Construction of a Cancer Center addition to the Robert Wood Johnson Hospital, in New Brunswick, NJ, required excavation to hard rock at up to 25 feet below existing grade. The 150-foot by 200-foot excavation site was bounded on two sides by hospital buildings and on the other two sides by an emergency access road and a busy thoroughfare. Earth retention and underpinning to protect the integrity of the structures and roadways was an integral part of the project design.
A soldier beam and lagging system, in conjunction with conventional concrete underpinning, is a common and effective approach to this type of work in the northern New Jersey area. However, evaluation of the subsurface conditions showed a soil profile of 10 feet of residual overburden soil with good stand-up time, below which weathered rock extended to bedrock. These conditions are ideally suited to soil nailing, a geotechnical technique that combines underpinning and excavation support in a single operation.
Value Engineered Alternate
For this site, Moretrench calculated that soil nailing would be as effective as conventional systems, yet less expensive and faster to install, and proposed this value engineered alternative design to the excavation contractor. An established, positive working relationship with the contractor, combined with Moretrench’s reputation for successfully implementing innovative solutions to difficult problems, resulted in award of the contract.
Design and Installation
In unobstructed areas of the site, soil nails were installed at 15 degrees from horizontal on a five-foot grid pattern. However, a 24-inch diameter sanitary sewer that served the whole hospital complex ran directly beneath existing foundations. Compromising this essential utility would effectively shut down all hospital services and lead to very large consequential damages. Moretrench investigated to determine the precise location and elevation of the sewer, adjusting the design accordingly.
Three to four tiers of soil nails were installed, depending of the depth at which hard rock had been encountered at the time of the geotechnical investigation. For each tier, a lift of four to five vertical feet was excavated, extending for a horizontal length that could be stabilized during the workday. Given the good stand-up time of the cut face, the soil nails were installed first across the entire face of the cut. Reinforcing wire mesh was then placed, and a three-inch thick shotcrete facing was applied to bridge the soil nails and complete the structural underpinning and retention system. In poorer soils, this process is typically reversed, with mesh and shotcrete being applied before soil nail installation.
Moretrench successfully completed the approximately 14,000 square foot soil nail system three weeks ahead of schedule, resulting in significant savings to the owner.