STRAIN MONITORING WITH OPTICAL SENSORS IN THE KAUNERTAL PENSTOCK

A. Hammer, P. Bonapace, A. Klarer

Abstract: A new steel lined pressure shaft was constructed for the existing Kaunertal high head hydropower plant in the years 2012 to 2015. Before starting operation initial filling tests up to 88 bar were carried out. The material strains were measured in several sections using optical strain gauges. Monitoring techniques, measurement results and conclusions regarding the interaction between steel lining and rock mass break new ground.

1 Project and Scope The pressure shaft built in the years 2012 to 2015 is part of the existing Kaunertal high head hydropower plant owned by TIWAG – Tiroler Wasserkraft AG. The project included the construction of a 1430 m long 4.3 m diameter inclined pressure shaft, a 325 m long penstock tunnel section, a new surge tank consisting of two chambers connected by a 150 m deep vertical surge shaft. [1] Before start of operation, initial fill-tests were carried out with water from the reservoir. Each test consisted of two loading and unloading cycles of the new facilities. The first fill-test was carried out before the interface between steel lining and backfill concrete was grouted. The second fill-test started after interface grouting was completed. This rather lengthy testing procedure should primarily facilitate the grouting. The rock mass / steel lining response to loading and unloading of the shaft and eventual benefits gained from interface grouting were recorded with optical strain gauges spot-welded to the steel lining. In particular the following scope was investigated

• adequacy of assumptions in the structural analysis • back-analysis of rock modulus “V” and thermal gap “u0” • behaviour of the shaft lining in case of several loading and unloading cycles • isotropic / anisotropic rock mass reaction • development of strains at transitions of embedded to the exposed penstock • redistribution of internal (cover-) pressure into the rock mass

2 Monitoring Arrangement Monitoring sections were situated at various locations along the alignment. Four sections “A” aiming to monitor a standard shaft section in varying geologic conditions and three sections “B” aiming to monitor the lining behaviour at transitions into the rock mass were installed.

Section “A” arrangements (Fig. 1) consisted of two identical monitoring cross sections at 1.0 m spacing to cover redundancy. Each monitoring cross section included seven monitoring points around the circumference to measure circumferential strains at 45° angles. Every second monitoring point was fitted with a longitudinal strain gauge to measure the longitudinal behaviour of the steel lining as well. For practical reasons no strain gauges were installed in the invert of the lining (danger of damage).

Fig. 1: Monitoring cross sections “A” in varying geologic conditions

Each section “B” arrangement (Fig. 2) was fitted with strain gauges at an angle of 45° left and right of the centre line in the invert of the steel lining. Six monitoring cross sections were placed at each transition of embedded to exposed penstock. Spacing of monitoring cross sections was selected to take measurements approx. 10 m deep into the rock mass along the penstock axis. Due to the long distances (up to 700 meters) between the monitoring cross sections and the feedthrough out of the penstock and the long measuring time, standard electrical strain gauges could not be used. Therefore optical strain gauges were applied. The optical sensors were mounted on thin metal strips which were spot welded onto the lining. The spot weld points were grinded and tested after removal of the strips to exclude surface damages.

Fig. 2: Monitoring cross sections “B” – measuring at transitions into the rock mass

3 Measuring principle of optical sensors The principle of optical strain sensors is that Light with the Bragg-wavelength λB is reflected by the grating and the wavelength is measured by an interrogator. Strain applied to a Fibre Bragg Grating (FBG) increases/decreases the distance between the “mirrors” of the grating and shifts the Bragg-wavelength of reflected light depending on compression-tension direction. This wavelength shift (Δλ) on a FBG subjected only to strain is a linear function of the applied strain and the initial wavelength (λ0). [2]

∆𝜀 =∆𝜆𝑘 ∙ 𝜆0

= ∆𝜆 ∙ 𝑆 ∆𝜀 𝑘 𝑆

… measured strain [µm/m] … gauge factor [-] … strain sensitivity [µm/m/nm]

(3.1)

Fig. 3: Measuring principle of optical sensors with FBG

The Fibre Bragg Grating (FBG) is “written” into a single mode optical fibre by activation of Germanium dopings or inscription of morphological defects into the core with high level energy lasers [3]. The advantages of the FBG technology are (compared to standard electrical strain gauges):

• Up to 20 sensors in one measurement chain • Possible length of chains up to some kilometres • Redundancy due to blank-cable at second end of measuring chain (ring line) • No effect of water-pressure onto FBGs • Small amount of cables and small diameters means small feed-through • Linear function to applied strain • No water effects on cables (no short circuit, no electrical issues)

As shown in Figure 4 a typical measurement chain consists of a connection fibre which can be almost as long as needed. At the end it is spliced together with the sensor fibre which is up to 200 meters long and has up to 20 FBGs with different nominal wavelengths in one line. The total sum of damping over the chain has to be smaller than the allowable value from the interrogator. Finally the emitted light reflects in every FBG and the wavelength of the ref is measured in the interrogator.

Fig. 4: Typical measurement chain with several FBGs

4 Initial Fill-Tests and Results In June 2015 the new shaft was filled for the first time. A pressure of 88 bar was reached at the shaft bottom after three days of filling and held for another three days until the facility was emptied again (Fig. 5). After a waiting time of approx. one day the shaft was filled again and emptied. At first loading the measured strains at sections “A” as predicted in the analysis responded pretty much isotropic (Fig. 6). Even without any interface grouting substantial interaction between rock mass and steel lining could be registered. On average the internal pressure was shared half and half between the steel lining and the rock mass. When emptying the facility for the first time a reduction of the original shaft diameter resulting in a sudden release of pipe friction along a substantial length of the shaft associated with substantial negative strains could be measured (e.g. Fig. 7, Fill-Test 1 at 25-20 bar).

Fig. 5: Measured strains at section A4-1 und A4-2 at both Fill-Tests (FT1 and FT2)

Fig. 6: Typical measurement cycle before (FT1) and after (FT2) interface grouting

At the second loading and unloading cycle (still before interface grouting), the thermal gap provided space for free deformation of the steel lining. This thermal gap closed after 200 to 400 µm/m (equals 0.02 - 0.04%) of radial strain increase and the steel lining was bedded in the rock mass again. The finding was blamed on an initial settling of the strain gauges and penstock, temperature difference, shrinkage of concrete and creep, which was, however, not predicted to such an extent. The gap had to be filled with grout to improve uniform bedding of the penstock in the rock mass. After interface grouting (loadings 3 and 4) longitudinal strains were zero. Circumferential strains responded elastically and returned to the starting point at the pressure-strain diagram (Fig. 7).

Fig. 7: Typical steel lining response before (FT1) and after (FT2) interface grouting

At sections “B” (Fig. 2) it was discovered that even without any shear rings the full cover pressure of up to 60 MN was redistributed at the transition after only 3 to 4 m (1.5 diameters) of embedded pipe length. [4] Remarkable stress redistribution was registered at pipe sections with moderate wall thicknesses as compared to thick walled pipe sections. A considerable improvement of stress redistribution into the rock mass was noticed in fill-test 2 after the interface grouting. After having redistributed the cover pressure close to the transition, the penstock was locked into the rock mass with restricted longitudinal strain equivalent to a rather constant longitudinal pipe force (e.g. Fig. 8, at 4.0 to 10.0 m distance from transition).

Fig. 8: Typical longitudinal strain and longitudinal force in pipe at transition into the rock

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Strains and Forces at Transition B3Pipe diameter 3100 mmInternal pressure 85 bar

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Force Fill-Test1 (KN) Force Fill-Test2 (KN)

5 Conclusions The measurements could prove that the contribution of the rock mass to bearing the internal pressure was at least the anticipated magnitude. Such the adequacy of assumptions in the structural analysis could be confirmed. Back-analysis of rock modulus proposes that using Vest* = 3840 MPa was adequate all along the shaft. A reduction of rock modulus in the Variegated Bündner Schist, which was predicted to be of poorer quality than the Grey Bündner Schist, would not have been required. The thermal gap was more pronounced than anticipated. Grouting of the concrete/steel interface however closed the gap entirely and reduced thermal effects such, that they were no longer measurable. The lining responded to pressure increase entirely elastically, although several loading and unloading cycles were applied. A separation of the pipe steel from the backfill concrete as noticed before grouting was no longer detectable after the interface was pressure grouted. A slightly anisotropic lining deformation could be recorded, but could not be related to anisotropy of the rock mass. Longitudinal lining strains at transitions of the embedded to the exposed penstock were redistributed by friction into the rock mass within 3 to 4 m of the transition. The remaining longitudinal strain further upstream was close to zero and left a constant force related to the steel thickness in the lining, independent of any cover pressure. Pressure grouting the interface after a first filling of the penstock was the right decision to justify favourable assumptions made in the analysis concerning rock mass contribution to bearing the internal pressure of the penstock lining. Also the applied measurement system including optical strain gauges provided very accurate and reliable data. It can be concluded that the following findings shall be used for design, analysis and testing to describe the behaviour of pressure shafts and embedded penstocks adequately and safe:

• Consideration of load sharing between rock mass and steel lining • Application of interface grouting after initial loading by a fill test • Consideration of the substantial load redistribution capacity by friction at

transitions of exposed to embedded pipe sections • Preference of optical strain sensors for measurements in and on the steel

pipe References

[1] P. Bonapace, The renewal of the pressure shaft for the high head hydropower plant Kaunertal in Austria: Civil engineering and civil works, 2012, Vienna Hydro

[2] D. Graham-Rowe, Sensors take the strain, Nat. Photon.1, (2007), 307-309 [3] E. Brinkmeyer, “Faseroptische Gitter” in: Optische Kommunikationstechnik, Eds.

E. Voges and K. Petermann, Springer, (2002) [4] H. Unterweger, A. Ecker, Abklingverhalten von Rohrlängskräften in

Druckschachtpanzerungen bei Berücksichtigung der Reibung zwischen Rohrwand und Hinterfüllbeton – Parameterstudie mit und ohne Schubringe, Institut für Stahlbau, TU Graz, 2016

Authors Andreas Hammer TIWAG – Tiroler Wasserkraft AG Eduard-Wallnöfer-Platz 2 - 6020 Innsbruck Phone: +43 50607 21392 E-mail: andreas.hammer@tiwag.at Paul Bonapace TIWAG – Tiroler Wasserkraft AG Eduard-Wallnöfer-Platz 2 - 6020 Innsbruck Phone: +43 50607 21311 E-mail: paul.bonapace@tiwag.at Andreas Klarer HBM – Hottinger Baldwin Messtechnik GmbH Lemböckgasse 63/2 - A-1230 Wien Phone: +43 1 865 8441-317 E-mail: andreas.klarer@hbm.com