Double Plane Direct Shear Apparatus for Determination of Adhesion Factor in Clays

摘要

A modified direct shear assembly with two parallel shear planes was designed to enable thesimultaneous and independent measurement of cohesive and adhesive resistance; it is namedthe “double-plane direct shear device.” The three-block specimen box of the device has topand middle blocks filled with soil specimen and the bottom block is made of solid steel. Themiddle box is laterally displaced while restraining the top and bottom box, leading to concurrentshearing along two parallel shear planes. Results from quick undrained testing of kaoliniteat five different consistency indexes (I_c) and three different vertical stresses (σ_v) are presented.The stress–deformation curves show greater mobilization of adhesive stress than cohesivestress at low deformations at all Ic and σ_v. The deformation required for the complete mobilizationof cohesive and adhesive stresses and their variation with I_c and σ_v is discussed. Aparameter crossover deformation (dco) is defined to identify the region where adhesive stressis greater than cohesive stress, which would be useful in applications where soil stickiness is ofconcern. The results show that the presence of partial drainage increases the dco, which resultsin an increased deformation range over which adhesive stress is greater than cohesive stress,thereby increasing the stickiness susceptibility. Furthermore, results show that the adhesionfactor (α, defined as the ratio of adhesion to undrained cohesion) always stayed below unityunder undrained conditions, whereas under partial drainage conditions, values of α greaterthan unity were obtained. The study concludes that for the application where soil stickinessis of concern, the determination of α and its usage to analyze stickiness susceptibility should beperformed under simulation of appropriate drainage boundary conditions.