Journal of Geotechnical and Geoenvironmental Engineering

Piles are extensively used as a means of slope stabilization. Despite the rapid advances in computing and software power, the design of such piles may still include a high degree of conservatism, stemming from the use of simplified, easy-to-apply methodologies. This paper develops a hybrid method for designing slope-stabilizing piles, combining the accuracy of rigorous three-dimensional (3D) finite-element (FE) simulation with the simplicity of widely accepted analytical techniques. It consists of two steps: (1) evaluation of the lateral resisting force ( $RF$ ) needed to increase the safety factor of the precarious slope to the desired value, and (2) estimation of the optimum pile configuration that offers the required $RF$ for a prescribed deformation level. The first step utilizes the results of conventional slope-stability analysis. A novel approach is proposed for the second step. This consists of decoupling the slope geometry from the computation of pile lateral capacity, which allows numerical simulation of only a limited region of soil around the piles. A comprehensive validation is presented against published experimental, field, and theoretical results from fully coupled 3D nonlinear FE analyses. The proposed method provides a useful, computationally efficient tool for parametric analyses and design of slope-stabilizing piles.

The effect of zeolite amendment for enhanced sorption capacity on the consolidation behavior and hydraulic conductivity, $k$ , of a representative soil-bentonite (SB) backfill for vertical cutoff walls was evaluated via laboratory testing. The consolidation behavior and $k$ of test specimens containing fine sand, 5.8% (dry weight) sodium bentonite, and 0, 2, 5, or 10% (dry weight) of one of three types of zeolite (clinoptilolite, chabazite-lower bed, or chabazite-upper bed) were measured using fixed-ring oedometers, and $k$ also was measured on separate specimens using a flexible-wall permeameter. The results indicated that addition of a zeolite had little impact on either the consolidation behavior or the $k$ of the backfill, regardless of the amount or type of zeolite. For example, the compression index, $C_c$ , for the unamended backfill specimen was 0.24, whereas values of $C_c$ for the zeolite-amended specimens were in the range $0.19\le C_c\le 0.23$ . Similarly, the $k$ for the unamended specimen based on flexible-wall tests was $2.4\times {10}^{-10}\mathrm{m}/\mathrm{s}$ , whereas values of $k$ for zeolite-amended specimens were in the range $1.2\times {10}^{-10}\le k\le 3.9\times {10}^{-10}\mathrm{m}/\mathrm{s}$ . The results of the study suggest that enhancing the sorption capacity of typical SB backfills via zeolite amendment is not likely to have a significant effect on the consolidation behavior or $k$ of the backfill, provided that the amount of zeolite added is small ( $\le 10\%$ ).

An experimental program was conducted to investigate the influence of geogrid as a reinforcement layer within the soil barrier of a landfill cover system under various levels of distortion. Centrifuge model tests were performed at 40 g on soil barriers subjected to continuous differential settlements using a 4.5 m radius beam centrifuge having a capacity of 2,500 g-kN available at IIT Bombay. Differential settlements were induced using a motor-based differential settlement simulator designed for a high gravity environment. Marker-based digital image analysis was adopted to estimate strain distribution along the geogrid layer and in the soil along the soil-geogrid interface at the onset of differential settlements. Various sensors, such as miniature pore pressure transducers, linear variable differential transformers, and strain gauges, were used to measure the water breakthrough, deformation profiles of the soil barrier, and the mobilized tensile load of model geogrids, respectively. Centrifuge model test results reveal that both 0.6 m and 1.2 m thick unreinforced soil barriers with an overburden pressure equivalent to that of a cover system experienced cracks extending up to the full thickness of the soil barrier and lost their integrity at low distortion levels. Soil barriers of 0.6 m and 1.2 m thickness with an overburden equivalent to that of landfill covers were reinforced with a suitable geogrid layer, and were found to sustain large distortions without any water breakthrough. An increase in the mobilization of the tensile load of the geogrid was noticed with an increase in the thickness of the soil barrier and overburden pressure equivalent to that of a cover system. This study also reveals the possibility of a reduction in the thickness of the soil barrier when it is reinforced with a suitable geogrid placed at one-fourth the thickness of the soil barrier from its top surface.

The relationship between soil suction and degree of water saturation (also known as the soil-water characteristic curve or SWCC) for a given soil depends on a number of variables such as the soil type, mineral type, fluid type, pore size distribution, and anisotropy. In addition, the SWCC is found to exhibit a form of hysteresis, in that it is different in wetting and drying. Design of structures founded on unsaturated soils requires an in-depth microscopic understanding of the variables controlling SWCC and the underlying hysteresis. Most of the past basic studies have focused on granular materials, with solid particles represented by spheres. The present study is focused on clayey soils involving platelike solids. The analysis method of molecular dynamics (MD) is used to understand the nature of clay-water capillary meniscus. On the basis of the observations from the MD analysis and the Young-Laplace equation, a model is proposed for predicting SWCC and the associated hysteresis. The microscopic parameters needed for the model are obtained by analyzing the fabric of numerical specimens prepared by the discrete-element method (DEM) of analysis of an assembly of clay particles. The results are shown to be consistent with experimental observations.

The part of effective stress resulting from soil moisture or soil suction variation can be defined by the suction stress characteristic curve (SSCC). For a given soil, the SSCC can be experimentally determined from shear-strength tests. Recent work shows that the SSCC can be uniquely linked to the soil-water characteristic curve (SWCC). The uniqueness of the SSCC determined from both shear strength and soil moisture retention tests is examined for several residual soils in Korea. The validity of the effective stress principle is demonstrated by showing that effective stress-based on the SSCC describes the same unique failure criterion as that for the saturated failure criterion. The measured SSCCs are also shown to predict the soil-water retention curves within a few percentage. The SWCCs of these residual soils, determined directly from soil moisture retention tests, also accord well with the SSCCs determined directly from triaxial shear-strength tests with the difference within several tens of kPa. Therefore, we show that the suction stress characteristic curve or soil-water retention curve alone can be used to describe both the effective stress and soil-water retention characteristics of variably saturated soils.

Direct measurement of a permeability function, $k(\psi )$ (where $\psi$ = suction), of an unsaturated soil is often tedious and time-consuming. Thus, researchers have proposed various semiempirical predictive methods to determine $k(\psi )$ indirectly from a soil-water characteristic curve (SWCC) or a particle-size distribution. Also, stress effects on $k(\psi )$ are generally not considered. To explore any stress effect on $k(\psi )$ , a new 1-m-high stress-controllable soil column is developed to measure stress-dependent SWCC (SDSWCC) and $k(\psi )$ of soils directly. The objectives of this paper are to investigate the effects of (1) two stress-state variables (matric suction and net normal stress) and (2) a drying-wetting cycle on both SDSWCC and $k(\psi )$ of a compacted decomposed silty clay. Each compacted soil column is subjected to an evaporation-ponding cycle under various vertical net normal stresses. The variations of pore-water pressure and volumetric water content (VWC, ${\theta }_w$ ) profiles with time are measured instantaneously. The $k(\psi )$ is hence determined by using the instantaneous profile method. At a given average vertical net normal stress, the measured $k(\psi )$ exhibits a remarkable hysteresis loop, whereas the $k({\theta }_w)$ seems to be less hysteretic. The effects of the vertical net normal stress appear to be more pronounced than that of the drying-wetting cycle on $k({\theta }_w)$ . As average vertical net normal stress is increased from 4 to 78 kPa, the measured permeability decreases by up to two orders of magnitude at a given VWC.

A 16–18 m deep excavation for a metro station with in-plane dimensions of approximately 20 m wide by 290 m long was constructed by using the cut-and-cover technique in soft clay in the Shanghai metropolitan area, in which many high-rise buildings and utilities exist. To investigate the performance of this deep excavation and the associated effect on the adjacent buildings and utility pipelines, a long-term comprehensive instrumentation program was conducted during construction. Field observations included deflections of diaphragm walls, vertical movements at wall tops, ground settlements, and settlements of surrounding buildings and utilities. Analyses of field data indicated that overexcavation (i.e., removing more soil than designed) and a long construction duration caused the diaphragm walls to develop substantial deflections. The base and middle floor slabs played dominant roles in suppressing postexcavation wall deflections and ground settlements. This excavation in general exhibited different behaviors than other excavation projects because of its relatively long shape. Behaviors of deep excavations in Shanghai soft clay are affected not only by construction duration (i.e., time effects), but also by their geometries (i.e., space effects). This project serves as a special case study and provides insights into the design and construction of a long and deep excavation that uses the cut-and-cover technique in soft soils and in a metropolitan environment.

The stability of tunnel faces supported by an umbrella of subhorizontal micropiles is analyzed by means of the upper bound theorem of plasticity. The micropiles are considered beams subjected to the kinematic motion imposed by the assumed failure mechanism. The limiting resistance of the supporting beams is first addressed. The failure mechanism imposes a displacement pattern on the beam, which is similar to the response of vertical piles subjected to horizontal load. The mechanism of beam deformation was included into the overall instability mechanism for the tunnel face stability. Limiting conditions of the beam under undrained conditions are used. A general minimization process associated with the upper bound formulation is developed. Stability conditions are described in dimensionless parameters and plotted in ready-to-use design charts. In particular, a dimensionless micropile coefficient, which includes all the relevant design parameters of the umbrella, could be isolated and plotted in undrained soil strength and tunnel geometry. A real face failure is finally described, and the required supporting umbrella is designed on the basis of the developed procedure.

The addition of cement is an interesting remediation technique when the project requires improvement of the local soil for the construction of pavement base layers, in slope protection of earth dams and canal linings, as a support layer for shallow foundations and to prevent sand liquefaction. The present study was carried out to quantify the influence of the amount of cement and the porosity in a cement-voids ratio, defined as the ratio between the volume of cement and the volume of voids of a mixture, on the stress-dilatancy behavior of an artificially cemented sand. A program of triaxial compression tests considering three distinct cement-voids ratios was carried out with two combinations of volumes of voids and volumes of cement at each cement-voids ratio. Results showed that the stress-dilatancy relationship is alike for a given cement-voids ratio and that the stress-strain behavior is also similar. The cement-voids ratio is therefore an appropriate parameter to assess stress-dilatancy of the sand-cement mixture studied.

This paper investigates the effectiveness of open trenches in reducing the deflections of sheet pile walls during deep dynamic compaction (DDC). This investigation included a series of comprehensive parametric studies using a two-dimensional (2D) finite-element (FE) method. Prior to investigation, a new dynamic approach based on the time history of tamper penetration depth was developed to simulate DDC impact, and its reliability and accuracy were verified by field measurements. Then, three open trenches with different cross-section shapes were examined. The dimensional effects of the trenches on reducing wall deflections were analyzed and discussed, and their reduction efficiencies were compared. For the investigated shallow open trenches, increasing trench depth or width can efficiently improve their isolation effects on reducing wall deflections. The cross-section shapes of open trenches do not impose apparent influences.