Simulating the membrane contact patterns of triaxial sand specimens

One of the common sources of error in triaxial tests is the penetration of the latex membrane into the peripheral voids of the specimen as confining p ressure is applied. Existing analytical solutions developed to predict the magnitude of this membrane penetration have all assumed that the peripheral sand particles in contact with the surrounding membrane are arranged so th at the centroids of the sand-membrane contact areas form a square pattern o f side length D-50 on a plane parallel to the undeflected membrane. Measure ments made on digital images of sand-membrane contact patterns for triaxial specimens of varying characteristics have shown this assumption to be inad equate. The analysis presented herein shows how actual contact pattern can be reasonably replicated by simulated patterns. These simulated patterns ar e generated by adding randomly selected distances to the centroid co-ordina tes of contact areas initially placed in a systematic grid pattern. The res ults indicate that it is possible to systematically model sand-membrane con tact patterns as a function of triaxial specimen characteristics such as re lative density and preparation method. The ability to simulate the real pat terns eliminates the reliance of existing analytical solutions on a single assumed square contact pattern and hence can serve as the basis for improvi ng the accuracy of future solutions to account for the effects of membrane penetration. Copyright (C) 2000 John Wiley & Sons, Ltd.