Aardal is situated between the high mountains of the Sognefjord region in central Southern Norway. The area provides the perfect setting for the archetypal Norwegian water power plant typified by unlined tunnels in good quality rock, high water heads and moderate water flows. To provide power to the area’s booming aluminium industry the Tyin Power Plant was constructed and put online at the end of the Second World War. During the early 1960’s the plant was upgraded to it’s current installed annual capacity of 1180GW to keep step with the growing industry’s power demands.
A third US$190M upgrade for Tyin is currently underway, designed to meet the further needs of the expanding aluminium industry by increasing Tyin’s annual output to 1398GW. Several alternatives were considered and it was concluded that building a parallel plant, the New Tyin Power Plant Project (NTPPP), would be more economically viable. This had a two-fold advantage in that it would enable power production to continue in the existing plant throughout the second project’s scheduled 4-year construction project whilst providing a location with a 35m higher head.

Project overview
As with the existing plant, NTPPP draws from the same water source as the original with the main reservoir at Tyin and the intake situated some 2.4km west at Torolmen. The major civil section of the new system includes construction of; 21km of tunnels between 12m² and 46m²; a 9,000m² surge chamber; a 4m diameter, 435m long raise drilled vertical surge shaft; six 1m diameter, 40-50m long brook inlet shafts; a 50,000m³ power station and connections to the existing tunnel system.
The system is based around an unlined pressure tunnel. In fact the client, Norsk Hydro ASA believes it may be setting a world record for water pressure in an unsupported tunnel as it exceeds 1,000m. Almost the complete headrace tunnel is unlined, with only the 150m from the cone to the power station being equipped with a steel lining.
In order to have rock conditions that withstand the high water pressure the power station had to be designed with sufficient rock overburden. The preliminary investigations concluded that 1.6km from the surface along the centre line of the access tunnel would be sufficient.
The old headrace tunnel will be used as both a surge system and an intake for several new 1m diameter brook inlets. A surge system is vital in such a set up as it acts as a ‘relief valve’ when water in the headrace tunnel is pending. If the turbines or power plant stops (and re-starts) the water pending could lead to massive blowouts. The system has to be designed to allow the water to pend inside the system whilst letting air in and out. There have been examples of massive blowouts with crushed gatehouses and brook inlets thrown 100m in the air in complex tunnel systems.
The brook inlets will be drilled from the surface, allowing the excavated material to deposit in the tunnel underneath. The flow during full operation of the old headrace will transport the finest materials through the old Pelton turbines, coarser particles in to sand traps and the coarsest material will be spread locally downstream of the inlet shafts.
The old and new headrace tunnels will be connected via Shaft 1 and Shaft 2. Shaft 2 will be reamed upwards with a 3.8m diameter from the pressure tunnel whilst shaft 1 will be drilled from the surface down to the new headrace. The shafts are located within a few meters of the operating headrace tunnel. This rock will be blasted at the time of connecting the system. Concrete plugs will be mounted at the old intake and the intake gates by the old penstocks and form the interface between the old and new systems.

The Tyin reservoir is situated on a plateau east of Upper Aardal at an elevation of 1,080m. From the reservoir the valley falls westwards to the valley of Upper Aardal. The headrace tunnel follows the southern side of this valley.
The stratigraphy of the area is rather complex. The eastern part of the area consists of mainly Cambrosilurian phyllite. These rocks are overthrusted by a large complex called the Jotun-Valdres Nappa Complex, consisting of Precambrian and Cambrosilurian rocks. The thrust zone between these two units can be followed on both sides of the valley. Just above this is a small zone of heterogeneous gneiss, quartzite and mica schist, followed by a large unit of meta-arkose and conglomerate. A new thrust zone divides the meta-arkose from a unit of the so-called Jotunheim Complex, consisting mainly of gneiss, gabbro, amphibolite and pyroxene-granulite. The power station area and the lower part of the pressure tunnel are located within these rocks.
The major weakness zone in the project area is a regional fault zone crossing the pressure tunnel approximately 3.5km from the power station. The need for heavy rock support is expected here.

Excavation and support
The civils contract, worth US$80M, was awarded to Selmer Skanska in September 2001 with the first tunnel blast carried out in October that year. The contractor is excavating the headrace tunnel from three faces whilst simultaneously constructing the power station and the tailrace tunnel.
The 300m long Biskopvatn adit leads to the attack point for the 4.3km long, 1:32 upstream drill and blast drive to the Torolmen intake and the 2.7km long, 1:13 downstream drill and blast drive towards the power station. The remaining 1:6, 2.5km is being driven by the same method, upstream from the power station. Access to this is via a connection tunnel from the access tunnel that leads to the power station (figure 2). This 1:6 section constitutes the pressure tunnel section of the headrace tunnel. Originally this was designed as a 45° drill and blast shaft, common in many older Norwegian power plants. This idea was shelved due to the adverse working conditions; also the 1:6 drive was shown to be the cheaper option.
The headrace tunnel varies in cross section from 27m² to 36m². The contractor is using three Atlas Copco 353 BC drilling jumbos, one per drive to excavate the tunnel as a full-face advance. Each drill pattern consists of around 80 x 6m long holes including the parallel cut of four unloaded 102mm holes and 11 loaded 51mm holes. The blasting is performed with slurry explosives for environmental reasons, although some of the smaller sections are being blasted with ANFO, both premixed and mixed on site.
The tunnel is being supported by 2.4m – 3m rockbolts and a shotcrete lining with the spoil being removed using CAT 980 and Komatsu W500 wheel loaders and CAT 963 tracked loaders. In the smallest cross sections Wagner and Toro loaders have been used. On average each cycle including transport and support is taking around 7 hours and the contractor is achieving the 60m per week advance rates predicted.
The power station is approximately 65m long x 16m wide x 40m high and is being excavated in six steps. Selmer Skanska is again using Atlas Copco 353 BC drilling jumbos (in combination with the tunnel advance) and Atlas Copco D7 surface drill rigs. During excavation of the access tunnel and the upper part of the power station, medium to heavy rock bursts have been experienced. To combat this the temporary rock support consists of systematic rock bolting and shotcreting – with fibre reinforced shotcrete and added alkali-free accelerator. The roof is permanently supported with 4m long bolts in a 1.5m x 1.5m pattern although 5m long bolts are installed along the centreline. Following this, the 10cm thick shotcrete layer is applied. The station walls are supported with a 2m x 2m bolt pattern, with 4m long bolts in the upper sections and 6m long bolts further down to cope with the increasing rock stresses. The maximum rock stress has been measured at up to 50MPa in the cone area. The power station is being provided with a final lining via a standard formwork system. The project’s 7,000m3 of concrete is being delivered from a local batching plant and contains some 450t of pre-cut and pre-bent reinforcement.
The project’s excavated material is being removed and placed in one of four designated areas. Two are situated in Aardal and will eventually be public function areas such as parks shorelines and parking lots. The other deposits are in the mountains, one will form the substructure for a future-widening scheme for an existing highway, whilst the other will be covered in topsoil and landscaped.

The teams are working a 12 day on 9 day off swing, with shifts running from 06.00hrs – 16.00hrs and 16.00hrs – 02.00hrs. After a year of construction approximately 4km of the headrace tunnel has been completed, along with the 1,540m long access tunnel, 700m of tailrace tunnel and 200m of pressure tunnel.
Concreting is underway, due for completion in December 2003, ready for installation works to be performed in spring 2004. Water filling is planned for July 2004 with the Power Station coming on-line in October 2004.

Related Files
The layout of the Tyin Hydropower Project showing the old and new systems
Diagram showing the access tunnel to the power station and the connection tunnel allowing pressure tunnel construction to begin independent of the station
Figure 3 – please see ‘fact file’