OTBe were engaged by FLO JV to develop an alternative construction methodology for the Falconbrook Pumping Station Interception Chamber as they were concerned with the buildability and programme impacts of the current proposed design. The proposed works need to overcome several constraints as summarised below:

• The interception chamber would have to be constructed around the operation LL1RS.
• Flows within the LL1RS were too great to enable a flume or over pumping.
• The works had to be carried out in close proximity (<1m) from a number of other operational Thames Water assets. The site was constraints in terms of working area and access.
• A number of existing obstruction and temporary works (sheet piles) were present in the ground, from the construction of the LL1RS.
• Works were taking place in an operational Thames Water Pumping Station.
• The LL!RS was located within the water bearing River Terrace Deposits, raising questions of how the proposed interception chamber could be constructed within a dry box and maintaining stability of the ground around and below the LL1RS.
• No support could be relied on from the permanent works in the temporary case.

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OTB in conjunction with FLO went back to the drawing board to come up with its proposed solution. The biggest single risk to the project was the connection of the interception chamber to the LL1RS (due to been located within the water bearing River Terrace Deposits and that this critical bit of Thames Water infrastructure had to remain operational. OTB's innovative solution was to split the excavation works of the interception chamber into three separate cells. This would isolate the more complex works of connecting the LL1RS to the interception chamber from the remaining works associated with interception chamber. The cells (1-3) were constructed using 750mm diameter hard soft secant piled walls. The secant piled wall provided a water cut off in cells 1 and 3 to enable to the excavation to be undertaken to its final level of 18m bgl. The secant piles in cells and 1 and 3 were supported by a perimeter R.C. capping beam and two levels of temporary steel frames.

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Completion of the Falconbrook interception chamber using OTB Engineering’s innovative design included:

• Development of the overall construction sequence for the chamber whilst maintaining flows through the existing sewer.
• Detailed design of the secant piles walls.
• Design of the temporary bearing piles, to support the cascade structures lab as the contractor excavated underneath it.
• Design of the temporary steel propping to the piles.
• Design of the temporary R.C. capping beam and upstand wall to support the piled wall.
• Design of the ground support around the existing sewer (jet grouting and timber ground support, supported off plunge column piles).
• Design of the timber headings beneath the cascade slab structure.

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