Manholes

Guidance for the minimum chamber diameters required for various sizes of sewer pipes entering the manhole is given in the Technical Design Guide. When a manhole is sited on a curve, or where additional pipes enter at the sides, a larger size may be required.

Traditional tongue and groove manholes may require a concrete surround complying the 'Design & Construction Guidance’ published by Water UK depending on the Adopting Authority. 

Modern sealed manhole systems with a precast base and wide wall chamber rings do not require an in-situ concrete backfill when being used under normal installation conditions. In these instances, excavated soil may be used as backfill. The use of an in-situ concrete surround may be necessary where the structure is not loaded equally i.e. located in the side of an embankment, or to reduce flotation risk due to high groundwater level. The same applies for manholes constructed with an in-situ concrete base and traditional thickness (i.e. not wide wall) concrete chamber rings, although a concrete surround may be a requirement if the sewer is to be adopted.

Standard CE and UKCA marked cover slabs are tested to be suitable for the heaviest loads permissible on UK roads. Units up to 1250mm diameter are tested to withstand a minimum crushing load of 300kN in accordance with BS EN 1917. Units greater than 1250mm diameter are tested with loads derived form BS EN 1991-2 Load Model 1 in accordance with BS5911-3. 

Yes soakaway rings are made to the same product standards, with the exclusion of the ‘watertightness’ requirement.

Concrete drainage products manufactured in accordance with BS EN 1917 are suitable for slightly aggressive chemical conditions. In the UK, some soils are more aggressive in nature. As a safeguard, the provision of a concrete suitable for ACEC AC-4 conditions as described in Building Research Establishment Special Digest 1 2005 is specified.

The preferred method to achieve AC-4 for a 100 year intended working life is the use of a DC-4 concrete with surface carbonation (i.e. precast concrete) and low permeability concrete to Clause 4.2. of BS EN 1917 without the need for additional protective measures. The provisions in the relevant product Standards are adequate for 100 years working life for the vast majority of discharges in normal conditions of use. However, further consideration should be given to suitable additional protective measures in the following cases:-

- where a manhole within the system is liable to carry untreated or corrosive trade effluents
- a rising main discharge
- septic sewage
- in situations without adequate ventilation