Settlement Monitoring Of A Building Underpinned With Micropiles And Soil Nails

Zietlow, Bill
Organization: Deep Foundations Institute
Pages: 10
Publication Date: Jan 1, 2008
A micropile and soil nail underpinning system was installed to support a building immediately adjacent to a 25-ft deep excavation in downtown Salt Lake City, Utah. Both the building and the underpinning were instrumented to monitor deflections, rotation and crack growth. The underpinned structure consisted of a two story concrete masonry unit (CMU) building founded on shallow spread footings attached to a concrete high-rise building on spread footing two floors below grade. The face of the excavation was 10 in. in front of the underpinned building foundation wall. The underpinning system consisted of small diameter hollow bar micropiles installed with injection grouting techniques below the bottom of the excavation and attached to the underpinned structure by a concrete pile cap doweled into the existing footing. The lateral loads imposed by the soil under the underpinned structure were designed to be supported by the soil nail wall installed through the micropiles during excavation. The settlement monitoring instruments consisted of crack gages, optical survey prisms, high sensitivity vibrating wire water levels and biaxial vibrating wire tilt meters. They recorded maximum movements of approximately 0.1 in. of settlement, 0.05 deg. rotation (top of wall rotating away from excavation) and up to 0.08 in. of crack opening during the excavation until one month after the soil nails and shotcrete facing were completely installed. This movement was well within both the estimated settlement and the estimated settlement tolerance of the underpinned structure.
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