Most underground services need to be accessed to carry out repair and maintenance works. They can be accessed through manholes, which consist of a vertical shaft allowing a person to get into a chamber below ground level. The type and size of manhole depends on the type of action required for repair and maintenance works to the service. Each entity has its own requirements for its own manhole.
Manholes can be divided into two categories:
- Series 1500 Manholes for motorway communication ducts.
- Series 500 Manholes for sewer and storm water networks, including catch pits, inspection chambers and draw pits.
Step 1 : Determine the Size of the Manhole
A manhole is a link between an underground service and the surface above it, through which the service can be accessed and maintained.
The size of a manhole depends on:
- the pipe diameter.
- the depth between the road level and the crown of the pipe.
Shallow manholes come in two sizes:
- 900x700mm, known as Manhole Type A which is used for small diameter pipes up to 500mm diameter.
- 1000x1050mm, known as Manhole Type B, for pipes ranging from 500mm up to 900mm diameter.
Step 2: Determine the Type of Construction
Shallow manholes can be constructed in block work, in-situ concrete, pre-cast concrete chambers, corrugated galvanised steel chambers. In this post we will be showing manholes constructed in blockwork and in-situ concrete.
Step 3: Excavate Terrain for Manhole Structure
After determining the minimum dimensions, excavate terrain for manhole and allow an extra 230mm on either side of the pipe for the manhole walls. The depth of excavation shall be a minimum of 150mm below invert level to allow for manhole foundations.
If the surface on which the foundation shall be placed is rock, then the surface has enough bearing capacity to construct the manhole structure. The surface shall be swept from debris and large stones.
If the surface is unbound material, then the surface shall be wetted and compacted using hand compactors.
Step 4: Lay reinforced concrete foundation
Lay A393 reinforcement mesh top and bottom as shown in the diagram. Use concrete spacers to minimum of 40mm cover around the mesh.
When you are ready to lay the concrete, wet the surface and pour a minimum of 150mm thick layer of C20 concrete (also known as ST4 concrete), up to the invert level of the pipe.
Step 5: Constructing the concrete walls
When the foundation concrete has set, lower the steel formwork into position. Make sure that there is a space of 230mm between the steel formwork and the exposed rocky face to ensure that the manhole walls are thick enough. Use an A393 mesh around the steel formwork ensuring that there is at least 40mm cover and adequate lapping, shaped into a rectangular shape. Cast the C20 concrete between the steel form and the exposed rock, up to road formation level and vibrate the concrete.
Step 6: Formation of benching
C10 concrete shall be used to form the benching next to the sides of an open channel as per above-drawings.
Step 7: Roofing the manhole
The manhole shall then be roofed using a pre-cast concrete slab, consisting of 180mm thick C20 concrete with A393 mesh top and bottom, having an opening of 600x600mm.
The orientation of the manhole roof shall be in line with road features such as kerbs etc.
- Avoid manhole covers from asphalted areas.
- Sometimes it is not possible to construct manhole covers out of asphalted areas.
- In this circumstance, refrain from installing manhole covers in the wheel path.
- They shall be installed next to the kerb, in the middle of the lane or on the centre line.
Step 7: Selection of manhole cover
Manhole covers shall be specified according to European Standard EN124, which classifies manhole covers into 6 groups, according to their intended use ranging from class A15 intended to be used in pedestrian areas only to F900 which are intended to be used in freeports and airports. Class A15 manhole covers can withstand 15kN while F900 manhole covers can withstand 900kN.
- Group 1 – A15, for light loads
- Group 2 – B125
- Group 3 – C250
- Group 4 – D400
- Group 5 – E600
- Group 6 – F900, for heavy loads
Manhole covers should be properly installed as per manufacturer’s recommendations to ensure that they perform satisfactorily. For more details you can follow the link below: