Geotechnical News June 2011
55
THESIS ABSTRACTS
testing indicated that suspended sediment in the SERF has little ef-
fect on bed shear stress.
Sponsor: Dr. David Bloomquist; Associate Professor, Department of
Civil & Coastal Engineering, University of Florida
Carbonate Diagenesis and Chemical
Weathering in The Southeastern
United States: Implications on
Geotechnical Behavior
Joan M. Larrahondo
Joan M. Larrahondo, email:
The Savannah River Site (SRS) soils below 30m are marine-
skeletal, calcium carbonate-rich, Eocene sediments with varying
clastic content and extensive diagenetic alteration. In addition, the
SRS’ coarse-grained surface soils are rich in chemical weathering-
derived iron oxides with various degrees of maturity. This research
is aimed to investigate the nature and origin of the diagenetically-
altered soft and hard geomaterials that coexist in the SRS area. An-
other objective is to use laboratory-prepared soil analogs to study
the effect of iron oxide coatings on coarse-grained soil mechanics.
Geologic surveys encountered meter-sized vugs closely associated
with hard limestone. EDS, XRD, SEM, K-Ar dating, P-wave ve-
locities, tensile strength studies, and solubility experiments with
ICP-OES analysis highlighted the contrast between hard and soft
facies and allowed calculation of kinetic parameters for modeling.
The SRS’ iron oxide-rich surface soils were laboratory-simulated
using Ottawa sands that were chemically coated with goethite and
hematite. Small-strain stiffness (Bender Element Technique), large-
strain strength, and surface properties (SEM and AFM) were studied
on the resulting soil analog. Results indicate that limestone and soft
carbonate soil bear distinct geochemical signatures: unlike the soft
soil, the limestone exhibits higher crystallinity, lower clastic load,
and freshwater-influenced composition. Iron-oxide coated sands de-
liver distinct inherent fabric parameters when compared to those of
uncoated sands. Likewise, small-strain stiffness and critical state pa-
rameters are enhanced upon iron-oxide coating. These results reveal
a carbonate diagenesis path driven by geologic-time seawater/fresh-
water cycles, though incompletely justified by inorganic processes
alone. Thus, microorganism-driven micritization (or the lack there-
of) in early, shallow-marine environment, followed by freshwater
micrite lithification are also proposed. Contact mechanics analyses
suggest that iron oxide coatings yield an increased number of grain-
to-grain contacts, higher surface roughness, and interlocking, which
are responsible for the enhanced stiffness and strength properties
observed. Abehavioral hierarchy associated with iron oxide thermo-
dynamic stability may also exist.
Advisor: Dr. Susan E. Burns, Georgia Institute of Technology
Sponsor: US Department of Energy
Gas Production from Hydrate-Bearing
Sediments: Geo-Mechanical Implications
Jong-Won Jung
Jong-Won Jung, Ph.D., Earth Sciences Division, MS 70108B,
Lawrence Berkeley National Laboratory, 1 Cyclotron Rd. Berke-
ley CA 94720. email:
or
,
Office: 1-510-486-4566, Fax: 1-510-486-7152
Gas hydrate consists of guest gas molecules encaged in water
molecules. The most common great molecule is methane. Methane
hydrate reserves around the world are estimated in 20,000 trillion
m3 of CH4. Methane hydrate can be an energy resource, as well
as a cause for global warming and the seafloor instability. Gas hy-
drates occur in sediments under high fluid pressure (~ 25MPa) and
low temperature (0 to ~25°C). The properties of hydrate bearing
sediments are poorly understood. In particular, gas production from
hydrate bearing sediments can cause problems related to over pres-
sure, clogging, fine migration, volume change, and seafloor instabil-
ity. Research documented in this thesis explores the formation and
growth of gas hydrates at the pore scale including wetting condition,
formation rate, particle-scale hydrate-mineral tensile strength, and
its impact on sediment-scale properties, and volume change during
hydrate formation and dissociation. Experimental studies, numeri-
cal simulations using PFC-3D, and macro-scale analysis of coupled
processing are complemented with particle-scale. Emphasis is
placed on identifying the advantages/disadvantages of different gas
production strategies such as depressurization, heating, and chemi-
cal injection; the later is focus of a fundamental study to enhance
the understanding of CH4-CO2 exchange as a unique solution to
recovery CH4 gas recovery while sequestering CO2.
Sponsor: J. Carlos Santamarina, School of Civil and Environmental
Engineering, Georgia Institute of Tehcnology
Micromechanical Analysis of Geosynthetic-
Soil Interaction Under Cyclic Loading
Anil Bhandari
Anil Bhandari, Project Manager, Terracon Consultants, Inc.
Charleston, SC, email:
Geosynthetics have been used to improve the performance of
civil engineering instrastructure in many projects. The interaction
between geosynthetics and soil is an important factor that governs
the performance of the geosynthetic-reinforced structures. Previous
studies on geosynthetic-soil interaction using laboratory and con-
tinuum based numerical approach were beneficial for studying the
overall behavior of the system, however those investigations did
not provide insight into microscale response. To improve the un-
derstanding of the geosynthetic reinforcement mechanisms, geosyn-
thetic-soil interaction was studied under a monotonic and a cyclic
loading using a micromechanical approach.
The micromechanical parameters of the granular materials and
reinforcements were calibrated using a biaxial test and a tensile test,
respectively. The behavior of granular materials was evaluated un-
der a monotonic and a cyclic loading and analyzed from force and
fabric orientation perspectives. Using the calibrated micromechani-
cal parameters, benchmark trapdoor experiments were simulated to
establish the simulation techniques for geosynthetic-soil interaction.
The micromechanical studies of three practical problems involving
geosynthetic-soil interaction were conducted. The practical prob-
lems were: geosynthetic-reinforced embankments overlying voids,
Geosynthetic-Reinforced Pile-Supported (GRPS) embankments,
and geosynthetic-reinforced bases.
In the trapdoor experiments, soil arching was observed as an es-
sentially meta-stable condition. The inclusion of reinforcement in
the embankments reduced the settlements measured on the top of
the embankments. Geosynthetic reinforcement increased the load
transfer to the piles and reduced the load on the compressible soils.
The anchorage failure of the reinforcement also controlled the load
transfer particularly in the low embankments. In the geosynthetic-
reinforced base simulation, the density of base course had a pro-
found effect on a rut depth. The tensile stresses developed in the
geosynthetic reinforcement helped distribute the contact forces wid-
er. A relatively small tensile stress developed in the reinforcement;
therefore, a very stiff reinforcement was not necessary to improve
the performance of the base. An optimum ratio between the aperture
