The process for building new concrete structures typically involves several construction techniques, this includes cast in-situ and prefabricated concrete structures. Once permits are in place, traffic management and services located and protected, sheet piles installed using a 30t excavator and vibro hammer, this facilitates a safe and effective environment to enable the structure to be built. To control groundwater and maintain a dry excavation, well-point dewatering is installed around the perimeter of the excavation. To control incoming flows from the live pipelines, a flow bypass system is installed by cutting each end of the pipeline and installing a wye and with bypass pipework to divert flows around the existing structure. Bypass setups require careful level control to ensure flows are managed without the need for pumping. As the diverted flows will continue to operate on similar grades to the existing pipelines and flow under gravity there is no requirement to use bypass pumps, removing the risk associated with pump failure.
Cast In-situ Concrete Structures
The cast in-situ technique involves building the concrete structure within the excavation. This technique involves placement of formwork and steel reinforcing followed by pouring of the concrete base. To build the structure walls, formwork shuttering is installed with steel reinforcing bar placed within the formwork wall cavities, with inlet and outlet connections boxed out. Concrete is poured to form the structure walls; formwork shuttering is removed once the concrete has been cured.
Pre-fabricated Concrete Manhole Structures
With existing pipelines disconnected and bypass pipework installed, the existing manhole is removed and disposed of. Bedding is placed at the correct level for the new manhole and compacted to specification. The new manhole base complete with riser is installed and holes cored in at the correct levels for the new connections to existing pipelines. Mechanical fittings are used to connect the new manhole inlets and outlets to the existing pipes. Whilst the bypass wye remains in operation, concrete haunching and plastering of the inside of the manhole is completed along with the concrete corbels to the exterior of the manhole outlets. Pipe bungs and bypass connecting pipework removed with the wyes capped and sealed and left in place for future removal. Excavation is then backfilled, compacted and surface restored.