Geotechnical News • June 2012
57
THESIS ABSTRACTS
Estimating the Soil–Water Characteristic
Curve using Grain Size Analysis and
Plasticity Index
Gustavo Torres Hernández
Gustavo Torres Hernández, 208E Baseline Road Apt 323, Tempe,
AZ 85283
The infrastructure is built in Unsaturated Soils. However,
the geotechnical practitioners insist in designing the struc-
tures based on Saturated Soil Mechanics. The design of
structures based on unsaturated soil mechanics is desirable
because it reduces cost and it is by far a more sustainable
approach.
The research community has identified the Soil–Water
Characteristic Curve as the most important soil property
when dealing with unsaturated conditions. This soil property
is unpopular among practitioners because the laboratory
testing takes an appreciable amount of time. Several authors
have attempted predicting the Soil–Water Characteristic
Curve; however, most of the published predictions are based
on a very limited soil database.
The National Resources Conservation Service has a vast
database of engineering soil properties with more than
36,000 soils, which includes water content measurements at
different levels of suctions. This database was used in this
study to validate two existing models that based the Soil–
Water Characteristic Curve prediction on statistical analysis.
It was found that although the predictions are acceptable for
some ranges of suctions; they did not performed that well
for others. It was found that the first model validated was
accurate for fine-grained soils, while the second model was
best for granular soils.
For these reasons, two models to estimate the Soil–Water
Characteristic Curve are proposed. The first model estimates
the fitting parameters of the Fredlund and Xing (1994)
function separately and then, the predicted parameters
are fitted to the Fredlund and Xing function for an overall
estimate of the degree of saturation. Results show an overall
improvement on the predicted values when compared to
existing models. The second model is based on the relation-
ship between the Soil–Water Characteristic Curve and the
Pore-Size Distribution of the soils. The process allows for
the prediction of the entire Soil–Water Characteristic Curve
function and proved to be a better approximation than that
used in the first attempt. Both models constitute important
tools in the implementation of unsaturated soil mechanics
into engineering practice due to the link of the prediction
with simple and well known engineering soil properties.
Supervisor: Claudia E. Zapata, Ph.D., Assistant Professor, Hon-
ors Disciplinary Faculty, School of Sustainable Engineering and
the Built Environment, Arizona State University, P.O. Box 875306,
Tempe, AZ 85287-5306, T: 480-727-8514, E:
Carbonate Diagenesis and Chemical
Weathering in the Southeastern United
States: Some Implications on Geotechnical
Behavior
Joan M. Larrahondo
Joan M. Larrahondo, Ph.D., Senior Geotechnical Engineer, IN-
GETEC S.A., Cra. 6 No. 30A-30, Piso 4, Bogota, Colombia,
T: 571-323-8050, ext. 325, E:
The Savannah River Site (SRS) deposits in the Southeastern
US between 30-45 m of depth are calcium carbonate-rich,
marine-skeletal, Eocene-aged sediments with varying clastic
content and extensive diagenetic alteration, including meter-
sized caves that coexist with brittle and hard limestone. An
experimental investigation including geotechnical (P- and
S-wave velocities, tensile strength, porosity) and geochemi-
cal (EDS, XRD, SEM, N2-adsorption, stable isotopes, K-Ar
age dating, ICP-assisted solubility, groundwater) studies
highlighted the contrast between hard and brittle limestones,
their relationship with cave formation, and allowed cal-
culation of parameters for geochemical modeling. Results
demonstrate that brittle and hard limestones bear distinct
geochemical signatures whereby the latter exhibits higher
crystallinity, lower clastic load, and freshwater-influenced
composition. Results also reveal carbonate diagenesis path-
ways likely driven by geologic-time seawater/freshwater
cycles, microorganism-driven micritization, and freshwa-
ter micrite lithification. The SRS surface soils are largely
coarse-grained and rich in iron oxides with various degrees
of maturity. These soils were simulated in the laboratory
using Ottawa sands that were chemically coated with goe-
thite and hematite. Surface (SEM, AFM, N2-adsorption) and
geotechnical properties (fabric, small-strain stiffness, shear
strength) were investigated on the resulting “soil analog”.
Results indicate that iron-oxide coated sands bear distinct
inherent fabric and enhanced small-strain stiffness and
critical state parameters when compared to uncoated sands.
Contact mechanics analyses suggest that iron oxide coatings
yield an increased number of grain-to-grain contacts, higher
surface roughness, and interlocking, which are believed to
be responsible for the observed properties.
Supervisor: Susan E. Burns, Ph.D., P.E., Professor, School of Civil
& Environmental Engineering, Georgia Institute of Technology,
