Converting crawl space to usable space in a 70-year-old school building turned into a major headache for the project's engineer and contractor. During excavation of the crawl space, workers discovered that pad footing elevations for two old columns were higher than the new basement floor elevation. With only 3 feet of headroom in the crawl space, workers then had to dig exploratory holes up to 12 feet deep to find the depth of every column footing. They found that footing levels within the crawl space were variable and all were at or above the proposed basement floor level for the new addition.
The engineer and contractor decided on two approaches for solving this problem. In some areas, existing column pads were substantially above finished floor grade. Engineers decided to leave soil in place beneath these footings and enclose it with a concrete wall. In other areas, the bottoms of existing stepped footings were at or near the finished floor elevation. In these areas, part of the stepped footing was removed and the rest was underpinned with below-floor-grade concrete.
Most exterior walls for the building were underpinned by casting concrete in successive 5-foot-long sections. Excavation was done by hand until workers got about 50 feet in from the exterior wall. Once inside, workers also underpinned interior foundation walls. Interior walls were permanently shored with steel columns where openings had to be cut to connect new and old sections of the basement.