Abstract:
By
Kenneth A. Pidgeon, PE, Engineers Construction, Inc., Williston, VT, USA, (802) 863-6389, kpidgeon@ecivt.com
Benjamin H. MacKinney, PE, Engineers Construction, Inc., Williston, VT, USA, (802) 264-2286, bmackinney@ecivt.com
Kenneth R. Allard, EIT, Engineers Construction, Inc., Williston, VT, USA, (802) 779-4780
kallard@ecivt.com
Adriane G. Boscardin, PhD, PE, Boscardin Consulting Engineers, Inc., Amherst, MA, USA, (413) 549-3804, adriane@boscardinconsulting.com
Marco D. Boscardin, PhD, PE, DGE Boscardin Consulting Engineers, Inc., Amherst, MA, USA, (413) 237-4852, marco@boscardinconsulting.com - (413) 237-4852
ABSTRACT
This paper presents a case history for a unique application of a system of vertical micropiles with horizontal soil nail bracing and shotcrete facing for underpinning. The system was used to support and protect an existing 2-story hospital building during demolition and renovation. The goal was to limit building movements and have the building remain in use during construction of an abutting addition. Soils at the site consisted of clean, loose to dense, sand and gravel. The original underpinning design was revised after initial excavation revealed unsuitable soil conditions for traditional underpinning pits. An alternative system consisting of vertical micropiles (hollow bars) through the footing, combined with shotcrete and soil nails was proposed, accepted by the owner, and successfully constructed. Excavation depths were up to 15 feet (4.6 m) below the existing footings. Wall movements were monitored at regular, horizontal and vertical intervals along the hospital wall, and less than 0.6 inches (15 mm) of settlement and 0.1 inches (3 mm) of horizontal displacement throughout underpinning and construction were reported. The challenges with the original proposed support of excavation design and other alternative approaches considered will also be discussed.
Key words: underpinning, grouting, permeation grouting, grouted column, micropile, soil nail wall, sand, gravel, shotcrete, hollow bar