Deformation and Stiffness Characteristics during Creep of Weathered Residual Soil in Korea

Abstract

Granular materials are prevalent sub-soils in which many geotechnical structures are constructed. Recent studies have revealed the importance of certain time-dependent behaviors, including time-dependent deformations and changes of the engineering properties of granular materials. The deformation of granular materials under constant loading influences geotechnical structures such as buildings, bridge abutments, earth-retaining structures, and earth slopes. Moreover, excessive deformations over time under a constant load can cause serviceability issues. However, time-dependent deformation, which may result in unexpected deformations of the ground or structural failures, has not been properly considered in geotechnical designs. Therefore, to solve existing problems in geotechnical engineering, it is important to investigate deformation behavior in detail as it occurs with time. Despite its geological predominance and the fact that it is the most common type of soil used as a construction material in South Korea, the long-term behavior of weathered residual soil has yet to be clearly investigated. The time-dependent behavior of a granular material depends on the material properties, confining pressure, and relative density. Weathered residual soils are used as construction materials with various degrees of field compaction, which are closely linked to the relative density. Thus, analyzing the effects of the relative density and stress conditions on the time-dependent behavior of soil is crucial for a proper investigation of the long-term behavior of weathered residual soil. The aims of this research are to investigate the influences of the relative density, stress ratio and effective stress on the creep behaviors of weathered residual soil and to study the effects of time on the variations of the elastic shear stiffness. To achieve these goals, a series of laboratory tests were designed such that the effects of the relative density, stress ratio, and effective stress on the creep/aging behaviors of weathered residual soil in Korea could be explored. Observations and evaluations of creep deformation under a constant load with various relative densities are discussed in detail. By performing a series of stress path triaxial creep tests under both isotropic and anisotropic conditions under every vertical effective stress level (100, 200, and 400 kPa) for 48 hours, creep strains were analyzed. For a comparison with triaxial test results, creep deformations were also evaluated under laterally constrained conditions using a one-dimensional consolidation testing apparatus. In addition, based on these experimental observations, the creep parameters for numerical modelling are provided to predict creep deformation in the field. The developed creep strains depend strongly on the initial relative density, current stress state, and stress ratio. Specimens with low and medium relative density levels showed contractive creep behavior, whereas specimens with high densities showed dilative creep behavior. The effects of the effective stress on the creep behavior also depend on the initial relative density. In specimens with low and medium relative density levels, high effective stress stimulates the development of creep strain. However, in specimens with high relative densities, high effective stress inhibits the development of creep strain. A high stress ratio promotes creep strain in axial direction, but impedes volumetric creep strain. The aging effects on elastic shear stiffness due to creep are also evaluated under various sample conditions with different relative density and shearing conditions and with different confining stresses. The elastic shear moduli during the creep period were measured using orthogonal bi-directional bender elements. By evaluating the aging effects on the directional stiffness as obtained from bender element tests, changes in the soil structure could be inferred during creep. Moreover, the relationship between the developing creep strain and the measured soil stiffness could be determined. Variations of the stiffness anisotropy during creep, an important factor in various problems related to ground deformation, are also investigated. Variations of the elastic shear stiffness and deformation during creep depend on the stress ratio and the initial relative density. The continuous change in the stiffness anisotropy during creep was found to be closely related to the patterns in creep deformation. In addition, the increase in the elastic stiffness is more sensitive to the direction of the major principal stress than it is to the direction of the minor principal stress during creep.