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allow for the provision of step irons at one end of the chamber. However, it is interesting to note that most water authorities in England now discourage the use of step irons because they are potentially dangerous when corroded. Given the fact that inspection shows that many step irons on existing manholes in South Asia are badly corroded, this philosophy has much to be said for it, provided, that ladders of a suitable quality are available as an alternative. |
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Benching was provided in North-east Lahore in accordance with conventional practice. There is a tendency for local government and community schemes to omit benching or provide it only up to the mid-barrel level of the sewer. Chambers in the early schemes in Faisalabad were constructed with their base dropped below invert level and the benching omitted, the aim being to ensure that any large solids entering the sewer would settle at a chamber rather than between chambers. After some months of operation, sludge has built up to above invert level on either side of the flow and, although there is no evidence of large solids being deposited in the sewer, it has been decided that conventional benching will be provided in future chambers. In any case, the dropped manhole invert system is only appropriate for shallow chambers which do not have to be entered for maintenance purposes. |
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The North-east Lahore chambers and manholes were rendered on their internal but not their external faces, on the ground that the sewers were above the water table so that there was no need to prevent the ingress of water. |
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4.3.4
Setting out Procedures |
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Perhaps the most difficult task associated with the implementation of sewerage projects is the laying of sewer pipes to the correct levels. This has traditionally been done in the West using profiles set a given distance above the invert of the sewer and travellers of the length necessary to fix the bottom level of the trench, the top level of any bedding and the invert level of the pipe. In Pakistan, a rather inferior version of this method is commonly used by government engineers. Measurements are made from a string line, which is stretched between profiles set across the sewer line at intervals. Both of these methods normally |
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