A total of 150 manholes have been installed in the first phase of a 900-house development in the Midlands. As 97% of the manholes were within the 1500mm-diameter range, contractor Newline Civils decided that Watertight precast concrete systems would be ideal for the majority of installations and an order was agreed with Stanton Bonna.
Once Newline’s project drawings had been prepared, the precast systems went into manufacture and the time from sign-off of
the designs to the systems appearing was two-to-three weeks. Manufacture and delivery were scheduled to ensure continual
production and efficient installation.
Newline carried out the foul and stormwater drainage works between April and October 2018, installing some 4,000m of pipework for the 200 houses being built in phase one.
The logistics for Stanton Bonna’s Watertight precast manholes is much simpler than onsite casting, where delivery of clay channels
coming from one manufacturer and concrete from another have to be managed. For the Lower Barden project, four or five Watertight manhole systems were delivered per load, in advance, including the chamber rings and cover slabs.
“The main benefit of precast concrete manhole systems is the speed of installation,” said John Fowler contracts manager of Newline Civils. “Delivery of ready-mixed concrete for casting manholes onsite can be unreliable, with workers sometimes left waiting around for a couple of hours. “Once the manhole is cast, you then have to wait until the next day for it to cure before pushing the next pipe into it. With precast manholes, all the materials are delivered to site in advance, you can lay a pipe-run and fit a manhole and go again.”
The installation process involves laying a sewer run and excavating the manhole to the required depth. A gravel bed is laid and the Watertight precast base is placed on top and fitted to the end of the pipe.
The Watertight precast manhole rings are lifted into place, raising the chamber to surface level. The Watertight concrete cover slab is fitted, which reduces the aperture to the diameter of the manhole lid, which is all that will be visible once the landscape is surfaced. The void around the manhole is then backfilled with stone and the process is repeated for the next pipe length.
Depending on the depths and distance between manholes, a three to-four person team can install approximately two precast manhole systems, with connecting pipework, in a day. This is significantly faster than casting onsite, where the concrete surround has to be built separately, making it necessary to wait while the concrete cures.
An additional advantage is that the factory produced units are already smooth and do not require a further skim where the pipes connect. This carries a health and safety advantage too.
Newline says, “Where manholes are cast onsite, workers have to re-enter the manhole to apply a skim of granolithic concrete to smooth out the rough benched concrete. Any confined space working carries a health and safety risk, so where this can be minimised, that is very welcome.”
He adds that the overall quality of precast units should be better as they are produced in factory-controlled conditions and installed more efficiently, with less risk of impact from adverse weather conditions. Variable ground conditions and the presence of groundwater also have less impact on the installation, with no need for extra groundwater pumping.
Daniel Cross, national sales manager, Stanton Bonna said, “Our Watertight precast concrete manholes systems are ideal for the vast majority of installations on housing projects like Lower Bardon. They offer clear logistical advantages over casting onsite and enable foul and storm water infrastructure to be installed more safely, significantly quicker and with a higher quality product.”