On Tuesday the 27th of November, a milestone was reached at the Búðarháls Hydropower construction site when explosives cut through the last remaining unexcavated section of the tunnel, merging the two tunnels from either side of the site. The headrace tunnel is 4km in length and the total cross section area is approximately 140 m2.
The tunnel is excavated from both ends and in two phases, as a result of a height of the tunnel section. The upper section is firstly excavated through the ridge (top heading) and then the lower section is completed (bench).
The event last Tuesday resulted in the top heading, breaking through the ridge of Búðarháls when the tunnelling workers on the western side blasting through to the eastern side, close to the the middle section of the ridge.
Búðarháls seems innocent enough on the surface but the geological conditions underneath are both varied and challenging. The powerhouse, located on the western slope of Búðarháls, at the western end of the headrace tunnel has been excavated in an area made up of rhyolite; a first in Iceland. The western part of the headrace tunnel is located in an area characterised by sedimentary deposit/clastic sediment and basalt layers. The eastern side of the tunnel is located in an area of thick, hard basalt rock.
The contractor for the project, Ístak ehf has been faced with the challenge of working within these varied conditions. Their powerful workforce and experience in the field has given them the tools to complete the task successfully. Work on the lower end of the tunnel has commenced and is expected to reach completion by the summer of 2013.
The Búðarháls Hydroelectric Power Station is located on the Tungnaá River and will take advantage of the drop in the river between the tail water of the Hrauneyjafoss Power Station and the Sultartangi Reservoir. The head will be approximately 40 metres, with a harnessed discharge of 240 m3/s and an installed capacity of 95 MW. The energy generating capacity of the Station is estimated at 585 GW/h annually.
The main proposal for the Búðarháls Hydroelectric Power Station is to build two dams to the east of Búðarháls, a short distance above its junction with the Tungnaá River and the Kaldakvísl Stream. One of the dams will be built across the Kaldakvísl Stream and the other across the outflow from the Hrauneyjafoss Power Station. The dams will both be approximately 25 metres at their highest point but will vary in length; one of the dams will be 1100 metres in length and the other 170 metres.
The dams will form the intake reservoir for Búðarháls and will be given the name ‘Sporðalda Reservoir’, with an estimated surface area of some 7 km2. The reservoir will mainly fill the channel of Kaldakvísl and also extend slightly into Þóristungur. A headrace tunnel approximately 4 km long will be constructed from the intake structure at the Sporðalda Reservoir, westward under Búðarháls, to a surge basin by the Sultartangi Reservoir. Two 60 metre long, steel pressure pipes will transport the water from the intake to the station’s turbines. The powerhouse will be mostly above ground, built into the western side of the slope of Búðarháls.
There will be two 47.5 MW Kaplan generating units.
Recently, up to 300 on- site workers have been involved in the project; 250 have been involved in construction work on behalf of Ístak and the rest have been working under foreign contractors, completing work on the set up of machinery and electrical equipment.
Construction work on the Búðarháls Hydropower Station has developed successfully and the station is expected to begin operation towards the end of 2013.