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Title: Investigation of plane strain shear testing. Report 2, Drained plane strain and triaxial compression tests on crushed NAPA basalt
Authors: Al-Hussaini, Mosaid M.
Keywords: Shear strength of soils
Soil mechanics
Issue Date: Jun-1971
Publisher: Soils and Pavements Laboratory (U.S.)
Series/Report no.: Technical Report;S-71-2, Report 2
Abstract: Abstract: Abstract: Although the triaxial compression test has been widely accepted for design problems in soil mechanics because of its adaptability to duplicate certain field conditions, it is not sufficiently versatile to simulate plane strain field conditions. Problems involving shear failure in one plane, such as strip footings, retaining walls, and slopes, may closely approximate failure under plane strain conditions and cannot be well represented in the conventional triaxial compression test. This report is devoted mainly to a comparison of deformation and strength characteristics of crushed Napa basalt tested under triaxial compression and plane strain conditions. Two types of shearing devices were used in the test program: the WES high-capacity plane strain apparatus for testing prismatic specimens, and a conventional triaxial compression apparatus with enlarged low-friction platens to test cylindrical specimens under axially symmetric stress conditions. The soil specimens were consolidated under either isotropic or Kₒ conditions and then sheared under drained conditions. Four series of tests were conducted in each shear device. The confining pressures (i.e., cell pressure minus back pressure) used for specimens tested under plane strain conditions were 60, 125, 300, and 425 psi, while the confining pressures used for specimens tested in triaxial compression were 60, 125, 300, and 500 psi. Both plane strain and triaxial specimens were prepared at two relative densities: 70 and 100 percent. The test results indicate that for same densities and confining pressures, specimens under axially symmetric stress conditions, in which the intermediate and the minor principal stresses are equal, undergo larger axial strain at failure and more crushing of particles than those tested under plane strain conditions. On the other hand, plane strain specimens compress more (i.e., exhibit larger negative volumetric strains) and show higher strengths and angles of internal friction, as measured by Mohr-Coulomb theory, than comparable specimens tested under axially symmetric stress condition. The difference in the value of effective angle between plane strain and triaxial compression tests is about 5 deg for dense specimens at a confining pressure of 60 psi and 2 deg for medium-dense specimens at a confining pressure of 425 psi. Test results for both plane strain and triaxial compression tests show that values of Kₒ, the axial strain at failure, and the negative volumetric strain at failure increase with increasing confining pressure and decreasing density of the material. However, the effective angle of internal friction increases with increasing density and decreasing confining pressure. For the material tested, the failure points fall between the theoretical Mohr and the extended Tresca failure criteria.
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