Delhi Metro is executing an MRT network of about 160 route kilometres in Phase Three which is a five year programme running from 2012 to 2017. Of the 160km over a quarter equivalent to 54km with 35 stations are underground. Out of the 54km route in underground sections, the length of single bored tunnel is 74.9km and rest of underground section/station is being built using cut and cover. Tunnelling for Phase Three has involved 36 TBMs from eight manufacturers, with 26 of them working simultaneously at the project peak in July 2014 to create the 74.9km of bored tunnel. By December 2015 62.5km had been completed. The tunnelling works have been executed under 13 construction packages. To build the enormous expansion to the metro a total of 53,700 concrete segmental rings are required to line Phase Three, and manufacturing is almost complete. The alignment has crossed below existing operational tunnels at three locations and passed beneath operational metro viaduct at six locations.

In addition, TBMs have crossed below the major vehicular outer and inner ring roads of Delhi at six locations, and below operational railway lines at two locations. Like any major tunnelling scheme the project has met with challenges along the way. Line Six of the network, which was built during phase 2 of the project, is being extended northwards by 7km through the heritage zone of Old Delhi, running adjacent to historical monuments such as the Red Fort, Jama Masjid mosque and Kashmere Gate. Here the geology comprises a silt or silty clay layer overlaying medium to highly weathered rock, with occasionally fractured and jointed hard quartzite at the lower levels, and with a water table at 10m below ground level. The alignment also overpasses the Phase One operating tunnels and was mined within 500mm of the foundation piles of an existing interchange station, with only 7mm settlement of the station structure recorded. The Jama Masjid to Delhi Gate station tunnel drives were originally planned with two TBMs. Due to the extremely hard rock experienced, progress of the first TBM was extremely slow with 39 interventions carried out to replace 204 single disc and 85 double disc cutting tools. Progress problems were compounded by a leaking bearing seal, and attempts to replace this met with limited success. It was therefore decided to deploy a third machine to drive from the opposite end at Delhi Gate station, and build a new retrieval shaft to lift all four TBMs. DMRC organised the evacuation and resettlement of 900 of the residents living along this area of Old Delhi, as the buildings were in various states of fragility, and vibrations from the rock tunnel drives, while within the allowable limit of 5mm/second, was considered too high a risk to the structures.

Line Seven

The new Line Seven in North West Delhi is an orbital route following the alignment of the Inner Ring Road, a major traffic arterial road circling the city. After departing from the Depot at the north west limit of this line, the alignment quickly changes to a three station underground stretch. The tunnels connecting the first of these stations, Azadpur, to the at grade ramp, cover a distance of 1,100m, and are at a depth of 21m at their low point. During tunnelling, both TBMs met with a large mass of intact rock which infringed the lower portion of the tunnel face and extended for approximately 30m. The TBMs used were both soft ground machines which had been used on a previous phase of the metro, and experienced high torque and overheating when attempting to cut through the rock zone. An attempt was made to break up the rock ahead of the cutter face by drilling and placing an expanding chemical to burst the rock and allow the cutter head to rotate and advance one ring at a time. The space within the chamber available to manage this exercise was very limited, and matters were made worse by the presence of wet running sand above the rock zone which dictated working under compressed air.

To add to the challenge this section of Line Seven is planned to be open by the end of 2016 and progress is therefore critical to the overall programme. The advance being achieved in the rock was very limited, and a decision was made to deploy two additional TBMs equipped with suitable cutter heads to tackle the rock, from the planned receiving shaft, working back to meet the stranded TBMs; all four machines would be removed from a new rescue shaft. Preparations are currently in progress to start these drives in January of 2016.

Line eight

In southern Delhi challenges were experienced along various stretches of Line Eight due to the presence of hard boulders located in a soft mica schist matrix. The boulders would often dislodge from the matrix after cutter face contact was made, and there was constant manual input required to break up or transfer these from the shield chamber.

In addition, the risk of grinding and movement of the boulders within the softer matrix often resulted in voided areas arising ahead of the cutter face with minor rock falls resulting from the filling of these voids.

Tight control of tunnelling was particularly required to review abnormal readings in the theoretical volume excavated, and cement slurry injection into the voided areas to lock the boulders in position and allow cutter head rotation and grinding of the rock proved to be successful.

On only one location along the alignment, a cavern became evident in the early stage of the drive east of Vasant Vihar station, with the space ahead and above the shield revealing a void which required 200m3 of cement/ fly ash mixture to be pumped in through the cutter head to infill the cavity before advancing the TBM. The full procedure is detailed in the box.

Lessons identified by DMRC

  • The contractors should be pressured to carry out a more intensive soil investigation program at the start of the contract. As a minimum, probing at regular intervals, say 25m, to establish the presence of rock, should be carried out before TBM selection is accepted.
  • There is a marked difference in the performance of TBMs from the range of manufacturers used for the project. The peak of 26 machines mining at any one time reflects the very high demand for equipment that had been expected from the outset. The client needed to be realistic in terms of his expectations, top of the range machines were unlikely to be available in numbers to satisfy the resource requirements of the project plan. Nevertheless, the versatility of the TBM to tackle varied ground conditions and it’s track record before deployment certainly require very close scrutiny for any major project attempting to deliver to a tight timeline. A simple example is that certain machines allowed for the easy replacement of main bearing seals within the tunnel, others failed to successfully carry out this work.
  • The joint ventures selected for the works should be closely examined for their international exposure. There have been examples on Phase Three of international partners failing to perform to expectations due to lack of experience in contracting in overseas environments.
  • Do not overlook the challenges of cross passages. If NFPA 130 is used as the guideline for tunnel safety, the number of passages can be large; there are over 140 required for Phase Three. The need for experienced mining personnel to manage the ground treatment, excavation and lining works in areas with soft, wet ground is paramount, and the time frame for executing these works can easily delay ongoing services installation, including track laying, in the tunnels.