Challenge

The condominium complex had a large trash compactor enclosed by three concrete block retaining walls and an access gate. The enclosure footprint measured approximately 52 feet long by 17.5 feet wide. Unbalanced earth pressures on the up to 10 feet high block walls caused the walls to lean in at the top or crack and bow near the center. The walls were generally leaning near the gate end of the enclosure and bowing in within the area of the trash compactor. The flared top of the catchment bin of the compactor was wedged within the sidewalls of the enclosure and now serving to brace the tops of the sidewalls and end wall. A tieback system was proposed to permanently stabilize the failing retaining walls.

The narrow enclosure and the immovable trash compactor limited equipment access to install the tiebacks. The trees and other vegetation above the walls created other concerns about installing tiebacks through potentially obstructing root systems. A soil investigation behind the retaining walls was not performed.

Solution

The wall stabilization detail included nine (9) Model 150 (1.5-inch round corner square bar) helical tiebacks. A mini-excavator could maneuver within the sidewalls of the enclosure to install the tiebacks away from the trash compactor. Hand held equipment would then be used to install tiebacks around the compactor and beneath the catchment bin. An 8”-10” double-helix lead section was selected to minimize the size of penetrations through the walls and to support the design working tension load of 8 kips. The tiebacks were installed in a single row approximately six feet from the tops of the walls and at a center-to-center spacing of approximately four feet. The tiebacks were installed at downward angles of 10 and 15 degrees from horizontal. Standard extensions advanced the tiebacks to lengths from 12 to 55 feet to achieve torque-correlated ultimate capacities exceeding the design working load by factors of safety greater than three (FOS > 3). Typical factors of safety for tension applications range from 2.0 to 2.5; however, a higher than typical factor of safety was specified by the engineer due to installing into unknown soil conditions. Thread rod adaptors transitioned the tiebacks to high strength thread rods for connection to a waler system of two-inch by four-inch tube steel. The holes in the walls were patched before installing the tube steel walers. PVC sleeves were included around the thread rod to allow pretensioning of the tiebacks. The tieback installation was completed in just two days.

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Project Summary

Structural Engineer:: Robinson Associates Consulting Engineers, Inc.

General Contractor: CS Miller Construction

Certified Tieback Installer: Foundation Supportworks® of Georgia

Products Installed: (9) Foundation Supportworks® Model 150 Helical Tiebacks, 8”-10” Double-Helix Lead Section, Installed to Approximate Lengths from 12 to 55 feet, Design Working Tension Load of 8 kips