Journal of Geotechnical and Geoenvironmental Engineering

This paper discusses investigations of potential block hazards in ten existing American concrete arch and buttress dams. Block theory was used to identify and describe all removable blocks, based on the study of construction photos and logs, and new mapping, often done on steep valley sides. Water forces on the block faces were calculated for stability analysis using a map of the phreatic surface or an estimated flow regime along the subsurface boundary of the block. In some cases, block hazards were shown to be nonexistent because there were no real intersections of significant discontinuities of sufficient extent to cut out a block, or there was no kinematically possible mode of failure with the prevailing force system. Several dams had blocks that were removable but judged sufficiently safe with the applicable friction angles. Three structures demanded attention: A spillway structure was armored to prevent further erosion that diminished the resistance of a key block in its foundation; a dome and buttress structure received a reinforced concrete buttress to add passive support against a block critically located in the abutment; and a drainage adit and deep drains drilled from the surface and from the adit were constructed to raise the safety factor of a large block beneath the abutment of an arch dam.

This paper presents a new approach that allows soil nailed walls to be analyzed using a trial wedge method. Most soil nailed wall analysis methods are rooted in traditional slope stability solutions with curvilinear or bilinear slip surfaces. This has led to limited access to these methods due to the cost of commercial software. In addition, there are at least two well-documented test walls brought to failure that indicated evidence of relatively steep, approximately linear slip surfaces instead of the more complex surfaces assumed by most software packages. The simplified trial wedge method is intended as a relatively simple and inexpensive method for preliminary or supplemental design calculations for soil nailed walls. The method stems from the existing Federal Highway Administration analysis guidelines. Procedures are outlined for implementing the trial wedge method using a spreadsheet-based approach. The method is applied to two test walls that were intentionally brought to failure, the Amherst Test Wall in clay, and the Clouterre Test Wall No. 1 in sand. In each case, the trial wedge analysis produces results consistent with the failure mode of the wall.

Results from drained triaxial compression tests on specimens of fiber-reinforced sand are reported. It is evident that the addition of a small amount of synthetic fibers increases the failure stress of the composite. This effect, however, is associated with a drop in initial stiffness and an increase in strain to failure. Steel fibers did not reduce initial stiffness of the composite. The increase in failure stress can be as much as 70% at a fiber concentration of 2% (by volume) and an aspect ratio of 85. The reinforcement benefit increases with an increase in fiber concentration and aspect ratio, but it also depends on the relative size of the grains and fiber length. A larger reinforcement effect in terms of the peak shear stress was found in fine sand, compared to coarse sand, when the fiber concentration was small (0.5%). This trend was reversed for a larger fiber concentration (1.5%). A model for prediction of the failure stress in triaxial compression was developed. The failure envelope has two segments: a linear part associated with fiber slip, and a nonlinear one related to yielding of the fiber material. The analysis indicates that yielding of fibers occurs well beyond the stress range encountered in practice. The concept of a macroscopic internal friction angle was introduced to describe the failure criterion of a fiber-reinforced sand. This concept is a straightforward way to include fiber reinforcement in stability analyses of earth structures.

Presented is a method of three-dimensional slope stability analysis for homogeneous and nonhomogeneous symmetrical slopes based on the upper-bound theorem of the limit analysis approach. A rigid-block translational toe, above-the-toe or below-the-toe collapse mechanism is considered, with energy dissipation taking place along planar velocity discontinuities. The approach can be considered as a modification and extension of the procedure proposed by Michalowski in 1989. An effective iterative algorithm is applied to find the optimum (least) upper bound in constrained or unconstrained problems. The present procedure removes some essential limitations of limit analysis methods in two- and three-dimensional stability analysis of nonhomogeneous slopes.

Excessive lateral diaphragm wall displacement and the associated ground settlement are often the primary cause of damage of nearby buildings. It is therefore imperative to minimize diaphragm wall displacement during basement excavation if the integrity of adjacent buildings is of concern. This paper describes the application of a jet grouting scheme to reduce the diaphragm wall displacement of a six-level basement excavation. Based upon field experience of similar projects, buildings adjacent to the construction site may settle well beyond an acceptable limit if excavation is carried out without any protection measures being taken. In this excavation project, the soil mass within the excavation zone was partially jet grouted in an attempt to increase its passive resistance as an effective measure to limit wall displacement. Numerical analyses were carried out to assess the effects of jet grouting. Field measurements on wall displacement and ground settlement confirm the effectiveness of the improvement scheme.

A quasi-linear elastic constitutive model is proposed to describe the behavior of sand well below failure. It is based on isotropic compression tests and captures the increase in stiffness due to confinement—a unique property that makes sand stiffer under certain applied loads. The model traces the increase in stiffness by describing the instantaneous bulk modulus in terms of the effective mean stress and two constitutive parameters: the initial bulk modulus and the ultimate volumetric strain. The validity of the model is examined by checking how closely the proposed mathematical formulation represents high quality isotropic compression tests reported in the literature.

When a water-saturated clayey soil is leached with an organic fluid such as heptane, it has been found that under some conditions the hydraulic conductivity (or the permeability) increases manyfold. While it is generally agreed that physicochemical changes (e.g., compression of double-layer thickness) and the consequent alterations to the internal fabric (e.g., shrinkage of clusters) are responsible in most cases for such an increase, the underlying mechanism is not clearly understood. Two possible mechanisms are (1) formation of a few macrocracks due to shrinkage of clusters and (2) uniform increase in intercluster porosity throughout the volume of the soil due to shrinkage of clusters. The objective of the study presented here is to examine the most plausible mechanism of permeability increase caused by leaching. With the aid of Olsen’s permeability equation based on the cluster model, Poiseuille’s law for laminar fluid flow between two parallel plates, physicochemical theories, and experimental permeability data, it is shown that the formation of macrocracks is the most plausible mechanism of permeability increase.

Four geotechnical centrifuge tests with different soil layered systems were performed to investigate the movement and entrapment of water and of light nonaqueous phase liquids (LNAPLs) in unsaturated layered soil deposits. The tests were performed at 20 g and a vadose zone condition was created during the centrifuge tests by lowering the water table from the initially water saturated condition. During the water drainage stage, the water distribution within the models and the dynamic air–water capillary pressure saturation relationships of the three sands were obtained using tensiometers and resistivity probes. After achieving the unsaturated condition, a model LNAPL (Soltrol 220® or silicon oil) was injected near the soil surface and the movement and entrapment were monitored during the redistribution stage until the LNAPL reached the top of the water table. Complex LNAPL preferential flow and entrapment patterns were observed in the layered models with different textural interfaces due to the relative movement of all three phases [water, nonaqueous phase liquid (NAPL), and air]. The centrifuge tests data coupled with the numerical analyses show that NAPL properties, subsurface soil structures, initial water saturation, and NAPL infiltration rate affect the variation in entrapment conditions in heterogeneous unsaturated soil deposits.