Caldwell & Walsh Building Construction, Inc.
Soil Nailing, Newtown Savings Bank | Newtown, Connecticut
Challenge: How to excavate a fifteen foot deep foundation for a new three story building with an active roadway for customers immediately adjacent to the dig, historic wood frame structures to the north and south of the project site, and create safe access for heavy equipment to remove more than 2,000 cubic yards of fill without shutting down access to the bank’s parking and drive-through areas.
Conventional Solutions: Over-cutting the excavation would provide the requisite slope stabilization by allowing the soil to reach its natural angle of repose. This method, however, would entirely shut down the entrance drive to the bank property for the duration of foundation placement. Driven soldier piles and wood lagging would also work as an effective soil retention system, but extremely dense soil characteristics and concerns over excessive vibration made pile driving less appealing.
Caldwell & Walsh’s Solution: Soil nailing. A technique pioneered in Europe and seldom used in the United States, became the optimal choice for meeting all of the project’s objectives. By excavating the deepest portions of the foundation in five foot increments, crews were able to install a 2”-3” layer of reinforced shotcrete over the exposed soil lifts. After curing, track-mounted pneumatic drill rigs drilled 4” diameter casings, up to 20 feet long, diagonally into the embankment. The casings were filled with a 1” diameter threaded rod and a high-strength pressurized grout, creating the soil “nail.” After the grout sets, a steel washer plate is affixed to the threaded end with a large hex nut. The nailing process is repeated at regular intervals to cover the embankment face for the full height of the wall.
Through careful coordination, planning, and in-house engineering, Caldwell & Walsh was able to reduce the amount of soil retention needed to only one side of the excavation. This reduced the original soil retainage estimates by more than two thirds, saving the owner $100,000. The entire process was completed in less than two weeks, as opposed to three to four weeks for conventional methods, and without any interruption to the bank customers.
© 2014, Caldwell & Walsh Building Construction, Inc. All rights reserved.