When fast-track new construction at an existing high school was halted due to slab cracking and settlement, rapid response to resolve the problem was critical. Moretrench was contacted by General Contractor W.G. Mills of Jacksonville, FL to provide a turnkey design, engineering and installation underpinning solution to resolve the problem. Within 14 days of the request for proposal being received and just two days after final document approval, Moretrench had completed the work, allowing W.G. Mills to resume construction.
Helical Pile Solution
The 3-story building, part of the expansion of Edgewater High School, had been designed as a steel frame and tilt- wall structure. With the frame and walls in place, cracking of the slab on the order of 1/8-inch wide and differential wall settlement occurred in the northeast corner of the building where two structural walls met. Distress was evident along a 40-foot length of one wall and an 8-foot length of the adjoining wall.
Following exploratory testing, Geotechnical Engineers Ardaman & Associates of Orlando, FL attributed the settlement to insufficiently dense foundation soils, possibly exacerbated by a high groundwater table, construction activity through what had previously been a high traffic area, and erosion and scour as a result of recent heavy stormwater run-off.
In close cooperation with Universal Engineering Sciences (UES) of Orlando, FL, which provided recommendations for pile capacity, Moretrench developed a helical pile underpinning solution (one of the key underpinning services Moretrench offers) that could be installed rapidly and with minimal disturbance.
Design and Installation
The heavier of the two tilt walls, was supported on a 4-foot wide footing and imposed loads of approximately 13.5 kips per linear foot. Loads on the second, 2-foot wide footing were approximately six kips per linear foot. Helical, or screw, piles featuring a triple helix configuration consisting of 10, 12 and 14-inch diameter helices were selected to obtain the minimum pile capacity for project loading.
For the more heavily loaded footing where wall settlement and cracking extended for a length of eight feet, the design called for a double row of three helical piles with an allowable design capacity of 14 tons each, spaced at 3-5 feet on center. For the more lightly loaded wall, 18-ton piles were designed to be spaced at six feet on center.
Based on the variable soils encountered during borings performed by UES, minimum torque required to achieve pile capacity were anticipated to be 2.9 foot-tons and 3.6 foot-tons for the 14-ton and 18-ton piles respectively. During pile installation, final torque measured at pile tip depths of 25 to 28 feet ranged from 5.1 to 7.2 foot-tons, verifying the design and installation procedure.
For more information about underpinning services, contact Moretrench.