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Problems arise with this arrangement when the depth to invert is less than about 1700 mm because the height available between the benching and the cover slab provides insufficient working space. There appears to be no entirely satisfactory way of dealing with this problem, but arguably the best response is to accept that chambers should be used for rather greater depths to invert than suggested above, perhaps to about 1500 mm. |
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4.2.5
Minimum Sewer Diameter |
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The flows which occur in tertiary sewers are low and rarely exceed 23 1/s. As the capacity of a 100 mm diameter sewer laid at a gradient of 1 in 100 is about 5 1/s, the flow to be carried is rarely the factor which determines the size required for such sewers. Most water and sewerage agencies in Pakistan specify a minimum sewer diameter of 9 inches (230 mm). This size is greater than that which is suggested by hydraulic theory and provides less favourable flow conditions than would occur in a smaller pipe. However, attempts to reduce pipe diameters are normally resisted on the grounds that, as some settlement of solids is bound to occur, the larger pipe diameter will enable the system to function even when it is partially blocked. This attitude is not confined to agency officials but is also widespread among community members. In Yusufabad, some short lengths of sewer, serving up to about 20 houses, are 150 mm diameter but residents definitely prefer to use 230 mm diameter pipes. The minimum diameter adopted in North-east Lahore was 230 mm. In Faisalabad, the first 100 m or so of the first conventional sewer constructed through the FAUP was 150 mm diameter and discussions are continuing as to whether some use of 150 mm diameter sewers will be acceptable to the government authorities. To date, after about 9 months use, no problems have been reported with this length of sewer. |
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The Peshawar Cantonment Busti interceptor tank schemes use 75 mm pipes to serve up to at least 25 houses. These appear to be working satisfactorily several years after construction, although no systematic evaluation has been carried out. The one interceptor tank sewer that has been constructed in Faisalabad is 150 mm diameter, reflecting the fact that government authorities and community members tend to be conservative on the subject. |
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