When inspection or maintenance of buried wastewater drainage systems is required a manhole is specified. They are most often 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 detail some advice for specifying manhole systems.
- Consider a precast concrete solution
It can take up to 40 hours to construct a manhole using traditional techniques. Construction in-situ like this can also involve working in wet and difficult confined spaces. This is where off-site manufactured precast concrete manhole base systems offer some big advantages. They are safer, quicker and cheaper to install. In addition, they are watertight, of consistently high quality, create less waste on site and have a lower carbon footprint.
- Each manhole should be correctly constructed for each specific location
Due to the precast concrete base being factory manufactured to a specific configuration, when the components arrive on site they can quickly, simply and safely be placed in position. This negates 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. Construction costs can also be reduced by 15 to 30 per cent, particularly when manholes are installed without a concrete or granular surround.
- 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 details the rules 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. These 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.
- Will you choose a round or square manhole chamber?
One of the biggest advantages 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. This enables circular precast manholes to be installed to a far greater depth than precast manholes of ‘equivalent strength’ with flat sides and corners.
- 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. These recommend a 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 no less than 1,050mm.
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.
- 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. 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.
- Save on carbon emissions
Factory manufactured precast concrete manhole base systems also offer 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 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.
- 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.
For more information about concrete manholes visit the British Precast Drainage Association website.