11 July 2020

Case Study-HCC

The CC30 package of DMRC is part of the 59 km long Majlis Park to Shiv Vihar Metro Corridor (Pink Line) of Phase III. The scope of work included design and construction of the twin tunnel between Shalimar Bagh and Subhash Place stations by Shield TBM, twin box tunnels by cut and cover method, underground ramp, architectural finishing of Shalimar Bagh station (underground) and Netaji Subhash Place station (semi-underground).


The notice to proceed with the work was issued on October 29, 2012 and HCC immediately undertook the detailed geo-technical investigation of the project along the alignment of the project. The soil was tested by drilling boreholes at nine locations and samples extracted were tested in the laboratory. The detailed investigation revealed that the soil along the project alignment was sandy silt and silty fine sand primarily. It was medium dense to highly dense at the depth of 30 meters. The ground water was encountered at about 11 to 15 meters depth.  The geology along the alignment of the tunnel was of mixed type.  (graphical representation of the geology).
Based on the geo-technical study done by DMRC during the tender stage, Earth Pressure Balance Tunnel Boring Machine was finalised for the tunnelling. Earth Pressure Balance (EPB) TBMs are used in excavation of soft ground or soil condition. The EPB method consists of cutting chamber located behind the cutter head of the TBM. This chamber is used to mix the soil with water foam. It is maintained under pressure by the mucking system. The ground at the cutting face is supported by earth pressure by balancing the advancement of the tunnel with the discharge rate of the excavated soil.

The underlying principle of the EPB method is that the excavated soil itself is used to provide continuous support to the tunnel face by balancing earth pressure against the forward pressure of the machine. The thrust force generated from the rear section of TBM is transferred to the earth in the cutter head chamber so as to prevent uncontrolled intrusion of excavated materials into the chamber. When the shield advances at the face of excavation, the excavated soil is then mixed together with a special foam material which changes its viscosity or thickness and transforms it into a flowing material. This muck is then stored and is used to provide support and to balance the pressure at the tunnel face during the excavation process.

The CC30 package orientation is north-south with Shalimar Station located on the northern end and an underground ramp on the southern end of the project. The northern boundary of the project is shared with CEC who is working on DMRC’s CC04 package and on the southern boundary where L&T is working on the elevated corridor package of CC28.



Construction sequence
The Shalimar Station location was the first area handed over to HCC for work. It is a complete underground station and goes up to 30 meters deep. After barricading the area, underground utilities shifting was the first task undertaken before commencing the excavation work. First the 1500 mm diameter PSC pipe line and MTNL Lines were shifted, after which the electrical lines of 11 KV and 33 KV were shifted. Prior to shifting, the permissions from TATA Power Delhi Distribution Ltd. were taken.

Shaft location next to the Netaji Subhash Place station on the southern part of the CC30 package, was the second area handed over to HCC for construction. The shaft is of 20 m in length, 17 mtr in width and is 12 mtr deep. Soldier piles are drawn at the periphery of the shaft to stabilize the ground. Between the shaft and Subhash Place station area is a 75 mtr long tunnel done by the Cut and Cover method. The entire length of shaft plus the cut and cover tunnel area was utilized for installation of TBM. After lowering the TBM part by part and assembling it in the Shaft and Cut and Cover area, it started its drive towards Shalimar Bagh Station. The Cut and Cover area was an added advantage to assemble the TBM in one-go before the start of the Initial Drive. The Subhash Place station was the next area handed over to HCC to begin work. This station is semi-underground as only 12 mtr of this station is below the ground level and the balance is above.  

In the Cut and Cover area there were three PSC pipe lines of 800mm, 900mm and 1100mm diameters which were to be diverted before the start of the excavation for which the approval from the owner agency was to be obtained. HCC initiated the documentation to seek approval. However, the permission formalities for shifting these utilities was taking considerable time. Hence, in consultation with DMRC, it was decided to hang these pipelines with the help of a temporary bridge to proceed with the work on the station and the excavation was completed. The station was built with the bottom-up approach where soldier piles are built first to stabilise the ground, then the excavation starts followed by the intermediate construction sequence.



Challenges encountered
While tunnelling in an urban environment, utmost care is taken so that the underground construction activities do not disturb the buildings on the ground. Along the alignment of the CC30 tunnel, there are various new and old buildings. A detailed study was undertaken to find out the status of various structures, their building foundations and adequate steps were taken including stabilisation of ground and continuous monitoring during the TBM drive so that these structures were not disturbed.  For instance, adjacent to the Shalimar Bagh station there is a shopping centre where the distance between the two is bare minimum. A rigorous scheme of instrumentation was set-up on this structure to measure deflections if any. Instruments like 3D tilt meters, Ground Settlement Markers (GSM), inclinometer...etc were set-up to measure the slightest variations as minute as few millimetres. These were monitored continuously during the construction phase.

The first major challenge encountered after commencing the TBM operations was crossing the via-duct. Around 138 meters from the TBM entry point the tunnel was crossing between the piers of the via-duct of an existing metro line. The depth of tunnel below the ground level under the viaduct was only 10 meters. While planning the project, DMRC had taken care to draw the tunnel alignment between the two pillars. The challenge was tunnelling between these pillars without disturbing the pillars in any way. HCC did a three dimensional analysis of the area using “Plaxis” software suggested by its Drawing Design Consultant (DDC). The instrumentation was in place to measure the volume loss during tunnelling and it was not allowed to cross 0.3 per cent. The soil condition was clayey with significant water presence. Hence the ground between the pillars was strengthened by TAM Grouting. TAM grouting is done by drilling boreholes in the soil and injecting cement slurry under pressure so that all cracks or fissures gets filled with the slurry. This process consolidates the ground so that there is no lateral deflection on the piles during tunnelling.  Around 90 boreholes were drilled between these two pillars to make the muddy ground hard for tunnelling. After consolidation of the ground a sample piece was tested for the required strength and then tunnelling process began under the viaduct. While tunnelling the vibrations caused by the TBM drive were measured. The vibrations during tunnel driving was less than the one caused by the movement of the train on the viaduct.  
The next challenge was tunnelling under an existing canal. The tunnel was passing under the canal at a depth of 14 meters. Though the canal had very less amount of water in that season, the lining of the canal was weathered. Due to seepage of water, the ground under the canal was muddy. A similar exercise was carried our while tunnelling under the canal by putting various instrumentation and regular monitoring of the soil conditions during tunnelling. Thus the TBM could successfully be used without disturbing the canal.

Rajesh Kumar, HCC’s Project Manager for CC30 project explains, “All along the tunnel alignment we installed intensive ground instrumentation and monitoring schemes such as ground settlement monitor and settlement markers in order to study the impact of TBM on above ground structures.The tunnel passed under some of the landmark structures such as Kasturba Polytechnic building, Kendriya Vidyalaya and even the slum area where the building conditions are very poor. In addition, while carrying out the tunnelling work, proper care was taken while finalising the alignment of the tunnel that it did not infringe the Pile area of the ‘Azadpur to Prem Bari fly over’.  TBM steering was difficult in the last 500 mtr excavation as the strata encountered in this stretch was clayey wherein driving of TBM was difficult. Despite these challenges we managed to complete construction of Tunnel 1 without causing any damage to the structures on ground and also without affecting the movement of the Traffic which runs over Ring road.”  

The tunnel boring began in October 2013 and completed the 1,247 meter long tunnel from Netaji Subhash Place to Shalimar Bagh consisting of 1,037 rings with a finished diameter of 5.7 meter in 111 days. The average monthly boring progress achieved during the construction was 337 meters with installation of over 9 rings per day.

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