Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | SEPTEMBER 2014 | 33 In the know Technically speaking: BS5911 Part 3:2014 T he latest changes to BS5911 Part 3:2014 Specification for unrein- forced and reinforced concrete manholes and soakaways, means that specifiers, contractors and asset own- ers will be able to take full advantage of the benefits gained from using pre-cast circular manholes for the first time along with some other significant changes. Off-site manufactured concrete manhole systems offer some big advantages compared with traditional in-situ solutions: they are quicker and cheaper to install, create less waste, have up to 43% lower carbon footprint and crucially they help improve safety on site. Until now the British Stand- ard has not included a specification for the precast base unit. That has changed with the latest revisions to BS5911. Circling the square The British Standard for the specification for manholes and soakaways has been updated. Stuart Crisp explains the most significant changes and outlines how they will impact users The revised standard, which was updated in March 2014, now covers the specification of precast base units, including provision for benching ar- rangements. Clause 5.4.4 notes "The provisions for benching arrangements shall be in accordance with BS EN 752: 2008". This includes a requirement that vertical benching to the main channel shall be to a height not less than the soffit of the outlet, as shown in figure 1. Another significant change to the standard is that Table 4 introduces two new precast rectangular (in this case square) manholes with preferred nominal sizes 1000mm x 1000mm and 1250mm x 1250mm Care should be taken to ensure the correct selection of functional equivalent circular and rectangular manholes. Two publications: BS EN 752: 2008 Drain and Sewer Systems Outside Buildings and Sewers for Adoption 7th edition, provide guid- ance on the minimum nominal sizes for circular and rectangular (includ- ing square) manholes based on the diameter of connecting pipework. It also helps users to identify circular and rectangular manholes deemed to perform as 'functionally equivalent'. Regardless of the shape, if a manhole includes a ladder or step irons then users need to refer to the Health and Safety Executive's (HSE) confined spaces regulations. These recommend a minimum 900mm clear- ance for personnel between the steps and the back of the shaž. Generally ladders and steps project more than 100mm. On that basis, a rectangular manhole 1000mm x 1000mm or a circular manhole of 1000mm diameter are not expected to comply with the HSE guidance. Instead contractors will need to use rectangular man- holes with preferred nominal size at least 1250mm x 1250mm or a circular manhole with a 1200mm diameter or greater for ladders, or a minimum of 1050mm diameter for steps. Users also need to be aware that the size of the manhole is critical if the sewer is to be adopted by water and sewerage companies. If a sewer is to be adopted then 1050mm diameter and 1000mm x 1000mm rectangular manholes have a cross section too small to comply with the nominal minimum internal dimensions. With the inclusion of square man- holes in the specification for the first time, users also need to be aware that the shape of the manhole can influ- ence its structural behaviour and the maximum depth at which the different shapes can be installed. Structurally, concrete is very strong when it is in compression. A circular manhole exploits this fact because its uniform, circular shape ensures that ground and hydrostatic pres- sure is evenly distributed around the circular structure. This places the entire manhole-ring in compression, which enables circular manholes to be installed to a far greater depth than for other shapes of chamber of the same strength class. For rectangular manholes, ground pressure can be an issue because stresses are created on the manhole's flat faces and at the corners, which could put the concrete under ten- sion. Annex A of the revised standard provides a mathematical calculation method to help users determine the maximum permissible installation Stuart CriSp BuSinESS dEvEloPMEnT diREcToR, concRETE PiPElinE SySTEMS ASSociATion Figure 1. Benching cutaway

• Cover