Direct Shear TestCEP 701 PG Lab

Mohr-Coulomb Failure Criterion(in terms of total stresses)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)f is the maximum shear stress the soil can take without failure, under normal effective stress of .u = pore water pressure

Mohr-Coulomb Failure CriterionShear strength consists of two components: cohesive and frictional.

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

Principal stressesor

Mohr Circle of stressResolving forces in s and t directions,

Mohr Circle of stress

Mohr Circle of stress

Mohr Circles & Failure Envelope

Direct shear testNEED AND SCOPEIn 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 testThis test is performed to determine the consolidated - drained shear strength of a sandy to silty soil. 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 soils shearing resistance. 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.

Apparatus1.Direct shear box apparatus2.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.

PROCEDURECheck 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 testPreparation of a sand specimenDirect shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays

Direct shear testPreparation of a sand specimen

Direct shear testTest procedure

Direct shear testStep 2: Lower box is subjected to a horizontal displacement at a constant rate

PROCEDUREMeasure the thickness of soil specimen.Apply the desired normal load.Remove the shear pin.Attach the dial gauge which measures the change of volume.Record the initial reading of the dial gauge and calibration values.Before proceeding to test check all adjustments to see that there is no connection between two parts except sand/soil.Start the motor. Take the reading of the shear force and record the reading.Take volume change readings till failure.Add 5 kg normal stress 0.5 kg/cm2and continue the experiment till failureRecord carefully all the readings. Set the dial gauges zero, before starting the experiment

Direct shear test

Direct shear testAnalysis of test resultsNote: Cross-sectional area of the sample changes with the horizontal displacement

Direct shear tests on sandsStress-strain relationship

Direct shear tests on sandsHow to determine strength parameters c and f

Direct shear tests on sandsSand is cohesionless hence c = 0Direct shear tests are drained and pore water pressures are dissipated, hence u = 0Therefore, f = f and c = c = 0

Direct shear tests on claysFailure envelopes for clay from drained direct shear testsIn 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)

Interface tests on direct shear apparatusIn 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)

Advantages of direct shear apparatusDue to the smaller thickness of the sample, rapid drainage can be achievedCan be used to determine interface strength parametersClay samples can be oriented along the plane of weakness or an identified failure planeDisadvantages of direct shear apparatusFailure occurs along a predetermined failure planeArea of the sliding surface changes as the test progressesNon-uniform distribution of shear stress along the failure surface