Slowing the Slide: Use of Deep Foundations to Stabilize Slopes in Washington, D.C.â€™s Challenging Potomac Formation-PPT Presentation

PPT Presentation

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The presentation examines the challenges of stabilizing slopes in the Potomac Formation around Washington, D.C. This Coastal Plain deposit contains highly plastic, heavily overconsolidated P1 clay interbedded with sand. Fissures and slickensides make its shear strength difficult to measure and can create weak planes. The clay may swell and soften, and its drained friction angle depends on its history: fully softened strength is relevant to first-time slides, while previously sheared clay may have much lower residual strength.<br /><br />Two case histories illustrate design approaches. For a new Washington, D.C., building set into a slope with no known history of movement, temporary support-of-excavation walls used soldier piles. Analyses considered multiple potential slip depths, calculated the required restraint, and checked pile response and structural stresses. The presentation notes that fully softened strength may be appropriate for temporary design where prior sliding has not occurred.<br /><br />At a 27-acre Cheverly, Maryland, site, slope movement threatened nearby rail infrastructure, including the WMATA Orange Line. Movement near a bridge abutment exceeded two feet, and back-analysis indicated residual friction angles around 9 degrees—consistent with laboratory results and published correlations. Stabilization used 59 drilled shafts, each five feet in diameter and extending to depths of up to 100 feet, together with regrading. Analyses and construction monitoring—including inclinometers, slope stakes, and building monitoring—supported the work and confirmed stability.<br /><br />Key lessons are that Potomac clay strength is uncertain because of fissures and prior shearing, making slope-stability predictions difficult. Site history is therefore essential: use fully softened strength where there is no evidence of prior slides, but consider residual strength where movement has occurred. Drilled shafts and regrading can successfully stabilize active slides.

Keywords

Potomac Formation

slope stabilization

Washington D.C.

Coastal Plain clay

fully softened strength

residual strength

drilled shafts

soldier piles

landslide analysis

Cheverly Maryland