DWR_2801682 - Laserfiche WebLink

Home Search

8 samples. This is conservative, but eliminates the need to perform moist CD tests, which are very complex. The CD test is applicable for both cohesionless soils (sands and gravels) and cohesive soils (clays and elastic silts), however the shear strain rates required for cohesive soils may be so slow as to be impractical, particularly considering that effective stress parameters for these soils can be obtained in a CU’ test, as discussed below. Refer to Figure 9 for the failure envelope corresponding to a CD test. Figure 9: CD test results.[Curved shear strength envelope with linear interpretation and apparent cohesion, c’] Consolidated-Undrained (CU or R) Test and Consolidated-Undrained (CU’ or R-bar) Test with Pore Pressure Measurements The CU’ test is probably the most common triaxial test. With the ready availability of pore pressure measurement devices, the CU test (without pore pressure measurement) is not commonly performed. In the CU and CU’ tests, drainage is prevented during back pressure saturation and consolidation, and a saturated soil specimen is consolidated under a specified confining pressure. Similar to the CD test, the CU or CU’ test can be performed using either isotropically consolidated (IC-U) samples or anisotropically consolidated (AC-U) samples. IC-U is generally the most common; however, AC-U tests are sometimes selected to model initial shear stress in soils. After pore water pressures generated by application of the confining pressure are dissipated, the specimen is sheared to failure with a closed drainage valve. Pore water pressures are either not measured during the consolidation shear stage (CU test) to produce total stress strength parameters (c,I) or measured during the consolidation shear stage (CU’ test) in which case both total and effective stress strength parameters (c’, I’) can be obtained. A backpressure is applied to the pore water to maintain saturation of the soil specimen. Maintaining saturation allows accurate measurement of pore water pressures. The CU and CU’ tests are suitable for both cohesionless soils (sands and gravels) and cohesive soils (clays and silts) and are performed at a faster shear strain rate compared to the CD test. Because both total and effective stress strength parameters can be obtained from a CU’ test, CU tests are not often done, as noted above. CU or CU’ tests on sands and gravels are usually performed on remolded samples because of the difficulty of obtaining quality, undisturbed samples of cohesionless soils. Refer to Figure 10 for failure envelopes corresponding to CU and CU’ tests. Figure 10: CU and CU’ test results. Examples of when to perform a CU or CU’ test include the following conditions: rapid load application to a soil previously consolidated under a smaller load, upstream drawdown of a reservoir and a saturated slowly permeable embankment, and shear strains causing sudden load application to a saturated, consolidated soil mass. Direct Simple Shear Test The direct simple shear (DSS) test is sometimes used when shear strength is expected to exhibit significant anisotropy, such as for some clay soils. The test typically measures the shear strength on the horizontal plane. DSS testing is performed as described in ASTM D6528. In the DSS test, the volume of a soil specimen is kept constant during shearing to simulate undrained V’ s I’ c’ V’1CV’3C V’1BV’3B V’3A V’1A CD Failure Envelope (Linear approximation of shear strength within stress range of interest) Three test results shown – A, B, and C c Shear Failure Envelope (Curved) CU’ Failure Envelope CU Failure Envelope