Lift Station Generator Building - West Fargo, ND

Monday, May 12th, 2014 by John Siwicki

Challenge

March 2012 - A 2,400 square-foot generator building was under construction at the regional storm water lift station when the general contractor observed settlement of the northeast corner of the foundation. The frost-depth foundation wall along most of the east wall and a portion of the north wall was poured separately from two other wall sections with deeper footings. The three sections of foundation walls were connected with continuous steel reinforcing bars that extended across the construction joints. Prior to building construction, weak unsuitable soils were reportedly removed below the proposed footings and replaced with approved fill that was tested for compaction during placement. A dynamic cone penetrometer test was completed near the northeast corner of the building to a depth of eight feet below footing bearing elevation. With the results of the cone penetrometer test and knowledge of the site development, Braun Intertec estimated approximately seven feet of Class 5 (sand and gravel) fill material underlain by silt. The project already had a tight completion deadline before discovering the wall settlement, so any option used to underpin and stabilize the structure had to be completed quickly.

Solution

The underpinning solution included five retrofit helical piers along the east wall line. Each pier would be installed to support a design working load of 15 kips. The helical pier configuration consisted of the Model 288 (2.875” OD by 0.276” wall) round shaft with a 10”-12”-14” triple-helix lead section followed by two 14” helix plates on the first extension. A mini-excavator was used to install the piers to depths of 21 to 52 feet below the bottom of the footing and to torque-correlated ultimate capacities of at least 30 kips (FOS ≥ 2). After all five piers were installed, a uniform seating load was applied with hydraulic cylinders to remove any slack in the system. The spring thaw created extremely wet and muddy conditions across the job site which hindered mobilization of product and equipment to the generator building. The multi-helix pier configuration was also a challenge to install as installation of retrofit helical piers is increasingly more difficult as the number of helix plates along the shaft increases. Proper installation techniques are critical to angle the pier at two to three degrees (pile tip toward the building) while still maintaining correct separation of the top of the pier to the footing. Even with the site and installation challenges, the five piers were completed in one day.

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