Journal of Geotechnical and Geoenvironmental Engineering

The behavioral modes to which a rock slope is susceptible are many and complex. This article discusses a long but not exhaustive list of common failure styles, highlighting observable characteristics that distinguish one from another. Slopes in both strong and weak rock masses are considered. Principles are stated to explain how and why different failure modes develop in different rocks. The discussion is illustrated with examples of diverse failure modes in foliated metamorphic rocks developed in the walls of an eroded spillway runout.

Traditional analyses of stability of slopes subjected to seismic loads entail global equilibrium considerations with seismic influence included as a quasi-static force. Such an analysis does not reflect the earthquake shaking process, and it does not provide any information about permanent displacements that may have occurred as a result of that process. Earthquake events in recent years have brought about renewed interest in analyses of slopes subjected to seismic loads. This paper focuses on displacement calculations of reinforced slopes. Design of reinforced slopes using the quasi-static approach may lead to an unrealistically long reinforcement for large ground accelerations. If slopes are allowed to move by even a small displacement, then the reinforcement length can be reduced significantly. Two mechanisms of failure of reinforced slopes subjected to seismic conditions are considered: (1) Rotational collapse; and (2) sliding directly over the bottom layer of reinforcement. Yield accelerations and integrals of seismic records are presented in charts for easy use in practical applications. An example is shown to illustrate the method.

Water balance simulations were conducted with the unsaturated flow model UNSAT-H to assess how layer thicknesses, unsaturated hydraulic properties, and climate affect the performance of capillary barriers. Simulations were conducted for four locations in semiarid and arid climates. Hydraulic properties of four finer-grained and two coarser-grained soils were selected to study how saturated and unsaturated hydraulic properties affect the water balance. Results of the simulations indicate that thickness and hydraulic properties of the surface layer significantly affect the water balance of capillary barriers. As expected, increasing the thickness or reducing the saturated hydraulic conductivity of the finer-grained surface layer reduces percolation. Unsaturated hydraulic properties of the coarser layer also affect the water balance, including the storage capacity of the surface layer as well as the onset and amount of percolation from the cover. Thickness of the coarser layer has a much smaller impact on the water balance. Climate also affects the water balance. Greater soil water storage capacity is required at sites where the season with more frequent and less intense precipitation does not coincide with the season having highest evapotranspiration.

A rotating block method is developed to calculate the rotational displacements of gravity retaining walls based on rigid foundations under seismic loading. The method is similar to the pseudostatic sliding block method of Newmark. When a threshold acceleration for rotation is exceeded, a rigid wall will start to rotate until the angular velocity for rotation is reduced to zero. The influence of ground motion characteristics on computed wall deformation was evaluated. The procedure was validated by data from centrifuge tests. This method is also applicable for the most complex cases when the sliding and rotation of a gravity wall are coupled.

The results from 30 nonlinear finite-element analyses of undrained deep excavation in stiff clay are used to support the use of a new displacement flexibility number in multipropped retaining wall design. The analyses address the effects of different initial stress regimes and various values of prop stiffness for the internal supports to the excavation. It is demonstrated that this flexibility number defines support systems that will displace to the same maximum lateral wall deflection and will result in the same profiles of vertical and horizontal ground surface displacement behind the wall. It is concluded that, as it is these movements that must be controlled to limit the damage to adjacent buildings, structures, and services, the new flexibility number gives the engineer more confidence in assessing possible support strategies to a given problem at a given site.

In the late 1940s, the principal levees along both sides of the Mississippi River from Alton to Gale, Ill., were raised to a 1-in-50-year flood level—a 500-year frequency in urban areas. In 1950, the St. Louis District initiated an investigation of seepage beneath these levees. As a result of this investigation, 2,480 relief wells were installed along 292 mi of mainline and tributary levees during the 1950s. During the flood of 1993, the stage of the Mississippi River along the mainline levees equaled or exceeded the design stage, the highest river level to which the middle Mississippi River levees had ever been subjected. The writers were responsible for collecting and analyzing the data and information relating to the condition, efficiency, and performance of the relief wells installed in the 1950s along the above stretch of the Mississippi River. The study showed that the relief well systems that were designed and constructed during the 1950s with limited design data performed successfully and prevented any significant sand boils or piping for a design river stage. The parameters involved in design and performance of the underseepage control systems were largely confirmed within the normal variations of the parameters involved and field conditions. From analysis of the data collected during the 1993 high water, a program for maintenance and repair of the relief well system was developed. This paper presents a brief description of the phenomena of underseepage, the parameters involved in the design and performance of underseepage control measures, and a comparison of design values with field performance for a 50-year high water against 300 mi of Mississippi River levees in 1993, of which 45 miles are now protected from underseepage by relief wells. This paper also describes the importance of not only building levees high enough to hold back the design high water of a river but also the design, construction, and maintenance of relief well systems to prevent failure of levees as a result of sand boils and/or piping for the various subsurface conditions that exist along levees in the alluvial valleys of major rivers.

This paper presents new developments of regression-based models to predict the saturated hydraulic conductivity of compacted soils from grain-size distribution. The models incorporate parameter values that adequately represent the distribution of grain sizes. Alternative representations of the grain-size distribution, the fractal dimension and entropy of the distributions, as well as porosity, soil density, and fines content are used in the models to estimate the hydraulic conductivity. These parameters that characterize the textural and hydraulic properties of the soil are combined and used in a multidimensional analysis to estimate the hydraulic conductivity. The predictions of the developed models are compared with those of existing models and laboratory measurements of hydraulic conductivity. The results suggest that the newly developed models outperform the existing models in predicting hydraulic conductivity using information from grain-size distribution. The presented models are suggested as alternatives to, for example, laboratory measurements of the hydraulic conductivity of certain soils that may be difficult to prepare or that may take several days or perhaps weeks to perform. In certain circumstances it may also be used to give first-hand information about the hydraulic properties in a field environment.

An effort has been made to demonstrate use of a small geotechnical centrifuge as a research tool to understand moisture migration in a silty soil. Such a basic study is essential for understanding the much more complex phenomenon of solute transport through a soil, where the physical, chemical, and biological properties of the soil-solute system play an important role. Present study indicates that the advection in soils is dependent on the state of the soil and, in particular, on its degree of saturation. The effect of pore structure on the advection process has also been demonstrated. The trends of experimental data indicate that modeling of models may be valid only for saturated soils.