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is to convince engineers to accept changes from universally applied 'conventional' standards. The figures in Table 4.6 show that the North-east Lahore manhole dimension standards were comparable with those used in similar situations in Britain. Some caution is required in view of the different sewer cleaning equipment available in the two countries. Purpose-made flexible drainage rods are available in Britain and in many cases it is possible to call upon high-pressure jetting equipment which can be much more effective than drainage rods in removing blockages. In Pakistan, as already indicated, the normal practice for small diameter sewers is to use lengths of bamboo, wired together on site in a rather crude way. One of the disadvantages of this practice is that the wired bamboo rods are likely to be much less flexible than purpose-made rods with screwed connections, thus requiring rather larger chambers to allow them to be bent and inserted into the sewer. The response to this situation should be to put more effort into the development of more effective methods and higher-quality equipment for clearing sewer blockages and this is certainly something that should be given priority in future externally funded schemes. However, it is likely to be many years before such improved methods and equipment become widely available and there is a need for more experimental work to establish the limits of what can be achieved with presently available equipment. |
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All manholes and chambers in Yusufabad and North-east Lahore had brick walls and in view of this it was felt that a rectangular shape was preferable for the smaller chambers. The 600 × 500 mm chamber is provided with a single cover slab while a split cover is required for the 800 × 600 mm chamber. The FAUP team has experimented with the use of circular concrete chambers using shuttering based on that developed by the OPP and it has been found that reasonable quality can be obtained using this method for 525 mm diameter chambers. The 750 mm diameter chamber has yet to be tested. In order to allow easy access, the removable cover should extend over the full plan area of a chamber and there may be problems with the cover on a 750 mm diameter chamber which, at about 900 mm in diameter and 75 mm deep, will weigh about 110 kg. |
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The normal practice for manholes designed for entry is to provide a fixed cover slab with a smaller opening, typically about 550 mm in diameter, through which the manhole can be entered. |
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