Rebuilding Dams in The Sand Hills: Challenges of Seepage Control and Embankment Stability-Paper

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Langley Pond Dam, a century-old embankment in South Carolina, faced serious seepage and stability risks. In 2014, seepage emerged as boils near the downstream toe, indicating potentially dangerous internal erosion. The dam also lacked sufficient spillway capacity. Its foundation consisted of permeable Coastal Plain sands over residual soils, while the embankment itself was generally loose and variable.<br /><br />Rehabilitation included a slag-cement-cement-bentonite (SCCB) cutoff barrier along the dam’s centerline to control seepage. The barrier extended up to 65 feet deep and was keyed into underlying residual soils. Its 2.5-foot thickness and low-permeability mix were designed to limit seepage without becoming overly brittle. Testing found average 28-day compressive strength of 79 psi and average permeability of 4.3 × 10⁻⁹ ft/s, meeting the project’s seepage-control objectives.<br /><br />During construction, crews uncovered a 16-foot-thick layer of very soft, loose alluvium beneath part of the proposed upstream slope. Because excavation and replacement or deep soil mixing would have been costly and difficult, seven SCCB shear walls were installed perpendicular to the dam’s centerline. Each was 3 feet wide, 45 feet long, and 25 feet deep, with 20-foot spacing. The walls were designed to improve slope stability and limit movement of the weak soil between panels.<br /><br />The project demonstrates that SCCB barriers and transverse shear walls can address distinct seepage and foundation-stability problems at dams built in challenging sandy and alluvial ground. Effective design depended on detailed subsurface investigation, including CPT soundings to identify soil layers and establish the cutoff’s depth. The rehabilitation works, including a new spillway, were completed in 2020.

Keywords

Langley Pond Dam

seepage control

embankment dam

SCCB cutoff barrier

internal erosion

spillway capacity

foundation stability

Coastal Plain sands

shear walls

subsurface investigation