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Geotechnical News June 2011
THESIS ABSTRACTS
Dynamic Properties of Colloidal Silica Soils
Using Centrifuge Model Tests and a
Full-Scale Field Test
Carolyn T. Conlee
Carolyn T. Conlee, 41 Pickle Brook Road, Bernarsville, NJ 07924,
Tel: 908-642-8731, email:
Traditional ground improvement methods to mitigate the effects
associated with liquefaction damage are often not feasible in devel-
oped areas. Commonly used soil improvement methods can have
adverse affects on the surrounding infrastructure and less invasive
methods are therefore required. Passive site stabilization is a non-
invasive grouting technique where a stabilizing material can be in-
jected at the edge of a site and delivered to target locations through
the groundwater. As the stabilizer flows through the subsurface, it
displaces the pore water and subsequently forms a permanent gel
that binds to soil particles, resulting in a stronger soil formation.
Based on its unique characteristics, colloidal silica has been se-
lected as an ideal material for passive site stabilization. For purposes
of liquefaction mitigation, the dynamic behavior of colloidal silica
soils was studied through centrifuge model tests and a complemen-
tary, full-scale field test. The centrifuge tests provided comparisons
of the response for untreated sands and sands treated with 4%, 5%,
and 9% colloidal silica concentrations subjected to a sequence of
dynamic shaking events. To complement the model tests, a full-
scale field test was conducted to compare the response of a liquefi-
able soil formation to a soil grouted with colloidal silica. Permeation
grouting techniques and field procedures were developed in order to
treat an approximately 1.5 m (5 ft) thick liquefiable soil layer.
The centrifuge model tests and field test both show that colloi-
dal silica soils reduce settlement, lateral spreading, and shear strains
induced when subjected to large dynamic loads. For purposes of
developing soil models, shear modulus degradation curves were de-
veloped and relationships that govern unloading-reloading behav-
ior were identified in centrifuge model tests. Amplification in the
acceleration response and increases in excess pore pressure ratios
were determined to be direct indications of treatment levels. Large
transient changes observed in pore pressure response were shown
to describe the behavior of stress transmittal between the soil and
gel during cyclic loading. Additionally, the hysteretic response of
colloidal silica soils exhibited greater hysteretic damping and cyclic
mobility consistent with dense sands. The response also revealed
a lower degree of cyclic degradation for higher concentrations of
colloidal silica.
Advisor: Patricia M. Gallagher, Dept. of Civil & Environmental En-
gineering, Drexel University
Investigation of Sediment Erosion Rates
of Rock, Sand, and Clay Mixtures Using
Enhanced Erosion Rate Testing Instruments
Raphael Crowley
Raphael Crowley, Post-Doctoral Associate; University of Florida,
Department of Civil & Coastal Engineering, 365 Weil Hall, Gaines-
ville, FL 32611; Tel: 352-392-3261; Fax: 352-392-9537; email:
Scour is the primary cause of bridge failures in the United States.
Although predicting scour depths for non-cohesive (sandy) bed ma-
terials is fairly well understood, much less is known about predict-
ing scour depths when cohesive materials such as clays, sand-clay
mixtures, and rock are present. A semi-empirical method exists for
predicting cohesive scour depths. This method relies on the input of
a sediment transport function or erosion rate as a function of shear
stress. Current design guidelines such as HEC-18 recommend mea-
suring sediment transport functions in a laboratory, but there has
been some question as to how to do this properly.
To answer this question, a series of improvements and enhance-
ments were made to the Sediment Erosion Rate Flume (SERF) at
the University of Florida (UF). A laser leveling system, a vortex
generator, a shear stress measuring system, computer updates, and a
sediment control system were designed and implemented. Using the
new shear stress system, a series of tests were run to assess the prop-
er way to measure shear stress in a flume-style erosion rate testing
device. Results showed that the pressure drop method will not mea-
sure shear stress properly, and in the absence of a shear stress sensor,
the most effective alternative method for estimating shear stress is to
use the Colebrook Equation (which describes the Moody Diagram).
A new material was developed for testing in both the SERF and
the Rotating Erosion Testing Apparatus (RETA) to serve as a basis
of comparison between the two instruments. Results were incon-
clusive because rock-like erosion described by the Stream Power
Model appeared to dominate erosion behavior. A database of results
from the RETA that has been developed since the RETA’s inception
in 2002 was used to verify that it is measuring the correct erosion
rate vs. shear stress relationships. Results showed that for the special
case where particle-like erosion dominates, the RETA appears to
produce correct results. Results also appear to indicate that when
rock-like erosion is present, it is generally an order of magnitude
lower than situations where particle-like erosion dominates. Fur-
ther analysis of the database showed that there may be a correlation
between material strength and erosion rate. Further research was
aimed at generalizing erosion rate vs. shear stress relationships for
sand-clay mixtures. A series of tests were conducted on a variety
of sand-clay mixtures. Results showed sensitivity to the method in
which the sand-clay mixtures were prepared. Rock-like erosion and
particle-like erosion were present in most sand-clay mixtures even
though typical sand-clay mixtures would not typically be described
as “rock-like materials.” Recirculating sediment during sand-clay
pretation of LWD data to characterize layer parameters is in its in-
fancy. This thesis aims to advance the understanding of layered soil
response to LWD loading and to develop an improved methodology
to obtain layer parameters from the LWD test.
The conventional LWD test includes a single sensor to measure
vertical deflection at the plate center wherein a static analysis is per-
formed using peak deflection and force to estimate a representative
deformation modulus. To improve the conventional LWD test, the
first topic of this thesis evaluates the use of radial offset sensors to
backcalculate layer moduli of a two-layer soil system using a static
analysis. The measurement depth for the LWD with radial offset
sensors was 1.8 times plate diameter versus the conventional mea-
surement depth of 1.0 to 1.5 times plate diameter. The second topic
of this thesis presents a dynamic finite element (FE) model of the
LWD test. Inertia and energy dissipation of the soil are neglected in
a static analysis, and it was found that results from a static analysis
can be substantially different than results from a dynamic analy-
sis. By performing a sensitivity analysis of the FE model, it was
found that damping ratio and Poisson’s ratio have similar influence
on peak deflections; however, their influence is an order of magni-
tude less than that of elastic modulus. The third topic of this thesis
presents a genetic algorithm (GA) to backcalculate layer parameters
without knowing top layer thickness a priori. The backcalculated
parameters from the GA agree with values that were expected, but
computational time of the GA was prohibitively long for field ap-
plication.
Sponsor: Dr. Michael Mooney, Colorado School of Mines
