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scheme, each chamber receives the flow from four houses and their spacing is typically about 16 m. Greater spacings were possible in the North-east Lahore scheme because of the approach taken to house connections, which will be described later. The maximum spacing adopted for tertiary sewers was 30 m, although spacings of up to about 100 m were used on collector sewers. The 30 m figure is considerably less than that which would normally be permissible in the UK where the minimum recommended spacings for manholes and inspection chambers are 90 m and 45 m respectively (BRE, 1984). However, the equipment available for clearing blockages is not of the same quality as that in the UK, consisting as it does of lengths of bamboo crudely wired together. Experiments in North-east Lahore proved that lengths of at least 25 m can be rodded with wired bamboo rods, but this appeared to be close to the limit of what was realistically possible. |
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Observation suggests that many sewer blockages in South Asia are caused by materials deposited into sewers at chambers and manholes, including pieces of broken cover. One report on Karachi (Balfours and Engineering Consultants, 1987) recommends that chambers should not be provided on shallow sewers on the grounds that the reduced possibility of blockages thus achieved more than compensates for the added inconvenience of having to dig down to the sewer and break into it to locate any blockage which does occur. This recommendation assumes that interceptor tanks are provided on house connections. This approach was independently adopted for the Peshawar Cantonment Busti schemes and blockages do not appear to have been a problem with these schemes in the first 23 years of their operation. However, they have not been systematically monitored. |
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Chambers have been omitted from some short privately built conventional sewers in Pakistan but the extension of the approach to public sector sewers is problematic because it is contrary to conventional wisdom and existing codes and regulations. There is, in any case, the problem of producing the special pipe fittings required to make connections at locations other than chambers. One possible compromise solution would be to make connections at chambers with covers located below ground level, as shown in Figure 4.1. Apart from reducing the opportunity for casual access to the sewer, this arrangement can also reduce costs by allowing the use of a simple unframed cover and should also |
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