Excavation of the second TBM project in Nepal is awarded to the same contractor as the first. After completing the Bheri Babai segmentally lined TBM drive water project in 2019, COVEC, China Overseas Engineering Co, is awarded the 13.3km long Sunkoshi Marin diversion tunnel, which is also specified as a segmentally lined TBM excavation.

Fig 1. Geological profile of the 13.3km x 6.4m o.d. water transfer drive

The COVEC bid for the project at NPR10 billion (about US$84 million) is the lowest of bids received and near a reported NPR 6 billion ($50 million) below the official Government estimate NPR15.84 billion ($134 million) for the project. Its proposal to reuse the Robbins double shield TBM as used on the Bheri Babai project and much of the equipment procured for that earlier TBM operation, together with the experience gained and the skilled managers and workers moving from the first to the second project, accounts for bidding advantage. The TBM shield and cutterhead and the segmental lining casting equipment will require modification to adjust from the 5.06m o.d. (4.2m i.d.) of the 12km long Bheri Babai project and the 6.4m o.d. (5.5m i.d.) of the Sunkoshi Marin project. COVEC is in discussions with Robbins to manage the TBM rebuild.

The Robbins TBM will require a new shield and cutterhead for its larger diameter second project

The 13.3km Sunkoshi Marin diversion project will divert part of the flow of the Sunkoshi River into the Bagmati River to irrigate farmlands in the Tarai district. According to project officials, the irrigation scheme, of which the diversion tunnel is part, is estimated at NPR 37.3 billion (US$311 milllion) with the associated multi-purpose project hydropower component estimated at NPR 46.19 billion (about US$385 million). Sushil Acharya, Project Director for the Nepal Government funded project reported that the tunnel has a strict construction period of 27 months.

Where TBM advance rates of more than 1,000m/month were achieved in more favourable ground conditions than expected on the Bheri Babai project, geological risks to the TBM drive for the Sunkoshi Marin project under a high overburden, are higher with the anticipation of shear zones, squeezing, slabbing and bursting conditions, high water ingress and fault zones. With its experience in hand, COVEC is equipped to manage the conditions as encountered providing the contract has the contingency funds and the mechanisms to cover the time and costs that will be required to overcome these anticipated challenges.

Nepal has released an invitation to prequalify for its second precast segmental lined TBM drive for the Sunkoshi Marin diversion multipurpose project. Building on the success of the first application in Nepal of a double shield TBM for the segmentally lined Bheri Babai diversion tunnel, the new application will excavate and line a 13.3km, 6.4m o.d. (5.5m i.d.) tunnel to divert 67m³/sec of water from the Sunkoshi River to the Marin Khola, a tributary of the Bagmati River, to augment the Bagmati irrigation scheme by an additional 122,000 hectares of land and generate 30 MW of electricity in a hydroelectric installation (Fig 1).

Prequalification applications, called for the diversion/headrace tunnel segment of the project by the Department of Water Resource and Irrigation of the Nepal Government, close on 4 November with invitations for the shortlisted applicants to bid expected to be released in December 2020. The overall contract period is 75 months, including a 24-month defect notification period, and the conditions of contract will be FIDIC Pink Book: The Multilateral Development Bank Harmonised Edition (2010). Prequalification is open to applicants from all countries.

Fig 1. Location of the Sunkoshi Marin project as the second TBM excavation project in Nepal after the successful Bheri Babai project

The overall project is budgeted at near NPR 100 billion, about USD$850 million, of which the tunnel excavation element is estimated at NPR15.84 billion ($134 million), the irrigation works at NPR37.3 billion ($314 million) and the hydropower component at NR46.19 billion ($390 million), all excluding taxes and duties. The project is to be funded by the Government of Nepal and is a designated Priority 1 project under the scheme of National Pride Projects which assures cash flow commitments from the Government of Nepal.

The irrigation element of the project includes construction of and outlet structures on the Marin River. The hydro power plant contract includes construction of a 12m high diversion dam and intake structure on the Sunkoshi River, a surface power house, a surge shaft and tailrace and installation of four generator turbines (Fig 2).

Located about 150km from the capital city Kathmandu, the 13.3km long TBM contract includes the construction of about 2km of access road, about 1km of highway diversion roadway, the TBM launch and working site, the TBM exit platform, about 2km of river retaining works, construction of a camp for the contractor, consultant and employer, and some geotechnical and geophysical investigation works before, during and after the construction. About 900,000m³ of earthworks are required for site establishment.

Progressing from the Marin River outlet portal, the 6.4m o.d. TBM is anticipated to pass through the series of competent and incompetent rock masses of the Siwaliks and the Lesser Himalaya and expected to encounter the conditions typical of young high overburden geology including shear zones, squeezing, slabbing, bursting, high water ingress and fault zones. The unconfined compressive strength of rock samples collected on the surface along the tunnel alignment range from tens of MPa to hundreds of MPa with the tensile strength logged at a few MPa to few tens of MPa (Fig 3).

Fig 2. Layout of the hydro plant and tunnel outlet

Fig 3. Plan of the tunnel alignment

The geology along the alignment was further described in details provided by Ajay Raj Adhikari, Senior Geotechnical Engineer for the project. About 4km of the alignment passes through fluvially originated, relatively young Siwaliks of the youngest Middle Miocene to Early Pleistocene mountain range of the Himalayas. While in the Siwaliks, the tunnel passes through all three formations of Siwaliks that consist mostly of interbedding sandstone, siltstone, mudstone and conglomerate. The overburden in in the Siwalik region varies from tens of metres to about 550m.

Except for about 1km in a slate formation that belongs to the Lesser Himalaya Region, the majority of the remaining tunnel alignment, or about 8.3km, passes through the relatively old Higher Himalaya Region mountain range formed during the Paleozoic Era. This region is mostly composed of medium to highly metamorphosed schists, quartzite, granite and gneiss with occasional calcareous beds of limestone and dolomite. Because of the long stress history of the mountain range, this region consists of several minor and major folds, faults and shear zones with the alignment passing through the major syncline of the Nepal Himalayas known as the Mahabharat Synclinorium. The TBM will have to bore through the core of the Synclinorium where the overburden is at its highest of about 1,320m, and through the anticipated granite rock type (Fig 4).

The two contrasting geological units of the alignment are separated by a low angle reverse fault known as the Main Boundary Thrust. On the surface, this fault is marked by the Dhanamana Khola where the overburden to the tunnel horizon is about 200m. In addition to that, another technic boundary, the Mahabhrat Thrust, is present immediately after the slate formation and between the Lesser Himalaya and Higher Himalaya. The thrust area is marked by a breakage in topography in the tunnel alignment. Another Mahabharat Thrust will be found at about chainage 13+020m. Another perennial river, the Tyan-Tyan Khola, crosses the tunnel at about chainage 10+550m and at an overburden of about 500m. As the rocks are dipping north or south at different angles, the TBM will have to bore through mixed face conditions of competent and incompetent strata.

Fig 4. Geological profile under the mountain range of the Himalayas

Due to uncertain geological conditions, systematic probing is a specification with the minimum length to match the average excavation rate of TBM. Ground engineering methods including cement/chemical pregrouting, forepoling and umbrella pipe roofing, are to be anticipated. Squeezing conditions may require profile over excavation and in extreme cases, TBM bypass headings.

Following the example of the previous 12km long x 5.06m o.d. (4.2m i.d.) Bheri Babai project, a double shield TBM is specified for the new 13.3km long x 6.4m o.d. Sunkoshi Marin drive erecting a specified 5.5m i.d. precast concrete segmental lining with the annulus backfilled with pea gravel and grout. A refurbished TBM is permitted with the prequalification documents requiring proponents nominate their TBM supplier and to include a signed TBM design and supply agreement document noting the TBM model and serial number, its year of manufacture, the projects on which it has been used, the design life of the main bearing and the remaining life of main bearing. The signed agreement is to also confirm that supply of a suitable TBM will be within not more than 15 months from purchase order and that the manufacturer will assist and support the operation and maintenance of the machine throughout the contract period.

In the case of a reengineered machine, the original equipment manufacturer will have to guarantee the following in the agreement with the proponent including:

• The condition of machine and declare that it can be fully reengineered and equipped to bore through all likely geotechnical conditions of the project;
• Has a minimum 7,500 hours remaining design life of the main bearing;
• Has not previously bored more than 15km of tunnel;
• Is not more than 7.5 years old from the date of the original factory acceptance test;
• Commitment of the original manufacturer to reengineer the proposed TBM; and
• Commitment of the original manufacturer to be responsible for excavation of the initial 500m of the tunnel.

Qualification requirements for supply of a new TBM requires that the manufacturer:

• Has at least 10 years of independent experience in the design and manufacture of a double shield TBM and its necessary ancillary facilities;
• Has independently designed and supplied at least five double shield TBMs in the last five years;
• Has supplied double shield TBMs to at least five successfully completed tunnels of 10km or more; and
• Its double shield machines have successfully completed at least an accumulated 200km of tunnel.

A similar signed agreement is to be part of the prequalification application with the manufacturer of the moulds and steam curing facilities for the precast segment production factory, stating that the supplies will be of a certified quality by a related institution and delivered to site within reasonable time period. The segmental lining will be about 30cm thick and will require up to 85,000m³ of cement, 13,000 tonne of steel reinforcement and 45,000m³ of pea gravel and annular grouting.

The TBM launch portal at the outlet end of the headrace is about 950km from the port of Kolkata, India, as the nearest entry sea port for the import of TBM components; and the nearest market town, Sindhuli, is about 15km from the working site for procurement of cement, reinforcing steel and other supplies. Aggregate is available from a deposit on the Marin River adjacent to the jobsite.

Interested eligible applicants may purchase the prequalification documents from the Department of Water Resource and Irrigation for a NPR 15,000 ($125.00) fee and a pre-submission meeting is to be held at noon on 11 October at the Department of Water Resource and Irrigation offices in Kathmandu. Applications are to be delivered electronically on or before 12 noon local Nepal time on 4 November and will be opened also on 4 November at 2pm local time in the presence of representatives who chose to attend the opening procedure.

Having one highly successful double shield project to its credit in the Bheri Babai project, and with a second now in procurement, there is news from Nepal that the second phase of the Melamchi water supply tunnel for the city of Kathmandu is also to use a TBM for excavation of the 10km long tunnel. The 27km tunnel of the Melamchi Phase 1 project was excavated by drill+blast. Despite working from eight headings – the two portals and in both directions from three intermediate adits – the project suffered serious difficulties, major delays and changes of contractor.

The benefits of a robust TBM in the hands of an experienced contractor are the advantages that Nepal is now seeking to repeat for its new long-distance tunnels, under high overburden and into geological conditions that are anticipated to hold serious challenges.