Top Tips for specifying concrete manholes
Stuart Crisp, business development director for the British Precast Drainage Association, gives his top tips for specifying manhole systems, and outlines the benefits of opting for a precast solution
Manholes provide access for the inspection and maintenance of buried wastewater drainage systems. In most situations, they are built at the point where one pipeline connects to another, where a pipeline changes size, direction or gradient and at a spacing that enables equipment to be used effectively. Here we present some top tips for specifying manhole systems.
1. Consider a precast concrete manhole solution
The construction of a manhole base in-situ using traditional techniques can take up to 40 hours and can often involve personnel working in wet and difficult confined spaces. Off-site manufactured precast concrete manhole base systems offer some big advantages compared to traditional in-situ solutions: they are safer, quicker and cheaper to install, are watertight, of consistently high quality, create less waste on site and have a lower carbon footprint. In fact, construction time from precast base to cover slab can be as little as one hour.
A precast concrete manhole base system comprises a bespoke precast concrete base unit supplied complete with main channel, benching and sealed joint connections for predetermined combinations of inlet(s) and outlet. A series of interlocking chamber rings are placed on top of the base unit to create the manhole chamber shaft – the ‘wide-wall’ rings incorporate profiled joints to accommodate flexible jointing material to form a watertight seal between the elements. The number of rings installed will depend on the depth of the pipeline/manhole base below ground.
The chamber is sealed by a precast concrete cover slab, which sits directly onto the chamber walls and provides a platform to distribute traffic loads from the surface through the walls of the chamber. There is no need to isolate the chamber from traffic loads, which means that longer, more complex and costly construction can be avoided and the risk of settlement and failure of the cover at the road surface is significantly reduced. The cover slab also provides a safe entry point into the chamber for inspection and maintenance.
2. Each manhole should be correctly constructed for each specific location
Because the precast concrete base is factory manufactured to a specific configuration, when the components arrive on site they can quickly, simply and safely be placed in position without the need for lengthy site-based operations.
The BPDA estimate that a contractor using a precast manhole base system could save up to 50 per cent on installation time and reduce construction costs by 15 to 30 per cent, particularly when manholes are installed without a concrete or granular surround.
3. Manholes should be compliant with appropriate Standards
The technical requirements for reinforced and unreinforced manholes are described in the European Standard BS EN1917:2002 Concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced and the British Standard BS5911 Part3: 2010+A1:2014 Specification for unreinforced and reinforced soakaways.
This British Standard is referenced in Approved Document H of the Building Regulations, which deals with drainage and waste disposal. This document sets out the rules with which construction of drainage and waste disposal systems must comply.
The BS is also referenced in Sewers for Adoption and the partner water utility publications throughout the UK, which require developers and installers to build drainage systems to a minimum standard and quality for adoption by the relevant water company.
The advantage of using a BS-compliant and Kitemarked precast concrete solution supplied by a member of the BPDA is that users can be sure that the product will comply with all necessary technical requirements.
4. Round or square manhole chamber?
A big advantage of using a concrete manhole is that concrete is very strong in compression. A circular precast concrete manhole exploits this trait – its circular shape ensures that ground and hydrostatic pressure is evenly distributed around the circular shaft, placing the entire manhole ring in compression. This enables circular precast manholes to be installed to a far greater depth than precast manholes of ‘equivalent strength’ with flat sides and corners.
5. Consider the need for access
Whatever the shape of the manhole, if it includes a ladder or step irons for access then users need to be aware that the Health and Safety Executive’s confined spaces regulations recommend 900mm clearance between the ladder/steps and the back of the shaft.
Ladders and steps usually protrude by at least 100mm, so for compliance users will need to consider a circular manhole with a diameter not less than 1,050mm.
Further, if the sewer is to be adopted, users also need to be aware that 1,050mm diameter manholes have a cross-section that is too small to comply with the Sewers for Adoption minimal nominal internal dimension requirements and the chamber will need to be up-sized to 1,200mm.
The minimum diameter of the chamber is also determined by the size of connecting pipework. For example, within Sewers for Adoption, pipes up to 375mm diameter require a 1,200mm diameter chamber; pipes between 375mm and 450mm diameter require a 1,350mm diameter chamber, pipes between 500mm and 700mm diameter require a 1,500mm diameter chamber; and pipes between 750mm and 900mm diameter require a 1,800mm diameter chamber.
Precast manhole systems offer great versatility and are designed to accommodate all standard pipe materials and sizes. Existing precast manholes can even be retrofitted with new connections from future development without the need to replace the entire manhole.
6. Minimise the need for granular backfill
The robust design and wide chamber walls of the factory manufactured precast concrete manhole base system means that the use of granular or concrete backfill can be eliminated, unless it is specifically required by the client.
Using a precast concrete manhole base system enables the construction to be completed quicker. The excavation can be backfilled sooner using the soil that was excavated, thus making the installation safer and faster whilst simultaneously reducing the cost of backfill and disposal of the excavated material.
7. Save on carbon emissions
A big advantage of the factory manufactured precast concrete manhole base system is that it offers a significant saving in embodied carbon for the installation. The BPDA estimates that carbon savings could be as much as 43 per cent per manhole compared with traditional in-situ construction. A notable part of this carbon saving is the avoidance of concrete or granular backfill and the use of excavated soil.
The environmental credentials of a precast concrete solution are further enhanced by the responsible local sourcing of raw materials and the appropriate use of recycled materials.
This ensures cradle-to-site miles are low and helps keep the product’s environmental impacts to a minimum.
The BPDA estimates that 15,000 tonnes of CO2 equivalent of embodied carbon would be saved annually if all manholes manufactured by the Association’s members changed from in-situ construction to the circular precast base system.
8. Consider durability
The precast manhole base system is manufactured under factory conditions by quality assured processes. This ensures the finish and quality is to a higher and to a more consistent standard than could have been achieved on site. The combination of durable precast concrete and quality controlled offsite manufacturing process will ensure that a precast manhole system has a long service life.
Their self-weight is an advantage in situations with high groundwater or in flooding where they can have a natural resistance to floatation and where additional anchorage is not required.
A major benefit of concrete drainage systems is that they have a long, proven service life, typically in excess of 120 years. This helps to keep down whole life and carbon costs.
9. Listen to what other users have to say
Natta Building opted to use the precast manhole base system from BDPA member Marshalls CPM to save time on site at its redevelopment of New Covent Garden Plaza project. The scheme forms part of the regeneration of Nine Elms in South West London. “We chose to use the Marshalls CPM Perfect Manhole system as the integral cost-in, high performance seals maintain water tightness and decrease the time taken to install the manholes,” James Hidderd of Natta Building said. “Because the bespoke inlet and outlet benching is made off-site, the manhole construction is closed quicker on site.”
For more information on precast concrete manholes, go to www.precastdrainage.co.uk
-This article appeared in the June 2018 issue of WET News magazine.
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