Design, Installation and Performance of Earth Retention System to Limit Ground Movements Around Sensitive Structures in Washington, DC-PPT Presentation

PPT Presentation

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The Scottish Rites Apartments project involved constructing a five-story building above street grade and four levels below grade behind Washington, DC’s House of the Temple. Schnabel designed and installed the temporary earth-retention system and deep foundations while protecting nearby structures and keeping public infrastructure accessible.<br /><br />Subsurface conditions included approximately 30 feet of urban fill and alluvium over dense Potomac Formation deposits containing sand, silt, clay, cobbles, and boulders. The retention system used soldier beams spaced eight feet apart, free-draining wood lagging, and up to three tiers of temporary tiebacks. An additional tieback tier was provided near the nearby townhouse and carriage house to reduce unsupported wall height. Design considered adjacent structures’ sensitivity and sought to limit ground loss during installation.<br /><br />Monitoring showed minimal soldier-beam settlement—about 0.25 inches—and negligible movement of adjacent buildings. The House of the Temple and a nearby townhouse experienced no settlement or tilt; the carriage house settled by no more than 1/32 inch. No visible structural distress or tieback creep was reported.<br /><br />The permanent foundation work comprised 28 micropiles beneath five column footings. A preliminary bond test informed the design, and seven-inch permanent steel casing extended through the free length to transfer loads below the lowest basement level. Installation methods addressed the dense, cobbly soils, including specialized drilling tools, tremie grouting, and one regrouting operation per pile. A non-production micropile was tested to twice the design load in accordance with ASTM D3689.<br /><br />The project successfully installed both the earth-retention system and micropiles without damaging adjacent structures, demonstrating the importance of movement-focused design, careful installation, and monitoring.

Keywords

Scottish Rites Apartments

earth-retention system

deep foundations

micropiles

temporary tiebacks

soldier beams

Potomac Formation

adjacent structure protection

subsurface conditions

ASTM D3689