Date of Graduation

12-2017

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Bernhardt, Michelle L.

Committee Member

Coffman, Richard A.

Second Committee Member

Wood, Clinton M.

Keywords

Aggregate Piers; DEM; Discrete Element Method; Stone Columns; Vibrated Columns

Abstract

Aggregate piers are an alternative ground improvement technique used for improving the bearing capacity and reducing the total and differential settlement of foundations supported on compressible soils. The piers are composed of a series of vibrationally compacted aggregate lifts from a desired depth up to the finished foundation level. The shear strength of the coarse-grained material comprising an aggregate pier is defined by its internal friction angle. There are several factors affecting the friction angle of the granular material that include: confining stress, particle shape, relative density and particle distribution (gradation). These factors are not directly considered in the design of aggregate columns. However, because the granular material friction angle is a fundamental design parameter used to estimate the bearing capacity of aggregate piers, these factors could play an important role in the mechanical behavior of isolated aggregate columns. Other factors responsible for altering the global performance of aggregate piers are column length and the undrained shear strength of the matrix soil. Full-scale 3D discrete element method (DEM) simulations were conducted to reproduce field plate load tests performed on small circular foundation resting on single aggregate piers in order to evaluate the effects of these factors on the global performance of aggregate columns. The numerical results demonstrate that incorporating the gradation, confining stress and relative density dependence of the aggregate friction angle in the DEM models provides better numerical estimation of the load-displacement curves observed in the field for both the foundation serviceability level and the ultimate load condition. The effects of column length on the load-displacement response are found to be a minimum for column slenderness ratios greater than four.

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