The Liberty Middle School is a single-story structure with exterior concrete block walls and brick veneer. A “bump-out” area of the building approximately 100 feet long with 14-foot returns had settled and rotated away from the main footprint of the structure. An interior block partition wall intersected the long exterior wall at its mid-length. The settlement and rotation of the exterior wall caused the two walls to separate, with the crack wider at the top than the bottom. The movement also raised concern that connections and adequate bearing of the roof joists to the top of the exterior block wall had been compromised. A steel pipe column had been installed within a classroom four feet from the exterior wall to support one of the joists. A subsurface investigation encountered lean to fat clay underlain by competent limestone. Auger refusal was noted on limestone bedrock at approximately 22.5 feet below grade.
A system of hydraulically-driven push piers was selected as the most economical method to stabilize and potentially lift the settled area of the building. A total of nineteen push piers were installed at a sevenfoot maximum center-to-center spacing. Fifteen piers were installed along the 100-foot section of wall and two piers were installed at each of the 14-foot returns. A 50 kip drive load was specified per pier with a minimum factor of safety of 1.5 after the lift.
A continuous excavation was made outside the building to expose the four-foot deep trenched and poured frost footing. At each pier location, the footing was prepared by removing excess concrete that extended beyond the plane of the vertical face. The underside of the footing was also trimmed smooth for uniform bearing on the retrofit bracket. The push piers were advanced to depths of approximately 19 feet below the bottom of the footing (23 feet below grade) to bear on the limestone and achieve hydraulic fluid pressures of at least 4,000 psi, corresponding to 56.7 kips of drive force. Hydraulic lift cylinders were then fitted to the installed pier assemblies and connected in series to uniformly lift the exterior wall approximately 1.75 inches back toward its original elevation. Lift forces averaged 24 kips for a factor of safety of 2.3 (FOS = drive force/lift force). The lift and slight rotation of the exterior wall closed the partition wall to exterior wall separation on the order of ½-inch at the tops of the walls. The interior pipe column and supported roof joist were monitored throughout the installation and lifting processes. When the lift was complete, the project engineer evaluated the joist to wall connections and allowed the pipe column to be removed. Interior wall cracks that did not close completely were filled with backer rod and caulk. Voids beneath the floor slab were filled with cement grout. Excavation to site clean-up was completed in just four days.
Architect: ACI/Boland, Inc.
Structural Engineer: Bob D. Campbell & Company
Geotechnical Engineer: Alpha-Omega Geotech, Inc.
Certified Pier Installer:Foundation Recovery Systems
Products Installed:: 19 Push Piers, Installed Depths of 23 feet; Driving Force > 50 kips