The planned building addition at Hawthorne Elementary School required a deep dig adjacent to an existing foundation. Digging deeper than the existing foundation meant that a combination underpinning and soil retention system would be needed to construct the new facility.

Two sections of an existing continuous wall footing (26’ total) needed to be underpinned and shored to allow for the new construction. The new wall footing was located at a depth approximately 14 feet below the existing foundation.

In consideration of the wall loads, a 4” diameter “sleeved” Atlas Resistance Pier was used for the underpinning. The resistance piers were spaced on 4’ centers for the portion of the wall having a 10 kip/ft. design load. The piers for the portion of the wall having a 7 kip/ft. design load were placed on 5’ centers. The pier sections that were exposed by the excavation were sleeved at the coupling to provide additional lateral stiffness.

Due to the relatively high wall footing loads and the height of the excavation, helical tiebacks were used to provide lateral stability to the existing foundation. These tiebacks were incorporated into each of the pier brackets.

The earth retention system consisted of four rows of soil screws. Work was completed in phases using the top down construction method. Each screw was installed at an inclination of approximately 5 to 10 degrees from the horizontal, using a minimum torque of 2,000 ft-lbs. The screws were installed to minimum depth of 26’. A wire mesh was then tied into the soil screw head assemblies and 10” shotcrete facing was used to retain the soils. After a row was completed, further excavation was done and the process repeated. The shotcrete face was designed for a minimum punching shear of 20 kips.