The problem
After installation, drains and their associated gullies and chambers must be surrounded by material that compacts easily and does not damage the drains themselves.
The material is often described as "selected fill" but this is an incomplete description and can lead to the use of builders' rubble, or the material excavated to form the drain trench which may, or may not, be suitable.
Different solutions for different component types
Drainage systems are made from a number of materials and whereas all must be back-filled with a material that compacts readily, some require a higher specification than others.
In general, clay and concrete are less prone to damage from back-filling than plastics and also require less support.
Plastic pipes should be surrounded with fill which has no sharp edges and which has a smaller maximum particle size than that used for clay pipes.
What to look out for on drawings and in specifications:
- For plastic pipes – reference to BS 5955-6:1980
- For clay pipes – reference to BS 8000-14:1989 or the Clay Pipe Development Association's specification – design and construction of drains and sewers using vitrified clay pipes.
What to look out for on site:
- "Selected fill" should never contain timber, frozen matter or vegetable material.
- For plastic pipes, the largest particle size should be 38 mm and 95% of material should pass through a 19 mm sieve (a maximum particle size of 25 mm may be required around chambers).
- For clay pipes there should be no material greater than 100 mm (smaller than half a brick) and no stones bigger than 14 mm.
- In all cases the material should be seen to compact easily and evenly with no apparent void.
The fail-safe solution
Surrounding pipes of 100 mm diameter with 10 mm pea shingle to a height of 150 mm above the top of clay pipes and 300 mm above the top of plastic pipes provides a satisfactory and easily verifiable solution. The trench can then be filled with any easily compacted material that conforms to the rules above.
Cost, sustainability and the environment are all factors which should be considered when using imported materials, the suitability of the excavated material available on site should also be investigated.
Remember, these are pointers to good practice, but they are not necessarily the only correct solution.
Source
Housing Today
Postscript
For more information contact Jeff Maxted, head of risk management services at Building LifePlans, on 020 7204 2466 or by email at jeff.maxted@buildinglifeplans.com
No comments yet