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Unexpected Groundwater Causes Stir at Pump Station

Fri July 07, 2006 - Southeast Edition
David S. Chartock



Project challenges can always be overcome. But, sometimes there are surprises that test the mettle of a team’s quickness to respond.

One of these surprises occurred at the $17.5 million pump station No. 357 in Miami, according to Project Manager Todd Palmatier of Atlantic Skanska Inc. the project’s Atlanta, GA-based general contractor.

One of the project’s first hurdles was dewatering.

“We anticipated 100,000 gallons per 24 hours. It turned into 1.7 million gallons of groundwater. This was a design error which we have overcome by utilization of two 10-in. hydraulic pumps and a formwork design that will allow us to place the structural walls underwater as we placed the tremie slab,” Palmatier said.

Included in the scope of the project, he said, is mitigation of the 8.5-sq.-mi. area, which would be accomplished by the construction of a perimeter levee around the area, a seepage collection canal through the middle of the area and a 575 cu. ft./sec. pump station south of the residential area, which will pump collected seepage into a storm water treatment area to be located southwest of the pump station.

Base offer work, he continued, consisted of production blasting and pre-splitting, rock excavation, dewatering and construction of S-357 pump station. The pump station includes four 125 cu. ft./sec. Morrison Pumps powered by Caterpillar 3406 diesel engines and one 75 cu. ft./sec. electric motor driven pump. The pump station also includes as sole source items a 60 kW diesel driven generator for backup power and telemetry equipment supplied by Pantropic Power for the station which includes a Cisco Ethernet switch, a Motorola MOSCAD unit, an Allen Bradley PLC, surveillance cameras, SCADA software, a security panel, fire alarm panels and a Dell personal computer.

Also included in the scope of work, Palmatier pointed out, will be rock excavation for the canal; construction of a seepage canal and interior levees with a canal invert elevation varying from minus 8.5 ft. to minus 6 ft.; interior levees with a 12-ft. crown, a 9.5-ft. elevation and 1:3 slopes along both sides of the canal; installation of two double barrel 10-ft. diameter corrugated aluminum pipe (CAP) culverts and two double barrel 12-ft. diameter CAP culverts; guard rails; and paving and construction of a perimeter levee with a 20-ft. crown, a 10.2-ft. top elevation and 1:3 slopes.

Palmatier said the work also calls for scraping of approximately 1.5 ft. of STA bottom to rock, flow-way and levee construction with channel invert elevation of 4 ft. NGVD, a bottom width of 320 ft., and 1:3 slopes. It also includes construction of levees with a 12-ft. crown and 1:3 slopes; a stormwater treatment area (STA) levee construction elevation of 13 ft. and 1:3 slopes; construction of a 400-ft. weir 2.5 ft. above grade at the end of the flow-way; two 350 ft. wide weirs at the bottom of the STA at 3.5 ft. and 4 ft. above grade; and construction of a new road along the flow-way.

Additional work will consist of non-hazardous waste and debris removal and disposal within the right of way at the base offer and at a designated area on the project site, he explained.

“All of the work,” Palmatier said, “will be performed in areas subject to frequent illegal dumping. The general contractor will schedule this work as close as possible in advance of the construction work to avoid repeating clean up of the same area.”

Continuing, he said, the “pump station structure is currently being constructed in a cofferdam which was excavated in the wet with a Caterpillar 385 and a Volvo long stick excavator. The excavation will be 27 ft. deep from the top of cofferdam. The bottom of the cofferdam required the installation of 56 rock anchors drilled by Hayward Baker from a barge with a rotary drill.”

After the rock anchors were complete, he continued, divers spent one week installing large plates and nuts on the anchor that would be cast in the six-ft. thick tremie slab. The tremie slab consisted of 1,300 cu. yds. (990 cu m) of 4,500 psi concrete that took six hours to place.

Crews ran into a challenge while constructing the 9 million sq. ft. (836,000 sq m) STA.

“It will be constructed by excavating the interior and constructing the levee with this material. Unfortunately the materials contain elevated levels of copper, DDE, barium, chlordane and atrazine. As a result, testing was conducted to insure our employees’ safety and now we are forced to cap the entire levee with 6 in. of imported clean material. This material will be hauled in by independent trucks, but the quantity and cost is very high. We are currently researching some alternate methods,” he said.

Palmatier also noted that the original contract duration was 13 months, making this a fast-track project, but with delays with FP&L and Bell South in relocating all the overhead lines and the restriction placed on the project’s blaster, the contract duration was increased to 16 months.

In addition, he said, “our team had work through five hurricanes over the past two years. Our current schedule is 50 to 60 hours per week, and Saturdays and Sundays when required.”

Palmatier said key heavy construction equipment used on the project include an LS 218 Link Belt crane, a Gradall 544D-10, four 10-in. hydraulic Holland Pumps, two Caterpillar 385s, a PC 750, two Powerscreen track-mounted crushers, four LS 210 Link Belt excavators, two Caterpillar LGP D6s and two rollers.

The project is scheduled to be completed in February 2007. CEG