in the soil and affect the drainage conditions. Three common test procedures have been devised to model or represent these changes: 1. Unconsolidated / undrained test („quick undrained‟) 2. Consolidated / drained test („drained‟) 3. Consolidated / undrained test

14 Shear Strength (Dr. P McMahon)

4.4.1 Unconsolidated/undrained test (“quick undrained”) Stage 1

Stage 2 .

F/A

1

.

3

3

.

.

3

3

3

3

3

3 1

F/A

No consolidation No drainage Valves „A‟ & „B‟ closed Valves „A‟ & „B‟ closed Valve “A” - drainage burette Valve “B” - pore pressure measurement This „quick undrained‟ test gives the short term „undrained‟ shear strength value of a saturated clay, cu. Increased pore water pressure in the short term carries the applied load, hence the angle of friction, u, equals zero. The rate of strain is selected to ensure failure occurs within 5 to 15min, i.e. a „quick‟ test. The main applications are in the design of shallow and pile foundations and in assessing initial stability of embankments and cuttings. As the axial load on the sample increases, a shortening in length occurs with an increase in the initial sample diameter (and area, Ao). A correction factor is applied to Ao, to determine the correct axial stress at failure; Corrected axial stress at failure,

1

=

Where; Axial strain,

=

F (1 - ) Ao

change in sample height (shortening) = original sample height

h ho

o

u

cu

=0

Test 2

Test 1

Test 3

3-2

3-1 1-1

n

3-3

15

1-2

1-3

Shear Strength (Dr. P McMahon)

NOTE: Compacted clay always has an air voids content, which means that be >0o (the value of u increases with >Av).

u

would

Class example 4 The results of quick (unconsolidated) undrained triaxial tests on three identical soil samples (76.0mm long x 38.0mm diam.) at failure are: Test No. 1 2 3 Cell pressure, σ3 (kN/m2) 200 400 600 Axial load (N) 222 215 226 Axial deformation (mm) 9.83 10.06 10.28 Determine the shear strength parameters of the soil with respect to total stress Ans: : cu = 85.0kN/m2 ;

u

= 0o

16 Shear Strength (Dr. P McMahon)

4.4.2 Consolidated / drained test Stage 1

Stage 2 .

F/A

1

.

3

3

.

.

3

3

3

3

3

3 1

F/A

Consolidation Drainage Valves „A‟ & „B‟ open Valves „A‟ & „B‟ open Valve “A” - drainage burette Valve “B” - pore pressure measurement In Stage 1 drainage is allowed via the burette and the initial excess p.w.p., u, dissipates to zero as the sample consolidates and reduces in volume, V and the cross sectional area of the sample. A correction is applied for this. In Stage 2 the loading rate is very slow (8 to 50hrs to failure depending on soil type) in order to prevent build up of p.w.p. in the sample. A check that pore water pressure is low or zero is made via a pressure transducer connected to valve „B‟. Any volume change in Stage 2 is measured as a change in burette water level during and up to failure. A correction is applied for the change in X.S. area. As the axial load on the sample increases, the sample becomes shorter, with a consequent increase in sample diameter (and area). A correction is applied for this. In order to take account of these changes, when calculating the vertical stress 1, the corrected cross sectional area of the sample, A, initially after consolidation and during axial loading up to failure, is found from: V 1-( ) V0 A = A0 h 1-( ) h0 Where; A0 = initial cross sectional area V0 = original sample volume V = change in volume h0 = original length h = change in length The total stresses 1 and 3 at failure are also the effective stresses 1‟ and 3‟ (since p.w.p. = 0) and so the Mohr‟s circles for effective stress are plotted direct, see diagram below. Test produces values of the effective stress strength parameters cd‟ and more generally termed c‟ and ‟). 17

d‟

(also

Shear Strength (Dr. P McMahon)

The results of the test are generally applicable to fill problems, e.g. embankment construction – slow build up allowing p.w.p. to dissipate as construction proceeds and the long term stability of cuttings.

Class example 5 Consolidated drained triaxial tests were carried out on three identical specimens (each 38.0mm diameter and 76.0mm long ) of the same soil sample and the following data was obtained; Sample Cell pressure (kN/m2) Failure load (kN)

1

2

3

100

200

400

0.297 0.458 0.794

Change in length (mm): During consolidation, ΔHc

0.73

During axial loading, ΔHa

9.38

1.77

2.82

12.24 15.38

Change in volume (ml): During consolidation, ΔVc

2.48

6.02

9.90

During axial loading, ΔVa

5.93

6.05

6.07

Determine the shear strength parameters of the clay with respect to effective stress. Ans: c’ = 35kN/m2 ; ’ = 25o

18 Shear Strength (Dr. P McMahon)

4.4.3 Consolidated/undrained test Stage 1

Stage 2 .

F/A

1

.

3

3

.

.

3

3

3

3

3

3 1

F/A

Consolidation Valves „A‟ & „B‟ open

No drainage Valve „A‟ closed Valve „B‟ open Valve “A” - drainage burette Valve “B” - pore pressure measurement

In Stage 1 drainage is allowed via the burrette and the initial excess p.w.p., u, dissipates to zero as the sample consolidates and reduces in volume, V and the cross sectional area of the sample. A correction is applied for this. In Stage 2 the loading rate, typically 0.05mm/min (with failure typically in under 3hrs), allows induced p.w.p to distribute throughout the sample, with valve „B‟ opened to measure the build up of p.w.p. A “back pressure” may be applied to the drainage circuit, to ensure complete sample saturation. Values of the undrained shear strength, cu and u (compacted clay always has an air voids content, which means that u would be >0o) may be determined. Pore water pressure during axial loading is recorded to enable the effective stress at failure to be determined. See effective stress strength parameters ccu‟ and cu‟ (also referred to as c‟ and ‟) below;

19 Shear Strength (Dr. P McMahon)

NOTE:The value of ccu‟ obtained by this test produces a higher value (>10kN/m2) for the same soil than the cd‟ value for the drained test (normally <10kN/m2) see previous section.

Class example 6 The results of the shearing stage of consolidated undrained triaxial compression tests on three samples of fully saturated clay are given below; Cell pressure Deviator stress Pore water Test at failure pressure (kN/m2) at failure (kN/m2) (kN/m2) 1

200

226

124

2

350

378

208

3

500

536

278

Each sample has been consolidated with a back pressure of 100kN/m2 prior to the shearing stage. This back pressure was maintained until the start of the shearing stage when the drainage valve was closed. Determine the shear strength parameters of the clay with respect to effective stress. 2 o Ans: : ccu’ = 18.0kN/m ; cu’ = 31 Summary of anticipated Triaxial Compression Test Results a) Cohesive/Frictional Soil

b)

(Clayey sand)

Cohesive Soil (Saturated Clay)

20 Shear Strength (Dr. P McMahon)

6.0

Summary

Any procedure which involves removing a sample from the ground will involve disturbance to the soil sample, and in general this will lead to a reduced strength. Modern thin walled piston samplers can minimise disturbance to the absolute minimum. Most of the disturbance comes from the initial sampling. Transportation of samples is usually not too much of a problem. Quick Clay is an exception when the effects disturbance may severely reduce sample strength. These effects can be minimized, however, by carefully cushioned transport arrangements. A vane shear test causes minimum soil disturbance and weakening. But, one the other hand it cannot measure strain or deformation, nor can pore water pressure changes be ascertained. In the shear box test in cohesive soil pore pressures cannot be measured. Also it is much more difficult to cut samples from the field to the correct shape without causing further disturbance and weakening. In the case of free draining granular soils pore pressures do not develop during testing, but it is difficult to compact the soil to its approximate in-situ value (if this is known). The unconfined compression test suffers from the same initial sample disturbance as the triaxial test, but has a much less accurate means of strain measurement. Furthermore it can only test samples at zero confining pressure, hence it is only possible to draw one Mohr‟s circle. The Mohr Coulomb envelope requires at least two Mohr‟s circles at different confining pressures. The drainage conditions in the triaxial test can be varied from the: completely undrained situation immediately after construction (e.g. a structure or excavation of a cutting in clay) ……… to fully drained test conditions (excess pore pressure is allowed to dissipate and volume changes as water is squeezed out/in are measured) representing of long term stability of a cutting in clay. NOTE: In consolidated undrained tests, the pore water pressure is measured (to enable effective stress strength parameters to be assessed) but no drainage is permitted.

21 Shear Strength (Dr. P McMahon)

Simplified assessment of bearing pressures of clay (For preliminary design purposes only) Consider a building with a raft foundation, bearing pressure q and foundation width B as shown below;

Assume that failure takes place along a circular surface.

Taking moments about the point “O”; Disturbing moment (per m. run); qB

B

x

=

qB 2

cu B

=

2

2

(approx)

Restoring moment (per m. run) = 2B 2

cu

B2

In site investigation, only a very small proportion of the ground is sampled. Therefore the degree of confidence in the results is much lower than for manufactured materials. Furthermore, soil which may not fail in shear may undergo unacceptable settlement if subject to stress close to the ultimate bearing pressure. For these reasons a factor of safety of 3.0 against failure is normally adopted. For a factor of safety of 3, Restoring moment = cu

B 3

qB 2

2

2

= cu B (approx) (since „ ‟ at 3.142 divided by 3 is approx 1 !!)

2

= cu B

2

(approx)

q = 2cu NOTE: For preliminary design purposes only Thus:

Approximate safe bearing pressure = 2 x shear strength

(relevant to shallow foundations on clay)

22 Shear Strength (Dr. P McMahon)

Implications of soil types on foundation design Shrinkage and swelling of clay soils Clay soils (those which usually contain over 35% of clay sized mineral particles) can absorb water and increase in volume, or swell. Reduction of the water, or more correctly the moisture content, will result in a corresponding decrease in volume, or shrinkage. In clay soils, foundations constructed at a shallow level i.e with the underside less than about 1.0m below finished ground level are liable to subside or be lifted as the clay‟s volume decreases with drying in summer or increases after wetting in winter. If the site is clear of trees, strip foundations taken down to a depth of 1.0m in clay are unlikely to move sufficiently to cause structural distress. The shrinkage potential is dependant upon the soils clay content and „plasticity index‟. Plasticity index (%)

Clay Fraction %

Shrinkage (or swelling) potential

> 35 32-48 12-32 < 18

> 95 60-95 30.60 < 30

very high high medium low

Additional effect of trees on clay soil movement Trees and heavy vegetational growth withdraw a considerable quantity of water from the ground during their seasonal growth. This is especially true in dry summers when trees growing in clay of medium to high shrinkage potential cause a large reduction in the volume of the clay in addition to that due to normal seasonal effects with consequent additional subsidence. With the return of heavy winter rain and the end of the growing season the clays volume increases and, on swelling, the ground surface rises. Even without trees, the grassed surface of an open field on clay can rise 20 - 40 mm. Consideration must be given to the following circumstances: i) ii) iii) iv)

Planting new trees on clay near to existing buildings. Building on clay near existing trees. Cutting trees down near to existing buildings. Building on a clay site newly cleared of trees and shrubs.

BRE (Building Research Establishment) and NHBC (National House Building Council) publish guidance on practical design approaches for shrinkable clay soil formations (for example NHBC Practice Note 3).

Foundations on sand and gravel Sands and gravels are not subject to swelling or shrinkage. However, in a sand or gravel soil it should be remembered that its „bearing strength‟ (simplistic terminology) will be approximately halved for a foundation based near to, or below ground water level. The underside of a foundation in sand or gravel soil should be kept as high as possible to remain above the groundwater. Formation level should though be at sufficient depth below that influenced by penetration of frost.

REFERENCES Bishop, A W and Henkel, D J (1972) The measurement of soil properties in the triaxial test 2nd Ed, Pub. Edward Arnold 23 Shear Strength (Dr. P McMahon)