Anchored Hollow Bar Tangent Pile System For Underpinning-Technical Paper Upload

Technical Paper Upload

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The paper describes an alternative underpinning system used to protect Brattleboro Memorial Hospital in Vermont during construction of an adjacent four-story addition. Excavation extended up to 15 feet below the existing hospital’s footings, and the building had to remain in use.<br /><br />The original design called for sequential pit underpinning. However, excavation exposed very clean, loose-to-dense sands and gravel that could not reliably hold unsupported vertical faces, creating risks of ground loss and building movement. Permeation-grouting trials were also evaluated, but coarse gravel seams absorbed large volumes of grout, while finer zones showed limited penetration. Repeated grouting cycles would have been required.<br /><br />The adopted solution combined closely spaced, grouted hollow-bar micropiles drilled through the existing footings with a shotcrete-faced soil-nail wall. The micropiles transferred foundation loads to deeper soil, while the soil nails and shotcrete supported the excavation. Installation proceeded in stages, using an excavator-mounted drill suited to the site’s restricted access. Continuous grout flushing during drilling improved grout penetration, and the hollow bars provided structural reinforcement.<br /><br />Construction and monitoring demonstrated the system’s effectiveness. Measured vertical movement was less than 0.6 inches (15 mm), and horizontal movement was less than 0.13 inches (3 mm). Load tests showed that hollow-bar elements achieved at least five times the required capacity, with little or no creep.<br /><br />The authors conclude that the composite micropile and soil-nail system was faster and less risky than the original pit underpinning approach and more practical than a soil-nail wall relying on permeation grouting. They recommend involving a qualified specialty underpinning contractor in system selection and detailing.

Keywords

underpinning

micropiles

soil nailing

shotcrete

foundation support

excavation support

grouted hollow bars

permeation grouting

hospital construction

ground movement