Construction of Head Race Tunnel of Vishnugad-Pipalkoti HE Project

Introduction

The 444MW Vishnugad-Pipalkoti HE Project is under construction on the Alaknanda River in Distt Chamoli, in the state of Uttarakhand. With an installed capacity of 444 MW, it is one of the major ongoing Hydro Electric Projects envisaged to harness the huge potential of river Alaknanda.


This Project will generate 444 MW Power by installing four turbines of 111MW each. The central feature of this project is its 13.5 km long head –race- tunnel with an 8.8m finished diameter. Of its 13.5 km length, 12.3km is to be bored by using a double- shield TBM of 9.86m diameter and the remaining 1.2km to be bored with the help of a DBM.

The Project shall utilize about 240 m of water head of the Alaknanda River, available in a stretch from Helong in Upstream to Birahi in Downstream. It is a Run- of- the River (ROR) scheme using a diurnal storage. The annual energy generation from the project is estimated to be about 1813 MU.


The main components of the project are as given below:

• Concrete Gravity Dam of 65 m height with four under sluices for passing 8004 cumecs of flood discharge.
• Three Power Intakes followed by 03 Desilting chambers each with a size of 390 m (L) x 20 m (W) x 17.5 m (H).
• One Head-Race-Tunnel of 8.8 m finished dia and 13.5 Km length, to be bored with the TBM and DBM methods.
• An underground Machine Hall sizing 146 m (L) x 20.3 m (W) x 50 m (H).
• An underground Transformer Hall measuring 142 m (L) x 16 m (W) x 24.5 m (H).
• An underground Surge tank measuring 120 m (L) x 16 m (W) x 35 m (H)
• One Tail Race Tunnel of 8.8 m finished dia and 3.07 Km length.

General Layout of the Project: A 3D view is shown in Fig.1:

Vishnugad- Pipalkoti Head Race Tunnel

One of the time-consuming activities in any hydropower scheme is excavation of the Head-Race-Tunnel and its eventual lining. This is generally true for the hydropower schemes with Head-Race-Tunnels expan- ding in kilometres.

The Tunnel-Boring- Machine (TBM) has to be used to bore the tunnel in order to completing the excavation and lining in a calculated time. Parallelly, the number of Adits, otherwise required, accessing the HRT decreases in number. The project under discussion is “The Vishnugad- Pipalkoti -Tunnel” is the part of the 444 MW Vishnugad Pipalkoti Hydro Electric Project (VPHEP) presently under construction, is located on river Alaknanda, Uttarakhand State of India. It plans out an excavation of a 12.3km of Head Race Tunnel using the Double Shield Unit Rock Terratec TBM. The HRT is proposed to be bored from the Surge -Shaft end of the HRT and will move up the slope towards the Dam-site end of the HRT. The owner is “Tehri Hydro Development Corporation India Limited” (THDCIL), and the World Bank is the financer.


The TBM has already been transported to the site and fixed at the site-location. It is expected to start boring, any day, in its full swing. Figure 1 shows the Project layout and location of the project, and the Table 1 summarizes the geometrical characteristics of the TBM tunnel.

Geology Along The Tunnel Alignment

The TBM will execute excavation entirely inside the Pipalkoti geological unit. This unit consists mainly of Dolomitic Limestone (with minor inter-beds slate) making up about 33.3% of the 12300 m. This formation is characterized by the following distribution of classes and rock strengths:

• 44% in RMR class II and for 56% in Class III; •51% very strong, 23% strong, 16% moderately strong.
• And 3% weak Slate (with occasional bands of dolomitic limestone) forming about 64.1% of the 12300 m. This formation is further characterized by the following distribution of classes and strengths:
• 74% in RMR class II and for 26% in Class III
• 21% very strong, 64% strong, 15% moderately strong.
• Along the tunnel are foreseen several critical geological sections, namely:
• “Thrust zones” – 150m long thrust zone in dolomitic limestone (Tapovan thrust) under shallow cover.
• Shear zones – Five shear zones with maximum individual length of 50m and cumulative length of 200m
• Fault zones- Three fault zones with max individual length of 30m and cumulative length of 70m.
• These zones, although limited in the total length (320 m, 2,6% of the tunnel), will represent the most critical sections by presenting weaker properties and by being associated with local groundwater inflows. Figure 4, Figure 5 and Table-2 summarize the foreseen seismic data at TBM Adit, geological profile along the tunnel geological data.

TBM Transportation, Construction of Launching Adit Cavern, and Erection

Details of transportation, construction of launching Adit and erection of 140m long TBM has been explained below:

Transportation of TBM to site

Transportation of such a large -dia TBM to the project site was itself a very tough task. More than 132 containers having different parts of TBM had to be transported to the site 225km away from Rishikesh yard. Some 8-10 transporting agencies having long trailers for this purpose were engaged. Transporting material in such a hilly terrain was very difficult as some of the stretches were very narrow and for safe passing the trailer JCB, hydra cranes and loading equipment’s were kept ready to coup with any eventuality. Special care was taken of the 6-7 numbers of heavy containers out of these 132 containers.

In spite of such precautions, one container carrying bearing of a cutter-head got slipped on road while transporting. And being a delicate part, the same was, first, transported to Tehri workshop of THDCIL; and then after a thorough fitness- checking, was further transported to VPHEP TBM site. Trailer movement was not allowed in the daytime as the highway remained occupied by the vehicles used in the Char Dham yatra, border supplies and continuous military convoy movement for border areas. Consequently, it took more than 3 months to transport the TBM parts to the site.

Construction of Launching Adit

The HRT is to be approached by an Adit having similar cross-section.

In the initial reaches of the TBM Adit, the geology was found to be extremely poor consisting of clay with quartzite boulders. Thus, it was proposed to construct the Adit with Conventional Drill and Blast Method (DBM) till the proper geological strata was encountered for launching the Double Shield Unit (DSU) Tunnel Boring Machine (TBM) from that point onwards. This DBM stretch of the Adit was worked out to be about 50m inside the hill based on Geological and Geophysical tests carried out in this area. After weighing various options of supporting the Adit, it was decided to design the Adit as a shell structure consisting of Self Drilling Anchors and Lattice Girders embedded in 350 thick SFRS layer. The design was executed ensuring availability of SFRS shell at all intermediate stages of excavation of the Adit.


The Adit was designed to be excavated in three stages; each heading of 7m was followed by benching of the remaining portion of the tunnel cross-section in two benches. The excavation of the heading was achieved by supporting the crown by a 114mm diameter pipe roofing. The face was kept in stable condition by FRP bolts/SDA anchors along with facial SFRS. The deflections at the crown and the invert of the Adit were kept under permissible values with the type of arrangement provided above. The 114mm dia pipe roofing were intuitively installed with the help of one arm Boomer, by adding an attachment to hold the pipe roof and its drilling bit. This saved us the cost of costly equipment normally required for putting in place a large diameter pipe-roofing. The heading was achieved at an average rate of about 20m per month. The Benching work was taken up after extending the side lattice girders and spraying SFRS as per specifications. After completion of the excavation work, in-situ, 20m long RCC lining was fixed near the face of the TBM Adit for launching the Tunnel Boring Machine.

Erection of TBM

After having the entire TBM at the site, erection of TBM started at the TBM launching pad constructed near Haat village. Suitable cranes, viz TATA 955, Cole’s crane, Hydra cranes; loading equipment’s, rail tracks, skilled manpower and experts were mobilised to the site for erection of the TBM. Entire TBM system got erected within a scheduled time of 3 months and all the components/units got checked properly. Then the conveyor and other arrangements were also installed to make the TBM operational and a cutter head with all cutters fitted in place. Figure 9 show the erected TBM ready to start boring.

Failure of TBM Launching Cavern

After erection of the TBM, it was pushed inside the adit cavern by way of placing below the bottom- precast- segment and rail lines. After taking the cutter head 50m inside, near the face, the boring started by mobilizing the cutter head. Initially it went up to 7m and muck came out through conveyors as per the plan. The entire team was very happy on this success. However, suddenly, we observed that several big, isolated boulders of more than 1m3 size were hitting the cutter head with a lot of noise. The Technical team had to open the shutter on the cutter head and found that so many big sized boulders had stockpiled in front of the cutter head. It was decided with the client’s technical team and the experts that we stopped the boring and took remedial measures to clear the boulders from the face.

Rectification of Failure of TBM Adit

It was decided to immediately rectify the TBM adit by inviting proposals from the specialised agencies and the other TBM parties available in India. It was a huge In-house mind-boggling exercise, to overcoming the critical situation. Proposals given by the specialised agency were very expensive. Therefore, the design- team of THDCIL, HCC and the consultants decided to construct an additional bypass adit to reach the face to clear the boulders and strengthen the face and the crown. One bottom adit, through surge shaft access adit, was also suggested to reach the centre of the TBM adit to know the exact geology of the zone, so that the TBM could be started at the earliest obviating all uncertainties in the operations, conforming with a planned progress of 15m per day and 450m in a month.

Construction of Additional Bypass Adit and Bottom Adit

In a high-level meeting, it was decided to construct an additional bypass adit (ADA) just above the present TBM adit and a bottom adit through the access adit for surge shaft, to reach the location ahead of TBM cutter head at RD140m. Fig 13 shows the bottom adit and ADA locations. With the help of the additional bypass adit, we could reach the face and could clear the damaged pipes used in fore-poling and boulders assembled in front of the cutter head. It was followed by the treatment of the face by using chemical-grouting with microfine cement and colloidal silica and thus restored the face. The entire strata was RBM only, with layers of sand surrounding it. A lot of seepage was also observed at the face and proper dewatering system was adopted to take out the water.


After strengthening the ADA face and the crown, the bottom adit excavation work started in full swing to reach at RD140m of the 200m weak strata reach of the TBM adit. Luckily, we noticed that at the junction of the bottom adit and TBM adit at RD140m, a soft rock line strata was encountered in place of the RBM strata. At this place, we tried upon 2 faces and continued strengthening them both ways at crown using fore- poling with 114mm-dia pipes. Maximum focus was given to the face towards the TBM cutter-face area, and finally maximum length of the TBM adit toward TBM cutter was completed. TBM adit is now almost ready to restart the TBM, and we hope that this time it will be a successful journey as planned towards completion of the HRT in 24 months time.


For the remaining 1.2 km length proposed to be bored by Conventional Drill and Blast Method from Dam site area, we have already completed the same and till we set to the TBM again, we are moving ahead with the help of DBM in the battery limit of TBM tunnelling area adhering to the instructions of clients.

Conclusion

The Himalayan geology is unpredictable, and some experts are of the view that TBM tunnelling is not viable in this region. But the VPHEP team has learned a lot from this setback and finally the team has started the TBM boring successfully and achieved considerable progress of 431m in the month of Jan’24. We had succeeded in the Kishan Ganga Project where similar geology was encountered for boring a 14km long stretch with the TBM. With this experience, we recommend that before starting any TBM project in the Himalayan region, one must focus on the following points:

1. Proper investigation of rock geology and mapping of the area
2. Logistics of the area and condition of access roads
3. Arrangement of specialised manpower
4. Installation of equipment with automation and developing casting yards and areas for stockpiling
5. Training of working crews
6. In-house innovations based on site conditions
7. Cooperation and coordination of clients, contractor, and consultants
8. Teamwork with complete focus on project completion.