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4.3.3
Manhole Construction. |
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In the Yusufabad and North-east Lahore schemes chambers are rectangular, constructed in brick on a concrete base with concrete covers. In North-east Lahore, the walls were originally half brick (112 mm) thick for 500 × 600 mm chambers and single brick thick (230 mm) for 800 × 600 mm chambers. It was found that half-brick thick side walls were unsatisfactory where more than one pipe entered a chamber along one of its sides and the design was therefore amended to allow a full brick thickness where this was the case. In both schemes, the covers were cast in an angle iron frame and located within a second frame, which was built into the surround at the top of the manhole. In North-east Lahore, some of these frames were galvanized, but the galvanizing was not found to be satisfactory and painting of the frames in bituminous paint was then adopted as standard. |
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In Faisalabad chambers on all but the first sewerage scheme implemented have been constructed using the methods developed by OPP. The base is laid in mass concrete, as for the other designs discussed, but the plan shape is circular and the walls are constructed in mass concrete, placed in-situ between steel shutters with box-outs left for sewers and connections. The wall thickness is 100 mm and given the fact that the cost of 1:2:4 concrete is roughly the same as that of brickwork in the Punjab, this gives some cost saving over conventional single-brick thick walls. The arrangement is shown in Figure 4.2. |
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The breakdown of tender prices for the small chambers used in North-east Lahore showed that the covers and their frame were priced at about twice as much as the rest of the chamber, about Rs1200 against Rs600 for the 800 × 600 mm chambers and about Rs600 against Rs300 for the 600 × 500 mm chamber. (1990 prices; US$1 approximately equivalent to Rs30.) The angle iron frames were the main reason for the high prices of covers. Most manhole covers provided by private individuals and community groups in Pakistan do not have frames, and OPP argue that this is quite satisfactory for their circular covers, although unframed rectangular covers tend to break at the corners. The OPP practice was at first followed in Faisalabad but it was found that even circular covers are susceptible to breakage when subjected to wheel loads. A modified design was therefore developed which included a vertical steel strip encircling the |
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