Notice: This blog piece was created prior to the formation of the British Precast Drainage Association.

Posted by & filed under Pipes & Manholes.

Traditionally, manhole construction has required the base to be formed in-situ where building of the channel, connections and benching occurs in the trench, often in confined spaces that can present a safety hazard for operatives frequently encountering wet and unpleasant conditions.

The process can take around 40 hours per manhole to complete and is not always successful, particularly in terms of water-tightness, where egress of waste water can lead to contaminated groundwater and where ingress can contribute to overloaded sewer networks plus increased energy usage, carbon emissions and running costs at treatment works and pumping stations.

These issues can be avoided by using the new circular precast concrete manhole base systems, which are available from all CPSA members. They comprise a circular precast concrete base unit, complete with channel and benching with predetermined/bespoke combinations of inlet(s) and outlet and new chamber rings with watertight flexible joints and no lifting holes, thereby removing points of possible leakage.

Base units and chamber rings are made with thicker, stronger walls. The robust design means that the requirement for a concrete surround is eliminated in most cases, considerably reducing the total volume of concrete required and resulting in up to 43% lower(*) embodied carbon for the installation. The excavation is backfilled sooner and there is less need for men to work in confined spaces. This reduces time spent in the excavation, further improving site safety and reducing installation time and costs

Benefits of the Precast Manhole
• Improved safety on site
• Reduced construction time; c50% (min 20%)
• Decreased installation cost; c30% (min 15%)
• Superior quality; factory made in a controlled environment
• Watertight; reduced leakage = reduced energy consumption, operational carbon emissions and running costs at treatment works and pumping stations
• Typically, no concrete backfill/surround required
• Reduced embodied carbon; up to 43% per manhole

(*) CPSA Manhole Systems Comparison Report

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