Date of Graduation

5-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Barry, Michelle L.

Committee Member

Coffman, Richard A.

Second Committee Member

Wood, Clinton M.

Third Committee Member

Millett, Paul C.

Keywords

Aggregate; DEM; Footing; Numerical Modeling; simulations; Stone Columns; Bearing Capacity

Abstract

The implementation of stone columns as a ground improvement technique has become more popular in geotechnical construction practice as a result of their ability to improve strength, stiffness and permeability characteristics of weak clayey soil deposits. There are several analytical and empirical approaches to estimate the bearing capacity of stone column foundation systems; however, there is notable variation in the performance of these existing methods when compared with full-scale experimental results. For very weak cohesive soils (i.e., undrained shear strength less than 15 kPa), the use of conventional stone columns becomes restricted because of the insufficient confinement that these types of soils can provide to the columns. Hence, the inclusion of cement-coated aggregate has been developed as an alternative method to improve the efficacy of stone columns in soft soils. Limited information is available regarding the global performance, load-transfer mechanism, and design of these types of cemented stone columns under various field conditions. Efforts to refine the accuracy of current design methods and reinforcement techniques for conventional stone columns naturally point to the need for improving the understanding of the fundamental load-transfer mechanisms of stone columns. Three-dimensional discrete element method (DEM) simulations of small- and full-scale footing loading tests were developed to investigate the effects of aggregate strength, pier length, aggregate Young’s modulus, area replacement ratio, cement content, and undrained shear strength of the matrix soil on the bearing pressure-displacement responses of isolated foundations supported on stone columns. The elemental responses of the aggregate and plastic matrix soil were calibrated against laboratory and in-situ test data from a well-characterized site and compared against the results of small- and full-scale footing loading tests. The column aggregate material was represented by discrete-deformable tetrahedrons in conjunction with strain-softening and strain-hardening models in order to improve the simulation of the nonlinear response of the cemented aggregate. Joined deformable blocks were employed to represent the continuous mechanical behavior of the surrounding clayey soil. The numerical results are in excellent agreement with the experimental laboratory and field data and provide improved estimates of the bearing pressure-displacement curves of the column-foundation systems investigated in this study. The Young’s modulus of the aggregate column and the area replacement ratio were found to have the greatest influence on the bearing pressure-displacement response. The DEM results also improve the understanding of the effects of granular material-cementation on the performance of stone columns. At low cement contents the stone column exhibits a type of bulging failure mechanism similar to uncemented stone columns, but at higher cement contents (10 % in this study), bulging is not observed, and the behavior resembles more like that of a concrete pile. These types of behavioral differences also have different implications for single isolated stone columns and group column behavior.

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