Simple Shear Tests on Clays

Abstract

This study evaluated the existing Berkeley type simple shear apparatus at the University of Sydney which had not been used for some time because of the failure of the old PC controller. During the course of working on the apparatus modifications to some mechanical parts, corrections to the software subroutines, and a comprehensive experimental investigation including series of monotonic as well as cyclic loading tests were conducted concurrently. The comprehensive experimental study was performed to investigate parameters including the specimen geometry, vertical boundary confinement systems, clay type, shearing strain rate, stress level, stress history, frequency of loading and cyclic stress ratio, which influence the stress – strain – strength behaviour of clays subjected to undrained simple shear loadings. To confirm the reliability of the test data, the results produced from each series of tests were compared with triaxial tests results for the same soils reported by other researchers and the other published data. Three types of soils including reconstituted Kaolin, natural Ballina clay and sediments obtained offshore from Australia’s continental slope were tested. The specimen confinement systems including flexible membranes, reinforced membranes and stacked rings; and the specimen geometry including the diameter, D, varying from 60mm to 80mm and the diameter/height ratio, D/H, varying from 2 to 4 were investigated on Kaolin. The shearing strain rate, ̊, investigated on Kaolin ranging from 1%/hr to 10%/hr. For the monotonic tests, the ranges of stress level covered were from 0.1MPa to 2.5MPa and the over consolidation ratio covered were from 1 to 25 for the Kaolin and offshore sediments. For the cyclic tests, the cyclic stress ratio (CSR) covered were from 0.20 to 0.80 for the normally consolidated (NC) as well as overconsolidated (OC) specimens of Kaolin and offshore sediments and the ranges of frequency covered were from 0.00056Hz to 0.1Hz for the NC specimens of Kaolin and offshore sediments. For the Ballina clay, the monotonic tests were conducted at normal stresses ranging from 15kPa to 90kPa and cyclic tests were conducted at CSR ranging from 0.24 to 0.65. In the tests with flexible membranes, the inclusion of a flushing unit was found to be an effective way to remove the entrapped air pockets inside the specimen assembly thus accelerated the saturation process of the clay specimens. Also, the use of two flexible membranes largely prevented the diffusion of compressed air into the specimen during the tests. The undrained strength is independent of the D/H ratio of the specimen within the range of 2 ≤ D/H ≤ 4. The influence of the diameter of the specimen on the undrained shear strength, su, is small, with the strength decreasing slightly with increasing diameter of the specimen. Measured values of the su and ΄ss, produced from the tests with stacked rings, reinforced membranes and flexible (constant  h) membranes are comparable with each other. However, the tests with flexible (constant v) membranes produced higher values of the su and ΄ss compared to the tests with stacked rings, reinforced membranes and flexible (constant  h) membranes. Gradually rising confining pressure to keep constant  v during shearing phase of the flexible (constant v) membranes tests is responsible to produce higher values of the su and ΄ss compared to the tests with other confinement systems. For a range of OCR between 1 to 8, the average value of su increased by about 5% and 10% when the shearing strain rate, ̊, increased from 1 to 5%/hr and 1 to 10%/hr, respectively. The shearing strain rate, ̊, has insignificant effect on the values of ΄ss in the tests conducted at higher level of ’vo of 800kPa compared to similar tests conducted at ’vo of 100kPa. The undrained shear strength parameters, su and ϕ’ss, for the NC soils are highly influenced by the level of vertical stresses, ’vo, with the values of su/’vo and ϕ’ss decreasing consistently with increasing ’vo. For the OC soils, the value of su/’vo increased with increasing OCR. During cyclic shearing of the soils, the number of loading cycles at failure increased with increasing frequency and decreased with increasing CSR. The degradation index is a measure of the stiffness of the soil. The reduction in degradation index and shear modulus were associated with the increasing excess pore pressure or decreasing effective stress with the loading cycles. The cyclic strength is a function of the amplitude of cyclic stress,c, and for a given value of c, the dilative tendency associated with the OC specimens resulted in them exhibiting stronger cyclic strength and higher shear modulus than the NC specimens. A study conducted by assuming many parameters on sediments of the Eastern Australian continental slope indicated that submarine landslides could be induced by earthquakes with Magnitude, M, of 6.5 at shallow depths while earthquakes of much higher magnitudes, M > 7.0 would be required to cause the observed failure of submerged slope at depths of 250m or greater.