The Emergence of Unsaturated Soil Mechanics

The first ISSMFE conference (International Society for Soil Mechanics and Foundation Engineering) in 1936 provided a forum for the establishment of principles and equations for what has been come to be known as saturated soil mechanics. A significant number of research papers were presented on unsaturated soil behavior at the first ISSMFE conference but it would take 3 to 4 decades before a sound theoretical basis would emerge for the practice of unsaturated soil mechanics. It took visionary, pioneer engineers such as Prof. Spencer Buchanan to organize the international series of conferences on expansive soils. This series of conferences would provide a focused forum for the later development of the broader discipline of unsaturated soil mechanics.

Combined Seepage and Slope Stability Analysis of Rapid Drawdown Scenarios for Levee Design

The strength of levees can be affected during fluctuations in the water table. It is also possible for the climate to have an influence on the position of the water table in an earth levee. Traditional methods have resulted in approximate methods for dealing with the transient fluctuations of the water table in a levee. These approximations are generally accepted in engineering practice but the question can be rightfully raised as to how these approximations compare to a rigorous transient combined seepage and slope stability analysis. Software technology has significantly changed in recent years and is now at the point where it is much easier to perform transient seepage analyses. There are new questions that can be asked. Does an effective stress analysis diverge significantly from the 3-stage Duncan (1990) analysis? If so, under what conditions?

This paper compares the Duncan (1990) three-stage methodology for analyzing rapid drawdown scenarios to a combined transient seepage and slope stability analysis. Traditional limit equilibrium methods will be utilized in the slope stability analysis and the accommodation of saturated and unsaturated pore-water pressures will be considered. Analyses of a number of typical cross-sections will be considered in order to determine the potential influence of geometry. The intent of the paper is to illustrate scenarios under which the Duncan (loc. cit.) methodology produces similar results to the results of a more rigorous analysis.

Formation, Evolution, and Stability of Coastal Cliffs–Status and Trends

The Ocean Studies Board of the National Research Council recently reviewed the U.S. Geological Survey’s Coastal and Marine Geology (USGS-CMG) program (National Research Council, 1999). One of the Board’s primary recommendations was that CMG prepare comprehensive assessments of the nation’s coastal and marine regions, drawing on expertise not only from within the USGS, but also from outside agencies and academic institutions. In response to that recommendation, this report assesses the status and trends of coastal cliffs along the shorelines of the conterminous United States and the Great Lakes. By “status” is meant the present distribution and character of coastal cliffs, as well as their current relevance to social issues such as coastal development. By “trends” is meant the changes in status caused by both geological forces and human activities.

Coastal cliffs are steep escarpments at the coastline. They commonly form during times of rising sea level, such as the present, as the shoreline advances landward and erodes the elevated landmass. Coastal cliffs are a common landform, particularly on the west, northeast, and Great Lakes coasts of the United States, as well as within large estuaries. The land adjacent to coastal cliffs has been heavily developed along much of the coast, particularly in urban areas where the natural instability and progressive retreat of the cliffs pose a threat to life and property. Coastal land is permanently lost when coastal cliffs collapse and retreat landward, which is an important national issue in coastal planning, management, and engineering.

Strength of Materials for Embankment Dams

This white paper presents the results of a survey initiated by the Committee on Materials for Embankment Dams to determine the “state of the practice” for selecting fill strength parameters used in static and pseudostatic analyses of earthfill embankment dams. A
questionnaire was sent to federal and state agencies, consulting firms, and private consultants experienced in the design and safety evaluation of embankment dams. Among the federal and state agencies contacted were: U. S. Army Corps of Engineers(USACE); U. S. Bureau of Reclamation (USBR); Federal Energy Regulatory Commission (FERC); U. S. Department of Agriculture, National Resources Conservation Service (NRCS); Tennessee Valley Authority (TVA); and California Department of Water Resources, Division of Dam Safety (DSOD).

The questionnaire asked each respondent to provide information as to their practice in determining the static strength of embankment materials and analyzing the static stability of embankment dams. The information provided included: approach to static stability analyses, loading conditions, shear strength parameters, field and laboratory testing used to determine shear strength parameters, procedures for interpreting test results, factors of safety, and methodologies used for static analyses.

The information provided by the respondents is summarized in this white paper, which is divided into the following sections:
~ Loading Conditions for Embankment Dams, including End of Construction, Steady-State Seepage, Rapid Drawdown, and Earthquake.
~ Determination of Shear Strengths, including subsections describing total and effective stresses, selection of shear strengths, and the use of cohesion. The information provided in this white paper regarding the determination of shear strengths applies primarily to earthfill materials: soils with a grain size less than 20 mm.
~ Static Analyses of Embankment Stability, including subsections on the limit equilibrium method of slope stability analysis, types of potential failures surfaces evaluated, location of potential failure surfaces for analyses, and recommended factors of safety.