Geotechnical Engineering Online Library

This paper presents a description of post-flood reconnaissance and geotechnical investigation of four earthen dams that were breached or damaged following an extreme flooding event in South Carolina in 2015. As a result of unprecedented rainfall and flooding that occurred during a five-day period at the beginning of October in 2015, a total of 51 earthen dams failed and nearly 200 were damaged. Many of these dams failed due to overtopping that led to a breach of the dam. Among the four dams investigated, full breach was observed at three dams; two of which were overtopped, and one was not. The fourth dam was not breached but was severely damaged. For each of these dams, the paper documents background information, pre-flood conditions, post-flood field observations and measurements, and laboratory testing results of collected soil samples. Impacts of vegetation on the dams and the effects of dam failure on critical infrastructure are also presented. The detailed descriptions and geotechnical investigations of the dam failures presented herein serve as case histories that can be used for dam breach modeling and risk assessment of dam failure.

Mangla Dam (Pakistan) is one of the largest Earth Core Rock fill Dam (ECRD) in the world in terms of volume of material used. The Sukian dyke of Mangla Dam Project is 5.33 km (17,500 ft.) in length and is founded on alternate beds of clay and weak (friable) sandstone. Raising of the dam subjected the project area including the dyke to an additional head of about 12.19m (40 ft.) in 2013. Before raising, the maximum seepage in the entire project was observed in the Sukian dyke area. An upstream blanket was selected to control seepage in this area after dam raising. FEM analyses were carried out to determine the efficiency of the upstream blanket in controlling seepage. After raising and impoundment, the FEM results were compared with instrumentation readings and seepage measurements and close agreement was observed.

The 1981 upstream slope failure in San Luis Dam in California, USA, is analyzed for stability and deformations in two- and three-dimensions using continuum-mechanics-based analysis procedures implemented in the computer programs FLAC and FLAC3D. The analysis results (failure surface, factor of safety, and displacements) from the continuum models are in general agreement with the field data. In addition, two-dimensional slope stability analysis results using a modified form of limit-equilibrium-based Spencer’s procedure for variable factor of safety implemented in the computer program SSTAB2 are included. Overall, the analysis results supplement the previously reported failure analyses. The paper serves three primary functions: (1) it documents results of a different analysis of the 1981 San Luis Dam slope failure; (2) it demonstrates the use of 2-D and 3-D continuum models to study the onset of instability, failure surface geometry and location, and permanent displacements associated with slope failures; and (3) it demonstrates the use of variable factor of safety to identify location of instability initiation, and progression of instability in a soil deposit. In addition, the paper demonstrates the benefits of three-dimensional analysis for complex conditions in dam engineering practice.

The paper presents two different projects: The first involves a case with limited rock cover on a side of an excavated surge shaft located near a steep slope. The second involves the assessment of design parameters of an existing masonry dam for use as input in dynamic analysis. The induced stresses in the surge shaft of Koyna Hydro Electric Project (K.H.E.P.) stage-IV were measured with flat jack. These tests were first performed in a 4 m diameter pilot shaft and after the shaft was excavated to its full diameter of 22.70 m. The stresses increased from 3.96 MPa to 5.09 MPa, when the 4m-diameter surge shaft was expanded to its full diameter of 22.70 m, in the case where significant rock mass cover existed at EL 651.00 m. However stress reduction or no variation in the induced stress was measured in the portion of insufficient rock cover. In the second case, to determine the design parameters of Kolkewadi masonry dam of K.H.E.P stage-III, flat jack tests were conducted at the upstream side of Kolkewadi masonry dam in masonry of 1:4 and 1:3 and at downstream sloping side in masonry of 1:5. It is impractical and difficult to obtain mechanical properties of masonry in laboratory from the extracted core samples, due to intrinsic nonhomogeneity of the material. The brick/stone and mortar layers caused anisotropic behavior of masonry. Average deformation modulus for 1:3 masonry was 32.8 GPa. Similarly, the average deformation modulus for the 1:4 and 1:5 masonry was 19.0 and 13.7 GPa respectively and were adopted for the dynamic analysis. Induced stresses in the masonry dam were found to be nearly equal to the overburden.