Raintree School - St. Louis, MO

Wednesday, February 1st, 2017 by Office Personnel

Challenge

A 5,200 square-foot, two-story addition was proposed for the existing 6,800 square-foot school building. The continuous footing and three column pad locations along the east wall of the existing building required underpinning to stabilize the structure during construction of the addition. However, the steep downward slope east of the existing building would limit access to construction equipment for these underpinning operations. The geotechnical investigation identified a soil profile consisting of rocky soils with cobbles above bedrock at a depth of 12 feet.

Solution

Push piers were selected as the optimal foundation support option based on the limited equipment access and the soil stratigraphy. Helical piles could have difficulty penetrating through the cobbly soil in order to bear on or within the bedrock and may not be able to provide torque correlated capacities if spin off were to occur at the bedrock surface. Additionally, the work would be performed adjacent to a classroom while school was in session and noise levels from helical pile installation equipment could have been disrupting. Ten (10) Model 288 push piers with 48-inch external sleeves were installed along the east wall of the existing building to average depths of about 12 feet. The estimated design load for the push piers was 15 kips. The push piers were installed to a minimum drive force of 48.1 kips and the piers were locked off at 28.9 kips. The final drive force provided a factor of safety of 3.2 for the 15 kip estimated design load. The ten piers were installed and the excavations were backfilled and compacted in one day.