Kazem Fakharian

The constant volume behavior of sands is substantially influenced by the initial stress anisotropy. This research aims to investigate the stress anisotropy effect by conducting a series of bidirectional direct simple shear tests that can apply initial shear stress on a sample in different directions during the consolidation stage. The experimental program provides insights into the impacts of the initial shear stress and the subsequent principal stress rotation (PSR) on some critical aspects of soil behavior, including the onset of instability, brittleness index, phase transformation, and the critical state line. The findings show that the onset of instability and the brittleness indices are significantly dependent on the initial stress anisotropy. In contrast, the critical state and phase transformation lines are not influenced by the initial anisotropy of the stress state, even in the presence of PSR. Furthermore, the study gives particular attention to the non-coaxiality between the major principal stress and strain rates to illustrate how the non-coaxiality decreases with increasing shear strain. The research also explores the non-coaxiality between the resultant shear stress and shear strain rate and suggests a predictive flow rule accordingly. The results reveal that a substantial level of non-coaxiality may exist between the resultant shear stress and the shear strain rate.

The main objective of this paper is investigation of clayey soil setup effects on axial “resistance” and “stiffness” of small-scale piles with special attention on “overconsolidation ratio” (OCR) and “installation method”. A consolidation chamber is developed and an instrumented model pile is either driven or jacked-in through saturated consolidated Kaolinite clay at different OCRs of 1.2 (normally consolidated, NC) and 4.8 (over consolidated, OC). The pile was static load tested at different time intervals of 1 h through 90 days. Higher pore pressures are induced during pile installation in NC compared to OC clay. The frictional resistance of OC soil is higher than NC at end of drive as well as end of setup in both driven and jacked-in piles. The frictional resistance of jacked-in piles is greater than driven piles at end of drive, but due to higher rate of setup, the long-term frictional resistance of driven piles is shown to be greater, attributed to more disturbance of the soil during installation of driven piles. Attempts are made to differentiate between the contributions of dissipation of excess pore pressure and non-pore pressure components for each consolidation condition and pile installation method. A relation is proposed to quantify the pile stiffness variations over time due to soil setup.