Journal of Geotechnical and Geoenvironmental Engineering

This paper presents the results of an experimental program to investigate recent stress-history effects on small-strain stiffness of lightly overconsolidated compressible Chicago glacial clays. Stress-probe tests with different recent stress histories were conducted on high-quality block samples taken from an excavation in Evanston, Illinois. The stress histories applied prior to probing represent in situ greenfield conditions and an unloading path associated with a common situation encountered when building in congested urban areas. Results of the stress-probe tests are compared in terms of shear, volumetric, and coupled components and their stiffness degradation. Strain-response envelopes were constructed in shear and volumetric-strain space to graphically show the significant difference in these responses between the two sets of stress-probe tests. The results clearly indicate that the recent stress history affects shear, bulk, and the two cross-coupled moduli of the Chicago clays as a function of the angle change between the previous and current stress paths. Consequently, the overall characteristics of strain-response envelopes of the two sets of stress probes are different.

Permanent sliding displacements are used to evaluate the seismic stability of earth slopes, and current practice utilizes a pseudoprobabilistic approach to predict the expected sliding displacement. The pseudoprobabilistic approach specifies a design ground-motion level based on a probabilistic seismic hazard analysis and a specified hazard level (e.g., 2% probability of exceedance in 50 years), but the displacement is predicted deterministically based on the design ground-motion level. The fully probabilistic approach develops a hazard curve for sliding displacement, and it is used to assess the displacement of the slope for a given hazard level. Comparisons of the fully probabilistic and pseudoprobabilistic approaches indicate that the pseudoprobabilistic analysis provides nonconservative estimates of sliding displacement in most cases. This paper presents a modification to the pseudoprobabilistic approach that provides displacement values more consistent with the fully probabilistic approach. This modification involves specifying a displacement greater than the median, in order to take into account the uncertainty in the displacement prediction. The appropriate value of displacement above the median is a function of the $k_y/\mathrm{PGA}$ value and the model used to predict the displacement.

In this paper, recently developed probabilistic elastoplasticity was applied in simulating cyclic behavior of clay. A simple von Mises elastic–perfectly plastic material model was used for simulation. Probabilistic soil parameters, elastic shear modulus ($G_{\max }$) and undrained shear strength ($s_u$), needed for the simulation were obtained from correlations with the standard penetration test (SPT) $N$-value. It has been shown that the probabilistic approach to geo-material modeling captures some of the important aspects—the modulus reduction, material damping ratio, and modulus degradation—of cyclic behavior of clay reasonably well, even with the simple elastic–perfectly plastic material model.

The performance of a slope, such as surviving a certain groundwater condition, can be viewed as the outcome of a full-scale test performed directly on the slope and may provide valuable information for safety assessment, upgrading analysis, and repair design of the slope. Performance information can be divided into two types: (1) the slope survived a certain state, and (2) the slope failed at a certain state. This paper illustrates two methods for slope reliability analysis considering site-specific performance information, i.e., an indirect method based on back-analysis of the performance information, and a direct method, in which the back-analysis procedure is bypassed. The two methods are theoretically the same but different in implementation details. As examples, an existing slope for safety assessment and a failed slope to be repaired are studied in this paper. Considering the past survival information increases the reliability of the slope. The increase in reliability is larger if the slope survives a more critical state. Thus, ignoring the survival information may result in uneconomical decisions. In contrast, ignoring the failure information may either underestimate or overestimate the reliability of a slope. As a result, neglecting the past failure information may lead to unsafe or uneconomical decisions.

A narrow mechanically stabilized earth (MSE) wall is defined as a MSE wall placed adjacent to an existing stable wall, with a width less than that established in current guidelines. Because of space constraints and interactions with the existing stable wall, various studies have suggested that the mechanics of narrow walls differ from those of conventional walls. This paper presents the reliability-based design (RBD) for external stability (i.e., sliding and overturning) of narrow MSE walls with wall aspects $L/H$ ranging from 0.2 to 0.7. The reduction in earth pressure pertaining to narrow walls is considered by multiplying a reduction factor by the conventional earth pressure. The probability distribution of the reduction factor is calibrated based on Bayesian analysis by using the results of a series of centrifuge tests on narrow walls. The stability against bearing capacity failure and the effect of water pressure within MSE walls are not calibrated in this study because they are not modeled in the centrifuge tests. An RBD method considering variability in soil parameters, wall dimensions, and traffic loads is applied to establish the relationship between target failure probability and the required safety factor. A design example is provided to illustrate the design procedure.

Buildings founded in proximity to shallow tunnels under construction may be damaged by the ground displacements induced by tunneling. This is a matter of concern for design, and a variety of protective interventions are currently adopted to prevent such damages. Among these, rows of piles or jet-grouting columns are widely diffused. In this paper, the effectiveness of a simple row of piles is computed by means of three-dimensional (3D) finite-element (FE) analyses, thus allowing the investigation of the relationship between performance and some simple geometrical parameters, such as the spacing among the piles. The results of centrifuge tests are reported and used as a benchmark. The potential damage has been quantified in this work, taking into account both the settlement profile and the horizontal strain induced at the ground surface by tunneling. It is shown that although the settlement reduction is significant only for very small spacing ($s=2–3$ pile diameters), even largely spaced piles ($s=5–6$ pile diameters) are useful to prevent damage to buildings because of the significant reduction of the average horizontal strain.

Distress in two housing developments was caused by a deep bedrock landslide triggered by placement of a large upslope fill. The large fill was placed to create a visual barrier between the upslope development and downslope housing developments and to balance the cut and fill quantities for the upslope site. This case history presents some of the ramifications of fill placement on natural slopes surrounded by urban areas, such as overstressing underlying weak material that may exist below the depth of borings typically conducted for single family residences and office complexes; the importance of surface and subsurface information in a formation known locally as Franciscan complex/mélange (which is a block-in-matrix rock formation common to the area); the shear behavior of serpentinite, which is part of the Franciscan complex; and the importance of natural and man-made changes to a slope, such as rainfall, surficial grading, home construction, and fill placement. It also illustrates the importance of locating the critical slope cross section before construction and the proper use of back-analyses in a landslide investigation.

Finite-element simulations demonstrate the influence of underlying weak soil on mobilization of passive pressures in cohesionless deposits. Traditional passive earth pressure theories with typical angles of interface friction may overestimate passive forces in such cases. Simple analytical models that incorporate the underlying weak soil using traditional passive earth pressure concepts are shown to agree reasonably with the finite-element simulations. The studies presented herein are relevant for cases in which cohesionless soil deposits overlie soft clay, liquefiable sand, or other weak layers.

The use of the plasticity theory offers an attractive framework to encapsulate the behavior of a pipe and the underlying soil in terminology consistent with pipeline structural analysis. Models that express the pipe-soil behavior purely in terms of the loads on a segment of pipe and the corresponding displacements have been suggested, although verification with geotechnical centrifuge experiments has been limited to relatively small lateral displacements (i.e., less than two pipe diameters). Over larger movements, the berms that build up alongside the pipe affect the load-displacement behavior, with existing strain-hardening plasticity models incapable of simulating this transition. This technical note provides experimental evidence of pipe-soil behavior for lateral displacements for up to five diameters. It further presents observations from 20 centrifuge experiments of a prototype 1-m-diameter pipe in calcareous sand. The results are used to validate the modification of a pipe-soil model to include the horizontal displacement hardening of the yield surface. Retrospective numerical simulations of the centrifuge experiments verify the modified model’s performances for lateral displacements of up to five diameters which was also the extent of the centrifuge experiments. This incorporation of large lateral displacements has significant application in on-bottom stability analysis as displacement-based design becomes more prevalent.

High-speed ballasted track design standards usually require the use of sand and gravel layers as subballast to fulfill an accurate protection of the formation layers, not only against traffic loads, but also against the effects of weather. Seasonal changes in soil water content, or suction changes, are responsible for cyclic volumetric strains on railroad trackbed layers, thus on the infrastructure. Being almost completely water-resistant when compared with granular-only layers, bituminous subballast offers a higher protection of the subgrade, consequently improving its behavior along the infrastructure life cycle. This question is investigated through the comparison of the performance of the track formation against atmospheric actions, taking into consideration the unsaturated state of the geomaterials. The method adopted consists of modeling the vertical displacements of both bituminous and granular subballast designs through a finite-element coupled thermo-hydro-mechanical (THM) analysis. The comparison of the two design solutions confirms that the adoption of a bituminous subballast layer might allow important reductions in seasonal vertical displacements.