Depth and Configuration
Depth measurement, cone and slab options, and level adjustment.
Maintenance hole depth is measured from the surface level to the invert or bench level. Depth determines the structure type, the wall or riser arrangement, and the access and confined space requirements.
Configuration ranges from a shallow structure with a cone or slab top, through standard precast riser rings, to deep structures with a working platform. The authority's standard drawings define which type applies at each depth.
Key design considerations
- Depth definition used by the authority, since it changes the structure class.
- Top configuration: cone, flat slab, or reduced-access slab, and the associated cover.
- Level adjustment rings and their limits, and whether adjustment is permitted at all.
- Base type: cast in situ, precast base, or a separate slab.
- Foundation requirements in soft or variable ground, and the risk of differential settlement.
- Deep structures needing a platform, intermediate access or forced ventilation.
Equations
depth = surface level − invert level
Confirm whether the authority measures to the invert of the outlet pipe or to the bench level.
Design checks
- Depth computed from final surface levels, not existing levels.
- Configuration matches the authority's standard detail for that depth.
- Level adjustment within the permitted range.
Standards and authority references
- Authority standard maintenance hole drawings
- Authority access and confined space requirements
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Depth thresholds for each configuration are set by the authority and are not reproduced here.
Related calculators
This reference is general engineering information. Confirm requirements against WSA 02, the current Australian Standards and the local water authority's supplement before use in design, and obtain project-specific advice for the conditions at hand.