Extension Hieflau HEP – Economical Solutions for the Headrace Tunnel

576 © 2008 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Geomechanik und Tunnelbau 1 (2008), Heft 6

The expansion project for the Hieflau hydropower plant involvesthe construction of a second headrace tunnel to enhance the ca-pacity from 60 up to 90 m3/s. The second tunnel, located betweenthe weir system Gstatterboden and storage reservoir Wag, isseen as the highlight of the enhancement scheme. The break-through of the 4,9 km long TBM-drive is expected by end of thisyear.

Erweiterung der Wasserkraftanlage Hieflau – WirtschaftlicheLösungen für den TriebwasserstollenDie Erweiterung des Wasserkraftwerks Hieflau sieht eine Er-höhung der Ausbauwassermenge von 60 auf 90 m3/s vor. AlsEngstelle der bestehenden Anlage wurde das Triebwassersystemzwischen Wehranlage Gstatterboden und Wagspeicher identifi-ziert. Somit wurde ein zweiter Triebwasserstollen zum Kernstückdes Erweiterungsprojekts. Der Durchschlag des 4,9 km langenTBM-Vortriebs wird gegen Jahresende erwartet.

1 Introduction

Due to environmental aspects and given EC-frameworkdirectives, which foresees an improved water protection,the realization of a new hydropower plant is limited.Therefore the enhancement of existing plants gains moreand more in importance. The 63 MW diversion HPP (hy-dro power plant) Hieflau, located along the river Enns inStyria, enhanced the power supply in a first stage about 50years ago by construction of a reservoir (Wag storage) fora daily storage. The next stage of the enhancement projectstarted in 2005 by carrying out a feasibility study, followedby site investigations and elaboration of an upgradedscheme. Environmental aspects play an important roledue to the vicinity to the Gesäuse National Park and someparticular ecological requirements had to be especiallyconsidered in the upgraded scheme. Only 13 weeks afterpublication of the tender, the energy supplierAustrian Hy-dro Power (AHP) awarded the contract to the Joint Ven-ture Jäger-Strabag-Frutiger.

The concept of the expansion project foresees an en-hancement of the present discharge capacity to the maxi-mum turbines capacity, which amounts 90 m3/s in total.Due to the fact that the existing headrace tunnel has alimited capacity of 60 m3/s a second tunnel had been con-sidered in the upgraded scheme to provide a combined ca-pacity of 90 m3/s. Beside the underground constructions

the expansion project for the Hieflau hydropower plantinvolves the extension of the existing weir system Gstatter-boden, including a mini power station, refurbishments ofthe upper headrace channel as well as a conduction sys-tem between the surge channel and the third pressureshaft to enable a direct water supply of the third machineby the headrace system. Furthermore environmental im-provements at the intake and the storage reservoir will becarried out (Figure 1).

2 Geology of the headrace tunnel

The alignment of the second tunnel runs parallel and in adistance of not more than 30 m to the existing one. Mostinformation about the geological conditions along thisroute is derived from the mapping of the first pressure tun-nel. Additionally some site investigations were carried outto gain a well mapped basis about the geology of the newheadrace tunnel. The large part of the TBM tunnel is ex-cavated in good rock conditions, predominantly limestoneand dolomite. In limestone some small fault zones andsome local karstic cavities with diameters up to 0.5 m arepredicted. The most challenging of the TBM-drive is thecrossing of a 40 m wide section in soft ground conditions(Hartelsgraben). Within this zone loose rock boulders,bedded in a matrix of fine materials such as sand and claywill be expected. In view to the hydrogeological conditionwater inflow of 2 l/s were predicted in the related section.

Extension Hieflau HEP – EconomicalSolutions for the Headrace Tunnel

Andreas WalterWolfgang WeberFranz Gappmaier

DOI: 10.1002/geot.200800064

Fig. 1. Overview enhancement schemeBild 1. Übersicht Erweiterungsprojekt

Fachthemen

577Geomechanik und Tunnelbau 1 (2008), Heft 6

A. Walter/W. Weber/F. Gappmaier · Extension Hieflau HEP – Economical Solutions for the Headrace Tunnel

The last part of the tunnel at the upstream was revealed tosoft ground (Figure 2).

3 DS-TBM and segmental lining system

The mechanized drive is managed by a hardrock doubleshield TBM with a specified excavation diameter of6,18 m. The heading unit including backup systemamounts to a length of about 200 m. The Robbins-TBM,owned by ITS (International Tunnel Services), was suc-cessfully applied on other projects in Switzerland, Slove-nia, Spain and Costa Rica.

The TBM-tunnel is lined with precast segmentalrings. The tunnel lining has an internal diameter of 5,6 mand a segment thickness of 20 cm. The segment rings are1.8 m wide. The tender design asks for a parallel ringsystem, composed of an invert segment, two similarsidewall segments and a key segment on top. The rectan-gular shaped invert segment is designed with a flat invert,fitted with a rail fixing system and contains a central water trench. The flat invert acts as a support system for the rails as well as a roadway for service tasks of thehydropower scheme. The inversely designed sidewall segments and the roof segment are trapezoidal shapeddue to installation requirements. The longitudinal jointsare convex-convex shaped to concentrate the load

transmission area in the centre of the relatively thin seg-ments.

In principle the lining is designed for straight line ap-plication. To manage the curve characteristic of the align-ment, with a minimum radius of 500 m, tapered segmentrings are used. To cope with slight deflections of the TBM-excavation within a certain range around the theoreticalaxis of the alignment small timber spacers are placed inthe circumferential joints.

The bedding of the lining is provided by filling the an-nular gap with pea gravel. Under bad ground conditions(e.g. Hartelsgraben or fault zones) contact backfill grout-ing is applied to improve the bedding conditions of thelining. Additionally every 200 m a two-ring wide groutingsection is carried out to avoid a washing out of the peagravel caused by fissure water inflows.

The reinforcement design is adapted to the statical re-quirements of the lining system. Beside the standard rein-forced segment, which is used under stable rock conditionsa segment type with stronger reinforcement is used to copewith extraordinary rock conditions like locally rock disinte-grations or in case of insufficient bedding conditions, whereadditional loads due to thrust forces have to be born.

To improve the accuracy of ring installation and tolimit the movements of the segments during regrippingphase additional adjusting systems are integrated in the

Fig. 2. Geological longitudinal sectionBild 2. Geologischer Längenschnitt

Fig. 3. Segmental lining systemBild 3. Tübbingauskleidung

578 Geomechanik und Tunnelbau 1 (2008), Heft 6


lining design. To minimize misalignment between invertand sidewall segments plastic pins are applied in the re-cessed part of the longitudinal joints (Figure 4a and 4b).To act against radial offsets in the upper part of the liningring steel pins are used as a connection between the side-wall segments and the key stone (Figure 4c). By applica-tion of these “adjusting features” and by sealing of the seg-ment joints with mortar, a hydraulically optimized liningsystem can be achieved.

4 Access tunnel

The 170 m long access tunnel is done by drill and blastmethod. In spring 2008 the TBM was moved to the start-ing position on heavy-duty dollies, running on steel pro-files. In the last part of the access gallery the shield-TBMwas shifted on a round shaped bottom slab for the startingposition of the heading.

5 Crossing of access gallery Hartelsgraben

The first challenge of the TBM-drive was the crossing ofthe existing access gallery Hartelsgraben. The rectangularshaped and unlined tunnel, about 3.2 m high and about2.5 m wide, has been used as an access to the existingpressure tunnel. It is situated in sound, stable limestone. Adrainage tube had been installed along this gallery fordraining the existing tunnel for inspection and mainte-nance purposes. Prior to the crossing of the TBM headingthis line had to be displaced in a lower position. Further-more an unreinforced bottom slap was foreseen to providea sufficient bedding of the shield-TBM. To avoid move-ments of the unbedded sidewall segments and to provide asufficient support of the segmental lining, additional dow-els were fitted to the circumferential joints of the segment.The annular gap between rock surface and segmental lin-ing was filled with expansion foam in order to prevent theflowing out of the pea gravel. While the access at the por-tal-side was given through the entrance of the gallery, aman-hole had to been cut at the dead end of the tunnel.After backfilling of the remaining part of the gap the drivecould be continued without loosing any mentionable time.The final works at the crossing of the headrace and the ac-cess tunnel will be finished after completion of the TBMheading (Figure 5).

Fig. 4. Adjusting systemsBild 4. Justierhilfen

c)

b)

a)

Fig 5. Access gallery crossing TBM TunnelBild 5. Fensterstollen Hartelsgraben

6 Crossing of a soft ground section Hartelsgraben

The most challenging part of the TBM heading might be thepassage of the 40 m long Hartelsgraben. According to thefirst tunnel blocks and rock boulders, measuring up to somemetres in diameter, bedded in a matrix of clay and sand areexpected. In addition layers of ravelling ground might oc-cur. Therefore the presence of ground water was identifiedas a hazard factor for flowing ground conditions.

In the tender design no active ground support wasforeseen. Grouting and forepoling measures performedahead of the cutter head were deemed appropriate to pro-vide a safe heading during the crossing of the critical zone.

Finally the following methods were foreseen to crossthe Hartelsgraben:– Pre-drainage of the ground: In case of water presence

the saturated ground shall be drained through boreholes carried out in advance. The hazard of flowingground and disintegration of the cohesive matrix shall

be reduced. Percussive drillings shall fulfil explorativefindings as well as the required drainage purposes.These drilling works can be performed by using the 22openings in the upper part of the tail shield respectivelygripper shield at an angle of 15° respectively 9° relativelyto the tunnel axis.

– Advance support measures (injection pipe umbrella;Figure 6): The concept of the advance support measuresforesees steel micropiles with a diameter of 89 mm. A se-ries of eight pipes per drilling stage with a length of ap-proximately 20 m each and an overlapping of thecanopy of about 8 m provides a systematical groundnailing and roof support. In a second step several mi-cropiles will be grouted with silicate foams in order toconsolidate the soft, porous ground. Prestressing of theweak ground sections will be achieved by applying highpressure grouting and using expansive foams, in order toestablish a supporting arch in the range ahead of thetunnel face.

Fig. 6. Grouting scheme (injection pipe umbrella): a) longitudinal section; b) cross section; c) drilling rigBild 6. Injektionsbohrschirm: a) Längsschnitt; b) Querschnitt; c) Bohrvorrichtung

a)

b) c)

579Geomechanik und Tunnelbau 1 (2008), Heft 6


580 Geomechanik und Tunnelbau 1 (2008), Heft 6


– Additionally reinforced segmental lining: For the 40 mwide Hartelsgraben a modified reinforced segment ringwith a higher load bearing capacity was designed. Con-sidering the higher external loads on the lining due torock mass disintegration and additional forces from thethrust jacks of the TBM, additional main reinforcementbars as well as tensile splitting reinforcement stirrups inthe joints are provided. The installation of such seg-ments has to take place approximately 10 m before en-tering the critical zone.

7 Site experiences during the crossing of the Hartelsgraben

The transition from stable rock to the loose ground sec-tion was exactly identified by exploration drillings aheadof the TBM. The first injection pipe umbrella was per-formed when only one metre was left to enter the looseground with the cutter head, in order to provide a save“dunking” of the TBM into the soft ground. Headingthrough the soft ground complex, the rock bouldersturned out to be small and well consolidated. As a conse-quence the foreseen heading respectively ground treat-ment concept was to be discussed. Finally the decisionwas taken to proceed with the heading without any pre-treatment of the ground. In any case all means for aneventual intervention had to be at hand and everythingpossible had to be undertaken in order to keep the TBM

running and provide a continuous advance of the ma-chine due to the time dependent behaviour of the ground.This challenging requirement for the TBM crew and all re-sponsible engineers from clients and contractors sidecalled for a high degree of attention to any indications ofrock destabilization. Additional observations about dis-placements of the segment lining in order to identify in-sufficient bedding conditions and to recognize extraordi-nary strains were performed.

Finally the remaining part of the critical zone of theHartelsgraben was excavated in a considerably reducedtime and without any mentionable difficulties. In a finalstep contact grouting was applied to improve the beddingconditions of segmental lining.

8 Final part

The upstream end of the headrace tunnel was done by aconventional heading. The reason, why this conventionalheading was done prior to the arrival of the TBM, was thecomplex, loose ground section on the final end of theTBM tunnel.

A reinforced shotcrete lining with lattice girders pro-vides the preliminary support in the soft ground. Thelength of this section was variable, as the transition fromloose ground to stable rock was not known exactly whenstarting the excavation works. Finally 130 m were excavat-

Fig. 7. Typical section upper tunnel sectionBild 7. Regelquerschnitt oberes Stollenportal

ed until good rock conditions were encountered. Another30 m were done by drill and blast for safety reasons.

The tender design foresees a cast in situ lining alongthis section. For economic reasons and in order to shortenthe construction schedule, the contractor presented an al-ternative concept with a segmental lining (Figure 7). Thislining shall be installed when pushing the TBM throughthe upstream end of the tunnel. In order to stabilize theTBM when cutting the concrete slab of the invert, shot-crete strips were applied where the grippers shoes are lo-cated. The circumferential bedding of the segmental liningsystem shall be provided by backfill grouting.

9 Review and site experience

Currently (October 2008) two-thirds of the TBM headingare done and the remaining part shall be completed byend of this year. Reviewing the last months a successfulTBM-drive with a maximum progress of 45 m/d and anaverage weekly advance rate of 250 m could be achieved.Worth mentioning of the drive is the successful passage ofthe challenging section at the Hartelsgraben as well as thecrossing of the existing access gallery. Before arrival at theupstream portal, pushing the TBM through the last 160 mof tunnel and the installation of the segmental lining willbe the last challenge for the tunnelling crew.

Franz GappmaierVerbund-Austrian Hydro Power AGAm Hof 6aA-1010 [email protected]

Dipl.-Ing. Wolfgang WeberJäger Bau GmbHBatloggstraße 95A-6780 [email protected]

Dipl.-Ing. Andreas WalterViglconsult ZT GmbHBatloggstraße 36A-6780 [email protected]


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Extension Hieflau HEP – Economical Solutions for the Headrace Tunnel