|
|
|
|
|
|
|
in Pakistan, is to construct chambers with no benching and with their floor level some way below the invert of the sewer so that they act as interceptors within the sewer itself. This practice is not in accordance with conventional thinking, but one sewer in Lahore to which it was applied was found to be operating satisfactorily over 2 years after construction even though its gradient was less than 1 in 800. Experiments with the practice conducted in Faisalabad are described later in this chapter. |
|
|
|
|
|
|
|
|
The available literature suggests some variation in thinking on the gradients which are allowable with interceptor tank systems. At one extreme (Otis and Mara, 1985), no limits are given on the minimum gradient; indeed it is argued that some sections can be laid below the hydraulic grade line, implying that some backfall sections are acceptable. The only stipulations are that there should be some overall fall across the system and that the hydraulic grade line during estimated peak flows should not rise above the invert of the outlet from any interceptor tank. Other recommendations are more conservative, reflecting concern that interceptor tanks will not be desludged, so that solids will eventually find their way into sewers. In a previous publication (Tayler and Cotton, 1993), gradients ranging from 1 in 150 for a 75 mm diameter sewer serving about 15 houses to 1 in 450 for a 225 mm diameter sewer serving up to 260 houses were suggested. The Peshawar Cantonment Busti systems have certainly been laid to much flatter slopes, although precise measurements of these slopes are not available. However, questions remain as to the long-term performance of such systems if desludging of interceptor tanks is neglected. |
|
|
|
|
|
|
|
|
4.2.3
Manhole/Chamber Spacing |
|
|
|
|
|
|
|
|
The manhole and chamber spacing adopted for the Yusufabad and North-east Lahore schemes was limited by the fact that the normal practice in Pakistan is to make all house connections directly into manholes. Indeed, this is required by most government departments and agencies which deal with sewerage. There are practical reasons for this. It is much harder to produce good quality concrete branch fittings than it is to produce straight spun concrete pipes and poor quality Y and T branches in a sewer line are potential causes of blockages. In the Yusufabad |
|
|
|
|
|