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Flushing cleans up its act Very deep, soft and compressible glacial lake deposits were part of the territory in the construction of a massive underground stormwater storage facility in New York.

USA FOCUS

Flushing Meadows in the New York borough of Queens is perhaps best known for the annual US Open 'Grand-Slam' tennis tournament.

Within New York, however, the area has a much less glamorous image. One problem over the last few years has been the falling water quality in Flushing Bay and the creek running into it, which contains, among other unmentionables, 'a significant discharge of floatables'.

Now, $240M is being invested in the Combined Sewer Overflow (CSO) project to improve the water quality. At the heart of the project is a huge stormwater storage tank at Avery Avenue on the eastern edge of Flushing Meadows valley, designed by primary design consultant URS Greiner Woodward-Clyde to control flow to the CSO's largest outfall in Flushing Bay.

New York based geotechnical engineer Mueser Rutledge Consulting Engineers was called in to advise on an earth support system for the structure as well as the temporary support and dewatering regime for the excavation.

The storage facility will cover 2.6ha and be capable of holding 130,000m3 of water (with an additional 70,000m3 stored in-line). It comprises several 'compartments' to hold the water, with screening facilities and a pumping station to control flow. The tank is designed to take heavy runoff for major storm events, typically every 10 years - at present there is no contingency for excess water and flooding does occur. Also, instead of being discharged directly into the bay, water will be treated before it goes to the outfall.

The excavation for the facility is 150m by 150m and for the most part 10m deep, with the excavation for the pumping station down to 20m deep.

The main challenge, says Mueser Rutledge Consulting Engineers partner Alfred Brand, was to design and build such a large structure below the water table in the soft compressible soils of Flushing Meadows.

The entire area is underlain by fill of varying thickness over an organic clay and silt layer up to 18m thick. This overlies glacial outwash sand which is underlain by stiff clay. Groundwater is only 3m below ground level.

Dewatering to allow excavation was not an option, Brand explains, mainly because of the induced settlements of the organic deposits and also because there were concerns that any extensive drawdown would cause movement of contaminants in the area.

The contract was flexible enough to allow tenderers to suggest different designs. The contractor, a joint venture of EE Cruz, NAB and Frontier, designed the excavation support system as a permanent cut-off, using piled walls keying into the impermeable clay at between 18m and 21m below ground level, with ground anchors installed for additional support.

The walls are formed by 1.3m diameter bored concrete piles at 1.5m centres, reinforced with steel I-beams. The gaps between the piles are sealed with jet grout columns. Brand explains that because the walls must provide support for the excavation and act as the water cut off, the concrete in the piles had to be strong, which meant that they would be difficult to secant, so jet grouting was used instead.

When work started in February 1998, the first job was to move an existing effluent pipeline crossing the site to the western edge of the excavation. Even so, Brand says, the position of the pipeline meant that design of the western wall was more complicated because tie backs could not be used and deflection criteria were tighter. Here, raker braces comprising steel pipe sections are being used to support the wall. Jet grouting was carried out in October and November last year.

Multi-strand ground anchors are used to tie back the wall. They an average of 23m long, with a bond length of between 9m and 12m and have five to seven strands. Installed at between 20degrees and 25degrees. They are all pressure grouted and 95% have been proof tested, with the remaining 5% undergoing performance cyclical testing load. There are four rows of anchors. Upper rows have a tieback at alternate piles; lower rows have tie backs corresponding to each pile.

While the combination of piled walls and jet grouting has met deflection criteria (monitored by inclinometers and strain gauges installed within some of the piles) and been largely successful in keeping the groundwater at bay, there has been a problem in the north east corner of the excavation, where leakage has occurred. Brand says that because of the high groundwater pressure, it has been difficult to go back down with jet grouting to repair the seal. Dewatering wells have been put down in this area, the location of the pump house, in an effort to ease this process. 'It is not an entire cut-off but still very tight, just 2.7m3 of water ingress a day is allowed,' Brand says.

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