Bonacci Infrastructure delivered the structural permanent works design for the Murray River Offtake Pump Station (wet well caisson) which formed part of the Wentworth to Broken Hill Pipeline project. Bonacci Infrastructure also provided complete temporary works design services as part of the project, including designs for the:
- Sheet pile cofferdam which facilitated installation of the wet well and associated mechanical infrastructure;
- Sheet pile driving frame and the methodology used to install the cofferdam. The driving frame was suspended from previously driven sheet piles to progress the cofferdam into the river. The frame was also used to suspend pipework during the mechanical installation;
- Crane platforms, including geotextile soil strengthening and slope stability assessments. Civil design drawings and earthworks quantities were produced as part of this work; and
- Access platforms and lifting checks.
The wet well itself was a 10.4 metre deep x 6 metre diameter segmental precast concrete structure which was sunk in stages using a jacking frame and a 60 tonne precast concrete ballast ring. The precast ballast ring was integrated into the permanent works to assist in resisting hydrostatic uplift forces on the base of the well. The lowest precast segment of the well incorporated a cutting shoe which aided the sinking operation.
The proximity of the well to the river and high groundwater flow rates made it impossible to dewater inside the well prior to construction of the floor slab. Construction of the mass concrete floor of the caisson formed a particularly high-risk item. The hydrostatic pressure at the underside of the floor slab was in excess of 110 kilopascals which required careful design to resist uplift pressures and to maintain internal water-tightness. Connection between the floor slab and the precast concrete walls also could only be performed using encapsulated diving techniques where visibility is extremely limited and the quality of construction workmanship is difficult to guarantee.
Bonacci Infrastructure’s solution was to use a temporary precast concrete floor which was lowered into the caisson and anchored to the inside of the walls by encapsulated divers. The precast floor incorporated a network of pressure pipes which were used to pump grout into the space beneath the floor. The grout worked in tandem with an innovative joint detail to form a complete seal around the perimeter of the precast. Water displaced during the grouting operation was expelled through bleed pipes to prevent the development of high fluid pressures.
The wet well was completely dewatered following installation of the temporary floor which permitted construction of the permanent floor (including tie-ins with the precast concrete walls) to be undertaken in the dry. This method proved to be far simpler than the normal industry practice of constructing the floor underwater.
Bonacci Infrastructure’s innovative approach to the wet well design delivered significant improvements to the overall quality and reliability of the permanent structure.
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