Seismic analyses of Wappapello Dam

Wappapello Dam is a rolled-fill, earth dam located on the St. Francis River, approximately 15 miles north of Poplar Bluff, in Missouri, USA. The dam is currently owned and operated by the US Army Corps of Engineers, St. Louis District (USACE-CELMS). The dam was constructed between 1938 to 1941 for flood control purposes. The dam is approximately 73 ft high and the crest is 30 ft wide at El. 419.74 ft (NGVD). The dam is approximately 2,700 ft long. The dam is founded on approximately 120 ft of alluvium underlain by bedrock material identified as dolomite. The upper 40 ft of the alluvial deposits consist of loose, fine sands and silts, less than 500 years old, identified as the Young Point Bar Deposits. This deposit has been identified as potentially liquefiable, and the USACE have initiated a number of studies over the years to determine the dynamic behaviour of Wappapello Dam. The first phase of the seismic study was carried out in 1982 by the USACE-Memphis District and consisted of a one-dimensional liquefaction assessment of Wappapello Dam and the underlying foundation soils. The main conclusions resulting from this work indicated that the Young Point Bar Deposits were indeed susceptible to liquefaction under the design earthquake loads. The second phase of the seismic study was carried out in 1988 b> the USACE-St. Louis District and consisted of limit equilibrium stability analyses, assigning post-liquefaction residual strength values to the liquefied deposits. The residual strength values were determined using an empirically-based correlation relationship with field measured penetration values. The postearthquake limit equilibrium factor of safety was about 1.0 for the estimated residual strength, and therefore, the main conclusion from the phase 2 studies was that the likelihood of an earthquakeinduced embankment slide causing a reservoir release was low. Since 1988, further advances have been made in the liquefaction assessment procedures, the assessment of residual strength and the evaluation of post-liquefaction deformation analyses. Therefore, the USACE initiated the third phase of the seismic study of Wappapello Dam. This phase included a critical review of the phases 1 and 2 work, to ensure that advances made in the liquefaction assessment techniques and the residual strength evaluations since 1988 have not changed the previous conclusions, and a more rigorous evaluation of the post-earthquake deformation analyses, to ensure that the magnitude of the movements are within acceptable limits. The post-earthquake deformation analyses was carried out using the finite element computer programs, TARA-3 and TARA-3FL, developed by Dr. W.D.L. Finn of the University of British Columbia, Canada. The phase 3 work deformation analyses indicated that the magnitude of the post-earthquake movements were in the order of 25 ft vertically and greater than 200 ft horizontally, if the same design parameters as used in the phase 2 work were assumed. The discrepancy between the results of the finite element analyses and the limit equilibrium slope stability analyses was attributed to the different failure mechanisms assumed in each analysis. In the finite element analysis, failure was a result of the squeezing out of the liquefied foundation deposits due to the high gravity loads of the dam and the sliding on the softened material as a result of reservoir loading. In the limit equilibrium slope stability model, the failure is modelled by a circular slip surface cutting through both the embankment having a high strength of 3,000 psf and the Young Point Bar Deposit with a low, post-liquefied strength of 115 psf. The seismic stability of structures with extensive zones of liquefiable materials in the foundation should not be assessed using limited equilibrium slope stability analyses as the conclusions resulting from this type of analysis may be misleading. Additional deformation analyses were carried out in the phase 3 work using more realistic fieldmeasured penetration test values for the Young Point Bar Deposit. Parametric analyses were carried out by assuming different residual strength correlation relationships and varying ( N^ values for the Young Point Bar Deposits located downstream of the dam toe (which was unknown during the phase 3 work). The deformation values ranged from less than about 3 ft of vertical and horizontal deformations to greater than 20 ft vertically and 250 ft horizontally for the varying strength assumptions. Therefore, the seismic stability of Wappapello Dam depends on two critical input parameters: the penetration test values of the Young Point Bar Deposits located downstream of the dam toe; and the evaluation of residual strength. The following recommendations are made following the phase 3 studies: further field investigations should be carried out to determine the (Nj)^ values of the Young Point Bar Deposits located downstream of the dam toe; further studies should be carried out to determine the residual strength of the Young Point Bar Deposits. Additional laboratory testing and review of the empirically-derived residual strength vs (N,)^ database should be carried out to establish appropriate Sur values for this site; after the ( N ^ values of the Young Point Bar Deposit have been established and the appropriate design Sur values have been assigned, further deformation analyses should be carried out to determine the post-earthquake deformations of Wappapello Dam. The maximum acceptable post-earthquake deformation limits to ensure seismic stability of Wappapello Dam should be established by the USACE. If the estimated deformations are not acceptable, potential remedial measures should be assessed and implemented.

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