J. Michael Duncan

Findings of Case Histories on the Long-Term Performance of Seepage Barriers in Dams

In a vast majority of cases, seepage barriers increase the reliability of dams. However, it is important to recognize that seepage barriers often drastically increase hydraulic gradients around the boundaries of the barrier, and through any windows or defects in the barrier. The result is increased water pressures and hydraulic gradients behind and around the barrier. These increased pressures and gradients have potential to provide the catalyst for initiation of several modes of internal erosion that were either unlikely or less likely without the seepage barrier. As a consequence, seepage barriers give rise to the potential for additional mechanisms of internal erosion and piping in the dam and the foundation. A compendium of 30 case histories of dams that have had seepage barriers in place for over 10 years has been assembled, and observations and insights garnered from these case studies were compiled. Finite-element seepage and deformation analyses have been performed to provide better understanding of the performance of seepage barriers and the mechanisms that affect their performance. This paper presents a summary of our findings from the case histories and the analyses performed.

Deformation and cracking of seepage barriers in dams due to changes in the pore pressure regime.

A procedure is presented for analyzing postconstruction deformation of seepage barriers due to changes in the pore pressure regime after seepage barrier construction. The procedure uses the changes in pore pressures calculated by finite-element seepage analyses to calculate changes in buoyancy and seepage forces that occur as a result of seepage barrier construction. When the buoyancy and seepage forces are applied to a finite-element soil-structure interaction model, the result is an effective-stress analysis that rigorously models seepage effects. This paper discusses application of the procedure to five dams to calculate postconstruction deformation and stresses in seepage barriers. The results of the analyses indicate that deformation due to pore pressure regime changes is a likely mechanism causing cracking in rigid seepage barriers.

Measuring Drained Residual Strengths in the Bromhead Ring Shear

A series of Bromhead ring shear tests were conducted to measure the drained residual strength of Rancho Solano Fat Clay. Tests conducted using dierent test procedures demonstrate the signicant eect that wall friction can have in the Bromhead ring shear device. This problem was addressed by beveling the top loading platen, which led to measurements of drained residual strength that were 21-23% lower than those measured with the unmodied platen. Using the modied platen, similar test results can be achieved independent of the details of the test procedure that is followed, giving greater condence in the measured residual shear strength.

The need for judgement in geotechnical reliability studies

ABSTRACT A core principle of geotechnical engineering practice is the need to exercise judgement in evaluating soil and site conditions and in performing analyses. The requirement for considering judgement applies equally to performing deterministic or probabilistic analyses. In addition, for probabilistic analyses, choices must be made among the possible methods of analysis and the method of characterising variables. Consequently, geotechnical reliability studies inevitably involve significant uncertainties, and judgement is needed to perform reliability analyses and to evaluate the results. Two case histories, briefly described here, show that judgement and experience are essential prerequisites for meaningful assessment of geotechnical reliability.

Identification of Failure Mechanisms Associated with Seepage Barriers in Dams

In a vast majority of cases, seepage barriers increase the reliability of dams. However, it is important to recognize that seepage barriers often drastically increase hydraulic gradients around the boundaries of the barrier, and through any windows or defects in the barrier. As a consequence, seepage barriers give rise to the potential for additional mechanisms of internal erosion and piping in the dam and the foundation. Mechanisms of erosion and piping that are uniquely related to seepage barriers are being investigated through review of measured performance and analytical studies, and are the subject of this paper. The information presented in this paper will be useful in 1) assessing the possibility of internal erosion and piping in dams with seepage barriers, 2) designing to minimize that possibility, and 3) assessing the risks associated with these mechanisms of erosion and piping.

Undrained Strength of Compacted Clay under Principal Stress Reorientation

AbstractConsolidated-undrained tests on saturated compacted clays are used to provide strengths for design scenarios such as rapid drawdown, rapid flood, and earthquake loading of dams and levees. Geotechnical engineers have long recognized the influence of stress path, particularly principal stress reorientation from consolidation to failure, on the undrained strength of clay. While stress reorientation effects have been explored for undisturbed soils, their influence on the strength of compacted clay has not been examined in detail. The results of triaxial tests with stress reorientation and direct simple shear tests on a compacted clay are presented and compared to results from isotropically consolidated triaxial compression (ICU-TC) tests. Triaxial extension (TE) tests are shown to impose a stress system very dissimilar to the plane strain field conditions. TE tests also have problems with necking. The undrained strengths from TE tests were 25–55% lower than those measured in ICU-TC. In contrast, tria...

Measuring “Fast” Shear Strengths Along Slickensided Surfaces in the Bromhead Ring Shear

Both fast and slow ring shear tests were performed in the Bromhead ring shear device to examine the effect of the displacement rate on the shear strength measured along slickensided discontinuities in Rancho Solano Fat Clay. For each test, initial drained shearing was performed at a displacement rate of 0.018 mm/min, fast shearing was performed at a rate of 44.5 mm/min, and drained shearing was recommenced at a displacement rate of 0.018 mm/min. Significant variations in measured post-peak shear strengths were observed, and problems with the pore pressure response in the soil surrounding the slickensided plane are discussed in this paper. As a result of these problems, it was concluded that fast Bromhead ring shear tests may not be suitable for evaluating the effects of fast shearing on the strengths of slickensided surfaces.

Effects of Initial Conditions on the Results of Transient Seepage Analyses

Measured profiles of soil suction beneath level ground, in river banks, and within slopes of silty sands, sandy gravels, clayey residual soils and compacted clay are presented. These show that under field conditions the variation of soil suction with depth is affected by soil type and climate (rainfall, evaporation), and is always less than 100% of the variation of suction with depth that would exist for hydrostatic conditions. The calculated rate of progression towards steady seepage occurs more rapidly when initial values of suction are smaller, and for this reason the initial conditions used in analyses are an important factor in transient seepage analyses. Example analyses described in the paper show the effects of initial conditions on rate of progression of the phreatic surface through a levee during a flood.