Refined True Triaxial Apparatus for Testing Unsaturated Soils under Suction-Controlled Stress Paths

Abstract

This paper introduces a servocontrolled true triaxial apparatus that has been developed to test cubical specimens [with 7.62 cm (3 in.) sides] of unsaturated soil under controlled-suction states and for a wide range of stress paths not readily achievable in a cylindrical cell. The apparatus is a mixed-boundary-type device, with the specimen seated on top of a 5-bar high-air-entry-value (HAEV) ceramic disk and between five flexible membranes on the remaining sides of the cube. The new cell is an upgraded, more elaborate version of one previously reported by the authors, featuring two independent pore-air and pore-water pressure control systems, as well as a fully computer-driven stress application/control system. Suction states are induced in the specimens via the axis-translation technique. The paper outlines the full development of the apparatus, including its main components; assembling process; and selection of appropriate compaction method, dry unit weight, and constant-suction loading rate for clayey sand. The suitability of the apparatus is demonstrated by repeatable results from two triaxial compression tests performed on identically prepared samples of compacted clayey sand under constant 200-kPa matric suction.