Excavation Granite Ledge

This new home on a tight lot in Belmont Hill specified an extensive list of below grade amenities, including a two car garage, an elevator shaft, a finished basement living space, plus a room for utilities. Given the design, the neighborhood and the space available, there was nowhere to go but down. The issue was that the site is located on an outcropping of granite ledge, so the excavation had to be made deep into solid rock.

The images don’t show it, because of the extensive mature plantings that we brought onto the site, but the neighbors are very close. This precluded any blasting of stone. The excavation was achieved entirely with excavator mounted hydraulic hammers and took a full month to complete.

The cavity was extended substantially beyond the perimeter of the foundation in order to accomodate an exterior surface water drainage system. This consisted of perforated pipe connected to several storage tanks for accommodation of high flow, ultimately draining into the city storm sewer. The concrete foundation footings were pinned and doweled into the bedrock and the exterior of the foundation covered with a  waterproof mesh and backfilled with gravel.

No building site is a blank slate. The natural geology below grade imposes certain conditions specific to every neighborhood. In Belmont Hill, the work to address this granite ledge was substantial, but the result is a very dry, healthy and comfortable basement area.