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Bombay Sewerage Project
Marine Investigations, 1998-1991 and
The Bombay Sewerage Project is an exceedingly important scheme for Mumbai (if unfamiliar to you, this is the new name for Bombay). Since 1988, on Behalf of Binnie Black & Veatch, advice on marine geotechnical investigations and rock and soil testing has been provided. Rock mechanics advice for tunnelling and excavation has been presented at outline design stage and continues during construction.
The works comprise of two 3.5 kilometre tunnels at 4.5 metre diametre. Tunnels are fully lined with precast concrete segments. Associated works include the major marine operations for placing twenty riser pipes with seabed structures. In addition Influent and Effluent pump stations, other pump stations and other feeder tunnels are to be either built or repaired.
The picture top picture shows some of the marine operations. Two jack-ups and a diving chamber is used to excavate and place diffuser unites, shown below.
The geology of the site is Basalt and Tuff (and intertrappean beds), which includes sedimentary units. The stratigraphy provides shallowing dipping units running out to sea.
On land the IPS station was partly completed in the early 1980's. Shafts excavated then now need de-watering and widening. The new design demanded that the new pump shafts are 2 metres wider 15 metres below the top.
Two pump shafts were originally seperated by five metres. After widening the 35 metre deep shafts they will be only two to three metres apart. All shafts have concrete retaining walls in-place, since the site is right next to the sea. Right is Pump Shaft No. 1 after de-watering but before widening works commenced. The old concrete base slab is seen at the bottom.
Pictured left is the top of the inlet Shaft again before widening and trimming works. The shafts are shotcreted and anchored prior to excavation. In the narrowest parts of the dividing rock wall, anchors are passed through the rock and anchored at each side providing a system of pre-reinforcement. The top of each anchor is sleeved to allow for the rock to release during excavation.
Right shows excavation in progress. Some anchor heads are visible and the shotcrete may be seen in the upper part of the neck. Rock excavation in the shafts are now virtually complete. Future visits are planned during the building of the internal walls and sub structures.
The first of the tunnels, at Worli, is well underway. During design stage the rock mass performance was reviewed. Below are two charts developed in-house for the design phase of the works in 1991-92.
The top chart shows rock mass interaction for an assumed in-situ stress condition in Tuff. The lower chart is from a program looking at sub-horizontal shears, joints or bedding interfaces (more is shown of this chart towards the end of this page).
Roof and floor failures have occured during tunnelling. Although relatively small failures production was slowed for some time due to the crowded work area. Straps and mesh provide security from rock falls, while the Swellex bars provide additional roof reinforcement.
The two photographs below show the lateral convergence in the roof (also seen in the floor). Loose blocks are held by the mesh (top).
The charts below show a sequence of analyses demonstrating the effect of sub-horizontal bedding interfaces.
The figures right at the bottom show the tunnel advancing towards a gently dipping interface which runs in the direction of the tunnel.
From one metre above the crown down to mid-height, the weak plane interface is seen to fail at differing locations. Ignoring (the significant) post-failure and 3D effects, the net result of this single failure is a bulb of failed rock stretching along the tunnel line. This occurs in each quadrant, so both roof and floor is affected.
This simple analysis and diagrams give an indication of the scale of possible failure. This enables a balanced view of the scale of support needed. The photographs shown above look dramatic but only confirm the general form of this mechanism at work.
Below is shown the tunnel lining in place with the back end of the TBM.
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