New mixed boundary, true triaxial loading device for testing three-dimensional stress–strain– strength behaviour of geomaterials Jian-Hua Yin, Chun-Man Cheng, Md. Kumruzzaman, and Wan-Huan Zhou Abstract: This paper presents a brief review of true triaxial apparatuses (TTAs) developed in the past and their advantages and limitations. Considering the limitations of previous designs, a new true triaxial loading device that provides mixed boundary conditions for a true triaxial apparatus (TTA) is introduced. This loading device consists of four sliding rigid plates and two flexible loading faces. The setup of the loading device together with the whole true triaxial system is de- scribed. Frictions between sliding plates and the soil membrane surfaces in the new loading device are examined. A three- dimensional finite element (FE) modelling study is carried out on the stress and strain distribution of a soil specimen sub- jected to loading from two different loading devices. It is found that stresses and strains of a soil specimen subjected to loading from the new sliding plates are far more uniform than those subjected to loading from nonsliding plates with pre- set gaps. Finally, the paper presents the applications of the present TTA with the new loading device for testing studies of a completely decomposed granite soil and a geofoam. Typical results are presented and discussed. It is found that the present mixed boundary loading device is very suitable for true triaxial testing on both soils and geofoam, especially under large strains or compression without corner contact problems. Key words: true triaxial, sliding plates, friction, stress, strain, strength, stress path. Re ´sume ´: Cet article pre ´sente une bre `ve revue des appareils vrais triaxiaux (AVT) de ´veloppe ´s dans le passe ´ ainsi que leurs avantages et limitations. En conside ´rant les limites des designs pre ´ce ´dents, un nouvel appareil vrai triaxial qui permet de choisir des conditions frontie `res mixtes est propose ´. Le syste `me de chargement est fait de quatre plaques rigides glissantes et deux faces de chargement flexibles. L’installation du syste `me de chargement et le syste `me vrai triaxial complet sont de ´- crits. Les frictions entre les plaques glissantes et les surfaces de la membrane du sol dans le nouveau syste `me de charge- ment sont examine ´es. Une e ´tude par mode ´lisation tridimensionelle par e ´le ´ments finis a e ´te ´ effectue ´e pour e ´valuer la distribution des contraintes et de ´formations d’un e ´chantillon de sol soumis a ` des chargements provenant de deux types de syste `mes de chargement. Il a e ´te ´ de ´montre ´ que les contraintes et de ´formations d’un e ´chantillon de sol soumis a ` un charge- ment par les nouvelles plaques glissantes sont beaucoup plus uniformes que dans le cas d’e ´chantillons de sol charge ´s a ` partir de plaques qui ne sont pas glissantes et avec des espacements pre ´de ´finis. Finalement, l’article pre ´sente des applica- tions de l’AVT avec le nouveau syste `me de chargement pour l’e ´tude d’un sol de granite comple `tement de ´compose ´ et d’une ge ´o mousse. Des re ´sultats typiques sont pre ´sente ´s et discute ´s. Il est de ´montre ´ que le syste `me de chargement a ` condi- tions frontie `res mixtes est tre `s approprie ´ pour les essais vrais triaxiaux autant pour le sol que pour la ge ´o mousse, particu- lie `rement lorsque les de ´formations sont grandes ou qu’il y a compression sans proble `me de contact dans les coins. Mots-cle ´s : vrai triaxial, plaques glissantes, friction, contrainte, de ´formation, re ´sistance, cheminement des contraintes. [Traduit par la Re ´daction] Introduction Applications of three-dimensional (3-D) analyses to deal with field geotechnical problems have become more desir- able. In most field geotechnical problems, soil is normally subjected to three different principal stresses. The direct measurement of the stress–strain–strength behaviour of soils under a 3-D stress state is necessary. A true triaxial appara- tus (TTA) is specially designed for the measurement of stress–strain behaviour of geomaterials under three principal stresses. The developments of a TTA and its use in testing studies have been an active research area with a long his- tory. For example, many researchers conducted studies on the influences of the intermediate principal stress (s 0 2 ) on the stress–strain–strength behaviour of geomaterials using a TTA (Lade and Mustane 1978; Yamada and Ishihara 1979; Ochiai and Lade 1983; Abelev and Lade 2004; Prashant and Penumadu 2004; among others). This paper will give a brief review of the TTAs devel- oped in the past, and then presents a TTA with a new load- ing device. To investigate the boundary influence on the stress–strain distribution, 3-D numerical modelling of a soil Received 13 May 2008. Accepted 11 June 2009. Published on the NRC Research Press Web site at cgj.nrc.ca on 24 December 2009. J.-H. Yin, 1 C.-M. Cheng, Md. Kumruzzaman, 2 and W.-H. Zhou. Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. 1 Corresponding author (e-mail: cejhyin@polyu.edu.hk). 2 Present address: Department of Civil Engineering, Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh. 1 Can. Geotech. J. 47: 1–15 (2010) doi:10.1139/T09-075 Published by NRC Research Press