Expanding underground

Among the many construction projects underway in the hydropower sector are those that involve expansions around existing infrastructure, at both pumped storage and conventional plants. These projects are being driven by increased demand for energy, especially clean, green or low carbon, but also the need for even more rapid-generation capability to help control the intermittent outputs of some renewables, such as wind and solar.

The construction works present a range of challenges, not least of which are the significant tunnelling works where underground enlargement is the way ahead, such as for the likes of the Linth-Limmern and Veytaux schemes in Switzerland, Limberg II in Austria, and the Theun Hinboun Expansion in Lao PDR.

The technical obstacles on those underground construction projects include: steeply inclined tunnels that are bored by TBMs or employing such a machine where it is the first time it has been used in a country; difficult geology causing rock spalling and crown collapse; and, insitu site investigation of insitu rock mass within confined space and with limited time.

Theun Hinboun Expansion

Another TBM success, again for a Robbins machine, was the recent hole through at the 280MW Theun Hinboun Expansion project in Lao PDR, which was also the first use of a TBM in the country.

The project is being developed to provide power domestically (60MW) but also for export to neighbouring Thailand (220MW). The developer is Theun Hinboun Power Co – a joint venture of Electricte du Laos, GMS Power of Thailand and also Stratkraft, the Norwegian energy utility.

Construction of the project is being performed by Italian firm CMC di Ravenna though it subcontracted Cavico to execute some underground works. Main tunnelling work was undertaken using a 7.6m diameter single shield Robbins TBM, which bored the 5.5km long headrace through varying sedimentary strata. A segmental tunnel lining was constructed to complete the 6.9m i.d. tunnel.

The shield was designed to allow for over-cutting should the soft ground, and overburden of between 26m-380m, cause some squeezing conditions. Probing ahead of the machine was done up to distances of 60m. A wet fault zone was encountered about three-quarters of the distance along the drive and was sealed ahead using foam delivered through a borehole, enabling boring to continue with relatively little delay.

While the TBM broke through in late 2010 – having progressed at 19m per day on average, peaking at 37m – the machine did not bore the entire tunnel. The last 100m long stretch would be opened up by drill and blast excavation after the wet season, and meanwhile act as a natural plug because the tunnel alignment is at a relatively low level.

Limberg II

Yet a further use of a TBM in a recent hydropower enlargement was the employment of a Aker Wirth shield on the Limberg II pumped storage project, in Austria. The wide range of excavations were required for the project – TBM as well as drill and blast method.

The 480MW Limberg II project is an extension to the 353MW Kaprun scheme, and is being developed as a peak-load plant by Austrian Hydro Power, part of the Verbund group. The new plants is due to be completed in 2012.

While the project will draw upon the existing Mooserboden and Wasserfallboden reservoirs, which are located in Kaprun valley, in the province of Salzburg, it requires significant underground construction for all key hydraulic conduits and access routes.

The powerhouse cavern is 62m long by 25m wide by 44m high, while the transformer cavern is of almost the same length but 15m wide by 16m high. Underground works also include a 4km long headrace, 750m long inclined shaft, 600m long tailrace plus a surge and also valve chambers.

The majority of the headrace was bored through basalt and gneiss at a steep incline by a 7.03m diameter Wirth open hard rock gripper TBM. The rock has an average Unconfined Compressive Strength (UCS) of 100MPa though in stretches the value did reach up to 150MPa. The TBM completed its drive in the second quarter of 2008. Other tunnelling works in tunnels and caverns were undertaken by drill and blast.

Much of the length of the headrace was unlined but along some sections there was support provided, including shotcrete. To help with the extra support for the pressure tunnels, and also secure the cavern excavations, there was a wide range of products provided to the project by the chemical division of BASF. A further challenge of the shotcreting and concreting works was logistical due to limited safe access to the high mountains for re-supply in the winter months, and so particularly careful planning of quantities of materials needed – cement, aggregate, chemical accelerators, and their storage – was vital at the batching facility.

In the inner lings of the pressure tunnels and also the shafts – which need to work in operational conditions with water pressures reaching up to 40 bar – a super-plasticising admixture GLENIUM SKY 582 was used to help achieve high concrete strength within the formwork carriage. BASF said the GLENIUM admixture is 3rd generation ‘hyper plasticiser’ based on polycarboxylic ether polymers, and key feature is the slow down in cement dispersion which in turns helps to better maintain the consistency of the fresh concrete with improved workability and no reduction in early strength gain.

For high early strength development in shotcrete used in the caverns and other tunnels, a liquid admixture MEYCO SA 180 was used as an accelerator. The product also helps shotcrete to be applied in thick layers, if necessary, and dosage can be varied to adjust setting and hardening times. A further product used, to enable concrete to stay open and usable until application, was the stabiliser DELVOCRETE.

Contractor on the tunnels and overall construction is a JV led by G Hinteregger, and includes Porr Tunnelbau.

Linth-Limmern

In Switzerland, two new large caverns plus associated hydraulic and access tunnels are to be constructed by a mix of excavation methods for the 1,000MW Linth-Limmern pumped storage project – aka Limmern (Linthal 2015) – which is being developed by Kratwerke Linth-Limmern AG.

The project is located in the Alps and will use the existing Lake Limmern as a lower reservoir, and the natural Lake Mutt – impounded to greater depth with construction of a long concrete gravity wall – as an upper reservoir. Depending on operations and management of the water resources, the scheme will benefit from a varying head range as the difference in levels of the lakes extends or contracts between 560m-710m.

Geology in the area is upper Jurassic limestone – Quintnerkalke – as established, initially, from excavation of an exploration tunnel and also geological mapping at the ground surface. The rock is generally massive, hard and stable, though can have local softer areas with schistosity as well as possible karst systems and fault zones, linked to regional joints. The rock mass weaknesses might be water-bearing – a challenge at the best to locations but the caverns are located approximately 500m below ground near part of Lake Limmern.

The powerhouse cavern will be approximately 160m long, more than 25m wide and up to 50m high, and the transformer cavern will only be slightly smaller in each dimension. The caverns will be excavated by drill and blast method. The powerhouse cavern will hold four reversible Francis pump-turbines, which are to be commissioned over 2015-16.

Construction began in late 2009, but shortly before then, as recently reported by Swiss site investigation company Solexperts, a number of hydraulic and geotechnical tests were performed underground, and under conditions of limited space and with relatively little time. Solexperts tested for various properties of the rock mass – structural (bedding, joints, faults, karst features), mechanical (elasticity and deformation moduli, minimum horizontal principal stress and direction) and hydraulic (conductivity, head).

Ten boreholes (50m-130m long; 96mm diameter) were drilled from an exploration tunnel and two cross-cuts to further examine the insitu geological conditions of the rock where powerhouse caverns will be excavated. Limitations of both time and space – the cross-cuts were 3.5m wide by 3.5m high – meant logistics were difficult and so no drill rig was used, it was necessary to adapt the drilling and testing steps to the available zone, and multiple tests in different boreholes were run at the same time. In addition, in preparing for the tests, due to the 500m overburden it was anticipated that, at worst, water pressures of up to 50 bar might be encountered.

The company also designed and manufacturered two custom-made, four-fold groundwater multi-level tests system to help estimate the hydraulic permeability in upwardly inclined boreholes. The system proved effective, in terms of both time and cost, it said, enabling hydraulic heads in the saturated boreholes to be monitored and tested.

A key outcome of the site investigation work, says Solexperts, is that the successful test programme proved that even under challenging conditions it is possible to perform timing and cost-effective insitu site investigation for large caverns, providing data to both help site safety and optimise construction costs.

In addition to the significant cavern works, a key and challenging part of the excavation on the project is the 4km access tunnel, which is part of Lot A1. The access tunnel is to be bored by a 8.03m diameter double gripper Aker Wirth TBM, at a constant steep upward gradient (24%) to the caverns – an unusual challenge for size of the machine and the distance.

While the TBM was manufactured by Aker Wirth, the backup equipment was provided by Rowa Tunnelling Logistics and due to the incline includes a hoist cable car arrangement to supply the heading. There is also an anti-slip back system, and spoil removal is by conveyor belt to the portal.

The TBM was transported to site in 2010 and successful completion of the bore will enable the principal underground structures to then be excavated. Tunnelling contractor is Arge Zugangstollen Limmern, which is a Swiss-German joint venture comprising local firms Rothpletz, Lienhard + Cie AG, G Lazzarini & Co, Andrea Pitsch AG and Ragotti+Weber Bau AG with Baresel GmbH and Wayss & Freytag from Germany.

Veytaux Expansion

Also in Switzerland, construction recently began on site for another pumped storage expansion project – Veytaux 2, which will double the installed capacity of the 240MW Veytaux plant when it comes into operation by the end of 2014.

The expansion project is being undertaken to help meet the growing demand in the west of the country, especially, for more grid balancing capability due to increased use of renewables with intermittent production for the grid, such as wind and solar. e

Both the existing facility and the expansion project are owned by Forces Motrices Hongrin-Leman (FMHL) – a special purpose company established in 1963 and held by Romande Energie (41.13%), Alpiq (39.3%), Group E (13.13%) and the Municipality of Lausanne (6.43%). The Veytaux plant is operated by Hydro Exploitation SA, a member of Groupement d’exploitation hydraulique du Chablis.

Veytaux is located near Hongrin, in Vaud canto,n, with Lake Geneva as the lower reservoir. The scheme also features the Hongrin dam – a double curvature arch structure connected by an abutment, and with a crest length of 600m – which impounds the upper reservoir. There is also significant tunnel infrastructure. The gross head of the scheme is 878m.

The facility was commissioned in 1970 and the powerhouse holds Pelton turbines and pumps, and average electricity generation is 530GWh annually – just over a third of which is from runoff. The output for 2009-10 was 424GWh per year. After the expansion project doubles the installed capacity, FMHL expects Veytaux to then generate approximately 850Gwh per year. FMHL is investing about SF331 million (US$) in the expansion project.

Expanding the scheme involves new tunnel and cavern excavation, and modification to the surge chamber. No alterations are required to either the penstocks or headrace as the main hydraulic conduits will supply both the existing and new powerhouse caverns. Equipment for the new facility included installation of two pump turbine groups with combined capacity of 240MW. Of that capacity, 60MW will be held as reserve.

Approvals for the project were obtained in stages through 2010. It is early in the construction phase and design studies are well underway for the underground enlargement. Construction work got underway in April this year.

Physical scale model tests have been performed at the Hydraulic Constructions Laboratory of Ecole Polytechnique Federale de Lausanne (EPFL). The model was built to a 1:30 scale, includes the reservoirs, two powerhouses and outlet channel, and is designed to simulate the basic hydraulic behaviour of the new infrastructure, in particular to test the interactions between the two power plants and examine how they function in borderline cases to anticipate problems that may arise.