This brick- and concrete-encased, two-story steel building (circa. 1924) lies within the Greenwich Village Historic District and is 700 feet from the Hudson River. A former parking garage, it has been converted into a single-family residence—one of the largest currently in New York City. Maps reveal that in the 1800s, this neighborhood was in the Hudson River and the area was built up over time with the addition of approximately 10-14 feet of historic fill. The building is wedged among other buildings of varied age and materials, and two of its four walls directly abut other buildings
Comprehensive interior renovations, including removal of floors and interior walls, as well as excavations up to 30 feet for a new cellar, required extensive interior bracing and support of excavation (SOE). The site’s shallow groundwater also necessitated substantial dewatering during construction. Because the building is in a historic district, all exterior walls were required to remain. Although the site was not subject to current local, state, or federal flood regulations, anticipation of climate change impacts was a priority for the owner and integral to the design.
GZA conducted subsurface explorations and reviewed reports of geotechnical investigations in the area completed by others. GZA worked closely with the project design and construction team to evaluate foundation, SOE, dewatering, and resiliency related design alternatives. In addition, GZA coordinated with the contractor and design team to design a complex temporary bracing system that allowed for all stages of demolition and construction.
Support of excavation (SOE) and underpinning
To achieve the planned excavation of up to 30 feet while leaving all existing site building walls in place, GZA proposed an innovative solution to SOE. Over 350 13-inch diameter tangent piles were installed within 12 inches of the interior walls to provide a continuous earth retention and groundwater cutoff for the project. Underpinning, secant and slurry walls were also considered, but were not selected due to schedule, cost, and constructability. The possibility of settlement and tilting of adjacent buildings, sidewalks and roadways was also considered before excavation, SOE and dewatering began.
The high-volume dewatering system GZA designed handled more than one million gallons of water per day for four months during construction. The deeper local excavations within the site also utilized sheet pile wall construction.
Given that the subsurface investigation revealed bedrock at depths of 70-75 feet below grade as well as the extent of renovation required for the project and the anticipated structural loads, foundations were generally designed to bear partially on the tangent piles with the cellar utilizing a mat slab on grade. Deep foundations consisting of drilled mini caissons were utilized to carry temporary construction loads and final buoyancy inducted final loads.
Above grade temporary support system
Working with the design and construction teams, GZA developed a sequence of work that allowed for construction to begin without removal of the existing second floor, roof, or walls. This was achieved by utilizing the permanent micropiles to temporarily support the existing columns. Another project innovation was the implementation of simultaneous “bottom-up” and “top-down” construction. To accomplish this, GZA designed a support system to temporarily hang the new ground floor slab from temporary beams that were supported by the permanent drilled mini caissons. The ground floor slab supports were designed to accommodate the load of the new second and third floor structures without the slab being placed on its permanent supports, allowing for work to proceed both above and below the ground floor slab simultaneously.
Although not required to do so by current code requirements, the owner was cognizant of the area being flood-prone and opted to take an “early adopter” approach to flood hardening. GZA evaluated the hydrologic and hydrogeologic risks of the site using calculations based on early climate change research of forecasts and resiliency needs in the next 50-100 years. This data informed project design and cost discussions with the owner and design team, with the building ultimately designed for flood elevations four feet about the current sidewalk level.
GZA’s multidisciplinary strengths in geotechnical, geo-structural, and hydraulic engineering allowed the owner and design team to achieve their design goals under challenging subsurface and schedule conditions and to proactively prepare the site for longer term impacts of climate change.