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10 <br />The torsional ring shear test can measure shear <br />stresses over any magnitude of displacement and is <br />therefore suitable for estimating residual shear <br />strength, as shown in Figure 13. The test is primarily <br />used for evaluating the drained residual shear strength <br />of cohesive soils (clays and silts). The specimen must <br />be sheared slowly enough that pore water pressures <br />do not develop. This is generally not a problem <br />because of the small sample height and resulting short <br />drainage path. A minimum of three remolded <br />specimens is generally tested under different normal <br />stresses that are representative of field conditions to <br />evaluate the drained residual failure envelope. <br />Figure 13: Torsional ring shear test results. <br />Laboratory Testing Shear Strength <br />Characterization Summary <br />The table below presents a summary of the <br />aforementioned laboratory tests and the shear <br />strength characterization evaluated by each test. <br />Laboratory Test Shear Strength Characterization <br />Direct Shear (DS) <br />Drained effective stress shear strength <br />parameters (I’, c’) for fine-grained <br />sands, clays, and silts <br />Unconfined <br />Compression (UC) <br />Undrained shear strength (Su) for <br />saturated cohesive soils (clays and silts) <br />Unconsolidated- <br />Undrained Triaxial <br />(UU or Q) <br />Undrained total stress strength <br />parameters (I, c) for cohesive soils (clays <br />and silts) <br />Consolidated- <br />Drained Triaxial <br />(CD or S) <br />Drained effective stress shear strength <br />parameters (I’, c’) for cohesionless soils <br />(sands and gravels) and cohesive soils <br />(clays and elastic silts) <br />Consolidated- <br />Undrained Triaxial <br />(CU or R) <br />Undrained total stress strength <br />parameters (I, c) for cohesionless soils <br />(sands and gravels) and cohesive soils <br />(clays and silts) <br />Laboratory Test Shear Strength Characterization <br />Consolidated- <br />Undrained Triaxial <br />with Pore Pressure <br />Measurements (CU’) <br />Undrained total stress strength <br />parameters (I, c) and drained effective <br />stress strength parameters (I’, c’) for <br />cohesionless soils (sands and gravels) <br />and cohesive soils (clays and silts) <br />Direct Simple Shear <br />(DSS) <br />Undrained shear strength (Su) most <br />suitable for soft cohesive soils (clays and <br />elastic silts) <br />Rotational Ring <br />Shear <br />Drained effective stress residual shear <br />strength parameters (Ir’, cr’) most <br />suitable for cohesive soils (clays and <br />silts) <br />Evaluation of Laboratory Test Data <br />Engineers should be capable of prescribing and <br />critically reviewing laboratory test data with regard to <br />soil strength characterization. Listed below are the <br />main categories of data that should be critically <br />evaluated: <br />x Method of sample preparation <br />x Initial sample data <br />x Backpressure data (when applicable) <br />x Consolidation data (when applicable) <br />x Final water contents <br />x Stress, strain, and pore pressure data <br />x Plotted data <br />Field Testing for Shear Strength <br />In-situ testing of embankment or foundation materials <br />for direct shear strength measurements is performed <br />by the following methods: <br />x Vane Shear Test (ASTM D2573) <br />x Pocket Penetrometer Test <br />x Torvane Test <br />The vane shear test is the most widely used field test <br />for measuring the undrained shear strength of soft to <br />medium stiff clays. In the field vane shear test, a four- <br />bladed vane is pushed into the soil and rotated until <br />the soil fails in shear along a cylindrical surface. The <br />resisting torque is measured to evaluate the undrained <br />shear strength. A schematic of the vane shear test <br />apparatus is shown in Figure 14.