56
Geotechnical News June 2011
THESIS ABSTRACTS
sizes to the aggregate diameter was identified for the improved per-
formance of the geogrid-reinforced base.
Sponsor: Prof. Jie Han, The University of Kansas
Analysis and Behavior of Preexisting
Landslides
Manzoor Hussain
Manzoor Hussain, Civil and Environmental Engineering, Univer-
sity of Illinois at Urbana-Champaign, 205 N. Mathews, Urbana, IL
61801, Tel: 217-953-4303, email:
The selection of shear strength parameters for the design and
repair of landslide is important and difficult. Skempton (1964)
concludes that if a failure has already occurred in clayey soils, any
subsequent movement along the preexisting slip surface will be con-
trolled by the drained residual strength. Skempton (1985) suggests
that the strength of a clay also will be at or close to the residual value
on slip surfaces in soliflucted slopes, bedding shears in folded strata,
sheared joints or faults, and after an embankment failure. Therefore,
the drained residual shear strength has been and still is being used
for analysis of slopes that contain a preexisting shear surface.
The main objectives of this research are to study the shear
strength and long term behavior of landslides and in particular pre-
existing shear surfaces. The research involves laboratory testing
to determine the strength recovery, if any, of cohesive soils with
varying plasticity and the applicability of the recovered strength
to remedial measures and the back-analysis of landslides. Some of
the issues addressed include (a) if the shear strength increases from
the residual value with time, (b) how long does it take to reach the
maximum recovered strength, (c) if the strength increases with time,
does the strength return to the residual value with additional shear
displacement and if so how much shear displacement is required to
reduce the strength back to the residual value, and (d) what is the
maximum shear strength that can be obtained from strength recov-
ery and used for design purposes.
Back-analysis of landslides is important for evaluating the mobi-
lized recovered strength and thus back-analysis procedures were re-
viewed and augmented. Empirical correlations for drained residual
and fully softened friction angles proposed by Stark et al. (2005)
in graphical form are being widely used in geotechnical practice.
These empirical correlations were augmented with additional test
data and mathematical equations were developed. These mathemati-
cal expressions were incorporated in a spreadsheet that can be used
as a tool to estimate the shear strength parameters of a soil by using
only two index properties, i.e., liquid limit and clay-size fraction.
Thus, the values of shear and effective normal stresses obtained
from the spreadsheet can also be used to model the residual failure
envelope in a stability analysis.
Sponsor: Timothy D. Stark, University of Illinois at Urbana-Cham-
paign
Three Dimensional Slope Stability Analyses
for Natural and Manmade Slopes
Kamran Akhtar
Kamran Akhtar, Civil and Environmental Engineering, University
of Illinois at Urbana-Champaign, 205 N. Mathews Ave., Urbana, IL
61801, Tel: 217 953-4303, email:
This study focused on the importance of three dimensional (3D)
slope stability analyses in practice. The results show that if the ac-
tual shear strength is used in the design of a slope, a 2-D analysis
will yield conservative results. However, a 2D analysis may lead to
an overestimate or unconservative value of back-calculated shear
strength. 3D analyses are important in back-analyses to accurately
assess the relative effects of slope changes, precipitation, shear re-
sistance and remedial measures.
Present 3D limit equilibrium (LE) software does not consider the
effects of shear resistance offered by vertical sides that parallel the
direction of movement of a translational landslide mass. Based on
results of a parametric study conducted herein using finite element
(FE) and finite difference (FD) analyses, it was found that the use
of an earth pressure coefficient (Kτ) that is in-between at-rest (KO)
and active (KA) earth pressure provides a reasonable estimate of the
side shear resistance and 3D/2D FS ratios that are in agreement with
FE and FD analyses. Charts developed herein can be used to deter-
mine the importance of performing a 3D slope stability analysis for
a translational failure. Failure surfaces for rotational landslides usu-
ally do not require the effects of side shear resistance to be included
because shear resistance is calculated along the non-vertical sides of
the failure surface.
A LE methodology for calculating the 3D factor of safety for
natural and manmade slopes and an accompanying user friendly
software package were developed. 3DDEM- Slope was developed
as part of this study to incorporate and/or verify some of the findings
of this study. A comparison of different 2D and 3D slope stability
methods e.g., LE, FE, and FD methods, was used to verify the meth-
odology. Using case histories, 2D and 3D slope stability analysis
were performed by LE, FE, and FD methods to investigate the ap-
plicability and/or limitations of each method to different field slope
stability problems and geometries.
Sponsor: Timothy D. Stark, University of Illinois at Urbana-Cham-
paign
Strength of Transversely Isotropic Rocks
Jianyong Pei
Jianyong Pei, Baker Hughes, 2001 Rankin Road, Houston, TX
77073, Tel: 713-966-3811, email:
This thesis proposes a newAnisotropic Matsuoka-Nakai (AMN)
criterion to characterize the failure of transversely isotropic rocks
under true triaxial stress states. One major obstacle in formulating
an anisotropic criterion is that it usually involves six stress compo-
nents, instead of three principal stresses. As such, anisotropic criteria
usually lead to complicated mathematical expressions, and cannot
be directly visualized in three-dimensional space. This problem is
solved by introducing the Material Normal Stress System (MNSS),
which is the space formed by the three normal stress components
reflecting the material anisotropy. Within this system, the failure be-
havior of transversely isotropic rocks in conventional triaxial tests
can be represented with geometrical features in the MNSS. These
features are then incorporated into the failure surface of the origi-
nal Matsuoka-Nakai criterion in the Material Normal Stress System,
resulting in the Anisotropic Matsuoka-Nakai criterion. This crite-
rion, combined with the Coulomb criterion, is validated against both
conventional and true triaxial test data, that are collected from an
extensive literature review. The combination of the AMN criterion
and the Coulomb criterion satisfactorily characterizes the measured
strength from an extensive program of true triaxial tests on a schist,
which confirms the ability of the proposed criterion. Finally, this
combination of criteria is applied to the borehole stability problem.
The necessary mud pressure against borehole collapse and the onset
of borehole failure are examined.
Sponsors: Prof. Herbert H. Einstein, Prof. Andrew Whittle, MIT
