Part 5 – Wastewater Drainage
5.11 Manholes
5.11.1 General
Manholes shall be located away from areas likely to pond water and away from potential building sites. They are to be designed and constructed to exclude groundwater for the life of the network.
Manholes up to 2400 mm deep shall be constructed using a single riser with a pre-cast external flange base. Manholes in excess of 2400 mm deep shall be constructed using a 2400 mm long pre-cast riser with external flange base and then completed to final ground level using no more than a single riser for manholes up to 5.0 m deep. Three risers are allowable for manholes in excess of 5.0 m depth.
In no case shall a series of short risers be permitted.
Manholes shall be a minimum of 1050 mm diameter for depths of 1.0 m or more. Manholes of 750 mm diameter may be used for depths less than 1.0 m (typically infill situation).
Manholes on pipelines less than 300 mm diameter shall be provided at each change of direction or gradient, and at each branching pipe, and at a spacing of not more than 100 m.
Manholes in pipelines 300 mm diameter and over may have the spacing increased with the specific approval of Council.
For infill developments, manholes shall not be required for a 150 mm connection on a 150 mm pipeline where a manhole is provided immediately inside the property being served and another manhole exists within 20 metres on the existing pipe and these provide adequate accessibility without needing another manhole.
Manhole lid rings may be used to a maximum depth of 150 mm. For depths over 150 mm, manhole risers shall be installed.
Drop manholes are to be constructed with internal drops only; external drops are not permitted.
5.11.2 Standard Manholes
These are to be circular manholes with a minimum internal diameter of 1050 mm and are to be used on pipelines up to and including 600 mm diameter. Manhole steps shall be provided.
Note: The MPDC Infrastructure Code of Practice provides a useful guide.
5.11.3 Specific Design Manholes
Where manholes are more than 4.0m deep they shall be specifically designed (wall strength, foundation support and adequate ballast to resist buoyancy).
Where a manhole is to be constructed in soft ground, the area under the manhole shall be undercut down to solid and backfilled with suitable hard fill to provide an adequate foundation for the manhole base. Where undercutting exceeds 1.5m, a special design will be required.
5.11.4 Hydraulic flow in Manholes
In addition to the normal pipeline gradient, all manholes for pipelines less than 1 metre in diameter shall have a minimum drop of 20mm plus 5mm per 10 degrees of the angle of change of flow within the manhole.
In addition to the normal pipeline gradient, all manholes on pipelines greater than 1metre in diameter shall have the drop through the manhole designed to a minimum of 20mm plus compensation for the energy lost due to the flow through the manhole at the deviation angle.
