Challenge

The project site is a former coal fired power station being renovated to receive crude oil delivered by rail from North Dakota. The crude oil will be transferred through a system of piping from the rail cars to barges, and then shipped on the Delaware River to various oil refineries. Two Air Eliminator Tanks associated with Pump Pits A and B were supported on 11’-0” by 12’-4” by 3-foot thick mat foundations (Pads A and B). The two foundations settled differentially when adjacent trench excavations were made. The pads were each originally constructed on four Geopiers (rammed aggregate piers); however, the Geopiers reportedly compressed and shifted laterally toward the excavations when lateral support was lost to the foundation soils. Although the excavations where quickly backfilled, the project engineer was concerned with long-term foundation alignment and stability, and recommended that both pads be underpinned. Pad B would require stabilization only, while Pad A, where settlement had been more significant, would be lifted as much as three to four inches, if possible. A geotechnical study performed across the site identified sandy and gravelly fill containing rubble, debris, and coal fragments over alluvial silts and sands, underlain by bedrock. Several of the borings also described possible fill to depths up to 34 feet. The nearest test borings to Pads A and B encountered approximately 8 to 14 feet of fill and bedrock at about 28 to 38 feet.

Solution

The foundation support detail included eight hydraulically-driven push piers at each mat foundation, two at each corner, to support a design working compression load of 15 kips per pier. Flush mount brackets were anchored to the sides of the concrete foundations with adhesive anchors to avoid excavation below the pads at the pier locations. The 16 Model 288 (2.875-inch OD by 0.165-inch wall) push piers were then driven from 22.5 to 37.5 feet below the bottoms of the foundations to achieve the target drive pressure of 4,000 psi (drive load of 38.5 kips). For added system rigidity, the pier tubes were filled with 5,000 psi bag mix concrete every six feet during the pier installation. Pad B was stabilized by reloading the piers to the design working load of 15 kips. Pad A was lifted 1 inch to 3.5 inches to achieve both proper elevations for surface drainage and equipment/piping alignment. The lifting operation was monitored and directed by a survey crew. The piers supporting Pad A were locked off at loads between 15 and 20 kips. The void created beneath Pad A was filled with PolyLEVEL® PL250, a two-part, rapidly setting polyurethane foam. The material was injected from the top of the concrete through eight PVC-sleeved ports to provide uniform coverage.

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Project Summary

Structural Engineer: MKEC Engineering Consultants, Inc.

Geotechnical Engineer: Duffield Associates, Inc.

General Contractor: Nooter Construction Company

Certified Pier Installer: JES Construction, Inc.

Products Installed: (16) FSI PP288 Push Piers, Depths from 22.5 to 37.5 feet, Design Working Compression Load of 15 kips; 95 lb of PolyLEVEL® PL250, 75 psi Compressive Strength, 4 pcf Typical In-Place Density