You will need to follow the guidelines provided here when enclosing our sewers in tunnels.
Enclosing sewers in tunnels should be considered a last resort.
Depending on the age and condition of the sewer, complete replacement of the sewer may be required as a condition of tunnelling, as well as an increased pipe diameter.
The steps below outline our requirements for enclosing sewers in tunnels.
1. Tunnel size
- Sewer less than 1.5m deep – 1200mm high x 900mm wide
- Sewer 1.5m to 3m deep – 1200mm x 1200mm
- Sewer 3m to 5m deep – 1800mm x 1800mm
- Deeper than 5m – individual assessment is required.
The minimum clearance between the underside of the access tunnel roof slab (tunnel soffit) and the top of service, or its concrete encasement, must be 0.9m. To achieve this clearance in a pipe tunnel, the minimum internal tunnel diameters are as follows:
- Sewer: 150–225mm, 300–450mm and 600mm
- Tunnel: 1200 mm, 1500mm and 1650mm
2. The tunnel must be designed by a professional engineer
Designs must comply with our Wastewater Manual. Email email@example.com for a copy of the manual.
3. Tunnels constructed below the maximum level of the water table, or in areas subject to flooding, must be fully watertight and designed to resist floatation at maximum water level and with the sewer removed
They must be graded to easily accessible sumps to facilitate pumping out from ground level. Sewers must be adequately restrained against uplift and lateral movement, and mustn’t be used as a means of preventing flotation.
4. Sewers in rectangular tunnels in dry ground must be laid on a sand underlay with side support compacted to the level of the pipe centre
PVC sewers in rectangular tunnels in wet ground and in all pipe tunnels must be laid on a 10:1 sand/cement bed up to the level of the pipe centre. Other sewer types must be supported on a concrete block placed behind the socket of each pipe.
5. The ends of sewer tunnels are to be formed with a structural wall and must be properly sealed off to prevent the ingress of soil, water and gas
Structural load on the sewer must be avoided by using suitable compressible material between the sewer and the end wall of the tunnel. The designer is responsible for assessing long term settlement of the building or structure. Typically clearances of 50 to 80 mm would be allowed, however, the actual dimensions could only be determined based on the structural engineer's assessment.
6. Access chamber access openings are to be provided at a spacing of no greater than 100m along the tunnel length
At least one access opening is required for each tunnel and either a vent of at least 230mm diameter or an access opening is to be provided within 1.5m of each tunnel end. Vents don’t need to be taken to roof level as forced ventilation may be used when access is required. Vent and access chamber covers should be trafficable where appropriate.
Tunnel access chambers, including ladders and covers, must meet the requirements for access chambers outlined in our Protection of Services Manual. Holes cut into the soffit of a culvert to allow access are not acceptable.
7. We need to be able to access tunnel openings at all times via the property where any of our access chambers, inspection shafts or inspection openings is located
An access way of at least 900mm width is required.
8. Where possible, existing property connections must be relocated so that they are in accessible locations outside of tunnels
Where this is not possible they must be located in the corner of a tunnel access chamber. Unobstructed access to the inspection shaft is then required from within the access chamber and more than one property connection may be provided in each access chamber.
You will need our approval to build over sewers of 300mm or greater in diameter before proceeding with detailed design
Please contact us on firstname.lastname@example.org or call 13 13 95 for more information.