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Geotechnical News • June 2012
March 2012
THESIS ABSTRACTS
Modeling Macro-scale Clay Behavior at
Micro-scale Clay Particle Interfaces
Dr. Laura Kosoglu
Dr. Laura Kosoglu, Department of Civil, Environmental and Infra-
structure Engineering, George Mason University, 4400 University
Drive MS 6C1, Fairfax, VA 22030, T: 703-993-5319,
E:
Clay consolidation has generally been considered from
a macro-scale perspective by measuring the macro-scale
compression of a clay soil over time. Clay particles in
consolidation tests experience shear and normal forces at
the inter-particle level due to force applied to the soil at
the macro-scale. These shear and normal forces cause the
particles to slide at the micro-scale and produce macro-
scale changes in soil volume and shape. By considering the
inter-particle interactions at the micro-scale, the shear force
- normal force - velocity relationship can be described by
the Rate Process Theory (RPT). This research investigated
the use of RPT for analyzing sliding at clay particle contacts
during secondary compression to describe macro-scale clay
behavior.
The novel micro-scale friction experiments conducted in
this research demonstrated that an Atomic Force Micro-
scope (AFM) can be used to obtain coefficient of friction
measurements for montmorillonite. This method allows for
measurements to be performed over spatial scales of several
microns, can be conducted under dry conditions or a wide
range of aqueous solutions, and requires no calibration
beyond a few microscopic measurements of the probe. The
micro-scale AFM and macro-scale triaxial shear, ring shear,
and direct shear experimental data of the coefficient of
friction as a function of velocity were found to match well
with those calculated using RPT. A discrete element method
(DEM) model was also developed to calculate clay particle
movement in three dimensions during compression using
RPT as a contact model.
This research provides evidence of the close correspondence
between macro-scale and micro-scale coefficient of fric-
tion measurements and contributes to multi-disciplinary
understanding of factors that control friction between clay
particles and deformation of clay masses. The results from
this work can be applied to a wide range of time-dependent
phenomena such as clay secondary compression, shear
deformation, and fault dynamics behavior.
Sponsoring Professor: Prof. George M. Filz, Virginia Tech, Civil
and Environmental Engineering, 120-C Patton Hall, Blacksburg,
VA 24061, T: 540-231-7151, E:
Critical Height and Surface Deformation of
Column-Supported Embankments
Michael P. McGuire
Michael P. McGuire, Ph.D., P.E., Research Associate, Charles
E. Via, Jr. Department of Civil and Environmental Engineering,
Virginia Tech, 120E Patton Hall, Blacksburg, Virginia 24061,
C: 540-357-4073, T: 540-808-2044, E:
Column-supported embankments with or without basal
geosynthetic reinforcement can be used in soft ground
conditions to reduce settlement by transferring the embank-
ment load to the columns through stress redistribution above
and below the foundation subgrade level. Column-supported
embankments are typically used to accelerate construction
and/or protect adjacent facilities from additional settlement.
The column elements consist of driven piles or formed-
in-place columns that are installed in an array to support a
bridging layer or load transfer platform. The bridging layer
is constructed to enhance load transfer using several feet
of compacted sand or sand and gravel that may include
one or more layers of high-strength geotextile or geogrid
reinforcement. Mobilization of the mechanisms of load
transfer in a column-supported embankment requires some
amount of differential settlement between the columns and
the embankment as well as between the columns and the
foundation soil. When the embankment height is low rela-
tive to the clear spacing between columns, there is the risk
of poor ride quality due to the reflection of the differential
foundation settlement at the surface of the embankment. The
minimum embankment height where differential surface
settlement does not occur for a particular width and spacing
of column is the critical height. The conventional approach
is to express critical height as a fixed ratio of the clear span
between adjacent columns; however, there is no consensus
on what ratio to use and whether a single ratio is applicable
to all realistic column arrangements. The primary objec-
tive of this research is to improve the understanding of how
column-supported embankments deform in response to dif-
ferential foundation settlement. A bench-scale experimental
apparatus was constructed and the equipment, materials,
instrumentation, and test procedures are described. The
apparatus was able to precisely measure the deformation
occurring at the sample surface in response to differential
settlement at the base of the sample. Critical heights were
determined for five combinations of column diameter and
spacing representing a wide range of possible column
arrangements. In addition, tests were performed using four
different column diameters in a single column configura-
tion with ability to measure the load acting on the column
and apply a surcharge pressure to the sample. In total, 183
bench-scale tests were performed over a range of sample
heights, sample densities, and reinforcement stiffnesses.
Three-dimensional numerical analyses were conducted to
model the experiments. The critical heights calculated using
the numerical model agreed with the experimental results.
The results of the laboratory tests and numerical analyses
indicate that critical height depends on the width and spac-
ing of the columns and is not significantly influenced by the
density of the embankment fill or the presence of reinforce-
ment. A new method to estimate critical height was devel-
