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| | Pipe diameter | | | | | Minor roads | | | | | | | | | Gardens | | | | | | | |
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produced plastic pipes in Pakistan, research is needed to determine standards for the covers to be provided over locally produced clay pipes. The minimum covers specified in Thirukatchur were about 700 mm. |
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4.2.2
Minimum Sewer Gradient. |
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In flat areas, the minimum sewer gradient has possibly more influence on the depth of a sewer throughout its length than the minimum permissible cover. Conventional theory is not suitable for the design of sewers near the head of the sewer system as the steady-state flow conditions which it assumes do not occur in such sewers. British codes overcome this problem by providing rules of thumb for such sewers. The building drainage code BS8301 (British Standards Institute, 1985), for instance, recommends the following minimum gradients: |
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100 mm diameter sewer: 1 in 40 if peak flow less than 1 1/s; 1 in 80 if peak flow more than 1 1/s] |
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150 mm diameter sewer: 1 in 150 if at least five WCs are connected. |
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Sewers for Adoption (WSA, 1995) recommends a gradient of 1 in 150 for sewers receiving flows from at least 10 dwelling units but this, as with the BS8301 recommendations appears to be a rule of thumb. |
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Various attempts have been made, mostly applying probability theory to the likely timing of discharges and using information on the flow produced when a WC is flushed, to develop a rational design theory for sewers which are subject to unsteady |
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