Evaluation of Settlement in Poorly Graded Sand with Gravel Under Strain-Controlled Cyclic Triaxial and Dynamic Centrifuge Model Testing Conditions

This research evaluates the dynamic behavior and volume change of sand with gravel subjected to cyclic loading. Historically, research on the dynamic behavior of cohesionless soils has primarily focused on the study of liquefaction due to its known implications on soil shear strength loss. Gravelly soil is generally assumed to be less susceptible to liquefaction since its large particle size facilitates drainage during cyclic loading. Gravel’s particle size also makes it difficult to test using standard cyclic shearing laboratory equipment. Thus, there is a lack of widespread research and predictive methods for the dynamic behavior of gravelly soil. Determining the shear strength loss associated with cohesionless soil liquefaction is essential in seismic analysis and design. Additionally, the estimation of seismic soil settlement is of critical importance. Most state-of-practice seismic deformation prediction methods are based on research performed on clean sand, with little research on the cyclic deformation of sand containing gravel particles. The primary motivation of this research is to improve understanding of the dynamic behavior of gravelly soil. In this study, the tested soil has a USCS classification of Poorly Graded Sand with Gravel (SP), containing 29.5% Gravel, 70.3% Sand, and 0.2% fines. To assess the cyclic volumetric strain of sand with gravel, nine (9) strain-controlled drained cyclic triaxial tests and three (3) dynamic centrifuge tests were performed. Strain-controlled, drained cyclic triaxial testing is an advanced laboratory testing method that controls axial deformation of a soil specimen through alternating cycles of contraction and dilation to measure progressive volume change within the sample. The cyclic triaxial tests in this thesis assess the effect of confining stress and cyclic axial strain amplitude on cyclic volumetric strain. It was found that cyclic volumetric strain decreases with an increase in confining stress. Distinct volumetric strain trends that are largely dependent upon the confining stress of the soil were observed and analyzed. This research also indicates that sample deformation increases with cyclic strain amplitude at all tested confining stresses. In the centrifuge modeling tests, 50 g centrifugal acceleration applied scaled stress to the soil specimen simulating an 8.0 m (26.2 ft) soil profile with an overburden stress of 150 kPa (22 psi) at the midpoint of the model. The shaking table simulates sinusoidal ground motion and allows for soil response to be analyzed. The dynamic centrifuge tests presented in this thesis assess the influence of soil drainage on the dynamic behavior and settlement of sand with gravel. Soils in the partially drained condition during loading were found to deform more than soils in drained and undrained conditions. Based on the results, the sand with gravel specimen in an undrained condition experiences the formation of a water lens in the top layer of soil. This conclusion was deduced based on the presence of a loose, liquefiable soil layer along the impermeable soil boundary.