Manholes Print

Design Outcome

​​Manholes or non-access chambers shall be provided at all changes of direction, gradient and pipe size, and at branching lines and terminations. 

Other suitable specifically designed structures may be used subject to Auckland Council approval. Manholes shall be provided at a distance apart not exceeding:
  • 90m for pipelines with diameter less than 750mm
  • 150m apart for pipelines with diameter of 750mm and above.
Manholes shall be easily accessible and shall be located at least 1.0m clear of any structure or boundary. All public mains shall terminate with a manhole, non-access chamber or catchpit at the upstream end.

Table 4.8 shows the types of manholes that shall be used and their reference drawing numbers. Drawings are in Appendix B.​

Table 4.8: Manhole Types ​

Type

Outlet Pipe Diameter Range

Depth Range*

Drawing No.

Standard manhole

≤1050mm

<4.0m

SW05

Non-access chamber

≤225mm

<1.0m

SW06

Manhole with in situ base

500 - 1200mm

<5.0m

SW07

Stormwater manhole offtake**

>900mm

<4.0m

SW08

Deep manhole

-

>4.0m

Specific Design 


*Depth is defined as the distance from the manhole cover level to the invert of the outlet pipe. **Lobster bend​ manholes are only to be used with specific Auckland Council approval.​

High Energy Manholes

Specific analysis is required for manholes with:
  • High energy inlets (e.g. with inlet/upstream velocities in excess of 4m/s)
  • Bends
  • A significant change in invert levels
  • Significant side inlets.
This is to guard against the impact of any extraordinary head losses and the potential "popping" of manhole covers.

4.3.10.1. Location of Manholes

The selection of a suitable location for manholes may influence the pipe alignment. A minimum clearance of 1.0m from the outside diameter of the manhole riser to any structures or boundaries shall be provided to facilitate maintenance and rescue. Auckland Council may determine other specific requirements subject to the individual site characteristics.

Manholes and pipelines shall be located to minimise interference with future building. Where they cannot be located in public land they shall be located within common accessways. If this is not practicable, the following preferences apply:
  • Residential areas: within the front, rear and side yards
  • Business areas: adjacent to road frontage boundaries
4.3.10.2. Manhole Material

Manholes are to be constructed of reinforced concrete in accordance with drawings SW05 to SW09 in Appendix B. Benching of the manhole shall be hard finished concrete as opposed to a plaster finish. Benching shall be to the equivalent of the full height to the level of the inlet and outlet pipes as shown in drawing SW05.

4.3.10.3. Manhole Design

Structural Design

Manholes shall be designed to be constructed as segmental, panel or one unit structures, made of precast or cast in situ reinforced concrete. The general shape of manholes may be circular, rectangular, or any other shape that satisfies all design requirements of this section. Manholes shall be designed so that structural integrity between its components is maintained during installation and normal service life. The effect of pipe entries on the structural integrity of the manhole risers shall be considered during design. 

All manholes shall be designed to support all existing and any predicted future dead loads. Design live loads for reinforced concrete manholes and cast iron covers shall be HN-HO-72 for motorways and arterial roads, HN for local streets and driveways, and 20kN wheel load for non-trafficable areas. Manholes shall also be designed to support lateral earth pressure, hydrostatic pressure and any expected high eccentric lateral pressure due to live load or surcharge load. Manhole foundations shall be designed such that the bearing pressure does not exceed the safe bearing capacity of the soil.

Reinforced concrete pipes produced to AS/NZS 4058:2007 may be used as manhole risers provided that the load class of the pipes is adequate to carry the service loads on the manhole. Use of pipes with oval reinforcement is not acceptable.

Durability Design

All components of manholes and access chambers shall be designed for an asset life of 100 years. Circular manhole risers shall be designed and manufactured to meet the 100 year design life requirements of AS/NZS 4058:2007 for normal environment when designed for installation in such an environment, and for marine environment when designed to be installed in marine tidal locations or aggressive soils. The durability design of other reinforced concrete components shall be in accordance with the durability requirements of NZS 3101: Part 1: 2006.

4.3.10.4. Size of Manholes

The minimum internal diameter of circular manholes is 1050mm. Manhole sizes shall be in accordance with the table included in drawing SW05 in Appendix B.

4.3.10.5. Non-Access Chambers

Non-access chambers may only be used where the depth to invert is less than 1.0m. The maximum outlet pipe diameter for a non-access chamber shall be 225mm. Non-access chambers shall have a maximum of two inlets of no greater than 100mm diameter as shown on drawing SW06 in Appendix B. Additional connections may be allowed on approval from Auckland Council. Any chambers outside of these criteria will require a standard manhole instead.

Non-access chambers shall be constructed in accordance with drawing SW06 in Appendix B. Benching within non-access chambers is not mandatory.

4.3.10.6. Hydraulic Flow in Manholes

Internal Falls through Manholes

In addition to the normal pipeline gradient, all manholes on pipelines of less than 1m diameter shall have a minimum fall through the base of the manhole of 50mm. The maximum allowable fall through the base of the manhole between inlet and outlet pipes is 300mm.​

Manholes on pipelines greater than 1m diameter shall have the fall through the base of the manhole designed to compensate for the energy lost due to the flow through the manhole at the design radius.

Where the outlet pipe diameter at a manhole is greater than the inlet diameter, the minimum fall through the manhole shall be not less than the difference in diameter of the two pipes, in which case the pipes shall be aligned soffit to soffit.

On pipes where the above criteria for internal fall across the base of the manhole is not achievable due to a large difference between the levels of incoming and outgoing pipes, then an open cascade may be able to be used (see Section 4.3.10.7). Manholes on steep grade lines and where a stormwater line changes direction or has a vertical drop shall be designed as an open channel flow, without surcharging the manhole.

Effect of Steep Grades on Manholes

To avoid excessively deep channels within manholes, steep grades (>7%) shall be designed out during the design phase where practicable. Where a pipe of grade >7% enters a manhole, the following precautions shall be taken if the topography and the connection pipes allow:

a) No change of grade is permitted at an inlet to a manhole

b) Steep grades are to be continuous through the manhole at the same grade.

c) Depth of manhole is to exceed:

o 1.5m to invert for 225mm diameter or smaller pipes

o 2.0m to invert for 300mm diameter or larger pipes.

d) Change of direction at the manhole is not to exceed 45 degrees.

e) No open cascades are to be incorporated in a manhole where the entry pipe grade is >7%.

f) The inside bending radius of the channel inside the manhole is to be greater than six times the pipe diameter.

g) The hydraulic gradient at design peak within the manhole shall be contained without surcharging.

Pipe Bending

For flexible pipelines laid at gradients in excess of 7%, it may be practicable to reduce manhole depths and water velocities by the installation of vertical bends at the approach to a downstream manhole. This will require specific design to the council’s approval.​

4.3.10.7. Manhole Connections

Connections to manholes shall be made following this order of priority depending upon practicability of the option:

1. At the manhole invert, with the soffit of the inlet pipe level with the soffit of the outlet pipe

2. At the top of manhole benching level, with a channel in that benching provided to direct the flow to the outlet

3. Via an open cascade in accordance with the cascade conditions below

4. In accordance with specific design to the council’s approval.

Cascades are only permitted under the following conditions:
  • Where the manhole is more than 2.0m deep
  • Where the cascade inlet pipe diameter will be a maximum of 300mm
  • Where the cascade will not discharge on to any steps or ladders
  • Where the drop height will not exceed 1.0m (from the invert of the cascade inlet pipe to the top of the benching within the manhole).

Where any of the above conditions are not met, specific approval is required.

The base of all manho​les shall be benched and haunched to a smooth finish to accommodate the inlet and outlet pipe. For manholes where there is more than one inlet pipe (excluding minor inlets), a specific design and detail drawing of the benching layout shall be provided and shall be subject to council approval. The detail shall include the location of the manhole cover and frame and steps in relation to the benching.

New inlet pipes shall be cut back to the inside face of the manhole and provided with a smooth finish. All chambers are to be made watertight with epoxy mortar around all openings.

Minor pipelines (less than 200mm diameter and not the main flow) connecting to a manhole shall do so at an angle of not greater than 90 degrees to the main pipeline direction of inflow. Where the minor pipeline connects above the design water level only, a connection at an angle in excess of 90 degrees may be permitted with the council’s approval.

The total number of connections to a manhole (other than the primary inlet and outlet pipes) shall not exceed four without the council’s approval.

4.3.10.8. Flotation

In areas of high water table, it shall be demonstrated to the council’s satisfaction that all manholes are designed against flotation.

The factor of safety against flotation shall be at least 1.25, excluding skin friction in the completed condition, with empty manhole and saturated ground. Where allowance is made for skin friction in the permanent condition the factor of safety shall be at least 1.4. Both factors of safety in the respective conditions shall be met.

Design against flotation shall consider the potential for post-construction ground conditions with saturated non-cohesive backfilling above the piping over the width of the trench.

4.3.10.9. Manhole Covers

Manhole covers shall be hinged with a clear opening of at least 600mm diameter, except for non-access chambers where a clear opening of 500mm diameter is allowable.

Manhole covers for 600mm diameter or over shall be ductile iron. Square and/or rectangular manhole covers are generally unacceptable to the council.

The class of manhole cover and frame to be used shall be determined by the potential loading on the manhole in accordance with AS 3996. Limits are as follows:
  • A minimum AS 3996 Class D, or better, heavy-duty manhole cover and frame shall be used in all areas.
  • In industrial areas, or for any application where abnormally high loads are likely to be transferred to the manhole cover, AS 3996 Class E or better will be required at the council’s discretion.
Manhole covers on roads shall be aligned so that a vehicle striking a hinged cover in a partially open position shall push that cover towards its closed position. Liaison with Auckland Transport is required for the location of all manhole covers in the carriageway.

Where a manhole is installed within a secondary flow path, the manhole cover shall be aligned to open with the direction of flow (hinge upstream). This will ensure that a cover opening under hydraulic surcharge will close again and will not be prevented from closing by trapped debris swept by the secondary flow.

Scruffy domes as per drawing SW21 in Appendix B should be used for manholes and manhole risers in stormwater ponds and stormwater management devices or in situations where overland flow is to be directed into the stormwater pipe network. More aesthetic alternatives may be considered/required and approved at the council’s discretion.​

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