CEEN Geotechnical Engineering Laboratory Session 7 - Direct Shear and Unconfined Compression Tests

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1 PURPOSE: The parameters of the shear strength relationship provide a means of evaluating the load carrying capacity of soils, stability of slopes, and pile capacity. The direct shear test is one of the methods for obtaining the frictional resistance component (generally termed the angle of internal friction), the cohesive component, c, and the shear stress-strain characteristics. Properly interpreted, these values may be used to determine the ultimate shear resistance of a soil. The unconfined compressive test of a cohesive soil is a useful method for determining the undrained shear strength of the soil. ASTM REF: D , D EQUIPMENT: PROCEDURES: Controlled strain direct shear testing machine, shear box, tamper, caliper, funnels, ring mold, steel plates, Harvard miniature compaction equipment, mixing equipment, compression testing device, computer data acquisition system, balance. Angle of Repose 1. Place the 12 in x 12 in glass plate on a level surface with the roughened side up. 2. Cover the small opening of the funnel with your fingers and fill the funnel with dry sand. Position the small opening of the funnel over the center of the glass plate and allow the sand to flow from the funnel into a conical mound. During flow, keep the opening of the funnel approximately ½ inch above the surface of the conical mound. 3. Using the caliper, determine the diameter of the conical mound at three separate locations. Use these measures to determine the average radius of the conical mound. Position the height indicator rod so that it just touches the top of the sand mound. Pour off the sand and reposition the glass plate under the indicator rod. Use the caliper to determine the height of the conical mound. Direct Shear Tests 1. Remove the shear box and mold from shear testing machine. Record the internal diameter of the mold, the height of each half of the mold, the thickness of the soil tamper, and the mass of the brass loading cylinder. Reassemble the mold and place the brass set rods in place so that both halves of the mold are prevented from sliding. 2. Fill the small aluminum tin with sand and strike flush. Loosely fill the mold by passing the sand through a small funnel. Compact the sand using the tamper. Record the depth to the top of the tamper. Place the brass loading cylinder onto the compacted sand. 3. Carefully position the shear box onto the direct shear testing machine. Adjust the counter balance so that the cross-arm applies minimal additional normal load to the specimen. Increase the normal load on the specimen to the desired value by adding weights to the suspended platform. Position the horizontal deformation indicator and zero appropriately.

2 4. Shear the specimen using a controlled strain rate of approximately 0.04 in/min (Setting 12). Obtain simultaneous readings of proving ring and shear box deformations until a total shear box horizontal displacement of 0.20 inches has been attained. 5. Remove the suspended weights and reverse the direction of movement of the shear device. Manually slide the top of the shear box towards the retreating load piston until it returns to its pre-sheared position. Continue reversing the load piston until a small gap between the piston and shear box is visible. Remove the shear box from the direct shear machine. 6. Repeat steps 2-5 for three additional trials, increasing the suspended normal load by 100% for each subsequent trial. Clay Strength Tests 1. Position the block of undisturbed clay soil on a stable surface. Hold the pocket penetrometer in a horizontal position and slowly push the penetrometer piston into the side of the block specimen until the piston has penetrated to the calibration groove machined into the piston. Observe the internal indicator during penetration and record the maximum scale reading of the indicator prior to full penetration as the unconfined compressive strength of the soil. 2. Insert the pocket shear vane device into the side of the block specimen until the exposed vanes are fully penetrating the soil. Apply a smooth, constant torque to the handle of the shear vane creating a clockwise rotation of the embedded vanes. Continue applying torque until the failure, noted by free rotation of the vanes withing the block specimen. Record the maximum shear vane reading indicated on the dial face of the handle as the undrained shear strength of the soil, which is approximately half of the unconfined comperssive strength. 3. Remove the cylindrical clay specimen from its wrapper and record the diameter and length of the specimen. Carefully place the specimen in the compression loading device so that it is centered on the bottom platen. Adjust the loading device carefully so that the upper platen just makes contact with the specimen. Zero the deformation and proving ring indicators. 4. Apply the axial specimen load by setting the speed indicator to 25, producing an axial strain of approximately 1% per min. This rate of strain is selected to ensure that the time to failure does not exceed about 15 min. Record simultaneous readings of the proving ring and specimen axial deformations until the proving ring deformation continually decreases with increasing strain or until a total strain of 15% strain is reached, whichever occurs first.

3 CALCULATIONS 1. Use the results of the angle of repose tests to determine the slope angle of the conical sand mound (angle of repose). Use this value as an estimate of the angle of internal friction for the loose sand, φ L. 2. Using the shear mold measurements, determine the area of the shear plane in square meters. Compute the effective normal stress (kn/m 2 ) on the shear plane for each test trial. 3. Using the recorded direct shear proving ring deformations, compute the shear load (kn) and shear stress (kn/m 2 ) acting on the soil. Prepare a plot of shear stress versus shear displacement for each test trial. From the data plots, estimate the peak and residual shear strength for each normal stress condition. Construct a plot of peak shear stress vs normal stress and determine the angle of internal friction for the dense sand, φ D. 4. Using the recorded unconfined compression proving ring and specimen deformation data pairs, compute the axial strain and normal stress of the specimen. Prepare a plot of normal stress versus axial strain and estimate the unconfined compressive strength of the soil specimen. Compare the results of all clay strength tests and comment on the similarities/differences noted. The axial strain e t, at any time t, is determined as: ε t = L / L o where: ε t = axial strain at time t, cm/cm (in/in)?l = length change of specimen at time t, cm (in) L o = initial length of test specimen, cm (in) The average cross-sectional area of the specimen A t, at any time t is determined as: A t = A o / (1 - ε t ) where: A t = cross-sectional area of specimen at time t, cm 2 (in 2 ) A o = initial cross-sectional area of the specimen, cm 2 (in 2 ) The applied normal stress σ t, at any time t is determined as: σ t = 10 P t / A t where: σ t = applied compressive stress at time t, kn/m 2 P t = applied load at time t, N

4 Angle of Repose Tests Measurement Average Radius, cm Diameter of Mound, cm Mound Height, cm Angle of Repose Direct Shear Tests Trial Diameter of Mold, cm Height of Bottom Mold, cm Height of Top Mold, cm Thickness of Tamper, cm Mass of Empty Mold, N Depth to Tamper After Compaction, cm Mass of Mold + Sand, N Mass of Brass Load Cylinder, N Added Normal Load, N 1 kn/m 2 = 1 kpa = 0.1 N/cm 2

5 Direct Shear Tests Shear Test Number A B A B A B A B A = Shear Box Displacement (0.01mm) B = Proving Rind Deformation (0.001mm) Load (N) = B 1 kn/m 2 = 1 kpa = 0.1 N/cm 2

6 Clay Strength Tests Trial Number Pocket Penetrometer Reading, tsf Pocket Shear Vane Reading, tsf Cylindrical Soil Specimen Height, cm Cylindrical Soil Specimen Diameter, cm A B A = Axial Displacement (0.001") B = Proving Rind Deformation (0.0001") Load (N) = B 1 tsf = kn/m 2 1 kn/m 2 = 1 kpa = 0.1 N/cm 2