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Levels and earthworks

Manhole or chamber depth and number of rings

H=CL−ILHr=H×1000+hb−ts−tfNr=⌈Hrhr⌉
where
H
depth to invert (m)
CL
cover level (m)
IL
invert level (m)
Hr
ring height needed (mm)
hb
invert down to the seating of the bottom ring (mm)
ts
cover slab thickness (mm)
tf
frame and adjusting courses (mm)
Nr
rings needed
hr
height of one ring (mm)
Explain this equation

This gives the depth of a manhole or inspection chamber from cover to invert, and how many precast rings make up the shaft.

CL and IL are the cover and invert levels, so H is the depth to invert. hb is the drop from the invert down to where the bottom ring sits, ts the cover slab thickness and tf the frame with its adjusting courses. Hr is the ring height needed, hr one ring’s height and Nr the number of rings.

The rings fill the space from their seating up to the underside of the cover slab; the slab and frame take up the rest of the depth. Rounding up with ⌈ ⌉ gives whole rings, and any excess is taken out with a shorter ring or a lower seating.

Sense check: 1.8 m to invert with a 150 mm seating, slab and frame needs 1.65 m of rings: three 500 mm rings leave 150 mm short, so four, or three plus a 250 mm ring.

Watch out: chamber and manhole sizes and access depend on depth under Approved Document H.

Depth from cover to invert and how many precast rings make up the shaft, allowing for the base, cover slab and frame.

Enter your values to see the result.

Good to know

Depth to invert = cover level − invert level. The ring stack fills the height between the seating at the bottom and the underside of the cover slab; the frame and its adjusting courses sit on the slab. If the rounded-up rings leave an excess, use a shorter ring for one of them or adjust the base. Approved Document H Tables 11 and 12 set chamber and manhole sizes by depth; adoptable manholes follow the water company’s Design and Construction Guidance. Ring heights, slab thickness and seatings come from the precast supplier (BS EN 1917).

Common mistake

Measuring depth to the top of the pipe or the benching instead of to the invert, or forgetting the frame height.

Limits

Simple stack arithmetic; does not check structural design, steps, ladders or safety chains.

Sources

Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.

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