Drainage below ground

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5.3.7Design to avoid damage and blockages

Drainage systems shall minimise the risk of damage and blockage. Issues to be taken into account include:

  1. ground stability
  2. pipe runs
  3. pipe sizes
  4. gradients
  5. access and connections
  6. drainage covers and gully grids
  7. ground water
  8. flooding.

Ground stability

Proper allowance should be made for ground movement.

Pipes should have flexible joints and additional precautions taken to prevent leakage where required. Where ground movement could be significant, for example in made-up ground or clay soils, the following issues should be taken into account:

  • the use of flexible pipes and flexible joints
  • design gradients that are steeper than the minimum requirements for flow rate and pipe size
  • a support system designed by an engineer in accordance with Technical Requirement R5
  • conditions where ground movement is likely to adversely affect the drain.

In non-uniform or saturated soils where movement at the trench bottom can be expected, soft spots should be removed and replaced with suitable material. Immediately after excavation, the protective blinding should be placed in the trench bottom.

Pipe runs

Pipe runs should be designed to maintain a self-cleansing velocity (0.7 m/s). They should be as straight as practicable with minimal changes of direction. Bends should only occur in, or next to, inspection chambers and manhole covers. Curves should be slight so that blocked pipes can be cleared.

Pipe sizes

Pipe sizes should be designed for the maximum peak load in accordance with BS EN 752.

Ground water drains and soakaways should be designed with sufficient capacity for normal weather conditions.


Design gradients should:

  • be as even as practicable
  • where flows are less than 1.0L/second, gradients for 100mm diameter pipes should not be flatter than 1:40
  • where peak flows exceed 1.0L/second, the gradients in Table 2 may be used:

Table 2: Minimum gradients

Pipe diameter (mm) Minimum gradient

Where peak flows are greater than 1.0L/second, 100mm pipes should serve a minimum of one WC and 150mm pipes should serve a minimum of five.

Access and connections

To ensure that every length of drain can be rodded, the design should include appropriately located access points, such as:

  • rodding eyes
  • access chambers
  • inspection chambers
  • manholes.

All access points should be located as shown in the design information and should:

  • be accessible for rodding and cleaning
  • not cross boundaries or kerb lines.

Inspection chambers and manholes should:

  • be of sufficient size for the depth of invert, and
  • the invert depth for the fitting or chamber should not exceeded those given in Table 3.

Table 3: Minimum dimensions for access fittings and chambers


1 The clear opening may be reduced by 20mm in order to provide further support for the cover and frame.
2 Drains up to 150mm.
3 A larger clear opening cover may be used in conjunction with restricted access. The size is restricted for health and safety reasons to deter entry.

Table 4: Minimum dimension for manholes


1 Larger sizes may be required for manholes on bends or where there are junctions.
2 May be reduced to 600 x 600 where required by highway loading restrictions and subject to a safe system of work being specified.
3 Not applicable due to working space needed.
4 Minimum height of chamber in shafted manhole 2m from benching to underside of reducing slab.
5 Minimum clear space between ladder or steps and the opposite face of the shaft should be approximately 900mm.
6 Winch only; no steps or ladders, permanent or removable.
7 The minimum size of any manhole serving a sewer, i.e. any drain serving more than one home, should be 1200mm x 675mm rectangular or 1200mm diameter.
8 Tables 3 & 4 have been reproduced from Tables 11 and 12 of Approved Document H by permission of HMSO.

Inspection chambers and manholes may be one of the following types:

  • Open, half-round section channel with suitable benching.
  • Closed access, where covers have to be removed to gain access to the pipe.

Side branches to inspection chambers and manholes should discharge into the main channel no higher than half pipe level. Connections should be made obliquely in the direction of flow.

Traditional construction

The minimum specification for traditional manholes and inspection chambers is as follows:

Clay bricks for manholes should comply with BS EN 771 and:

  • be of low active soluble salt content
  • have a minimum compressive strength of 48N/mm2.

Engineering bricks are also suitable.

Concrete bricks for manholes should:

  • comply with BS EN 771
  • have a minimum crushing strength of 48N/mm2 with a minimum cement content of 350kg/m3 for foul drainage.

Calcium silicate bricks should comprise strength class 20 or above for foul drainage situations.

Proprietary systems

Proprietary systems should be installed in accordance with manufacturers’ instructions.

Proprietary manholes should not be used at a depth greater than the manufacturer’s instructions.

Adaptors, couplers and sealing rings should be:

  • installed correctly and in accordance with the manufacturer’s instructions
  • treated using the lubricants and solvents specified.

Drainage covers and gully grids

Manhole covers and gully grids should be of the correct type for the proposed location in accordance with Tables 5 and 5a.

Manhole covers used within buildings should be airtight and mechanically secured. Covers used for septic tanks, cesspits and settlement tanks should be lockable.

Manholes should be constructed or installed at the correct level so that the covers will align with the adjacent ground. Gullies should be adequately:

  • bedded
  • set level
  • square and kerbed.

Table 5: Type of covering and grid required for inspection and manhole covers and frames

Group 1Areas which can only be used by pedestrians and cyclists.
Group 2Footways, pedestrian areas and comparable areas, car parks or car parking decks.
Group 3For gully tops installed in the area of kerbside channels of roads which when measured from the kerb edge, extend a maximum of 0.5m into the carriageway and a maximum of 0.2m into the footway.
Group 4Carriageways of roads, including pedestrian streets, hard shoulders and parking areas, and suitable for all types of road vehicles.

Proprietary items, e.g. covers to plastic manholes, should be in accordance with manufacturers’ recommendations.

Table 5a: Gully grids in carriageways

Grade BFor use in carriageways of roads with cars and slow-moving normal commercial vehicles.
Grade A class 2For use in carriageways of roads.
Grade A class 1For use in carriageways of roads (gully grids of permanent non-rock design).

Ground water

Foul and surface water drainage systems should prevent the ingress of ground water.


Where there is a risk of flooding, the advice of the relevant river authority should be followed.