Specifications Part 1 - Water Supply, Sewerage and Tube Well Works

14.7.4 Heavy reinforced concrete covers with suitable lifting arrangements could alsobe used instead of C.I manhole covers. However pre-cast cement concretecovers reinforced by materials other than mild steel should be used providedthat those are properly tested & certified for use by competent authority. Fibrereinforcement plastic covers (FRP) conforming to relevant IS: may be usedwherever such covers are available.14.8 Inverted siphon14.8.1 The main purpose of installation of inverted siphons is to carry the sewer linebelow obstructions such as ground depressions, streams, rivers, railway etc.14.8.2 In the course of laying sewers, at times it is found necessary to crossobstructions like nallah etc. Such obstruction shall be crossed by means ofas much elevation as possible after the obstruction is passed .14.8.3 As the siphons are depressed below the hydraulic grade line, maintenance ofself cleaning velocity at all flows is very important. Two considerations, whichgovern the profile of a siphon are (i) provision for hydraulic losses and (ii)Provision of cleaning.14.8.3.1 It is not possible to give side connections to the inverted siphon.14.8.4 Design aspect:14.8.4.1 The inverted siphon usually consists of siphon tubes or pipes which may be ofcast-iron, concrete of steel.14.8.4.2 The inverted siphon is constructed between inlet and outlet chambers. It isgenerally made up of two sloping lengths which are connected by a flat length.14.8.4.3 If the length of siphon is more, it is essential to provide a ventilating shaft alongthe siphon to avoid the formation of air-lock in the siphon.14.8.4.4 The siphon should be so designed that a self-cleansing velocity of about 900mm per second is achieved during the period of minimum discharge.14.8.4.5 For this purpose, the siphon is usually made of three pipe sections-one forcarrying minimum discharge, the other for maximum discharge and the third forcombined flow in monsoons. The inlet chamber contain three channels, one foreach pipe section. When channel no. 1 overflows, the sewage enters channelno.2 and pipe no. 2 comes into commissions, similarly, when channel no.2 alsooverflows, the sewage enters channel no.3 and pipe no.3 comes in tocommission.14.8.4.6 For sanitary sewer, only two sections shall be required-(i) for minimum dryweather flow (ii) for maximum dry weather flow.14.8.4.7 The changes of direction in inverted siphons should be easy and gradual.Urban Administration & Development Department Page 166