Indian firm Afcons is preparing to start boring risers at the Bandra outfall site, and is nearing completion of works at Worli. The monsoon is the contractor's biggest problem - heavy seas make off-shore work difficult or impossible. A time overrun of days can hold work back five months if it extends into the monsoon season.
Working from a jack-up barge, 1.6m diameter riser shafts are bored using a Wirth rig and 1m diameter GRP liners are installed and grouted. Plugged at the top and sealed at the bottom, the liners are filled with dye which tells Dyckerhoff & Widmann whether or not they are on target when probe- drilling prior to excavation of the connecting adit. Closing error on the Worli risers was between 0.2m-0.7m.
The discharge structure is composed of precast concrete spacers, each 7m diameter, 1m high and weighing 50t, placed from a jackup barge to bring the diffuser head just above sea-bed level. Gravels are used to fill around the spacers, and a 1m layer of rock is placed as protection around them.
The diffuser heads have 10 discharge ports, half of which will remain sealed until there is sufficient flow and demand to open them. Design capacity for the Worli outfall is currently 18m3/second and for Bandra 22m3/second.