Slope Stabilization in the Potomac Group Formation with Continuous Flight Auger Piles-PPT Presentation

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The project involved stabilizing MSE walls on a slope in Southeast Washington, DC, where global stability was inadequate in the Potomac Formation’s hard clay. Schnabel Engineering designed the stabilization, and Wagman Geotechnical Construction installed the piles.<br /><br />Slope/W analyses identified critical slip planes and estimated the shear reinforcement required. LPILE modeling then evaluated pile resistance at a limited soil movement of one inch, accounting for mobilized shear, structural shear capacity, and bending capacity. The design used one to three rows of 24-inch-diameter auger-cast piles, 55–95 feet long, with full-length reinforcing cages. In total, about 30,000 linear feet of piles were installed.<br /><br />Construction challenges included concrete communication between nearby piles and difficulty inserting cages into the bottom 10–20 feet of some piles. Initial Thermal Integrity Profile (TIP) reports suggested pile diameters of 28–32 inches, but test pits found actual diameters of only 24–25 inches. The team discovered that grout volume had been estimated using calibration under zero pressure, so reported grout factors—and TIP-derived diameters—were unreliable.<br /><br />The underlying problem was excessive grout pressure. Stable, uncased holes in the hard clay did not expand under pressure; grout instead packed around the auger, preventing cages from reaching the pile tips. Rock-auger drilling and low-volume air had helped because they improved cage installation.<br /><br />The team revised procedures to minimize grout pressure rather than target high grout take: they modified grout admixtures for stability, reduced pump power, and began grouting with the auger tip 10–20 feet above the hole bottom. These changes resolved the cage-installation and concrete-communication issues. The case also highlighted the importance of clearly specifying integrity-testing plans and accurately measuring grout flow.

Keywords

MSE walls

slope stabilization

Southeast Washington DC

Potomac Formation

global stability

auger-cast piles

Slope/W analysis

LPILE modeling

Thermal Integrity Profile

grout pressure