A Simple Measurement of Membrane Penetration in Gravel Triaxial Tests Based on Eliminating Soil Skeleton Plastic Deformation with Cyclic Confining Pressure Loading

摘要

Previous investigations of membrane penetration mainly focused on uniform sandy soils, and few studied gravelly soils. A common problem in most previous investigations was that unverified assumptions were made to evaluate or mitigate the membrane penetration of a cylindrical specimen in triaxial test. In this study, a simple measurement of the membrane penetration of gravelly soils was proposed, based on small-amplitude cyclic confining pressure Delta sigma(3) loading, in which the cyclic membrane penetration amplitude Delta V-m is recoverable, and the cyclic soil skeleton volume deformation amplitude Delta V-k gradually decreases with increasing number of cycles and becomes elastic. The accuracy in the proposed method is related to the accuracy in measuring Delta V-k. Because of the natural limitations in measurements of the radial strain of a cylindrical specimen, Delta V-k is calculated based on the isotropic assumption. The proposed method provides sufficiently accurate Delta V-m values for gravelly soils because of the small portion of Delta V-k in the total cyclic volume change amplitude Delta V-c (=Delta V-m+Delta V-k), and the accuracy increases as the coarse fraction increases. Furthermore, the method can be easily implemented for individual specimens to remove inconsistences in specimen preparation. The test results indicate that Delta V-m values corresponding to the same Delta sigma(3) exponentially decrease with the confining pressure sigma(3). Membrane penetration increases with increases in the void ratio and coarse fraction, but the trend is becoming weak with decreasing coarse fraction and void ratio, respectively. It also demonstrated that there exists a clear difference between the results of Nicholson's empirical formula and the test results.