Potomac Clays - Landslide Susceptibility and Design of Various Earthen Structures-PPT Presentation

PPT Presentation

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The presentation examines the landslide susceptibility of Potomac Clay and the design of earthen structures in Fairfax County, Virginia. These Early Cretaceous coastal-plain deposits are highly plastic, moisture-sensitive, and very low in permeability. They are often overconsolidated but have low shear strength, and may weaken through wetting-drying cycles or prolonged exposure during excavation.<br /><br />Past engineering challenges include slope and retaining-wall failures, identifying soil layers and weathered zones, assessing groundwater, and selecting appropriate soil-strength parameters. Investigations may combine SPT sampling, inclinometer monitoring, groundwater observations, laboratory plasticity testing, and site or lidar reviews of historic failures. Designers must distinguish intact from failed soils and weathered from unweathered materials.<br /><br />A Fairfax County data-center project provides examples of four structures: post-and-panel walls, a segmental-block wall with geogrid, a hybrid wall, and a reinforced soil slope. Stability evaluations considered short- and long-term conditions, groundwater, shear strengths, and global factors of safety. Analyses integrated SLIDE2 and RSPILE to model slope movement and the resulting forces on piles.<br /><br />Design measures included deeper pile embedment and tiebacks for post-and-panel walls, and toe keys plus excavation and replacement of weak clay with select fill and reinforcement for mechanically stabilized earth walls and reinforced soil slopes. These measures help deepen potential global failure surfaces and improve resistance.<br /><br />Key lessons are that relying solely on residual strength may produce overly conservative designs, while shear piles, tiebacks, or deeper foundations may be needed to address global instability. Practical design also depends on early coordination with the agency having jurisdiction.

Keywords

Potomac Clay

landslide susceptibility

Fairfax County

earthen structures

coastal plain deposits

soil stability

retaining wall failures

groundwater assessment

shear strength

reinforced soil slope