Advances in production techniques for precast manholes offer multiple advantages for the construction sector. Innovations championed by BPDA
members mean manhole installation can be completed in about an hour, with a much higher success rate when compared with traditional techniques.
Precast manhole systems are easier to install, but they also improve onsite safety and raise the bar on quality and performance as well as lowering costs and reducing waste to landfill. To help contractors deliver best practice on-site, BPDA has now produced a handy guide to installation.
The Pocket Guide to Installing Concrete Manholes can be downloaded from the BPDA website as a PDF or is available as a printed document. It lists seven simple steps to successful precast manhole installation (with minimum 125mm wall thickness) and the full version can be found at https://www.precastdrainage.co.uk/uploads/downloads/manhole_guide_interactive.pdf
Precast concrete manhole systems are suitable for a wide-range of pipe connections and can be retrofitted without complete replacement of the chamber. The systems comprise a precast concrete base unit with channel and benching and predetermined combinations of flexible and watertight inlet(s) and outlet. Base units and chamber rings are made with thick strong walls and lifting points, eliminating the need for a concrete or granular surround unless specifically required by the client. High-performance seals and extra-thick chamber walls ensure long-term watertightness and durability. The excavation is backfilled sooner than with traditional techniques, minimising the health and safety risks associated with open excavations, and there is less need for work in confined spaces, which also lessens risk to workers. By reducing project time, overall costs are also brought down.
Sourcing constituent parts from local suppliers and a rise in the use of recycled materials keeps embodied carbon impact to a minimum. Production techniques for precast manhole systems continue to advance and the use of modern logistics ensures excellent and consistent product quality and reliable service.
In summary the seven steps are:
Safety must always be the first priority for any construction project and all site activities must be preceded by an appropriate risk assessment. Typical activities include vehicle offloading, movement of components, excavation, backfilling and the lifting and positioning of components.
Excavate a trench of appropriate dimensions to accommodate the manhole structure. The trench must allow sufficient working space outside the chamber for access and backfilling to the required specification, taking into account the ground conditions, depth of excavation and any other relevant factors. The heights of the manhole components supplied by the manufacturers are nominal, so it is beneficial to measure the units prior to installation in order to assist with obtaining the required height of the completed chamber.
- Installing the precast manhole base
Prior to lowering into the trench, the precast base unit may be pre-fitted with a lubricated outlet if required. A plastomeric sealing strip/elastomeric seal is used to form a waterproof joint between units. It may be fitted before lifting into position or after
each unit has been individually placed. Concrete to concrete contact between units must be avoided.
Place the base unit onto the prepared granular bed and mate the stub pipe with the installed outlet pipe. Check the base position for alignment, level and inverts. Note that precast bases have an inbuilt fall across the main channel and can be installed level.
- Fitting the chamber rings
Make sure that the joints are clean and free from foreign objects before fitting the next chamber ring unit. The plastomeric sealing strip/elastomeric seal should already be in place on the installed unit and ready to receive the next chamber ring unit. Repeat with further ring units until the chamber has been constructed to the required height. Ensure that the steps are correctly aligned.
- Fitting the cover slab
Place the cover slab directly on the last chamber ring with the access opening lined-up with the steps. Apply slight pressure onto the cover slab using suitable protection, such as timber, to seal the chamber.
When using wide-wall precast concrete manhole chamber rings, the excavated soil can be returned as backfill unless an alternative arrangement is specified by the client. Compact the backfill soil as specified in the design.
- Operation and maintenance
Precast concrete manhole base systems are strong and durable and eliminate the risk of inconsistent quality from site-based operations. They are designed to remain watertight and maintain their structural integrity for over 120 years.
Fact Zone: Concrete Manholes
Up until the 2000s concrete manhole construction has required manhole bases to be constructed onsite from ready-mixed concrete. This required the channels, connections and benching to be constructed in confined spaces with works often carried out in wet and hostile conditions. Additional external contractors were required to supply and pour the concrete and the process for each installation would take several hours due to routine logistical and operational challenges. Furthermore, construction was not always successful. In 2011, a revision of Part 3 of BS 5911, the main standard for precast concrete manholes, introduced a new type of precast manhole system. This comprised a factory-made precast base with elastomeric or plastomeric seals on all joints and connections to ensure permanent watertightness.