Keller has provided excavation support for a US$439M (£399M) sewage treatment plant project on the coast of Long Island, New York, in the US.
The South Shore Water Reclamation Facility is undergoing a major upgrade to redirect treated effluent away from the Western Bays, where nitrogen rich effluent has had a detrimental effect on the water quality.
In a design-build partnership with Western Bays Constructors, Keller has provided excavation support for 14 shafts for the the tunnel boring machine (TBM) working on the new pipeline.
Each shaft is up to 21m deep and has an internal diameter of up to 9m. The project also included excavation support for a diversion structure and a wet well – a tank that holds effluent until it reaches a certain level, after which it's pumped out.
To select the right techniques for each location Keller spent nine months working with the client before construction. Working close to the ocean with a water table only a few feet below grade, the design had to be optimised to meet stringent shaft wall strength requirements.
For each of the shafts Keller opted for cutter soil mixing (CSM), an innovative and relatively new technique for this area.
This specialised deep mixing technique uses a machine with rotating cutting wheels. As these submerge, Keller pumped slurry to create a series of strengthened panels that form the shaft walls, ready for excavation.
Half of the shafts required a single layer of panels, and the other required a double ring of panels to support the jacking frames used for the TBM launch.
"The choice of CSM really brought value to the table," said Keller senior project manager James Myers. "From a technical perspective, it creates fewer panel joints and more overlap than secant piles, which reduces the risk of water inflow.
“From a production standpoint, it's also faster, more cost-effective and saves wear on the TBM. With CSM, the TBM machine must chew through less than 1,000psi of soilcrete; with piles, it can be as much as 5,000psi and 3ft (0.9m) thick.”
To achieve watertightness and the required levels of strength, which depended on the depth, Keller focused on ensuring high levels of quality control. To do this, the CSM rig used a sophisticated data acquisition system, generating detailed reports that assured the crew and the client.
However, the CSM was unsuitable for some shaft locations. The heavy organic peats and clay soils in some areas meant the CSM would struggle to construct panels with the necessary strength. For these shafts, Keller switched to bored piles.
The scope included over 400 cased continuous flight auger (CFA) piles, 750mm diameter drilled to 18m, plus around 40 traditional bored secant piles with 850mm diameters where extra depth was needed. The piling team also installed around 60 micropiles.
As well as the piles and CSM, approximately 450 jet grouting columns were constructed to form water-control bottom seals in the shafts and wet well.
Keller finished work in June after 12 months.
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