Geotechnical News June 2011
59
THESIS ABSTRACTS
Use of Non-Steel Fiber Reinforcement in
Concrete Tunnel Lining
Sang Yeon Seo
Sang Yeon Seo, GS E&C Research Institute, 417-1 Deokseong-
ri, Idong-myeon, Yoingin-si, Gyeonggi-do, 449-831, Korea,
email:
, Tel: +82-10-2772-4728
Fiber reinforcement is being widely used in concrete tunnel lin-
ings these days. Using fiber reinforcement can save not only cost,
but also labor and time spent on construction. However, many own-
ers hesitate to incorporate fiber reinforcement in tunnel lining due
to lack of experience with and knowledge of the behavior of fiber
reinforced concrete (FRC)
In this study, fiber reinforced concrete was made with various
kinds of fibers such as steel fiber, macro-synthetic fiber and hybrid
fiber (a blend of macro-synthetic fiber and glass fiber). Many experi-
mental tests were performed to investigate the compressive, flexural
and shear behavior of fiber reinforced concrete. In addition to the
structural capacity of FRC, the distribution of fiber reinforcement
inside the concrete matrix was investigated. Test results of these ex-
perimental tests were thoroughly examined to compare and quantify
the effects of fiber reinforcement. Next, the test results were used
to generate axial force-bending moment interaction diagrams based
on current design approaches. In addition, the current design ap-
proaches were modified to estimate the accurate and exact value of
bending moment. Fiber reinforcement clearly improved the struc-
tural performance of tunnel lining. The post-peak flexural and shear
strength was significantly influenced by the type and amount of fiber
reinforcement.
Sponsor: Fulvio Tonon, The University of Texas at Austin
Analysis of Spatial Variability in
Geotechnical Data for Offshore Foundations
Jeong Yeon Cheon
Jeong Yeon Cheon, Ph.D, Tel: 512-786-2375,
email:
Deep foundations, such as piles and suction caissons, are used
throughout an offshore oil and gas production facility in deepwater.
Ideally, the values of geotechnical properties for foundation design
are determined by results from geotechnical investigation programs
performed at the site of the foundation. However, the locations for
facilities are not known exactly when soil borings are drilled and the
footprint of a facility in deepwater can be very large with numer-
ous foundation elements spread out over miles. Therefore, it is not
generally feasible to perform a site-specific investigation for every
foundation element.
The objective of this research is to assess, analyze and model
spatial variability in geotechnical properties for offshore founda-
tions. A total of 97 geotechnical investigations from 14 offshore
project sites covering the past twenty years of deepwater devel-
opment in the Gulf of Mexico are compiled into a database. The
geologic setting is primarily a normally to slightly overconsolidated
marine clay, and the property of interest for the design of deep foun-
dations is the undrained shear strength.
The magnitude and characteristics of variability in design und-
rained shear strengths are analyzed quantitatively and graphically.
Geostatistical models that describe spatial variability in the design
shear strength properties to the distance away from the available
information are developed and calibrated with available information
from the database. Finally, a methodology is presented for incorpo-
rating the models into a reliability-based design framework to ac-
count for spatial variability in foundation capacity. Design examples
are presented to demonstrate the use of the reliability methodology.
Based on the design undrained shear strength profiles for the past
20 years in this Gulf of Mexico deepwater area, the design und-
rained shear strength varies spatially but does not depend on the
time or method for site investigations. There are nonlinear spatial
relationships in the point shear strength laterally and vertically due
to stratigraphy such that depth-averaged shear strengths are corre-
lated over further distances than point shear strengths. The depo-
sitional forces are an important factor causing spatial variations in
the undrained shear strength, with greater variation and less spatial
correlation in the more recent hemipelagic deposits (about upper 60
feet) than the deeper turbidite deposits and along the shelf versus off
the shelf. The increased conservatism required in deep foundation
design due to spatial variability when site specific strength data are
not available is generally small with less than a five percent increase
required in design capacity in this geologic setting.
Sponsor: Robert B. Gilbert, Ph.D., P.E., Professor, Civil, Architec-
tural and Environmental Engineering, The University of Texas at
Austin
A Multi-Axial Tension Test for Geotextiles
Theresa Louise Andrejack
Theresa Louise Andrejack, Bucknell University, Tel: 570-577-2340,
email:
The use of geotextiles as reinforcement is well-established in
geotechnical applications. However, some uses of these materials
occur in geotechnical systems where the in situ loading and bound-
ary conditions on the geosynthetic vary greatly from laboratory
testing conditions that are used to characterize their constitutive
behavior. In this work, the development of a new large-diameter
experimental device capable of applying multi-axial, out-of-plane
loading to relatively large geosynthetic specimens (48 cm diameter)
is presented. Although similar in concept to the types of appara-
tuses typically used for the established Multi-Axial Tension Test for
Geosynthetics, this newly developed device is unique in that load is
directly applied to the circular specimen using a rubber membrane,
thus allowing pervious materials such as geotextiles to be tested. A
key advantage of the device is that it mimics the in-service loading
conditions of geosynthetics used in a range of design applications
including the spanning of subsurface voids and geosynthetic-rein-
forced pile-supported embankments.
Constant strain rate, multi-axial tension tests were completed on
a range of seven geotextiles that varied in mass per unit area, resin
type, anisotropy, fiber type, fiber density, and weave. Two methods
for interpreting the results from the multi-axial test, the constant-
thickness and constant volume methods, are derived and compared.
Uniaxial and fiber tension tests were also performed to provide a
better understanding of the multi-axial test results. Three dimen-
sional models, constructed using photogrammetry, were created to
evaluate the assumptions that are used in the interpretation of the
multi-axial test results. These models also provide insight into the
micro-level behavior of geotextiles in multi-axial tension.
The constant strain rate, multi-axial test results indicate that there
is a significant deviation in the response of geotextiles in multi-axial
tension compared to their response in uniaxial tension. Although
ultimate strength values were found to be comparable using the
constant volume interpretation of stress, the ratio of secant modu-
lus values from the multi-axial test over the uniaxial tension test at
2%, 5%, and 10% strain are consistently on the order of 0.6 – 0.9.
