Soil structure changes during drained and undrained triaxial shear of a clayey soil

Orientations during drained and undrained triaxial shear tests on an artificially made clayey soil were studied. The results show that the orientation pattern before shearing is nearly random, although there may be some degree of preferred orientation caused by the overburden pressure. Becoming higher towards the failure plane, the degree of preferred orientation sharply increases in both tests as the compression increases until failure. It was discovered that in the consolidated undrained tests, failure occurs at higher levels of strain than previously believed, while in the consolidated drained tests, failure occurs at much lower levels of strain than previously believed. The results indicate the formation of a wider deformation zone towards and at failure in the drained tests. This is probably because particles in the drained tests have enough time to respond to the applied shear stress and change their orientation accordingly. This may explain the formation of wider deformation zones along creeping (aseismic) faults and narrow zones along seismic faults whose mechanisms are analogous to the drained and undrained tests, respectively. © 2013 The Japanese Geotechnical Society. Production and hosting by Elsevier B.V. All rights reserved.This study was partly supported by the Çukurova University Research Foundation (Project no. MMF.2006.D15 ). Thin sections were made at the Thin Section & Micromorphology Laboratory of the School of Biological & Environmental Sciences at the University of Stirling in Scotland, UK. Thanks to Mr. George W. MacLeod for helping to make the thin sections and for shipping them to us. The soil mechanical tests were performed at the Soil Mechanics Laboratory (the only internationally accredited Turkish laboratory) of the Technical Research and Quality Control Department (TAKK: Turkish acronym) of the General Directorate of State Hydraulic Works (DSI: Turkish acronym) in Ankara, Turkey. Thanks also to Dr. Vehbi Özaydın and Burhanettin Ünal, for their unlimited support during the triaxial testing program, and to Fatih Karaoğlan, for his help in taking the photographs using the department's digital microscopic camera (Leica DFC) under his responsibility