Shear Strength of soil Direct Shear Test MohrCoulomb

Shear Strength of soil Direct Shear Test

Mohr-Coulomb Failure Criteria – Total stress – Effective stress – Cohesion and friction angle Normal and shear stresses Principal stresses Mohr Circle of Stress and failure envelope Direct Shear Test : Apparatus and procedures

Mohr-Coulomb Failure Criterion (in terms of total stresses) re u l i a f pe o l e env Friction angle Cohesion f c f is the maximum shear stress the soil can take without failure, under normal stress of .

Mohr-Coulomb Failure Criterion (in terms of effective stresses) re u l i a f Effective cohesion f c’ ’ pe o l e env ’ u = pore water pressure Effective friction angle ’ f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.

Mohr-Coulomb Failure Criterion Shear strength consists of two components: cohesive and frictional. f ’ c’ ’f tan ’ c’ ’f sive e h co co nt e n mpo ' frictional component

Normal stresses and shear stresses on any plane can be obtained with the following equations

Principal stresses or

Mohr Circle of stress s’ 1 s’ s’ 3 Soil element s’ 3 t q s’ 1 Resolving forces in and directions,

Mohr Circle of stress t s’

Mohr Circle of stress t (s’, t) q s’ PD = Pole w. r. t. plane

Mohr Circles & Failure Envelope Failure surface X Y ’ Soil elements at different locations Y ~ stable X ~ failure

Direct shear test NEED AND SCOPE In many engineering problems such as • design of foundation, • retaining walls, • slab bridges, • pipes, • sheet piling, The value of the angle of internal friction and cohesion of the soil involved are required for the design. Direct shear test is used to predict these parameters quickly.

Direct shear test 1. This test is performed to determine the consolidated - drained shear strength of a sandy to silty soil. 2. The shear strength is one of the most important engineering properties of a soil, because it is required whenever a structure is dependent on the soil’s shearing resistance. 3. The shear strength is needed for engineering situations such as determining the stability of slopes or cuts, finding the bearing capacity for foundations, and calculating the pressure exerted by a soil on a retaining wall.

Apparatus 1. Direct shear box apparatus 2. Loading frame (motor attached). 3. Dial gauge. 4. Proving ring. 5. Tamper. 6. Straight edge. 7. Balance to weigh upto 200 mg. 8. Aluminum container. 9. Spatula.

PROCEDURE • Check the inner dimension of the soil container. • Put the parts of the soil container together. • Calculate the volume of the container. Weigh the container. • Place the soil in smooth layers (approximately 10 mm thick). If a dense sample is desired tamp the soil. • Weigh the soil container, the difference of these two is the weight of the soil. Calculate the density of the soil. • Make the surface of the soil plane. • Put the upper grating on stone and loading block on top of soil.

Direct shear test is most suitable for consolidated drained tests specially on granular soils (e. g. : sand) or stiff clays Preparation of a sand specimen Porous plates Components of the shear box Preparation of a sand specimen

Direct shear test Preparation of a sand specimen Leveling the top surface of specimen Pressure plate Specimen preparation completed

Direct shear test Test procedure P Steel ball Pressure plate Porous plates S Proving ring to measure shear force Step 1: Apply a vertical load to the specimen and wait for consolidation

Direct shear test Test procedure P Steel ball Pressure plate Porous plates S Proving ring to measure shear force Step 1: Apply a vertical load to the specimen and wait for consolidation Step 2: Lower box is subjected to a horizontal displacement at a constant rate

PROCEDURE 8. Measure thickness of soil specimen. 9. Apply the desired normal load. 10. Remove the shear pin. 11. Attach the dial gauge which measures the change of volume. 12. Record the initial reading of the dial gauge and calibration values. 13. Before proceeding to test check all adjustments to see that there is no connection between two parts except sand/soil. 14. Start the motor. Take the reading of the shear force and record the reading. 15. Take volume change readings till failure. 16. Add 5 kg normal stress 0. 5 kg/cm 2 and continue the experiment till failure 17. Record carefully all the readings. Set the dial gauges zero, before starting the experiment

Direct shear test Shear box Dial gauge to measurevertical displacement Proving ring to measure shear force Loading frame to apply vertical load Dial gauge to measure horizontal displacement

Direct shear test Analysis of test results Note: Cross-sectional area of the sample changes with the horizontal displacement

Direct shear tests on sands Shear stress, t Stress-strain relationship Dense sand/ OC clay tf tf Loose sand/ NC clay Expansion Compression Change in height of the sample Shear displacement Dense sand/OC Clay Shear displacement Loose sand/NC Clay

Direct shear tests on sands Shear stress, t How to determine strength parameters c and f Normal stress = s 3 Normal stress = s 2 tf 3 tf 2 tf 1 Normal stress = s 1 Shear stress at failure, tf Shear displacement Mohr – Coulomb failure envelope f Normal stress, s

Direct shear tests on sands Some important facts on strength parameters c and f of sand Sand is cohesionless hence c = 0 Direct shear tests are drained and pore water pressures are dissipated, hence u = 0 Therefore, f’ = f and c’ = c = 0

Direct shear tests on clays In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish) Shear stress at failure, tf Failure envelopes for clay from drained direct shear tests Overconsolidated clay (c’ ≠ 0) Normally consolidated clay (c’ = 0) f’ Normal force, s

Interface tests on direct shear apparatus In many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood) Where, ca = adhesion, d = angle of internal friction

Advantages of direct shear apparatus q Due to the smaller thickness of the sample, rapid drainage can be achieved q Can be used to determine interface strength parameters q Clay samples can be oriented along the plane of weakness or an identified failure plane Disadvantages of direct shear apparatus q Failure occurs along a predetermined failure plane q Area of the sliding surface changes as the test progresses q Non-uniform distribution of shear stress along the failure surface