The Bangalore metro project boasts the first TBM to be used in a southern Indian state and the first slurry TBM to be used anywhere in the country. As with many Indian cities, crowded, narrow streets define an outsider’s view of Bangalore. But being around 1,000m above sea level and armed with a booming IT trade, the city is more affluent and also cooler than many of the other cities clamouring for metro access.
The project comprises a north-south and an east-west line, the latter being constructed by a JV of Taiwanese contractor Continental Engineering Corporation (CEC) and India-based Soma. CEC’s Indian arm CICI is carrying out the tunnelling works and Soma is excavating the stations.
Lay of the land
The Peenya precast yard supplies the metro project with tunnel lining segments. The plant is 20km from the site. “We estimate that it would be economical up to a maximum distance of 35km if we are only supplying segments and not our ready mix,” says CEC project manager Russell Brown. After that the land prices do not change by much, and the increasing transport costs would be the only factor.
“We are also central to the delivery of raw materials, so there is no benefit in moving closer to one of them at the expense of the others.”
Brown adds that the two batching plants – Plant One supplying segment concrete and Plant Two acting as a backup as well as supplying concrete for civils work, including ramps, track bed concrete and cut and cover tunnels – take deliveries only during the night shift to avoid traffic congestion, so moving further away would be undesirable for this reason too.
India-based batching plant manufacturer Polytech Automation supplies the plants that were purchased new by CEC some three years ago. The plants were installed in November 2010 and operated in house by CEC personnel. In addition to Brown, Albert Chiang is CEC/CICI deputy project manager, Kasem Reinpracha is CEC/CICI factory manager and William Chan is factory manager for YaLi.
Plant setup
Segment production is subcontracted to Taiwanese segment manufacturer YaLi, the same company used by CEC for segment production for the Delhi metro project. It is one of the largest such contractors in Taiwan and China.
“The moulds we use were manufactured by CBE in China and are steel,” says Brown. “They are built to French standards and are checked by Mott MacDonald.” Static moulds are used, six for the 5.6m internal diameter rings for the Hitachi Zosen TBMs on the project and two for the 5.8m internal diameter rings for the Robbins machine.
Static moulds are in use because of the low capital expenditure compared to a carousel system (see feature, page 52). It is beneficial to lower investments when continuity of projects is unknown. Brown adds that CEC is not currently considering supplying concrete or segments to other contractors. Indeed, due to the rate of progress, Peenya would currently be unable to supply other contracts.
The production rate is two segments per day per mould segments as T&TI goes to press. Brown says that they have only just started to cast the Robbins. The site is ahead of schedule with its segment manufacturing.
The factory process works six days per week on a 24-hour cycle. This equates to 72 rings per week, with a daily production rate of 12.
Steam curing is used to accelerate the process and, upon stripping, the segments are sprayed with a resin-based curing compound. Rate of heat loss is closely monitored to ensure it remains in the 22°C heat loss curve. Actual heat loss is approximately 7°C per hour, says Brown.
Segments are handled by scissor clamps and suction pads. The suction pads are more ideal as the stripping strength is 12N/mm2 as opposed to the 15N/mm2 offered by scissor clamps. The suction pad, supplied by CBE, allows the segments to be stripped earlier. T&TI was onsite in June when the vacuum lifter was delivered.
Converted tipper lorries transport the segments from the yard to the tunnel site. Three segments are carried per lorry and are stacked with the concave facing up.
Segment design
The mould conceptual design came from Chris Smith of CRS consultants and Mott MacDonald’s UK office. Yogini Vimalanathan of Mott designed the segments, which were checked and approved via Bangalore Metro Rail Corporation’s chief engineer for design and underground stations N.P. Sharma.
Vimalanathan details the general design criteria of the segmental lining for the Bangalore metro. “On the dimensions, the 5.6m internal diameter is dictated by the train structure gauge, provision of a third rail, services and fittings of a raised walkway. A surcharge load of 50kN/m2 is stipulated in the client’s requirements and ground water leakage must not exceed 0.1l/m2 per day for the segmental lining with no more ingress than 0.2l/m2 per day for any 10m length of tunnel.
She adds that casting tolerances are very tight, with a tolerance on radial joint surface of +0.5mm/-0mm and a tolerance on segment width and circle joint surfaces of +1mm/-0mm. Tolerances on the trial ring were +8mm/-0mm for both the outer and inner diameter and +/-20mm for the outer circumference, which is measured on three levels.
“The precast concrete segments are subject to short term and long term loads,” explains Vimalanathan. “Short term includes handling, stacking, erection, TBM shove forces and annular grout pressure. To resist these loads, two layers of reinforcement are provided along the intrados and the extrados of the tunnel lining.
“There are also load conditions when the segments would not have reached the 28 day cure strength of 50N/mm2.”
Vimalanathan continues, “De-moulding of segments takes place within 24 hours of casting. At this stage the concrete strength has to be a minimum of 15 N/mm2 and stacking segments can only take place when the cure strength has reached 25N/mm2. Timber battens are used to separate the segments when they are stacked with a tolerance on the batten position of +/-100mm.
“When the segments are in the ground the ring is in compression and the segments are designed as short columns subjected to axial forces and small bending moments. The most highly stressed zones in a segmental concrete lining are the circumferential joints, through which the TBM shove forces are transferred and the radial joints, through which large compressive hoop forces due to ground loading are transferred.”
Brown adds that only steel reinforcement is used, and no fibres are added.
The full circle
“The ring consists of a 72° trapezoidal segment, four 67.5° rhomboidal segments and an 18° wedge shaped key,” explains Vimalanathan. “The angled radial joint across the ring is sloped at 1:7. This arrangement ensures that cruciform joints never occur. The tight interlocking configuration of the segments forces the tunnel ring to be built circular and prevents ring ovalisation due to bad build.
“The 1.5m-long ring minimizes manufacturing cost and maximises the rate of construction. To negotiate the alignment curvature a suitable ring is needed to correct for line and level during construction. The tightest curve on the bored tunnel alignment is 270m. The tunnel ring has a 55mm taper to accommodate a 170m minimum curve, which allows for error corrections if the TBM goes off line.
“The radial and circumferential joints have flat surfaces with chamfers and rebates along the edges and corners of the segments.”
“The chamfer and the rebate prevent edge contact, which could result in spalling during ring erection, TBM shoving and subsequent loading of the lining. A 3mm plywood packer is glued to the trailing edge of the ring to assist in smoothing out any irregular surfaces caused by lack of fit of segments and to spread and assist in absorbing TBM thrust ram loads.
“The circumferential joint contains dowels at 22.5°. The dowels are anchored using embedded sleeves in the pre-cast concrete segments. The dowels act as locating devices speeding up the ring erection and enable the ring to be built circular.
“The universal rings are rolled to achieve the required alignment with the key taking up any of the 16 positions around the circle. “The radial joints have two spear bolts [by Cooper & Turner] per segment.”
Vimalanathan adds, “It is normal practice to reduce the moment of inertia of a segmental lined tunnel ring depending on the number of joints and treat it as a flexible ring. This is advantageous as it attracts less bending moment under non-uniform ground loading.
“However, on the Bangalore Metro, due to the trapezoidal and rhomboidal segments, which nearly locks all the joints preventing articulation about the radial joints, the ring was considered to be rigid.”
Co-extruded Datwyler composite gaskets are used, consisting of an elastomeric carrier (EPDM) and hydrophilic facing material. The gaskets resist a working pressure of 2.5 bar and a test pressure of six bar at a 10mm offset and 8mm gap.
Quality control
Brown tells T&TI that quality control is handled by having a close working relationship with their client Bangalore Metro Rail Corporation and the client’s agent general consultants Rites-OC-PBISystra, who compliment the CEC quality control and assurance team. He reports 0.5 per cent rejected segments so far, saying that every raw material wagon is tested for conformity of cement, sand and gravel. Initially the site had a high failure rate of raw aggregates, but with the suppliers now aware of the standards demanded, Brown says that the failure rate has become insignificant. He credits this to the onsite CEC quality control department, which includes client engineers, general consultants and is managed by general consultant project manager Nigel Butterfield for underground works.
Challenges
One complication that has to be worked around on a daily basis is that of water supply. “With water shortage and mains supply issues, we have to provide water from our own bore wells,” says Brown, “These draw water from 200m below and we constantly test for compliance. It actually needs to be chilled during the summer months to avoid overheating during the initial setting of concrete.”
Vimalanathan concludes, “With tapered rings and trapezoidal and rhomboidal segments, calculating the segment dimensions proved extremely arduous on the Bangalore Metro. It is important to note that when calculating segment dimensions the segment width should always be calculated using the projected length of the segment on the plane containing the minimum and maximum ring widths and not the arc length.”
A resin based curing compound is applied upon stripping Peenya precast factory makes use of CBE moulds. Segment production is subcontracted to YaLi The steel CBE moulds were built to French standards
