Laserfiche WebLink
6 <br />Figure 5: Unconfined compression test results. <br />Triaxial Shear Test <br />The triaxial shear test is the most common and <br />versatile of the five tests, but also one of the most <br />involved and time consuming. In the triaxial test, a <br />horizontal confining pressure is applied to the <br />specimen and drainage conditions are controlled. The <br />triaxial shear test can be performed on “undisturbed” <br />samples (samples obtained from a constructed <br />embankment or its foundation, typically using tube <br />samplers) or remolded samples (soil samples <br />compacted to specified design density and moisture <br />content, e.g. to replica fill placement). Strength tests <br />on undisturbed samples rely on the integrity of the <br />sample being maintained during sampling, <br />transportation, and specimen preparation. Sample <br />disturbance can have a significant effect on strength <br />results. Care in preserving the tube samples <br />throughout collection, transportation, and storage is <br />critical. Samples should be extruded only in the <br />laboratory. The engineer needs to rely on the lab to <br />identify signs of sample disturbance or be present <br />during sample extrusion and specimen preparation. <br />Triaxial shear testing is performed as described in <br />ASTM D2850 (UU test) and D4767 (CU’ test). <br />In the triaxial shear test, a cylindrical soil specimen is <br />encased in a rubber membrane and placed in a triaxial <br />test chamber that is filled with a fluid (usually water). <br />The specimen is subjected to an all-around confining <br />pressure laterally by pressurization of the fluid in the <br />chamber. An axial load is applied by means of a loading <br />piston through the top of the chamber. The confining <br />pressure is held constant while the axial load is <br />increased (compression testing) or decreased <br />(extension testing) until shear failure of the soil <br />specimen occurs. A drainage system consisting of <br />porous stones and drainage lines is connected to the <br />sample on the bottom and/or top to allow for drainage <br />and pore pressure measurement. Figure 6 illustrates <br />the principles of the triaxial compression test. A <br />schematic of the test apparatus is shown in Figure 7. <br />Figure 6: Principles of triaxial compression tests: (a) <br />application of stresses, (b) representation of principal <br />stresses, (c) usual arrangement for effective stress <br />tests, (d) representation of total and effective stresses. <br />Figure 7: Triaxial shear test apparatus. <br />Total stress Mohr’s <br />circle at failure <br />V3 = 0 <br />Su <br />V1 = qu <br />V1 <br />V1 I = 0 <br />V <br />W <br />Reference:Das (2006) <br />Reference:Head (1986)