Journal of Geotechnical and Geoenvironmental Engineering

T-walls in New Orleans survived Hurricane Katrina whereas I-walls obviously failed in several sections. However, it is still unclear whether these T-walls truly survived the hurricane with a fair amount of safety margins or barely survived it with undetected damages. The initial design of T-walls was based on simplified loading conditions with limited consideration of soil-structure interaction. In this study, three-dimensional (3D) numerical analyses were conducted, incorporating realistic loading conditions and soil-structure interactions but with time-saving techniques to evaluate the detailed behavior of T-walls. This paper addressed the procedure of innovative 3D numerical analyses and important findings by using special structural elements in ${\mathrm{FLAC}}^{3\mathrm{D}}$. From this study, T-walls were found to have adequate stress levels in H-piles and concrete walls. However, it showed that the major factor that may cause the instability of the T-wall was the slope instability-type unbalanced force. This unbalanced force, however, was counteracted by the batter piles so that the overall stability of the T-wall system could be maintained.

An investigation was conducted on the relationship between the internal shear strength of hydrated needle-punched (NP) geosynthetic clay liners (GCLs) and the interface shear strength between hydrated NP GCLs (nonwoven side) and high-density polyethylene (HDPE) textured geomembranes (GMXs). New large-scale direct shear data are presented and compared to previous results obtained using similar materials and procedures. The data indicate that both GCLs and GMX/GCL interfaces display large postpeak strength reduction, even at high normal stress. Peak and large-displacement failure envelopes are nonlinear; except for the GCL internal residual strength envelope, which passes through the origin and has a friction angle of 4.8°. GMX/GCL interfaces can be expected to have lower peak strengths and higher large-displacement strengths than GCL internal shear specimens. However, the failure mode for GMX/GCL specimens can change from interface shear to GCL internal shear as normal stress increases. Design for peak strength conditions should be based on the lowest peak strength interface in a liner system, and design for large displacement conditions should be on the basis of the residual strength of the same interface.

The BART Transbay Tube (TBT) is an immersed cut-and-cover subway tunnel that runs from Oakland to San Francisco, California. The loose sand and gravel backfills placed around the tunnel are considered to be liquefiable, and the clays under the backfill are soft in some zones along the alignment. These conditions could potentially result in uplift of the tunnel during strong earthquake shaking. This paper describes centrifuge model tests performed to verify numerical methods used to assess the stability and to evaluate the potential uplift mechanisms of the TBT. The observed mechanisms of uplift were a ratcheting mechanism (sand migrating under the tunnel with each cycle of relative movement), a pore water migration mechanism (water flowing under the tunnel), and a bottom heave mechanism, involving soft soils below the base of the trench. A fourth potential mechanism, viscous flow of liquefied soil, was not observed. The volume of the tunnel relative to the volume of the trench and the densities and permeabilities of the nonhomogeneous backfill were important parameters affecting the uplift of the tunnel. From the experiments reported here and analyses reported by the designers, it was concluded that the magnitude of uplift is limited and, hence, that an expensive ground improvement project to densify the backfill was unwarranted.

This paper presents the results of an extensive program of laboratory model wellbore tests that have been performed to study wellbore instability in saturated clays. The tests were conducted on resedimented Boston blue clay (RBBC) anisotropically consolidated to vertical effective stresses up to 10 MPa by using two custom-built thick-walled cylinder (TWC) devices with outer diameters $D_o=7.6$ and 15.2 cm. The experimental program investigated the effects of specimen geometry, mode of loading, strain rate, consolidation stress level, and overconsolidation ratio (OCR) on deformations of the model wellbore measured during undrained shearing. Results indicate that for normally consolidated clays most of the change in cavity pressure occurs at volumetric strains less than 5% after which the borehole becomes unstable. Increases in outer diameter and strain rate led to a reduction in the minimum borehole pressure. Stress-strain properties were interpreted by using an analysis procedure originally developed for undrained plane strain expansion of hollow cylinders. The backfigured undrained strength ratios from these analyses for normally consolidated specimens range from $s_u/{\sigma }_{vc}^{\prime }=0.19–0.22$. Overconsolidation greatly improves the stability of the borehole, and interpreted undrained strength ratios from the TWC tests are consistent with well-known power law functions previously developed for elemental shear tests.

This paper deals with the active landslides that are controlled by pore water pressure changes owing to groundwater fluctuations. These landslides are usually characterized by low displacement rate with deformations essentially concentrated within a narrow shear zone, above which the unstable soil mass moves as a rigid body. Taking advantage of some original analytical solutions, a method is developed for a preliminary prediction of the landslide mobility. This method is based on a simple sliding-block model and allows the landslide velocity to be readily evaluated once pore water pressure measurements are available. An application to a case study documented in the literature is also shown.

In constructing a vertical cutoff wall, bentonite-water slurry is frequently used to maintain the stability of sidewalls during excavation before backfilling the trench with less permeable materials to complete the cutoff wall construction. This procedure leads to a thin but relatively impermeable layer, called a filter cake, on the excavation surface. The aim of this paper is to examine the effect of a filter cake on evaluating hydraulic conductivity of the cutoff wall backfill through a slug test analysis with the aid of the verified numerical program, Slug_3D. As an upper bound solution for evaluation of the hydraulic conductivity of the cutoff wall backfill, no-flux boundary conditions for the boundaries of cutoff walls are imposed to consider the effect of filter cakes. The type-curve method and modified line-fitting method are employed to reanalyze the case of EMCON/OWT, Inc., as an example. The previous analysis, without consideration of a filter cake, is compared with the current results that consider the filter cake to reveal the necessity of considering the effect of a filter cake in the slug test analysis. The comparison shows that the hydraulic conductivity of the cutoff wall backfill will be underestimated in a slug test analysis if the filter cake is not properly considered.

Material damping is a fundamental parameter required for dynamic analysis of geotechnical and civil infrastructure. The material damping ratio is very difficult to measure in situ. A new methodology for in situ measuring of material damping using surface waves is presented in this work. This methodology is successfully evaluated on laboratory scale models and numerical simulations. Ultrasonic waves are used in this work because of the size of the laboratory models. The output force of an ultrasonic piezoelectric transmitter is modeled by using a Morlet function. The wave attenuation and phase variation of propagating surface waves with distance are analyzed using the wavelet transform. Numerical results show that the material damping ratio calculated using the wavelet transform gives a global value that represents an average damping ratio for the frequency bandwidth imposed by the seismic or ultrasonic source. Experimental results, from tests on a cemented sand and a concrete plate, show good agreement with published damping values.