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Tunnelling by EPB Tunnel Boring Machine in DMRC

Jitendra Tyagi, Director (Works) DMRC, Saleem Ahmad, AGM/Civil, DMRC

Background

TBM at Factory
TBM at Factory
Delhi Metro Rail Corporation has been entrusted with the task of providing an MRTS network to the city of Delhi and National Capital Region. The network is partly at grade, elevated and underground. At present, 190 km of metro network is operational out of which 48 km is underground. In Phase III also, 45 km of section is planned to be underground. The underground network comprises stations built by cut and cover method, while tunnels are constructed by Cut & Cover, NATM (New Austrian Tunnelling Method) or earth pressure balance or slurry type shield machine depending on the geological strata of the project. Selection of TBM depends on ground conditions, surface features and dimension of the tunnel. DMRC used 14 tunnel boring machines in Phase-II to construct the tunnel of finished diameter of 5.8 meter.

Soil Investigation

The geological strata of Delhi varies along the project alignment and consists of compacted alluvium soil which is also known as Delhi silt, this is generally a fine grained material consisting of different variation between clay and silt with variable fine sand content. The strata consist of coarse sand, gravel and kankar. Soil investigation was carried out throughout the tunnel alignment before the start of the project to finalise the type of machine to be deployed for the project. The cross section of the tunnel varied in level above and below the ground water table. The overburden was between 3.5 to 22 meter as water level varied from 4 metre to 18 metres.

Earth Pressure Balance (EPB) Tunnel Boring Machine (TBM)

Depending on the geological parameter and construction programme tunnel boring machines of M/s Herenknecht, Mitsubishi, Kawasaki and OMC etc. were selected for the project. The time taken for the delivery of the machines ex-works to site arrival was around 10 to 12 month (manufacturing time being 8-10 months and shipping and road transportation of 2-3 months).

TBM-X Section
TBM-X Section

Specification of the TBM

TBM's used for the project were of the same specification with an external cut diameter of 6640 mm, 35000 KN thrust force and 630KW available power. TBM's were provided with an articulated tail shield to maneuver the sharp radius of 300m as per the alignment. Other additional features included soil condition and grouting system which was injected behind the tail shield during advancement of the TBM. The cutting wheel consisted of scrapers and bucket teeth for soft ground tunneling and disc cutters to ease the cutting of boulders and diaphragm walls on exit and entrance to/from the TBM launch and receiving shafts. A VMT guidance system was selected for survey to control the TBM alignment. The TBM parts consist of cutter head, front , shield, middle section, tail kin, manlock, screw conveyor, erector, erector carrier beam, bridge & back up gantries/systems.

Construction of Launching and Retrieval Shaft

Launching Shaft
Launching Shaft
Before the arrival of the TBM at site, the launching shaft upto the drop base slab was made ready for lowering of the TBM. Shafts were constructed by Diaphragm Wall with RCC/structural steel waler beams and struts at various levels as per design to support the D'walls against Earth and water pressures. Shafts were constructed at the ends of each station. In D'walls, glass fibre reinforced polymer (GFRP) rebars were placed at the location of TBM breaking and break out points known as the Soft Eye, instead of the usual steel bars, to avoid damaging the cutter head of the TBM. In addition, a false concrete wall of M10 strength was cast behind the D'Wall to limit ground forces and water pressure during TBM break out and break in. Similarly retrieval shafts were made ready before the TBM arrival out at the other end of the tunnel.

Tunnel Lining

Segment Yard
Segment Yard
The tunnel lining was provided with precast segments of 5.8 meter internal diameter with each ring comprising 5 segments and 1 key segment, each 1.2 and 1.5 m long and 275 mm thick. The segments were precast with M50 grade concrete at the casting yard at the extreme limit of the city, MUndka, 30 kms away from the site. The transportation of the segment was only allowed at night time and added a challenge to the tunneling works. PVC sheet was used in plastic sheath in tunnel segment as a conduit to accommodate curve bolt installation. Curve bolts were used to connect two segments. Grout pipes with lifting socket were used for grouting material & lifting during tunnel segment installation. Segments were cured by covering with tarpaulin sheet and in winter season steam curing was carried out. The segments were stored in maximum 8-10 layers. A combined Gasket made of an EPDM profile with hydrophilic top layer of size 33x16.1mm rated at 3bars working pressure was used at segment joints to prevent ingress of water. A trial Master Ring was formed at the casting factory before fabrication starts in full swing.

Arrangements in Launching Shaft for Lowering and Asse- mbly of TBM Parts

RCC Entrance packing was installed around outer periphery of the circular soft eye of the D'Wall for guidance of the TBM during the start of excavation. A Cradle frame fabricated from structural steel was placed on the drop base slab for supporting the TBM components and alignment of the Shield machine. 2 Nos. 100 T hydraulic jacks at the bottom of the shaft were used to push the assembly through the Safe Eye. After completion of TBM assembly, a shoving frame made from structural steel members was installed on the back side of the TBM. The shoving frame was erected to provide reaction to the forces required to propel the TBM into the tunnel during excavation. The backup gantry was set up at the ground level along with the power unit and was connected to the assembled TBM in the launching shaft through a number of hoses. Test run of TBM was done before actual starting tunnel excavation.

Assembly of TBM
Assembly of TBM

TBM Initial Drive

Ring segments were prepared (after installing gasket and timber packing) and inspected on the surface before sending down to the Tisted down from the surface into the shaft bottom by 15T Gantry crane. First, temporary segments & (dummy ring) were installed behind the TBM with the help of an Erector placed in the middle section of the TBM shield machine to make contact with the shoving frame. The TBM was pushed forward by means of TBM thrust Jack for a distance of approximately 1.5 m by taking reaction from the shoving frame. The shove rams are retracted and another temporary ring installed. In this way, by the 3rd temporary ring, the TBM cutter face almost touches the D'wall face entrance rubber packing is installed. The TBM then started cutting the D'wall through special type of cutter installed on the cutter head upto a length of approximately 1.5 m. The TBM was stopped and another temporary segment was installed behing dhe shield machine. In this way, total 7-8 numbers temporary rings were installed.

After installation of 7-8 number segment rings, excavation for the first permanent segment ring started. After TBM advanced of about 1.5 m the first segment ring was installed and grouted from the shield tail to fill the voids (annulus) behind tunnel lining by means of a pump located at the backup gantry. Grouting material consisting of a mixture of bentonite and cement with stabilizer (Liquid A) was mixed at the ground level in a grouting plant. Sodium silicate (Liquid B) was mixed with grout material at the tail skin of the TBM.

Muck was removed by means of a small muck car and transported to the muck pit at ground level a 35 T Gantry crane. A muck pit of capacity 900m3 was constructed beside the shaft wall at the ground for each tunnel drive. One excavator was used for each tunnel to load the excavated spoil inside the muck pit onto dump trucks for off-site disposal.

TBM at Initial Drive Stage
TBM at Initial Drive Stage
The muck car was working with the help of electric winches. A segment car was used for transportation of segmenting rings inside the tunnel. Two segment cars could take one full ring consisting of five segments plus one key segment. It was pushed inside the tunnel for erection of rings by erector of TBM. In the mean time, a temporary belt conveyor was installed at the outlet of the screw conveyor for handling of muck. Sleepers were installed at 1m intervals, laid on the invert segment. The lateral movement of the sleeper is restricted by the invert segment. Safe and elevated pedestrian walkways were also erected inside the tunnels.

A ventilation duct was provided for circulation of air from tunnel to atmosphere. The ventilation fan was installed at the surface and was provided with silencer in order to reduce sound pollution.

After excavation of about 80-100 meters (which was subject to design), excavation was stopped when the initial drive was completed. Average progress during the initial drive was 2-3 rings/day. Sufficient ring segments were stored at the shaft surface to ensure continuous TBM advancement. The segment erection plan was developed by the engineer prior to starting excavation on each shift. Drainage or waste water from the tunnel was pumped from the sumps in the shaft to the water treatment plant located on the surface so that muddy water or grout is not directly discharged into the public drains.

A maintenance team was assigned to perform daily inspections of the condition of the track (including walkway) and maintenance in the tunnel. At the end of a shift, tunnel workers were deployed to clean muck spilled over from the muck skips inside the tunnel invert.

In order to minimize disturbance to the ground surface, it was important to control the excavation method during TBM advance. Adequate earth pressure balance in the TBM mixing chamber was maintained and the quantity of spoil being removed during excavation for a complete shove was closely monitored and recorded. The TBM operated in untreated ground with a pressure greater than or equal to hydrostatic pressure at all times.

Arrangements for Transition from Initial to Main Drive

Temporary segments, cradle and shoving frame were taken out of launching shaft. Then, 05 nos. back up gantries consisting of the following were installed behind the TBM: Gantry No. 1 Grout Tank Gantry No. 2 Power Unit and Bentonite Tank Gantry No. 3 Electrical panel and Grease Pump Gantry No. 4 Transformer and cooling unit Gantry No. 5 Cooling unit and First aid kit The above Gantries were transported into the tunnel and connected to the Bridge Gantry. Before back-up gantries were transported into tunnel, segment hoists and other accessories were fitted on the top of back up gantries. The belt conveyor was then installed on the top of the rear gantry. On completion of installation, all power cables and hydraulic hoses were connected from back up gantries to the TBM. A car shifter (for changing of tracks) and working platform were installed in the shaft. The locomotive along with the Muck car and segment car were driven by diesel locomotive inside the tunnel, N and Y points were installed at the initial length of the tunnel and the single track was converted to double track from Y-point to the shaft bottom. A walkway was constructed and a water pipeline attached for circulation of water for the cooling unit as well as for heat generated by the TBM. A cooling tower was provided at the ground level. A permanent belt conveyor was installed at the discharge point of the screw conveyor of the TBM for carrying muck from the screw conveyor to the muck car placed beneath the conveyor belt. Separate supply lines for grouting made of MS 3" pipe for Liquid A and 1" for Liquid B were laid from the grout plant to the holding tanks at back up gantries in tunnel. The ventilation duct was 'hung' at the crown of the tunnel. The communication cables, lighting cables, CCTV cables, data management, the system cable and VMT guidance system cable were hanged on separate brackets fixed with the curved bolt of the segment. For 6.6 KV high voltage cable, separate hooks were provided at higher level along the tunnel. Extension of High Voltage (HV) cables in the tunnels was carried out by Licensed Cable Jointers.

TBM Main Drive

Segment Loading on Segment Car
Segment Loading on Segment Car
After installation and testing of TBM back up and all other associated equipments, TBM was restarted to commence excavation of the main drive. Soil was excavated until the space at the shield tail sufficed for the erection of one ring 2x3 muck skips (2 No's of train, each train consisting of 3 muck skip) having capacity of 12m3 excavated soil was enough for a ring of segments erection. During excavation, segments on the 2 segment cars were unloaded to the stock area near the shield tail. Excavation was done concurrently whilst grouting from the shield tail. Once the excavation sequence was finished, the locomotive hauled the skips and segment carrier cars out to the shaft. The erection sequence started while the locomotive was being hauled out. The segments were assembled individually by the erector from the bottom until the last segment piece. The next train was hauled into the tunnel consisting of segment cars and empty skips after unloading muck as explained earlier. Initially the progress of work was quite slow. About 7-8 rings were erected in one day. However, the progress of the work picked up and 12-15 rings average were erected in one day. Surveyor also checks the alignment and level to enable control of the next tunnel sequence to be made. The trigger value on alignment of TBM was set to be 50 mm.

TBM Retrieval Shaft

When the TBM approached, the retrieval shaft earth pressure on TBM was gradually reduced to zero. TBM proceeded to cut through the M10 concrete panels cast behind the M35 D'wall. After the TBM excavation stopped, the D'wall face was partly broken and TBM pushed through the shaft on to the arrival cradle placed on the drop base slab of the retrieval shaft. Before disassembly of the TBM, a Gantry crane of 125 MT with 2 numbers of crab of 100T capacities were installed in the shaft at GL. The TBM was dismantled section by section and shoved into the retrieval (arrival) shaft. The TBM parts were then transported through trailers to the next launching shaft for starting another TBM drive as described above.

Settlement Control and Monitoring System

TBM Breakthrough
TBM Breakthrough
As per the project requirement, ground movements were to be kept to a minimum and during design stage all existing structures needs to be assessed. Extensive instrumentation and monitoring plans were installed by forming arrays at regular distance intervals along the project route to check the actual settlement values obtained during TBM excavation. During the design phase a volume loss of 1% was used for the calculations of predicted settlement along the route. Maximum allowable settlement of 15 mm was proposed. Following instruments were used for general instrumentation: Ground settlement markers were used based on survey of the actual ground conditions:
  1. Soft ground – a rebar 300 mm long was fixed into the ground and the top part surrounded by concrete and if protection required then a plastic or metal cover was provided.
  2. Hard ground – the majority of ground settlement markers was placed in existing roadway by using a simple Hilti nail and this is hammered into the road surface or concrete.
The inclinometers and logging system were used for recording movements in both directions (i.e biaxial). Piezometers to monitor pore warer pressure (eg. Grount water drawdown) and water pressure. Crack meters were used to monitor propagation of existing cracks. The tunnel passed under important heritage structures and structures of national importance along the alignment. The tunnel crossed below buildings with basements, multi-storeyed building, railway crossing etc. Some of the structures were less than 1 tunnel diameter above the tunnel crown and hence additional investigations were done to minimize the settlement under the structure which included the following:
  • Extensive monitoring of the building including settlement points and reflective targets on the building were fixed. Glass slides were fixed within the structure to give an early visual indication of any movement within the building. An extensometer was modified with a dial gauge to give continuous readings. In addition a series of sub-surface settlement points were drilled in the basement which were attached to automatic dial gauges that display the settlement continuously on an analogue display.
  • Calculations of the TBM parameters to be adhered to while tunneling under these structures included i) earth pressure balance criteria ii) thrust pressure iii) excavation speed iii)rpm iv) grouting pressure and theoretical volume pumped into the area – in normal operations around 100% of the theoretical volume was used but in the case of tunneling under the structures this was increased to 140 to 150%.
  • Additional measures such as injection of bentonite around the front shield to limit settlement around the TBM.
  • Extensive technical support including survey teams with senior personnel around the clock deployed during excavation.
  • Additional precautions for safety with emergency call out set up in case the alarm value was exceeded or other associated problems occurred.
  • Precautions and discussions were held with the relevant personnel involved in these operations and this included the residents of the properties.

Conclusion

Tunnelling under varying geological conditions was carried out successfully without causing any disturbance to the city especially within the tight construction programme. In addition, settlement control and other precautions taken while tunneling under sensitive structure proved successful. Thoughtful planning and effective communication including identifying problems and proposing realistic solutions together proved a positive factor in successful tunneling operation.

NBMCW June 2012




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