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Geotechnical News June 2011
THESIS ABSTRACTS
A Study of Passive Earth Pressure in
Anisotropic Sand with Various Wall
Movement Modes
Achmad Bakri Muhiddin
Achmad Bakri Muhiddin, YP Adhiputeri, Jl Arief Rate 15A,
Makassar 90113, Indonesia, Tel: 62-411-587661,
email:
This study investigated the effect of anisotropy on passive pres-
sure in sands by developing computer simulation utilizing FLAC
code for plane strain condition. A series of wall movement modes
was applied namely translation, rotation about a point below the
wall, RBT, and rotation about a point above the wall, RTT.
From comparisons with other FLAC model in translation with
isotropic material, the coefficients of passive pressure Kp were simi-
lar except for combinations of zero dilation angle (ψ), low wall fric-
tion, and high angle of internal friction (φ). ψ has less effect on Kp
than the effect of φ. ψ equals ½φ could be used without significant
effects on Kp.
When comparing simulations with anisotropic material and
model wall experiment in translation, peak Kpx (Kp in x direc-
tion) from simulations were higher for loose sand, close for me-
dium dense, and about the same for dense sand. Strains at maximum
Kpx were less for loose sand, close for medium sand, and higher for
dense sand. In RBT modes, Kpx were higher for low “n” (ratio of
distance of center of rotation to wall height), and close for high “n”.
In RTT mode, Kpx were higher from simulation with low “n”, and
close for high “n”. For all modes, points of application of resultant
of lateral earth pressure at large wall displacement were practically
similar. However, in the early stage of wall movement, there exist
some differences.
From simulations with increasing “n” with various relative den-
sities, Kpx for RBT and RTT reached similar maximum at “n” about
2 and 15 respectively. For simulations with various φ in translation,
RBT (n=0), and RTT (n=0), Kp of anisotropic simulations were sig-
nificantly smaller than isotropic simulations. Increasing wall high
from 0.5 m to 4.0 m resulted in lower Kpx in anisotropic simulations
with 13% average reduction.
Sponsor: Isao Ishibashi, Professor P.E., Ph.D., Department of Civil
and Environmental Engineering, Old Dominion University
Groundwater Inflow into Rock Tunnels
Ran Chen
Ran Chen, Ph.D., 2880 Landmark Dr., Marietta, GA, 30060,
Tel: 512-769-7770, email:
Prediction of groundwater inflow into rock tunnels is one of the
essential tasks of tunnel engineering. Currently, most of the meth-
ods used in the industry are typically based on continuum models,
whether analytical, semi-empirical, or numerical. As a consequence,
a regular flow along the tunnel is commonly predicted. There are
also some discrete fracture network methods based on a discon-
tinous model, which typically yield regular flow or random flow
along the tunnel. However, I observed that in hard rock tunnels, flow
usually concentrates in some areas, leaving much of the tunnel dry.
The reason for this is that, in hard rock, most of the water flows in
rock fractures; fractures typically occur in a clustered pattern rather
than in a regular or random pattern. I develop a new method in this
work that can model the fracture clustering and reproduce the flow
concentration. After an elaborate literature review, a new algorithm
is developed to simulate fractures with clustering properties by us-
ing geostatistics. Next, I discuss a discrete fracture network that
was built and simplified. In order to solve the flow problem in the
discrete fracture network, an existing analytical-numercial method
must be improved. Two case studies illustrate the procedure of frac-
ture simulation. Several ideal tunnel cases and one real tunnel proj-
ect are used to validate the flow analysis. I determine that fracture
clustering can be modeled and flow concentration reproduced by
using the proposed technique.
Sponsor: Fulvio Tonon, Ph.D., P.E. (Texas, Italy), Assistant
Professor, Department of Civil Engineering, The University of Texas
at Austin
Analysis of Performance and Reliability of
Offshore Pile Foundation Systems based on
Hurricane Loading
Jiun-Yih Chen
Jiun-Yih Chen, The University of Texas at Austin, 1 University Sta-
tion C1792, Austin, Texas 78712, U.S.A., 1-512-657-7182,
Jacket platforms are fixed base offshore structures used to pro-
duce oil and gas in relatively shallow waters worldwide. Their pile
foundation systems seemed to perform better than what they were
designed for during severe hurricanes. This observation has led to a
common belief in the offshore oil and gas industry that foundation
design is overly conservative.
The objective of this research is to provide information to help
improve the state of practice in designing and assessing jacket pile
foundations to achieve a consistent level of performance and reli-
ability. A platform database consisting of 31 structures was com-
piled and 13 foundation systems were analyzed using a simplified
foundation collapse model, supplemented by a 3-D structural model.
The predicted performance for most of the 13 platform founda-
tions is consistent with their observed performance. These cases do
not preclude potential conservatism in foundation design because
only a small number of platform foundations were analyzed and
only one of them actually failed. The potential failure mechanism
of a foundation system is an important consideration for its per-
formance in the post-hurricane assessment. Structural factors can
be more important than geotechnical factors on foundation system
capacity. Prominent structural factors include the presence of well
conductors and jacket leg stubs, yield stress of piles and conductors,
axial flexibility of piles, rigidity and strength of jackets, and robust-
ness of foundation systems. These factors affect foundation system
capacity in a synergistic manner. Sand layers play an important role
in the performance of three platform foundations exhibiting the larg-
est discrepancy between predicted and observed performance. Site-
specific soil borings are not available in these cases. Higher spatial
variability in pile capacity can be expected in alluvial or fluviatile
geology with interbedded sands and clays.
The uncertainties in base shear and overturning moment in the
load are approximately the same and they are slightly higher than
the uncertainty in the overturning capacity of a 3-pile foundation
system. The uncertainty in the overturning capacity of this founda-
tion system is higher than the uncertainty in shear capacity. These
uncertainties affect the reliability of this foundation system.
Sponsor
: Prof. Robert B. Gilbert, The University of Texas at Austin
