Dam Seismic Hazard and Embankment Response
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Project Objective

This earth-rock dam, including embankment and powerhouse facilities, was constructed between 1923 and 1925. The dam impounds a 3,600-acre lake, which is the largest freshwater lake in a state in Northeast United States. For seepage control, the dam contains a thin-section concrete core wall beneath the upstream portion of the crest. The embankment, which has a maximum height of 86 feet and length of approximately 1,300 feet, was constructed using the semi-hydraulic fill method whereby earthen materials were dumped on either side of the core wall and then sluiced towards the core. The dam is State-regulated and is classified as a high-hazard structure.

Geosyntec’s Scope of Services

Previous studies had shown the potential for liquefaction beneath the upstream portion of the embankment. Geosyntec was retained to perform a more rigorous analysis for evaluating the liquefaction potential and seismic deformations at dam, develop site-specific ground motions, and analyze the dynamic response of the dam. Specifically, Geosyntec: (i) provided a recommendation for the ground motion return period to use in seismic analyses of the dam; (ii) developed ground motion time histories for seismic analyses of the dam; (iii) performed 2D dynamic response analyses using QUAD4MU to evaluate permanent deformations and the extent of potentially liquefiable soils within the embankment and foundation; (iv) evaluated residual strength for liquefied soils considering several different methods; (v) conducted post-earthquake stability analyses that reflect the potential for liquefaction; and (vi) assessed the seismic performance of the dam in the context of regulatory criteria. To support this analysis, Geosyntec helped coordinate a LiDAR survey of the dam and a bathymetric survey of the upstream slope to better define the geometry for analysis.

In an ancillary project, Geosyntec was also selected to perform the annual inspection of Deep Creek Lake Dam as required by the state regulatory agency. The inspection included observation of the powerhouse, intake structure, spillway, embankment dam, and downstream drainage outlets and weirs.

Notable Accomplishments

Historically, the embankment dam has been modeled by others as a homogeneous material throughout the entire cross-section, with shear strength defined based on limited laboratory strength testing on samples not representative of the entire dam. Geosyntec reviewed historical construction photos and leveraged our experience with other semi-hydraulic fill dams constructed during a similar time period to develop a more accurate, zoned cross-section for analysis. Geosyntec developed bounded parameters considering degree of conservatism, uncertainty, and sensitivity to determine which parameters presented the most uncertainty and which parameters had the most influence on the results of the seismic stability analysis. Geosyntec is currently developing recommendations for reducing uncertainties associated with the embankment zoning, strength parameters, and overall dynamic analysis.

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Project Summary

  • Location: Northeast US
  • Client: Confidential
  • Project Practice Areas: Environmental Management
  • Services Provided: Coastal and Inlet Management; Beach Renourishment; Coastal Stabilization; Design, Engineering, Permitting, and Bid Documents; Post-Construction Monitoring; Environmental and Habitat Monitoring