High in the avalanche-prone and seismically active Greater Caucasus mountain zone of the Upper Svaneti, close to the northern border with Russia, contractor preparations are under way for excavation of what will be – at 31km – the longest TBM-excavated tunnels in Georgia’s history.
The Black Sea republic, which gained independence in 1991 following the collapse of the old Soviet Union and now stands on the cusp of integration into NATO and the European Union, holds a wealth of untapped water-based energy resources that in recent years have led to the consideration of variety of hydro schemes now at varying stages of development.
With political stability has come increased foreign investment in the country – notably from Japanese and Chinese equity funds, as well as the European Bank for Reconstruction and Development (EBRD) and various Indian and European hydro development companies – the result of which has been the availability of funding and loan guarantees to assist State financing of long-term energy projects, including numerous hydro schemes.
Some 80% of Georgia’s 2011 energy consumption was met by hydropower (now estimated at 85%) – one of the largest ratios for domestic hydro uptake in the world. With at least 300 major rivers identified as having hydro generation potential the country is estimated to have a capacity from this source of approximately 40 TWh (40,000 GWh) annually.
In 2011 total hydro generation in Georgia stood at 7.9 TWh – up nearly 30% from 6 TWh in 2004 but still barely 25% of the country’s total estimated capacity. Additionally, in 2007 the country became a net energy exporter, and since 2010 Georgia has exported electricity to all its neighbours: Russia, Azerbaijan, Armenia, and, principally, Turkey.
The business case for hydro investment is made even more compelling by the fact that peak hydro production comes in the Georgian summer – at a time when peak excess domestic supply coincides with the peak demand season in all its neighbouring countries. This business case is all the more attractive to foreign companies specialising in hydropower development since Georgia does not impose any form of taxation or VAT on energy exports, making it significantly cheaper to produce than the domestic electricity tariffs chargeable by most of its neighbours. Furthermore, schemes that make it to construction are currently bound to supply only 20% of generated power to serve the domestic Georgian market.
Against this backdrop, the latest of some 70 or so planned Georgian hydro schemes to go into construction is the 280 MW Nenskra Hydroelectric Project (NHP), located at an altitude of 1,400m asl and upstream of the existing Khudoni Dam which is itself subject to an environmentally controversial US$780 million redevelopment proposal for a three-powerhouse, 700 MW cascade system.
With design-build NHP construction awarded by the client at the end of last year (2015) to Salini-Impregilo of Italy for a contract sum of US$575 million, project scope for the latest scheme includes a 135m asphalt-faced rockfill dam – the highest of its kind in the world – a downstream surface-level powerhouse, surge tank, and penstock. For the contractor, successful completion of the NHP is likely to open up strong opportunities for future underground construction works in what is likely to become a key strategic market.
At the core of the Nenskra Project lies the 15.6km x 4.5m i.d. diameter headrace tunnel from the new Nenskra Dam and Reservoir, down through 679m of head to a powerhouse located just short of where the Nenskra River flows into the Khudoni Reservoir further downstream; and a 14.4km x 3.5m i.d. diameter transfer tunnel from an intake structure that will add to the Nenskra Reservoir’s capacity by feeding extra water from the Nakra River to the west (Fig 1).
Preliminary design and secondary stage preliminary design were completed by the client, the JSC Nenskra Hydro Consortium comprising the Korea Water Resources Corporation (K-Water) and the Georgian Government, with final design completed by the contractor’s engaged design consultant Lombardi SA Ingegneri Consulenti.
“The owner’s invitation to bid design specified open-face TBM excavation with a bolted and sprayed concrete lining for the transfer tunnel, and a double shield TBM with precast concrete segmental lining for the headrace tunnel,” explained Salini-Impregilo Project Manager Paolo Leoni. The contractor – unusually for a Georgian hydro project – proposed a segmental lining for virtually the whole of the underground works, using two double shield machines each on a single heading and with no intermediate adits.
Most hydro excavations in Georgia – like the run-of-river US$400 million 187 GW Shuakhevi scheme currently under construction by Turkish contractor ACT in the autonomous Georgian republic of Adjara, close to the southern Turkish border – feature networks of unlined tunnels excavated by drill+blast through hard rock. In the case of the Shuakhevi project – the first of what is planned as a three-scheme cascade system – some 38km of underground tunnels and adits are specified, along multiple headings.
For the Nenskra scheme drill+blast excavation is restricted to only minor portions at either end of both TBM headings; namely the headrace access adit downstream (which will serve as the TBM1 starter tunnel) as far as just short of the bifurcation to the penstock; its counter-excavation from the upstream portal (the headrace inlet) up to the gate shaft bottom chamber; the transfer tunnel access adit downstream (TBM2 starter tunnel) in the proximity of the dam area; and the transfer tunnel upstream access ramp down to the dismantling chamber at the Nakra Weir site.
“This method will allow us to construct the headrace tunnel gate shaft prior to TBM breakthrough, and will also enable us to recover a significant amount of good rock for the dam aggregate preparation, while along the transfer tunnel inlet drill+blast enables us to solve issues related to the gradient as well as the foreseen crossing of a potential major fault near the River Nakra intake,” Leoni told TunnelTalk.
Excavation on both headings will proceed downstream to upstream, net of the minor drill+blast preparatory excavations; the surge shaft at the penstock top end is disjoined from the headrace tunnel TBM excavation, and the gate shaft will be completed prior to the TBM breakthrough in order to clear it from the critical path.
Rather than allowing the geology to determine the type of excavation method, Salini-Impregilo has adopted an excavation approach that assumes mechanised excavation from the outset but which determines the specific requirements for the TBM in terms of thrust, torque, geometrical conicity, and the loading cases for the design of the precast segments.
“Over such long tunnels, especially the transfer tunnel which will be driven across a top elevation ridge, intermediate access adits are not possible and the portals remain the sole excavation origin,” said Leoni. “The preference for mechanised excavation derives from the much higher progress rate expected, making a single heading approach a worthwhile consideration.”
In addition to a number of Georgian hydro schemes already in construction, or bid to construction, six further major schemes of 100MW installed capacity or greater – two of them multiple cascade systems – are in the project pipeline.
According to Georgian Ministry of Energy figures obtained by TunnelTalk, as at 16 February this year a total of 115 hydro schemes are either under construction or subject to an ongoing or planned future feasibility study – for a total hydropower investment value of more than US$7.7 billion and an installed capacity of 4.16 TW (4,1600 MW).
The seven major (100 MW+) schemes that are listed by the Georgian Ministry of Energy as being most imminent include the planned 282 MW three-dam Oni cascade project which will require headrace and diversion tunnels totalling nearly 40km in length. Most projects will need at least some element of underground construction, with some requiring cavern excavations for underground powerhouses.
To make foreign investment in the many smaller (under 50 MW) schemes more attractive the Georgian Government is bundling them into groups and committing to selling state-owned land on which they sit at much-reduced prices.
Since the plan of attack includes drill+blast excavations at the opposite ends to each of the TBM headings, counter excavation remains an open option for minimising the impact of possible delays should unforeseeable conditions or events occur during a 62-month construction schedule that starts from contract award in September 2015.
Negotiations with a world-leading TBM manufacturer for supply of the 5.1m and 4m diameter double shield machines plus continuous conveyor system are at an advanced stage. "The procurement of the two TBMs is almost complete," Leoni told TunnelTalk. "All we are missing now is the actual confirmation of the order due to some further clarifications required by the client." This is ahead of a scheduled arrival on site date of 1 June 2017 for the 4m diameter double shield TBM (transfer tunnel); and 14 May 2017 for the 5.1m diameter double shield TBM (headrace tunnel). An excavation period of 24 months is anticipated for the 15.6km headrace tunnel, following scheduled launch on 29 June 2017; and 27 months for the 14.4km transfer tunnel, following a scheduled launch date of 26 July 2017. Advance drill+blast excavations at either end of both headings are scheduled to begin in November and December this year (2016).
Water ingress is expected to be “minimal” along alignments that follow a minor gradient but which are under mountain cover of up to 2,000m along the transfer tunnel and 1,300m along the headrace tunnel that runs along the left bank of the Nenskra River. “It is expected that the water table during operational conditions will be restored to its natural level, which is expected to be higher than the tunnel elevation,” explained Leoni. “During maintenance [emptying of the tunnels], the external pressure cannot be higher than the maximum sealing pressure, approximately 5 bar. This loading condition, however, has no relevance to the strength of the precast ring.”
The 4+1 design for the precast segmental lining of both tunnels features the same concept of a trapezoidal invert and key segment, plus three rhomboidal segments, in each ring.
“In the invert segment the rails for the service trains and TBM backup will be installed on precut notches,” said Leoni. “The rings will have two basic geometries – left and right turn – which when combined will make it possible to excavate in either a straight alignment or along horizontal curves. The invert element includes support pads in order to be stably laid on the rock, and these are able to support the whole service loads.”
For the headrace tunnel individual segments will measure 300mm thick x 1,400mm long for an o.d. of 5.1m and an i.d of 4.5m; for the transfer tunnel the segments are of a 250mm thick x 1,300mm long design for an o.d. of 4m and and i.d. of 3.5m. The segments will be joined by connectors and sealed using an EPDM sealing system designed to withstand up to 5 bar of pressure, with relevant grouting of the annular gap between the segment ring and the excavation diameter.
A single segment casting yard, serving both TBM jobsites, will be established at the Dam site. "Salini Impregilo will produce the segments itself, and transport them to the two TBM locations using 30-tonne capacity trucks, with two complete rings being transported on each trip," explained Leoni. Production is scheduled to commence in February 2017, at least four months ahead of the start of TBM excavation. A network of local roads will be built specifically to service the jobsites, and the last 20 km of the existing national road network prior to reaching the entrance to the valley, close to Kaishi Village, will be upgraded and refurbished in order to facilitate transportation of the TBMs and other heavy equipment.
TBM excavation, although relatively unusual for a Georgian hydro scheme is not unprecedented. In October 2014 a refurbished 5.3m diameter Robbins main beam TBM holed through to complete hard-fought excavation of a 5km-long headrace tunnel for the Dariali Hydropower Project in the mountains of Kazbegi near the town of Stepantsminda on the Russian border.
Driving through a geology comprising slate, sandstone, limestone and malms, and across a number of fault zones and areas of weak and fractured rock, the excavation took nearly 32 months – with progress hampered further by temperatures of up to -40°C and landslides that blocked the exit portal and the main highway serving the jobsite. Temperatures were so low that setup of the TBM using Onsite First Time Assembly (OFTA) was restricted to 1-2 hours at a time to enable workers to warm up.
Similar extremes of temperature, as well as rockslides and possible avalanches, are among the project risks for the Nenskra project, along with the logistical constraints of operating in a high-altitude remote location and to a demanding TBM excavation schedule. TunnelTalk will report on developments as they occur.