Bangalore metro’s two lines total 8.82km underground and 33.48km elevated. One line runs north to south, the other east to west, intersecting at Majestic Station. There will be 40 stations in total, of which seven will be underground.
Bringing experience in a similar role on Delhi Metro, we were appointed as contractor’s designer at short notice, replacing the original design firm. Our local presence and international track record provided further reassurance to client Bangalore Metro Rail Corporation in the face of challenging ground conditions. We were appointed to develop the contractor’s design in July 2010, with a deadline for submitting our proposals in October. We had to pull out all the stops.
Slurry TBMs debut in India
The route selected for the tunnels mainly follows roads. Ground cover varies from 6m to 17m. The challenging geology is broadly formed of three layers, with recently deposited fill on top, followed by soil consisting of silts and sands with varying amounts of clay and gravel. Beneath the soil is granitic gneiss, varying from weak fragmented to hard rock. Added to the changeable ground conditions, the groundwater table varies between 2m and 10m below ground level and is significantly affected by rainfall.
The mixed face conditions, shallow depth of the tunnels and proximity to other structures made protection against subsidence a key issue over a section of the eastern end of the alignment. Marking a significant first in the Indian tunnelling sector, slurry TBMs were selected to support the tunnel face during excavation and to minimise the danger of subsidence while tunnelling.
The tunnels were excavated using two Hitachi Zosen TBMs. These machines use bentonite slurry at the cutting face to exert an equal and opposite pressure to that of the ground. There is a bulkhead behind the rotating cutter head, creating a chamber that is filled with slurry, excavated soil and water. An Archimedes screw mechanism prevents the slurry from escaping and allows pressure at the face to be controlled.
Slurry tunnelling is well established in many parts of the world with mature infrastructure, but is relatively complex and previously untried in India. We brought a wealth of experience – in fact, the first bentonite slurry TBM, the forebear of today’s machines, was patented by us in 1964.
A first for cross-passages
Three cross-passages link the running tunnels to enable passenger emergency evacuation, each with 3m internal diameter. Conventionally they would be constructed using the sprayed concrete lining system. This method involves incremental excavation. Concrete is sprayed onto the tunnel face and walls after each advance to provide temporary support. But there were concerns here about whether the ground would ‘stand up’ long enough for the concrete to set. So the contractor asked us to come up with an alternative method.
In what we believe to be an industry-first application, we designed a jacked box solution. Steel box sections are driven through the ground. As the box is pushed forward, earth is excavated – with the box itself protecting workers from falling debris or potential tunnel collapse.
This is an innovative use of a proven process and not without risks. Jacking had to take place from within the confines of the 5.6m diameter running tunnel – conventionally such an operation would be performed from a much larger purpose-built shaft. This required a number of short sections to be joined, one behind the other, to create each 9m long passage. Sections had to be welded together in very confined spaces. The break out and break in points between the jacked box and the segmental tunnel lining had to be sealed to prevent influx of soil and water. By highlighting risks early and through thorough detail design, we aim to create a new industry benchmark for jacking applications.
For six underground stations and associated tunnels we provided detailed engineering services for all electrical and mechanical works including LV distribution, lighting, hydraulics, fire, SCADA, UPS and DG.
We were also design engineer for the Majestic interchange station between the north south metro and east west metro lines. Formerly a major bus terminal, the interchange station is designed to provide two levels of interconnected platforms, a dedicated underground subway and a main concourse area.