Sand was tested under triaxial conditions, plane strain conditions, and conditions where the intermediate principal strain (ε2) varied between the limits for triaxial and plane strain. Tests were performed using a variety of specimen sizes and shapes, a range of densities from loose to dense, and a range of confining pressures from 70 to 3450 kN/m2 (10 to 500 psi). The results of these tests provide useful guidelines for plane strain testing of cohesionless soils: (1) Plane strain and triaxial test results differ most for dense specimens tested at low pressures, (2) Plane strain equipment need not be elaborate. Specimens with square cross sections and simple lubricated end plates can be used for plane strain tests, and (3) The end plates need not enforce a condition of perfect plane strain. Tests with values of ε2 as large as 40 percent of the axial strain at failure result in essentially the same value of the angle of internal friction (φ) as do perfect plane strain tests.
A new simple shear apparatus is described which can apply bi-directional simple shear cyclic and monotonic loading. Cylindrical specimens are placed in a pressure chamber so that confining pressure and back pressure can be applied. Bi-directional shear loads are applied to the specimen's base, which is mounted on a series of two horizontal rolling tables which can be independently loaded in two orthogonal directions while preventing cap and base platen “rocking.” A computer-automated feedback-loop-controlled pneumatic servovalve system provides excellent control of loads and displacements. Displacement control during cyclic tests on softening specimens is improved by the inclusion of a parallel stiffness. Procedures and test results are presented for three different undrained cyclic simple shear loading conditions and for a strain-controlled monotonic loading condition.
This paper reviews some of the current systems in use for the application of cyclic loads to soil specimens. Their main features and principles of operation are described. A relatively low-cost electropneumatic system recently developed at the University of California at Berkeley is described in detail. This system provides sinusoidal and other shape load traces using normal compressed air controlled electronically and is an assembly of commercially available components.
Initial feasibility findings of a research program directed toward the development of a practical tool for rapid in situ determination of material properties (modulus and damping) of soft to medium-stiff clays over a wide range of strain (0.0001% to 10%) are presented. The motivation for this work is discussed in terms of both the sensitivity of earthquake ground response analyses to input material properties, as well as the limitations of existing design methodologies for specifying material behavior. The scope and objectives of the research program are outlined. Results are presented from numerical analyses used to establish typical stress and strain fields caused by borehole installation. The merits of several tool-design alternatives are considered, and preliminary design concepts for a new tool called the “Freestanding Torsional Shear (FTS)” device are introduced. Critical issues and future directions of the research program are discussed.
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