Sewage Works
Network design and construction for sewage systems. El-Eman Office provides surveying and contracting for drainage projects.
Hydraulic Design: Pipe Sizes and Gradients
Sewer design starts by estimating the flow expected from the building or community, then choosing the pipe diameter and gradient so that water moves fast enough to carry solids without depositing them, but not so fast that it damages the pipes. A gradient that is too flat causes blockages, and one that is too steep lets the water run ahead of the solids, which are left behind and build up. The designer therefore sets minimum and maximum gradients for each diameter.
- Define the flow direction and the final connection point to the public network.
- Divide the network into main and branch lines, with manholes at changes.
- Set the pipe invert level at each manhole.
- Avoid reducing the pipe diameter in the direction of flow.
Excavation, Bedding and Pipe Laying
The trench is excavated to the required width and depth, with side support in weak or deep soil to protect workers. The trench bottom is prepared with a layer of sand or suitable material so the pipe rests along its full length and no single point carries excess load. The pipes are then lowered and jointed according to their type, with level and gradient checked by laser or survey instrument after each joint or every few joints.
Backfilling is best done in stages: a layer around the pipe in fine material free of sharp stones, then the rest of the trench in compacted layers, so the road or floor above the line does not settle later.
Testing and Handover
Before final backfill and handover, the network is tested for leaks and blockages. Tests may include visual inspection inside larger pipes or by camera, water or air testing of pipes and joints, and inspection of manholes. The results should be documented, and concealed works photographed before they are covered.
- Check the straightness of lines and the evenness of gradients.
- Test joints for leakage.
- Confirm the lines are clean and free of construction debris.
- Check manhole cover levels against the road or floor.
- Prepare an as-built drawing showing the route and actual levels of the lines.
Factors That Affect Cost
The cost of a network varies with excavation depth, soil type and groundwater level. Deep excavation in soft or waterlogged soil needs costly shoring and dewatering. Pipe type and diameter, the number of manholes and pumping stations also have an effect. Reinstatement of paving or finishes after backfilling is added, and it is sometimes forgotten. To get comparable offers, the bills of quantities must be standardised between contractors.
Frequently asked questions
Why do sewer odours appear in a building even though the network is sound?
The cause is usually a dried-out water trap in a basin, missing ventilation in the system, or a fault at a joint. Diagnosis needs checking point by point instead of guessing.
Why are manholes placed at set intervals?
To allow easy inspection and cleaning, and to allow changes in direction, diameter or gradient. They are usually placed at every change of direction or junction, and at regular distances along straight runs.
When is a pumping station needed?
When ground levels do not allow gravity flow to the public network, for example when the building is lower than the main line. The water is collected in a tank and pumped up to a higher level.
