Update on Obervermunt II19 August 2015
Christian Schönlechner, JÄGER BAU GMBH, provides details on development of the 360MW Obervermunt II pumped storage project, currently under construction in Austria
Vorarlberger Illwerke AG is constructing the Obervermunt II pumped storage plant in Montafon/Vorarlberg in the Silvretta Alps between the Silvretta and Vermunt reservoirs. The station is designed to exploit the cascade between the Silvretta and Vermunt reservoirs for recirculated pumped storage, thereby making a significant contribution to hydropower efficiency increases by VIW AG.
The 360MW plant will feature two highly flexible, fast and widely adjustable hydroelectric generating sets with separate turbines (180MW each) and pumps (2x180 MW). Obervermuntwerk II will be constructed in parallel to the existing Obervermuntwerk project. In the course of the construction of OVW II, a new headrace duct for the existing OVW will be created, with the existing surface penstock removed as a compensatory measure.
The Obervermuntwerk II project will see construction take place inside the mountain - a powerhouse cavern will be built with a rock excavation volume of approx.120,000m³. This will include a transformer cavern and machine cavern, to which access will be provided by means of an adit roughly 800m in length.
The upstream routing of the headrace water from the Silvretta reservoir to the plant will require the construction of a headrace tunnel (with a 9% decline) approximately 3km in length and a 110m pressure shaft with a gradient of 48°. Below the cavern the tail water will be discharged into the Vermunt reservoir via a tunnel roughly 380m in length.
The construction of the 270m-high surge tank, which will have a diameter of 18m in its upper third, will require the construction of a 1km access tunnel.
A 1.3km pipe gallery will also be excavated to connect the existing Obervermuntwerk project.
For the purpose of energy transmission from the powerhouse area, a 300m-long cable gallery and a 30m high cable duct will need to be constructed.
All in all, this will require the excavation of tunnels totalling approximately 8 km in length, 500m of shafts and 120,000m³ of caverns, amounting to a total excavation of approximately 500,000m³ of rock.
The installations of the Obervermuntwerk II lie entirely in the central part of the upper Eastern Alpine Silvretta nappe, which has been thrust over deeper but more recent tectonic units. The planned installations are in contact with most of the rock types of the Silvretta nappe, which is characterised by a variety of crystalline rocks and transitions between them. The powerhouse cavern and its associated tunnels, the tailwater tunnel and the lower part of the Silvretta tunnel and the Krespa surge tank lie in the hanging granite gneiss series, which is composed mainly of granite gneisses. The upper part of the planned Silvretta tunnel and the inlet gallery will pierce the horizontal amphibolite and para-gneiss series, which consists of amphibolites, hornblende gneisses, migmatite gneisses, granite gneisses, aplite gneisses, slate gneisses and mica schists, where, in the tunnel section in question, amphibolites, hornblende gneiss and slate gneiss predominate. The youngest rocks are diabases, which penetrate the other rocks discordantly in slender aisles.
The Bau Obervermuntwerk II consortium, consisting of the Austrian companies Jäger Bau GmbH, Porr Bau GmbH, Östu-Stettin Hoch- und Tiefbau GmbH and G.Hinteregger & Söhne Baugesellschaft m.b.H, was awarded the contract for the main construction works (especially excavation and shoring-up works and concreting works for individual galleries, shafts and caverns) in February 2014.
Progress of the works at the end of April 2015
The consortium started work on the construction of Obervermuntwerk II in May 2014.
Due to the tight schedule work started immediately on both the pre-cuts for the tunnels and the construction site set-up.
The main construction site facilities, including two workshops, two warehouses, a site office, water protection equipment, two concrete mixing plants and a gravel processing plant, had been built close to the Vermunt reservoir by the autumn of 2014. For environmental (reduction in lorry use) and logistical reasons (the site is accessible in the winter half of the year only by means of a passenger cable car and is supplied with materials by means of a purpose-built goods cable lift built by the contracting authority), the excavated material from the galleries is processed in situ and used for the production of concrete.
Deliveries of concrete and aggregates were necessary only in the first few months; since autumn 2014 concrete and shotcrete have been produced in situ.
In the vicinity of the Silvretta reservoir, two additional residential camps for the workers and a canteen were built, in addition to the contracting authority's existing worker accommodation. This provides accommodation and meals for up to 350 workers.
In June 2014, the tunnelling works began at three points:
Area I - powerhouse area
The powerhouse access adit was excavated to provide access to the underground cavern area. The first stretch of approximately 320m, consisting of granite gneiss, was excavated by means of drill and blast. The production of the blastholes involved the use of a twin-arm drilling rig equipped with a drilling control system (type AtlasCopco EC2). The explosive used was emulsion explosive in cartridge form. The muck was removed using wheel loaders and dump vehicles. The shoring-up work involved steel construction mesh, lattice arches and shotcrete, which was applied by mobile spraying devices using a wet spraying technique.
In December 2014, the frontal connection of the powerhouse cavern was successfully completed with the conclusion of the tunnelling work on the powerhouse and tail water adits.
The excavation of the cavern was subdivided into several excavation horizons. The uppermost horizon, or roof section, was excavated in both the transformer and machine hall caverns by the end of April.
This excavation was subdivided into an anticipatory first tunnel and the two respective side tunnels. The cavern was shored up using a 3-layer reinforced sprayed concrete construction with up to 20m long GEWI anchors, D = 57mm. The muck was again removed using wheel loaders and dump vehicles.
Area II - surge tank area/foxhole tunnel
June also saw the start of excavation work for the 650m-long foxhole tunnel with its 13% gradient, which provides access to the 350m-long upper chamber of the surge tank. This excavation was also carried out by blasting using a twin-arm drilling rig and cartridge explosives. The geology in this gallery was characterised by alternating sequences of solid granite gneisses and loose intermediate layers, forcing a constant switch between tunnelling classes. At roughly the 550m mark the excavation broke into the 30m-long Madertäli fault, a main fault in the project zone, in which fault rocks consisting of mylonitic clay predominated.
The excavation of the following upper chamber and of the surge tank shaft head, with a diameter of 20m, was completed by December 2014.
In January and February 2015, the muck hole for the 270m-high surge tank shaft was executed by NU Edilmac using a raise boring technique. This required the initial sinking of a targeted pilot hole with a diameter of 13 ¾ which reached the foot of the shaft at the end of the connecting tunnel with a deviation of only 17cm. The borehole was then widened to a diameter of 3m from top to bottom and now serves as a muck hole for the subsequent widening of the shaft to 18m.
The installation work for the widening work (setting up portal crane etc.) is currently being carried out.
Area III - connection to Obervermuntwerk I
An adit of approximately 1300m in length was excavated in the period from June to December 2014 to connect the existing Obervermuntwerk project to the new headrace. This latter was characterised by consistently compact geology, with only two minor faults encountered only in the last 200m.
Area IV - access tunnel, penstock and Silvretta penstock
Starting from the junction of the powerhouse access tunnel at the 360m mark, work began in October 2014 on the excavation of the approximately 520m-long access tunnel to the Silvretta penstock. The shaft head area of the pressure shaft was then reached in February 2015 and excavation work began on the Silvretta gallery itself.
This work has been advancing from two target points since February, from the start of the tunnel leading from the pressure shaft and from the 750m mark at the confluence of the Obervermuntwerk I penstock.
Both sets of tunnelling work are characterised by abrupt shifts in geology, as a result of which pull lengths must often be reduced to 1.3m, and the tunnels are being shored up using lattice arches, steel construction mesh and shotcrete. The mountain water ingress points reach peaks of up to 20l/sec.
Area V - tail water ducting, turbine discharge tunnel / pump inlet tunnel and outlet construction
In the autumn of 2014, the level of the Vermunt reservoir was lowered to allow the construction of the pre-cut for the tailwater tunnel. This required the initial construction of a bored pile wall consisting of 225 bored piles to provide protection for the excavation ditch for the pre-cut. After the completion of this preliminary work, a further 50m of the descending tailwater tunnel was excavated and shored up before the reservoir was refilled shortly before Christmas. In the next low water period in autumn 2015, the water level of the reservoir will once again be lowered, and work will continue on the construction of the inner shell and the outlet construction.
After Christmas the work on the tail water section then continued from the tail water access tunnel, i.e. the lower 110m of the tailwater gallery was excavated along with the turbine discharge tunnels and pump inlet tunnels for both sets of machinery.
Area VI - auxiliary equipment, cable and escape tunnels for energy transmission
Energy transmission from the transformer cavern will be effected in later operation via a 30m-high cable and escape shaft connected to the roof of the transformer cavern, and a 300m-long cable and escape tunnel before the lines are ducted along an underground route to the existing Trominier tunnel.
This cable and escape tunnel was likewise excavated in summer 2014; the shaft will be excavated in spring 2015.
Obervermuntwerk II is a complex construction project, in which a series of tunnels, shafts and caverns need to be excavated within a short period of time. Accordingly, the excavation work will be carried out simultaneously at no more than four tunnelling points. This requires the use of significant amounts of equipment and personnel with corresponding logistical challenges. Additional difficulties are presented by the high altitude and remoteness of the construction site in the winter.
Roughly 5000m of tunnel and the uppermost horizon of the powerhouse cavern tunnelling were excavated between the start of tunnelling in mid-June 2014 and the end of April 2015.
The excavation work will be completed in the current year 2015; the beginning of 2016 will see the start of concreting work in the cavern and inner shell work in the individual galleries.
Report by Christian Schönlechner, Civil Engineer, JÄGER BAU GMBH