Journal of Geotechnical and Geoenvironmental Engineering

Hydraulic properties of soils used for water balance covers measured at the time of construction and one to four years after construction are compared to assess how the hydraulic properties of cover soils change over time as a result of exposure to field conditions. Data are evaluated from ten field sites in the United States that represent a broad range of environmental conditions. The comparison shows that the saturated hydraulic conductivity $\left(K_s\right)$ can increase by a factor of 10,000, saturated volumetric water content $\left({\theta }_s\right)$ by a factor of 2.0, van Genuchten’s $\alpha$ parameter by a factor of 100, and van Genuchten’s $n$ parameter can decrease by a factor of 1.4. Larger changes occur for denser or more plastic fine-textured soils that have lower as-built $K_s$ , $\alpha$ , and ${\theta }_s$ and higher as-built $n$ , resulting in a reduction in the variation in hydraulic properties that can be attributed to compaction. After two to four years, many water balance cover soils can be assumed to have $K_s$ between ${10}^{-5}$ and ${10}^{-3}\mathrm{cm}∕\mathrm{s}$ , ${\theta }_s$ between 0.36 and 0.40, $\alpha$ between 0.002 and $0.2{\mathrm{kPa}}^{-1}$ , and $n$ between 1.2 and 1.5. The data may be used to estimate changes in hydraulic properties for applications such as waste containment, where long-term maintenance of hydraulic properties in shallow engineered soil layers is important.

Subsurface leachate recirculation or liquid injection methods for municipal solid waste (MSW) landfills are horizontal trenches, vertical wells, and permeable blankets. In this study, results of field-scale testing and numerical modeling of a recently developed subsurface leachate recirculation system called permeable blankets have been presented. In the field, at a MSW landfill located in Michigan, the travel of injected leachate in a 60-m-wide by 9-m-long by 0.15-m-deep blanket made up of crushed recycled glass was measured using an automated sensing system consisting of sensors embedded in the blanket. Leachate injection rates used in the field and simulated in this study ranged from 1.1 to $3.6{\mathrm{m}}^3∕\mathrm{h}$ per meter length of the injection pipe embedded in the permeable blanket. HYDRUS-2D was used to simulate the travel and pressure head of injected leachate in permeable blankets. The influence of the following parameters on the hydraulic performance of permeable blankets was evaluated: (1) hydraulic properties of permeable blanket and waste; (2) geometry of permeable blanket; (3) settlement of permeable blanket; (4) leachate dosing frequency; and (5) initial degrees of saturation of permeable blanket and waste. The key findings of the study are: (1) the rate and maximum distance of travel of injected leachate are a strong function of the relative hydraulic properties of the permeable blanket and underlying waste and the rate and frequency of leachate injection; and (2) the maximum pressure head in the blanket due to liquid injection does not exceed the injection pressure. The field data and the numerical modeling results indicated that permeable blankets can be designed to inject liquids or recirculate leachate in MSW landfills. Long-term performance of such blankets needs to be evaluated.

The technical feasibility of a new liquefaction mitigation technique is investigated by introducing small amounts of gas/air into liquefaction-susceptible soils. To explore this potential beneficial effect, partially saturated sand specimens were prepared and tested under cyclic shear strain controlled tests. A special flexible liquefaction box was designed and manufactured that allowed preparation and testing of large loose sand specimens under applied simple shear. Partial saturation was induced in various specimens by electrolysis and alternatively by drainage-recharge of the pore water. Using a shaking table, cyclic shear strain controlled tests were performed on fully and partially saturated loose sand specimens to determine the effect of partial saturation on the generation of excess pore water pressure. In addition, the use of cross-well radar in detecting partial saturation was explored. Finally, a setup of a deep sand column was prepared and the long-term sustainability of air entrapped in the voids of the sand was investigated. The results show that partial saturation can be achieved by gas generation using electrolysis or by drainage-recharge of the pore water without influencing the void ratio of the specimen. The results from cyclic tests demonstrate that a small reduction in the degree of saturation can prevent the occurrence of initial liquefaction. In all of the partially saturated specimens tested, the maximum excess pore pressure ratios ranged between 0.43 and 0.72. Also, the cross-well radar technique was able to detect changes in the degree of saturation when gases were generated in the specimen. Finally, monitoring the degree of partial saturation in a $151\mathrm{cm}$ long sand column led to the observation that after 442 days, the original degree of saturation of 82.9% increased only to 83.9%, indicating little tendency of diffusion of the entrapped air out of the specimen. The research reported in this paper demonstrated that induced-partial saturation in sands can prevent liquefaction, and the technique holds promise for use as a liquefaction mitigation measure.

A simplified semiempirical predictive relationship for estimating permanent displacements due to earthquake-induced deviatoric deformations is presented. It utilizes a nonlinear fully coupled stick-slip sliding block model to capture the dynamic performance of an earth dam, natural slope, compacted earth fill, or municipal solid-waste landfill. The primary source of uncertainty in assessing the likely performance of an earth/waste system during an earthquake is the input ground motion. Hence, a comprehensive database containing 688 recorded ground motions is used to compute seismic displacements. A seismic displacement model is developed that captures the primary influence of the system’s yield coefficient $\left(k_y\right)$ , its initial fundamental period $\left(T_s\right)$ , and the ground motion’s spectral acceleration at a degraded period equal to $1.5T_s$ . The model separates the probability of “zero” displacement (i.e., $⩽1\mathrm{cm}$ ) occurring from the distribution of “nonzero” displacement, so that very low values of calculated displacement do not bias the results. The use of the seismic displacement model is validated through reexamination of 16 case histories of earth dam and solid-waste landfill performance. The proposed model can be implemented rigorously within a fully probabilistic framework or used deterministically to evaluate seismic displacement potential.

This paper describes a precise numerical technique to compute the limit state exceedance probability of geosynthetic reinforced soil (GRS) slopes with normally distributed backfill and foundation soils by using the low-discrepancy sequence Monte Carlo (LDSMC) and importance sampling with LDSMC (ISLDSMC) methods. The LDSMC and ISLDSMC methods can effectively compute an accurate limit state exceedance probability of GRS slopes with a limited number of simulations. By using importance sampling, random variables can be generated in an expected failure region, thereby enabling enumeration by the Monte Carlo simulation. The failure region can be searched by the conventional first-order reliability method. To increase the computational efficiency, a low-discrepancy sequence, which is a sequence of quasi-random numbers with uniform distribution, is adopted in this study. The numerical simulation in this study revealed that the LDSMC and ISLDSMC methods can effectively compute an accurate limit state exceedance probability of GRS slopes by performing comparatively fewer simulations than the conventional crude Monte Carlo simulation.

A detailed experimental study on behavior of single column and group of seven columns is carried out by varying parameters like spacing between the columns, shear strength of soft clay, and loading condition. Laboratory tests are carried out on a column of $100\mathrm{mm}$ diameter surrounded by soft clay of different consistency. The tests are carried out either with an entire equivalent area loaded to estimate the stiffness of improved ground or only a column loaded to estimate the limiting axial capacity. During the group experiments, the actual stress on column and clay were measured by fixing pressure cells in the loading plate. Finite-element analyses have also been performed using 15-noded triangular elements with the software package PLAXIS. A drained analysis was carried out using Mohr-Coulomb’s criterion for soft clay, stones, and sand. The numerical results from the FEM are compared with the experimental results which showed good agreement between the results. Columns arranged with spacings more than 3 times the diameter of the column does not give any significant improvement. Based on the results, design charts are developed and a design procedure is suggested.

Low plasticity silts are liquefiable and the dissipation of pore pressures after an earthquake will be accompanied by densification and compression of the soil skeleton. Anisotropic rather than isotropic stress distributions are commonly found to exist in slopes or silty fills placed under $K_0$ conditions and this can be enhanced further by the weight of overlying structures. Compression after an earthquake generally increases soil resistance but it can still be liquefied by aftershocks. The postcyclic recompression of silt, and postdrainage monotonic and cyclic strength and stiffness have therefore been investigated with respect to the effect of initial anisotropic consolidation. The compressibilities during postcyclic recompression were similar to those for isotropic consolidation. Samples with a greater initial anisotropy had less volumetric strain but larger axial strains during postcyclic drainage. Under stress reversal conditions failure occurred as a result of the development of double amplitude cyclic strains, whereas under nonreversal conditions compressive axial plastic strain was accumulated. Postdrainage second loading cyclic strength increased with increasing anisotropy. For isotropically consolidated samples failure under reversal cyclic loading resulted in a weaker soil structure even after postcyclic reconsolidation.

While improvements in equipment and sampling methods have enabled collection of better quality samples of soft clays for more reliable engineering design and performance prediction, current sample quality assessment methods typically require destructive laboratory testing performed long after samples are taken. This paper describes a nondestructive technique for sample quality assessment of soft clays using shear wave velocity. A portable bender element device was used to measure shear wave velocity $\left(V_{vh}\right)$ in the field immediately following collection of Sherbrooke block, tube, and split spoon samples of Boston blue clay. $V_{vh}$ values were compared to in situ values from seismic piezocone $\left(V_{\mathrm{SCPTU}}\right)$ tests. The ratio $V_{vh}∕V_{\mathrm{SCPTU}}$ was compared with results from a conventional, laboratory-based assessment method. Results indicate a consistent correlation between laboratory-based methods and the $V_{vh}∕V_{\mathrm{SCPTU}}$ ratio, which ranges from $V_{vh}∕V_{\mathrm{SCPTU}}=0.77$ for the block samples to 0.28 for split spoon samples. The portable bender element device and nondestructive assessment technique offer the potential for field quality assessment and allow for real time adjustments to sampling techniques and/or more effective selection of samples for laboratory testing.

Various aspects of the mechanical behavior of kaolin clay are discussed in light of experimental observations from a series of strain controlled true triaxial undrained tests performed on cubical kaolin clay specimens with flocculated and dispersed microfabric, using a fully automated flexible boundary experimental setup with real-time feedback control system. The laboratory procedures used to prepare flocculated and dispersed microfabric specimens are presented. Mercury intrusion porosimetry is used to evaluate the pore structure of these specimens. The influence of microfabric on the consolidation behavior of kaolin clay is evaluated based on the data obtained from $K_0$ consolidation during constant rate of strain tests and the isotropic consolidation during true triaxial tests. Undrained tests on kaolin clay show that the following vary with microfabric of specimen: The shear stiffness, excess pore pressure generated during shear, and strength and strain to failure. For both microfabrics, the observed strength behavior using cubical triaxial testing shows a similar pattern of variation with applied stress anisotropy; hence, only a marginal influence of fabric-induced anisotropy.

It has been observed in many tunneling projects that surface settlement troughs caused by twin tunnels have a variety of shapes unlike single tunnels, where symmetric surface settlement troughs are usually observed. The surface settlement troughs observed over twin tunnels can be symmetric with respect to the midpoint between the two tunnels or symmetric but shifted toward either side or they can also be asymmetric. Settlement troughs both over single and twin tunnels (when symmetric) can often be described by a Gaussian curve. Most of the cases reported in the literature do consider the effect of ground conditions, tunnel size, and depth on the surface settlement. However, these cases do not consider the effect of construction operation. A study of settlements above tunnels driven with earth pressure balance shields in Bangkok made it possible to include operational parameters such as face pressure, penetration rate, and grouting pressure. This is possible by comparing the effect of twin tunnels which are geometrically and geologically identical but differ in the operational characteristics. These operational differences make it, however, more difficult to describe the resulting settlement troughs over twin tunnels using existing methods. Therefore, this paper introduces a superposition technique to describe surface settlement troughs over twin tunnels. It appears that one can construct settlement curves induced by the first shield and the second shield using the Gaussian function and combine the curves to obtain a total settlement trough as a result of the twin tunnels. Using extensive data from the Bangkok Subway Tunnel project, this approach was found to be suitable both for twin tunnels excavated side-by-side and also for stacked twin tunnels. Right now this superposition technique is basically descriptive. Eventually it will be possible to use this approach to predict settlement troughs over twin tunnels.

In this study, the behavior and scale effect of shallow circular anchors in two-layered sand is evaluated by comparing the results of a conventional $1\mathrm{g}$ model test with the results of finite-element analysis. The finite-element analysis is performed using an elastoplastic model in which progressive failure with shear band effect is introduced into the constitutive equation. The model is able to successfully predict the anchor behavior in two-layered sand. The direction of shear band propagation, within a sand layer, was observed to be a function of the density alone, regardless of the position of the layer. When the thickness of the lower layer was increased, maximum uplift resistance was found to increase with increasing thickness of a dense layer above a medium bed, and to decrease with increasing thickness of a medium layer above a dense bed. The scale effect was remarkable in the lower layer of dense sand.

The objective of this study was to characterize relative changes in waste shear strength parameters during waste decomposition. Twelve direct shear tests ( $100\mathrm{mm}$ diameter by $50\mathrm{mm}$ thickness) were performed on waste specimens ranging from fresh to well-decomposed residential refuse. In addition, nine direct shear tests were performed on selected waste components including fresh paper, partially decomposed refuse, and plastics. Results indicate that the friction angle of refuse decreased with decomposition. As refuse decomposed, the plastic content increased, which contributed to a decrease in friction angle as the friction angle of plastics is $18–{19}^{\circ }$ as compared to $33°$ for fresh shredded waste. The extent of refuse decomposition was characterized by the cellulose plus hemicellulose to lignin ratio $\left[\right(C+H)∕L]$ . The measured friction angle decreased from 32 to $24°$ as $(C+H)∕L$ decreased from 1.29 to 0.25. The shearing pattern for decomposed refuse showed a peak, followed by residual, which was then followed by a steady increase in shear stresses with displacement; the final rate of increase was similar to that observed in fresh paper specimens. Results from this work were comparable to data from previous reports, though it is important to characterize the extent of solids decomposition for a valid comparison with published studies.