Settlement of Compacted Clay in a Cyclic Direct Simple Shear Device

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Courtesy of ASTM International

The results of 36 cyclic strain-controlled direct simple shear tests on a low-plasticity compacted clay are presented and analyzed. The tests were conducted in a Marshall Silver-type device, which utilizes the Norwegian Geotechnical Institute (NGI) type of short cylinder specimen confined in a wire-reinforced rubber membrane. Three degrees of compaction achieved by the modified compaction test are encompassed: (1) below the optimum moisture content, (2) at the optimum, and (3) above the optimum. All specimens were sheared under the same vertical consolidation stress. In each test the cyclic shear strain amplitude, ?c, was controlled, i.e., the cyclic tests were strain controlled. The range of ?c was between 0.008 and 4.6%. The test results show a very consistent behavior of the clay at all three moisture contents. At small cyclic shear strains below ?c ? 0.1%, the stress-strain behavior is slightly nonlinear, i.e., close to linearly elastic, and the vertical settlement is negligible. In some tests such nondestructive behavior was recorded up to ?c ? 0.2%, showing that for the clay tested, the volumetric threshold shear strain, ?tv, ranges between 0.1 and 0.2%. At ?c larger than ?tv the cyclic stress-strain behavior becomes nonlinear and a continuous settlement with the number of cycles, N, occurs. The results do not show a clear relation between the rate of settlement with N and the degree of the compaction and moisture content. For different moisture contents, similar settlements were obtained for given ?c and N. The study also shows how to arrange the specimen setup and measuring system to eliminate the effect of the simple shear apparatus compliance. In such an arrangement very small shear strains can be applied and successfully measured.

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