Liquefaction of Earth Embankment Dams

Paul C. Rizzo Associates, Inc. (RIZZO) has developed efficient computational tools to evaluate the liquefaction potential of earth embankment dams located within high risk seismic regions. Two relevant case histories are the Saluda Dam and the Santee Cooper East Dam Extension, both structures susceptible to liquefaction, and located in South Carolina. Saluda Dam presents liquefaction risks within the materials that comprise the embankment itself. The earth structure is resting on a layer of non-liquefiable residual soil. On the contrary, the Santee Cooper case presents liquefaction risks within the foundation soils. These facts translate into post-earthquake slope stability processes that are quite different in nature. Remediation strategies are different as well and need to adapt accordingly. RIZZO has evaluated the liquefaction potential and established remediation plans and designs at each of these two sites. Computational tools developed by the industry and complemented with RIZZO expertise were utilized to identify liquefaction susceptibility both at the foundation and/or within the embankment soils. The remediation concepts between both cases significantly differ. This paper presents the two case histories by comparing their liquefaction analyses results, their feasible remediation alternatives, and their selected remediation concepts. The same in-house computational tool is used to analyze the two cases. 1 International Project Manager, Paul C. Rizzo Associates, Inc., 105 Mall Boulevard Suite 270-E, Monroeville PA, 15146 LIQUEFACTION OF EARTH EMBANKMENT DAMS TWO CASE HISTORIES: (1) LIQUEFACTION OF THE EMBANKMENT SOILS, AND (2) LIQUEFACTION OF THE FOUNDATIONS SOILS Paul C. Rizzo Associates, Inc. (RIZZO) has been involved in the evaluation of the liquefaction potential of earth embankment dams located within high risk seismic regions, specifically in the state of South Carolina, where the seismic criterion is controlled by one of the largest earthquakes ever to occur in the northeastern region of the United States, the 1886 Charleston Earthquake. Today, engineers anticipate that a similar earthquake, with an estimated Richter scale magnitude between 7.1 and 7.3, could occur again sometime in the next 650 to 1000 years. Two relevant cases of dam liquefaction potential are the Saluda Dam (Saluda) and the Santee Cooper East Dam and East Dam Extension (East Dam), both structures susceptible to liquefaction, and located in South Carolina, within the influence area of the 1886 Charleston Earthquake (Figure 1). Figure 1 – Location of the Saluda and East Dam Projects; for the sites, the Charleston 1886 earthquake’s Mercalli magnitude is estimated between X and IX, and it has been established as the Design Seismic Event (DSE). Saluda Dam presents liquefaction risks within the materials that comprise the embankment itself. The earth structure is resting on a layer of non-liquefiable residual soil. On the contrary, the East Dam case presents liquefaction risks within the foundation soils. These facts translate into post-earthquake slope stability processes that are quite different in nature. Though both Saluda and the East Dam are earth-fill retention structures, their geometry and characteristics are quite different in nature. Table 1 presents the main features of both projects, including also the volume of water retained. Both reservoirs are essential for local economic activities and should any of them fail, a considerable amount of people downstream of the facilities would be in jeopardy. Liquefaction failures, such as the 1925 Sheffield Dam in Santa Barbara or the 1971 Lower San Fernando Dam, should be prevented at these facilities. Table 1 – Saluda and East Dam project main features Concept Saluda East Dam Type Earth Fill Dam Earth Fill Height (ft) 220 36 Length (ft) 7,800 3,1680 Lake Murray Moultrie Lake Surface (acres) 50,000 60,000 Live Capacity (ft) 6.8 x 10 3.2 x 10 Dead Storage (ft) 1.6 x 10 1.8 x 10 Total Capacity (ft) 8.4 x 10 5.0 x 10 Year built 1929 1941 Main Use Recreation, Cooling Recreation, Water Supply RIZZO has evaluated the liquefaction potential and has established remediation plans and designs at each of these two sites. Computational tools developed by the industry and complemented with RIZZO expertise were utilized to identify liquefaction susceptibility both at the foundation and/or within the embankment soils. The remediation concepts between both Saluda and the East Dam significantly differ. The following text presents a summary of liquefaction analysis for each case and their results, their feasible remediation alternatives, and their selected remediation concepts. Liquefaction in Dams The term liquefaction is used to describe large deformations occurring in a soil mass caused by monotonic, transient disturbance of saturated cohesionless soil (Mogami, 1953). Generation of excess pore pressure under undrained loading is the main ingredient for liquefaction, since it is directly related with a sudden decrease of the effective shear strength of the soil. In the evaluation of the liquefaction hazard it is required to (a) determine the soil susceptibility to liquefaction, (b) determine the potential for liquefaction to be triggered, and (c) estimate the extent of damage. The previous concepts apply to any earth retaining structures that is contact with water and located in seismic regions, such as an earth fill dam. In such cases liquefaction potential may exist either in the embankment material itself, the foundation alluvial (in case the dam is resting on soil), and/or the abutments. Saluda and East Dam: Susceptibility to Liquefaction Saluda and the East Dam are both earth fill water retaining structures, but quite different in nature, as shown by the numbers of Table 1. Both dams give origin to high capacity impoundments, and are therefore classified as high hazard dams. If Saluda Dam would fail, more than 120,000 people would be in jeopardy and water and power provided by the project would be immediately lost. In the case of the East Dam, the damage would reach state wide concerns. Figure 2 – Schematic cross section of Saluda Dam; each of the embankment materials (fills and sluiced core) is susceptible to liquefaction Susceptibility to liquefaction is determined by means of a geotechnical exploration program. The results of such programs indicate that Saluda is a sluiced-core, earth fill dam that is resting on top of a dense, highly compacted, and geotechnically competent residual soil, as shown schematically by the cross section in Figure 2. Some portions of the dam are lying directly on top of bedrock. The height of the largest section is approximately 220 ft and both upstream and downstream slopes have an inclination ranging from three (3) horizontal to one (1) vertical. The fills adjacent to the core are washed fills while the exterior portions remained unwashed. The embankment materials are susceptible to liquefaction, presenting (N1)60 values below four (4). The cross section shown by Figure 2 is obtained form the original 1929 construction drawings, complemented by various RIZZO’s geotechnical subsurface exploration programs. The relatively elevated location of the piezometric line is the result of the loss of effectiveness of the impervious core and another indication of the susceptibility to liquefaction. Figure 3 – Typical cross section of the East Dam that is susceptible to liquefaction; in this case, the foundation will present the largest potential. The East Dam and East Dam Extension (Figure 3) are earth embankment structures constructed from rolled-fill consisting of competent clayey soils. The foundation soils are generally unconsolidated sediments and contain low-blow-count sand and clayey sand layers which are susceptible to liquefaction. The thickness of the low-blow-count sand layers underneath the East Dam and East Dam Extension ranges from 5 feet to 25 feet. A continuous very loose sand layer with (N1)60 less than 4 was encountered along the entire length of both Dams. This layer generally alternates with lenses of medium stiff to stiff clay and layers of medium dense to dense sand. A typical cross section is shown by Figure 3. Potential for Liquefaction There exists the potential for liquefaction in both dams, as shown by the dynamic quasilinear FEM analysis shown by Figure 4. The figure presents a graphic representation of the Factor of Safety against Liquefaction (FSL) obtained from the coefficient Cyclic Resistance Ratio (CRR) and the Cyclic Stress Ratio (CSR):