Creative Secant Pile Excavation Support System-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes the contractor-designed secant pile shoring system for a basement excavation at a new weapons storage facility at Malmstrom Air Force Base. The facility is supported by 526 drilled shafts, some up to 91 feet deep. Condon-Johnson Associates developed an internally braced system and worked with Brierley and Associates on the final design.<br /><br />The main design challenge was providing excavation support without obstructing mechanical equipment installation or risking damage to the foundation shafts. Four concepts were considered: top-down construction, a slab spanning the excavation opening, tiebacks, and tierods with a waler. The first three were unsuitable: top-down construction prevented the required open excavation, the slab edge could not adequately restrain the piles, and tieback drilling risked striking foundation shafts.<br /><br />The selected tierod-and-waler system confined the tops of the secant piles and spread loads laterally, reducing the number of tierods required. Tierods could be installed in trenches rather than drilled horizontally, avoiding the foundation-shaft risk. The approach required no additional equipment and allowed basement excavation to begin after the south slab was poured and cured. Once the basement and upper slab were complete, the tierods could be de-stressed so the slab carried the secant-pile loads.<br /><br />Construction relied on a guidewall to maintain pile location and overlap. Piles were installed in a primary-secondary sequence: primary piles were unreinforced, and secondary piles were reinforced. The tierods were locked off at 250 kips.

Keywords

secant pile shoring

basement excavation

Malmstrom Air Force Base

drilled shafts

internally braced system

tierod and waler

excavation support

foundation shafts

primary-secondary pile sequence

guidewall construction