Micropiles: Key To The Renovation Of The 161 First Street Building-Paper

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The renovation of the 1907, four-story building at 161 First Street in Cambridge required a new foundation system after inspections revealed that the existing timber piles beneath the walls and columns were severely decayed and overstressed. Underpinning was impractical because of construction constraints and PCB-contaminated basement soils, so the team developed a deep micropile (DMP) solution.<br /><br />The final design included 76 micropiles rated at 40 kips to support columns and 87 rated at 160 kips to support exterior walls, all connected to a structural mat. Initial soil-bond assumptions were conservative. A tension load test on an instrumented micropile reached 561.2 kips—3.5 times the highest design load—and met ASTM acceptance criteria. Strain-gauge data showed that the pile’s load was shared between the sands and clays and the glacial till. The test established substantially higher allowable bond values, enabling the team to shorten the piles and reduce total drilling by nearly 2,100 feet, or 21.4%.<br /><br />Contaminated soils created major cost, safety, and disposal challenges. A modified drilling method used permanent casing as a sleeve to keep contaminated upper soils separate from clean natural soils. A soil-separation system processed wet drill spoils, recycled water back to the rigs, and helped manage work in the confined basement.<br /><br />Installation was further complicated by granite-block foundations, hidden grade beams, limited access, and the absence of channels for moving spoils. Two specially configured drill rigs worked in separate areas, with close coordination among the contractors.<br /><br />The final pile was installed in December 2022. Despite delays related to the pandemic and difficult site conditions, the micropile work was completed in three and a half months. The project’s success depended on the load test and resulting design optimization, effective separation and processing of contaminated spoils, adapted drilling equipment, and daily coordination among the project teams.

Keywords

micropile

foundation renovation

load test

contaminated soils

underpinning

deep micropile

structural mat

drilling optimization

cement bond

Cambridge building