Kops II Pressure Tunnel Technical Concept, Geotechnics and Construction
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By Herbert Schnetzer. Alois Vigl and Helmut Wannenmacher
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he Kops [[ hydro power plant is a parallel scheme to the existing 247 MW HPP Kops I of the Vorarlberger llIwerke AG in Austria which is under full power operation since 1970. The new HPP comprises a pumped storage hydro power plant with a power capacity of 450 MW in turbine mode and pumped mode as well. The Kops [[ scheme utilizes the existing Kops reservoir with a capacity of 42.9 Mill. m 3 and a lower baJancing reservoir. A new intake and the headrace tunnel (Lot 1), a surge system and an inclined pressure shaft (Lot 2) and a powerhouse cavern together with a tailrace tunnel and an outlet to the lower balancing reservoir (Lot 3) had to be constructed within the Kops [[ construction programme. The construction of the new power plant extension started at the end of 2004 and the complete start of power production is planned for the middle of 2008.
Special requirements for the headrace tunnel The Kops II HPP will be operated as a pumped storage hydro power plant desigoed for a rather
short starting up time from zero to ful1 power turbine production and for short time changes from turbine mode to pumped mode as well. These features require for a special surge system which is able to die down oscillations within shortest time. At the other hand this is linked to a more or less sudden increase of internal pressure in the headrace tunnel according to the special characteristics of the surge system. In terms the hydrodynamic internal pressure could increase from about 50 to 150 m within 20 seconds and could decrease within the same range in the same time. Another special feature is, that the external water table can be considered to stay amply above the maximum internal pressure but could reach up to 450 m above the tunnel, resulting in a maximum external water pressure of about 45 bar in case of complete depletion . During operation an effective alternating external pressure between 30 to 40 bar must be considered as amaxlmum. For those special reasons from the very beginning it was clear, that spezial attention had to be spent to achieve an "open" lining system, more permeable than the surrounding rock mass, in
Kopswerk II Druckstollen - Technisches Konzept, Geotechnik und Bauausf iihrung Das Kopswerk II ist ein Parallelwerk zum. bestehendell Kopswerk I. Es ist elite I10chdruck Wasserkra/tanlage der Vorarlberger Illwerke AG und bringt eine Leistungssteigerung um 450 MW im Turbinen- und Pumpbetrieb. Fiir das Kopswerk II ist ein Iwehbelasteter Druekstollen, der Versalstollen II mit einem Innendurchmesser von 4, 9 m und einer Lange von rund 5500 Tn, zu errichten. Der 4 780 m lange TBM- Abschnilt des Slollens wurde steigend im Friisvortrieb mit einer DS-TBM (D",. = 5.54 m) auJgeJahren and mit hexagonalen Tiibbingen (st = 23 em) ausgekleidel. Der Stollen durchorterl Serien versehiedener Gneise und Glimmerschiefer bel Uberlagerungshohen von 150 bis 700 m. Der Bergwasserdruck wurde mit bis zu 45 bar abgeschiitzt, der Innenwasserdruek ist mil 0 bis 12 bar anzuselzen. Aufgrund dieser Rahmenbedingullgen wurde eine Tiibbingauskleidung in Verbindung mit durehgiingiger Niederdruek-Konlaktilljektion und bereiehsweise einer Hochdruck-Konsolidierungs- und Abdichtungsinjektion gewiihll. Bei Station 1 893 m war ein Nachbrueh zu iiberwinden, der tuller Einsatz von Sehiiumen und Injektionsbohrankern bewiiltigt werden konnle. Die WasserzutriUe wahrend cies Vo rtriebs erreiehten 150 I/s iiber die Slollenlange.
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Letztendlich konnte der Slollen mit einer DurehschniUsgesehwindigkeil von 15 bis 18 mid und Spilzengesduvindigkeiten von bis zu 37 mid erfolgreich a,ufgefahren und durehgeschlagen werden. Kops II is a 450 MW parallel scheme of the existing HPPKops I ofthe Vorarlberger IIIwerke AG in Austria. The Kops II pressurised headrace tunnel "Versalstollen II" comprises an inner diameter of 4.9 m and a length of about 5 500 m. The 4 780 m long TBM portion of the tunnel was excavated with a DS-TBM through gneiss and schist formations and is lined with honeycomb segments with a thickness of 23 cm. The overburden was ranging from 150 to 700 m. An external water table up to 45 bar and an internal pressure between 0 to 12 bar had to be considered within the technical concept. For that reasons a segmental lining in combination with obligate low pressure contact grouting and high pressure consolidation and sealing grouting was foUowed up during the desigo. At chainage 1 893 m an overbreak had to be overcome. This was achieved with the help of foam and self drilling grouted anchor bolts out from the cutterhead. The water ingresss to the tunnel was 150 lis. Finally tbe tunnel could be excavated at an average advance rate of 15 to 18 mid and maximum advance rated up to 37 mid . FELSBAU 24 (2006) NO.6
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SCHNETZER. VIGL AND WANNENMAC HER: KOPS II - TECHN ICAL CONCE PT, GEOTECHNICS AND CONSTRUCTION
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order to provide pressure relief towards the tunnel and full pressu re communication with the external water table as well. It was defined as a special target to improve the rock mass surrounding the tunnel and to keep the water ingress rate low in order to co pe with the highly variable pressure relief requirements in view to long term operation . The constru ction of the head race tunnel and all the related underground works and civil works of Lot 1 has been awarded to the JointVenture Swietelsky TUllllelbau (A ustria) and Tornofforn? (SwisslIlaly). The design was carried out by ViglConsult ZT (Austria) in close collabora tion with the engineerin g team of the client - the Vorarlberger 1llwerk e AG. The site supervision and the geological supervision were carried out by the client himself.
Geological/geotechnical prognosis The construction site is situated in the central part of the silvretta nappe, the crystalli n rocks are metamorphical graded, in contrast to the a lpidic metamorphism which was responsible for the tedonical prozesses and its related genesis of brittle fault zones in this area. The various rock types encountered are main ly amp hibolites, hornblende gneiss, quartzite-gneiss and mica schist, with transitions in between (Figure 1). The geomechanical model for Ule geotechnical design
Table 1 Rock mass parameters of the individual rock mass types
Tabelle 1 Gebirgsparameter der einzelnen Gebirgsarlen. Rock Mass
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20 FELSBAU 24 (2006 ) NO.6
was mainly based on the geological investigations carried out for the pressure gaUery or Kops I and the experiences gained from its excavation carried out by drill a nd blast in the early 1960ies. The different rock types of the project area strike east west and form a wideranging synclinalstructure with mid steep north dippin g. The pressure gallery was excavated in a wide arc and therefore the same rock types were excavated twice but at diITerent angles to the schistosity planes. Geotechnical design The Vorarlberger lllwe rke AG runs a spacious gallery system in the Silvretta, with a well documented geological a nd geotechnical mapping or these structures. The experiences gained from the excavation or the "Ve rsalstollen I" and additionally performed rock mechanical tests were used to develop a reliable geological and geotechnical model of the Versalstollen n. The geotechnical design describes the basic steps performed to derive a reliable geotechnical design for identification of o The application range of TBM performance, o The estimation of radial displacements of the rock mass and demand of rock s upport and the bearing capability of the support, o The description of the excavation and support classes, o The application of necessary special measures to overcome rau lt zones. On the basis of similar geological and geotechnical conditions ten different homogenous ranges (HR) were identified. For the estimation of the rock mass behaviour and for the determination of the rock support required, the "characteristic line method" was applied. The determination of the rock mass parameters (Table 1) was mainly based on the investigation programme consisting of several uniaxial compression and tensile rock tests as well as an additional carried out radial jack test in the range of the power house cavern (1). Predicted geotechnical settings Over the entire length fair to very good tunnelling conditions were expected with some potential of
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unfavourable geotechnical settings, mainly related to the occurrence of the faulted and disturbed mica schist zones. In these ranges some mild to severe squeezing conditions we re expected as well as in transition zones from competent to incompetent layers under high overburden.
Excavation and lining concept Selection and features of the lining system During design studies it was derived, that two lining systems, a conventional douhle shell lining conta ining a preliminary NATM support togeth er with a precast invert segment and an in situ concrete final lining, as well as a single shell rull precast segmental lining could co pe with the special requirements. For that reasons both systems have been tendered and the precast segmental lining was finall y awarded du e to the best offer. The salient features of the lining system have worked out as follows (Figure 2): o Internal diameter Din l = 4.9 rn , o Lining thickness st = 23 cm, o "nnu la r gap ag = 9 cm, o Excavated diameter D(lxc. = 5.54 m, o Segment width sw = 1.4 m, o Concrete grade C40/50 according EC2 , o Steel grade BST 550. The lining system was designed fo r aU handling loads and to carry an external grouting pressure of 15 bar and local rock mass disintegration in the order of one tunnel diameter acting in the roof. Since the HHT site was rather exposed with its portal at 1 600 m altitude, and the site access was rather restricted during wintertime from Decemher to March/April (Figures 3 a nd 4), a majo r benefit in the "precast segment solution" was seen to be able to store the segments in a utumn and to provide for independent su pply during wintertime. Another benefit of precast elements was seen together with high and frequent water inflow which would have raised the efforts of in situ concrete application essential1y.
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Excavation system By th e choice of a segmental liuing more or less the selection of a shielded TBM was determined. A used Robbins hard rock double shield TBM, which has bored in China right before, fin a lly was applied (Figure 5): o TBM type Hobbins 1617-290-1 hard rock -DSTBM, o Diameter: Dexc. = 5. 54 m, o Length/weight: 12 mI 380 t, o e lm-powe r: 5 x 315 kW = 1 575 kW,
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FELSBAU 24 (2006) NO.6
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SCHNETZER , VIGL AND WANNENMACHER: KOPS II - TECHNICAL CONCEPT , GEOTECHNICS AND CONSTRUCT ION
SCHNETZER, VIGL AND WANNENMACHER: KOPS II - TECHNICAL CONCEPT, GEOTECHNICS AND CONSTRUCT ION
Experiences during excavation
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Excavation and lining in general TBM tunnel excavation was started in August 2005 from an access tunnel and a starting tube respectively. After a certain learning curve which was affected by the exposed site location at the one side but by starting problems with the used TBM too, a mean advance rate of 15 to 18 metres per working day could be achieved (Figure 6). The TBM was operated with two working shifts each 10 h and a maintenance shift with 4 h. The segmental lining was erected under protection of tbe tail shield of the TBM (Figure 7). The invert segments were fitted with invert pads proving for initial bedding immediately after installation. Within a next step the annular gap between the surrounding rock and the segmental lining was backfIlled with pea gravel (washed broken gravel 8 to 12 mm grain size) immediately after the relief from the shield. The remaining invert section, where the pea grave] could not flow, was grouted with an invert bedding mortar stepwise over a set of the last three to four invert segments (Figure 8).
Fig. 3 Stock of precast lining segments during wintertime at 1 800 m altitude (Photo: VigIConsult). Bild 3 Tubbing/ager wahrend des Win ters auf 1 800 m Hdhe (Foto: VigIConsuft).
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Fig. 4 Transport of the lining segments from stock to the portal at 1 600 m altidude (Photo: VigIConsult). SUd 4 TObbingtrans port vorn TObbing/ager zum Portal auf 1600 m Hdhe (Foto: ViglConsult).
Geotechnical observations and special events With increasing experiences adequate parameters in terms of revolution of the cutterhead and thrust could be found for the specific rock mass types. In general the variations of the rock mass behaviour showed no major impact on the progress of the heading with the DS TBM. Geotechnical adverse conditions were encountered in the mica schist zone crossing the Verbella valley. The minor fault zones observed in the old "Versalstollen I" in this range showed no re markable influence on the TBM excavation. Problems sometimes were related to zones where rock mass disintegration occurred. But in such zones, the lining segments showed no visu-
Fig. 5 Assembling of the OS-TBM Robbins 1617-290-1 in front of the portal (Photo: VIW). Sild 5 Aufbau der DS-TBM Robbins 1617-290- 1 vordem Portal (Foto: V/W).
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CHD-thrust: 10 955 kN, Total thrust: 14 000 kN, Disks (17"): 4 x double (17") + 25 x single (19"), Backup: 280 m; 375 t.
Fig. 6 Actual advance of the T8M-heading. Bild 6 Aktuefler Vortriebsfortschritt des TBM-Tunnelvortriebs.
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DEC 2005 -JAN 2006
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The hydro-geological conditions were of'major influence on the geotechnical design of the pressure gallery. The various mica schist zones are dividing the Verbella ridge into a set of three major water tables with a water pressure to be anticipated up to 45 bar. Initial water ingress at the face at some chain age reached up to 30 Vs with a later decrease to some Vs. The only parts observed with really dry conditions were the mica schist, showing a more or less impermeable behaviour. The gneisses and amphibolites although really massive showed a high persistent joint net, with up to fOlu dominant joint sets. TBM Progress In order to analyse tbe influence of' different rock mass types on the advance rate of the DS TBM some basic correlation analyses were carried out, based on a modified cutting coefficient. The cutting coefficient is dermed hereby as the necessary thrust per cutter for 1 mm of excavation. Still a direct link from the cutting coelllcient to general rock mass characteristics could not be
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22 FELSBAU 24 (2006) NO.6
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al cracking due to local loads related to disintegrated rock. The most exciting behaviour showed the over-break at chainage 1 893 m, related to a fault zone at the end of the mica schist zone of tbe Verbella valley with an overburden up to 250 m. Tbe rock mass there was built up by sheared mica schist with incompetent behaviour due to mechanical disturbance. An over break continuously developed, whicb resulted finally in a blockage of the cutterhead and a collapse in front of the face . By probe drilling it could be observed that tbe rock mass approximately 10 m in front of the face showed good rock mass conditions again with no indication of loosening. The method applied to overcome that situation was a piped, grouted roof and face grouting applied through the cutter head. The pipes were self drilling groutable glass fibre anchors. The grout applied was a two~components silicatefoam. To overcome this overbreak. a TBM stop of a couple of working days was required only. Other measures like a roof tunnel or a bypass tunnel could be avoided. Hight a certain number of small overbreaks starting in the roof section above and in front of the cutterhead could be controlled by foam application and further cave development could be prevented with success. The conducted grouting showed a sulllcient stabilization of the partly blocky rocks above the crown with the result that already within a first attempt such zones could be mastered. During slow excavation some progressive overbreak development in front of the face and some local progressive failure at the crown could be observed. The overbreaks were filled immediately after the installation of'the segments with pea gravel as far as possible, to stop further de ve lopment. Where indicated, in addition mortar was injected in order to provide for a primary stabilization of the lini ng ring. In a later stage a consolidation grouting campaign wiJ] be conducted especially in such zones to provide the necessary long ~ term bedding conditions for the segments.
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FELSBAU 24 (2006) NO.6
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