Water and Effluent Treatment Magazine
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Caption if needed sdkvjb sdkvjnsdlk dvsdvdvsdvsdvsdv dv skjbdv lskdjvbsdkljvbsdkv M anholes 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 con- crete manhole solution The construction of a manhole base in-situ using traditional techniques can take up to 40 hours and can o•en involve per- sonnel working in wet and diffi- cult 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 water- tight, of consistently high qual- ity, create less waste on site and have a lower carbon footprint. ONSITE MANHOLES 14 WET NEWS JUNE 2018 | wwtonline.co.uk CPM 1,200mm Perfect Manhole system Top tips for speciing 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 3. Manholes should be compliant with appropri- ate Standards The technical requirements for reinforced and unreinforced manholes are described in the European Standard BS EN1917:2002 Concrete manholes and inspection chambers, unre- inforced, steel fibre and rein- forced and the British Standard BS5911 Part3: 2010+A1:2014 Specification for unreinforced and reinforced soakaways. This British Standard is refer- enced 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 publica- tions throughout the UK, which require developers and install- ers to build drainage systems to a minimum standard and qual- ity for adoption by the relevant 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 pre- determined 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 sha• – 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 construc- tion can be avoided and the risk of settlement and failure of the cover at the road surface is sig- nificantly 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 man- hole base system could save up to 50 per cent on installation time and reduce construction costs by 15 to 30 per cent, par- ticularly when manholes are installed without a concrete or granular surround. water company. The advantage of using a BS-compliant and Kitemarked precast concrete solution sup- plied by a member of the BPDA is that users can be sure that the product will comply with all nec- essary technical requirements. 4. Round or square manhole chamber? A big advantage of using a con- crete 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 pres- sure is evenly distributed around the circular sha•, plac- ing the entire manhole ring in compression. This enables cir- cular 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 rec- ommend 900mm clearance between the ladder/steps and the back of the sha•. 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 mini- mal 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 pipe- work. For example, within Sew- ers for Adoption, pipes up to 375mm diameter require a 1,200mm diameter chamber;