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117. Unloading joints in the central Aar granite of the Schöllenen Gorge (canton Uri, Switzerland)

Author

Bucher, Stefan and Loew, Simon

Abstract

Zusammenfassung: Talklüfte können im Zusammenhang mit Strasseneinschnitten, Wasserkraftanlagen und geologischen Massenbewegungen in steilen Talflanken eine wichtige Rolle spielen. Die Schöllenenschlucht im Reusstal (zentrale Schweizer Alpen) bildet ein ideales Untersuchungsgebiet für das Studium der Eigenschaften und Entstehungsmechanismen solcher Talklüfte. Die Schöllenen- Schlucht liegt am Südrand des Zentralen Aaregranits, welcher vor rund 300Mio. Jahren intrudierte, während der Alpenbildung nur lokal duktil deformiert und anschliessend an die Oberfläche gehoben wurde. Die bis zu 600 m tiefe Schlucht bietet einen idealen Einblick in das komplexe Trennflächengefüge des Plutons. Einen Einblick in das Berginnere erlauben zudem die Stollen des Kraftwerks Göschenen. Die Kartierungsarbeit führte zur Unterscheidung von zwei vermutlich alpin-syntektonischen Kluftfamilien (S- und Q-Klüfte) sowie drei Generationen von Entlastungsklüften (subhorizontale Dehnungsklüfte (L-Klüfte) und zwei Generationen von mittelsteilen bis steilen Talklüften (T1- und T2- Klüfte)). Die zahlreichen S-Klüfte verlaufen meistens parallel zur alpinen Schieferung, d. h. ungefähr E-W streichend und steil gegen Süden einfallend. Die weniger häufigen Q-Klüfte fallen steil gegen SW ein, der Winkel zwischen den beiden Kluftfamilien beträgt 60 bis 80 Grad. Die räumliche Lage der verschiedenen Entlastungsklüfte wurde primär in Querprofilen durch die Schöllenen erfasst. Die ersten Entlastungsklüfte verlaufen horizontal, sie bildeten sich unabhängig von der Topographie möglicherweise während einer spätalpinen miocänen Hebung des Aarmassivs. Eine erste Generation von Talklüften (T1) fällt mit 30-45 Grad beidseitig talwärts ein und wurde vermutlich während einer frühen pleistozänen Eintalung gebildet. Die jüngsten Talklüfte (T2) sind parallel zur heutigen Taloberfläche der Schöllenenschlucht orientiert, d. h. sie liegen parallel zu den mit spätpleistozänen Gletscherschliffen markierten Felsoberflächen. Die Frequenz sowie die Grösse dieser Klüfte nehmen zumindest in einem Stollen gegen das Berginnere ab und konnten bis rund 150 Meter ins Berginnere nachgewiesen werden. Fraktographische Untersuchungen, d. h. die Untersuchung von Bruchstrukturen auf Kluftoberflächen (hauptsächlich Federmarken mit einer Grösse von 5 bis 10 m) bestätigten diese relative Alterabfolge. Ebenso konnten dadurch jeweils der Ursprungspunkt einer Kluft und die Bruchfortpflanzungsrichtung bestimmt werden. Es zeigte sich dabei stets, dass grosse Talklüfte (bis über hundert Meter Länge) aus mehreren 5-10 Meter grossen Extensionsklüften bestehen, welche durch primär subhorizontale stabile Rissausbreitung entstanden sind. Grosse Talklüfte bilden sich bevorzugt in Graniten mit nur geringer alpin-tektonischer überprägung. Die Talklüfte der Schöllenen bilden sich nicht als Scherbrüche, wie dies von früheren Autoren angenommen wurde, sondern als Extensionsrisse in einem kompressiven Spannungsfeld. Das räumliche Auftreten und die beobachteten Grössen der Talklüfte können teilweise mit quantitativen Bruchkriterien, wie sie für hohlraumnahe Extensionsrisse in Tunnelwerken entwickelt wurden, erklärt werden. Wesentlich für den Ort und die Grösse der Talklüfte sind neben den Deviatorspannungen und Zugspannungen auch die σ3/σ1 Hauptspannungsverhältnisse. Erste numerische Modellrechnungen zu einer mehrphasigen glazialen Eintalung vermögen die beobachteten Bildungsbereiche und Eigenschaften der Talklüfte grob abzubilden

Published
2018

119. The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: Open questions tackled in a decameter-scale in situ stimulation experiment

Author

Amann, Florian, Gischig, Valentin, Evans, Keith, Doetsch, Joseph, Jalali, Reza, Valley, Benoît, Krietsch, Hannes, Dutler, Nathan, Villiger, Linus, Brixel, Bernard, Klepikova, Maria, Kittilä, Anniina, Madonna, Claudio, Wiemer, Stefan, Saar, Martin O., Loew, Simon, Driesner, Thomas, Maurer, Hansruedi, and Giardini, Domenico

Subjects
ddc:550
Abstract

Solid Earth, 9 (1), ISSN:1869-9510, ISSN:1869-9529

Published
2018

125. Monitoring Surface Deformation over a Failing Rock Slope with the ESA Sentinels: Insights from Moosfluh Instability, Swiss Alps

Author

Manconi, Andrea, Kourkouli, Penelope, Caduff, Rafael, Strozzi, Tazio, and Loew, Simon

Subjects
Radar interferometry, Science, digital image correlation, landslide deformation monitoring, Rock slope failure
Abstract

We leverage on optical and radar remote sensing data acquired from the European Space Agency (ESA) Sentinels to monitor the surface deformation evolution on a large and very active instability located in the Swiss Alps, i.e., the Moosfluh rock slope. In the late summer 2016, a sudden acceleration was reported at this location, with surface velocity rates passing from maximum values of 0.2 cm/day to 80 cm/day. A dense pattern of uphill-facing scarps and tension cracks formed within the instability and rock fall activity started to become very pronounced. This evolution of the rock mass may suggest that the most active portion of the slope could fail catastrophically. Here we discuss advantages and limitations of the use of spaceborne methods for hazard analyses and early warningbyusingtheESASentinels,andshowthatincriticalscenariostheyareoftennotsufficientto reliably interpret the evolution of surface deformation. The insights obtained from this case study are relevant for similar scenarios in the Alps and elsewhere., Remote Sensing, 10 (5), ISSN:2072-4292

Published
2018
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132. The seismo-hydro-mechanical behaviour during deep geothermal reservoir stimulations: open question tackled in a decameter-scale in-situ stimulation experiment

Author

Amann, Florian, Gischig, Valentin, Evans, William, Doetsch, Joseph, Jalali, Reza, Valley, John W., Krietsch, Hannes, Dutler, Hans, Villiger, Linus, Brixel, Bernard, Klepikova, Maria, Kittilä, Anniina, Madonna, Claudio, Wiemer, Stefan, Saar, Martin O., Loew, Simon, Driesner, Thomas, Maurer, Hansrudolf, and Giardini, Domenico

Subjects
Induced seismicity, In-situ experiments, Deep geothermal energy, Hydromechanical coupling processes in EGS, EGS
Abstract

In this contribution we present a review of scientific research results that address seismo-hydro-mechanical coupled processes relevant for the development of a sustainable heat exchanger in low permeability crystalline rock and introduce the design of the In-situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to study such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydro-mechanical processes that would enhance process understanding in a way that aids future stimulation design. Small scale laboratory experiments provide a fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from 1) difficulties and uncertainties in upscaling the results to the field-scale and 2) relatively homogeneous material and stress conditions that lead to an over-simplistic fracture flow and/or hydraulic fracture propagation behaviour that is not representative for a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales, and for which access to the target rock mass with a comprehensive monitoring system is possible. Only few intermediate-scale hydro-shearing and hydro-fracturing experiments have recently been performed in a densely instrumented rock mass. No such measurements have been performed on faults in crystalline basement rocks. The In-situ Stimulation and Circulation (ISC) experiment currently performed in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland) is designed to address open research questions, which could not be investigated in the required detail so far. Two hydraulic injection phases were executed to enhance the permeability of the rock mass: a hydro-shearing phase and then a hydraulic fracturing phase. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation and the micro-seismic response were monitored at a high spatial and temporal resolution., Solid Earth Discussions, ISSN:1869-9537

Published
2017

135. Hydro-mechanical processes and their influence on the stimulation effected volume: Observations from a decameter-scale hydraulic stimulation project.

Author

Krietsch, Hannes, Gischig, Valentin S., Doetsch, Joseph, Evans, Keith F., Villiger, Linus, Jalali, Mohammadreza, Valley, Benoit, Loew, Simon, and Amann, Florian

Subjects
HYDROELECTRIC power plants, SHEAR zones, SHEAR flow, CRYSTALLINE rocks, SEISMIC response, COMPOUND fractures, BOREHOLES
Abstract

Six hydraulic shearing experiments have been conducted in the framework of the In-situ Stimulation and Circulation experiment within a decameter-scale crystalline rock volume at the Grimsel Test Site, Switzerland. During each experiment one out of two different shear zone types were hydraulically reactivated. An extensive monitoring system of sensors recording seismicity, pressure and strain was spatially distributed in eleven boreholes around the injection locations. As a result of the stimulation, the near-wellbore transmissivity increased up to three orders in magnitude, while jacking pressures of the stimulated structures reduced during most of the experiments. Transmissivity change, jacking pressure and seismic activity were different for the two shear zone types, suggesting that the shear zone characteristics govern the seismo-hydro-mechanical response. The elevated fracture-fluid-pressures associated with the stimulations propagated mostly along the stimulated shear zones. The absence of high-pressure signals away from the injection point for most experiments (except two out of six experiments) is interpreted as channelized flow within the shear zones. The observed deformation field within 15 m-20 m from the injection point is characterized by variable extensional and compressive strain, produced by fracture normal opening and/or slip dislocation, as well as stress redistribution related to these processes. At greater distance from the injection location, strain measurements indicate a volumetric compressive zone, in which the strain magnitude decreases with increasing distance. This compressive strain signals are interpreted as a poro-elastic far-field response to the emplacement of fluid volume around the injection interval. The exceptional hydro-mechanical data reveal that the overall stimulation effected volume is significantly larger than implied by the seismicity cloud, and can be subdivided into a primary stimulated and secondary effected zone. [ABSTRACT FROM AUTHOR]

Published
2020
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140. Geothermal reservoir characterization through active thermal testing

Author

Jung, Martin, Klepikova, Maria, Jalali, Mohammadreza, Fisch, Hansruedi, Loew, Simon, and Amann, Florian

Abstract

EGU General Assembly 2016, EGU2016, Vienna, Austria, 17 Apr 2016 - 22 Apr 2016; Geophysical research abstracts : GRA 18, EGU2016-12475 (2016). special issue: "EGU General Assembly 2016", Published by European Geophysical Society, Katlenburg-Lindau

Published
2016
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141. The seismo-hydro-mechanical behaviour during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in-situ stimulation experiment

Author

Amann, Florian, primary, Gischig, Valentin, additional, Evans, Keith, additional, Doetsch, Joseph, additional, Jalali, Reza, additional, Valley, Benoît, additional, Krietsch, Hannes, additional, Dutler, Nathan, additional, Villiger, Linus, additional, Brixel, Bernard, additional, Klepikova, Maria, additional, Kittilä, Anniina, additional, Madonna, Claudio, additional, Wiemer, Stefan, additional, Saar, Martin O., additional, Loew, Simon, additional, Driesner, Thomas, additional, Maurer, Hansruedi, additional, and Giardini, Domenico, additional

Published
2017
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148. Time-Dependent Behaviour of Brittle Rocks Based on Static Load Laboratory Tests.

Author

Paraskevopoulou, Chrysothemis, Perras, Matthew, Diederichs, Mark, Loew, Simon, Lam, Tom, and Jensen, Mark

Subjects
ROCK mechanics, BRITTLE materials, DEAD loads (Mechanics), GRANITE, DATA acquisition systems
Abstract

Cumulative elastic and inelastic strain and associated internal stress changes as well as damage evolution over time in brittle rocks control the long-term evolution of the rockmass around underground openings or the land surface settlement. This long-term behaviour is associated with time-dependent deformation and is commonly investigated under static load (creep) conditions in laboratory scale. In this study, low Jurassic and Cobourg limestone samples were tested at different static load levels in unconfined conditions to examine the time to failure. Comparisons are made with longterm testing data in granites and limestones associated with the Canadian nuclear waste program and other data from the literature. Failure typically occurred within the time limits of the test program (4 months) with axial (differential) stress levels near or above the crack damage threshold (CD) estimated from baseline testing. The results also suggest that the time to failure of limestone is longer than that of granite at a given driving stress. Further insight into samples that did not reach failure was investigated and it was found that there was a clear division between failure and no failure samples based on the Maxwell viscosity of the samples tested (indicating that viscosity changes near the yield threshold of these rocks. Furthermore, samples showed a clear tendency towards failure within minutes to hours when loaded above CD and no failure was shown for samples loaded below CI (crack initiation threshold). Samples loaded between CD and CI show a region of uncertainty, with some failing and other not at similar driving stress-ratios. Although such testing is demanding in terms of setup, control of conditions, continuous utilization of test and data acquisition equipment and data processing, it yields important information about the long-term behaviour of brittle rocks, such as the expect time to failure and the visco-elastic behaviour. The information presented in this paper can be utilized for preliminary numerical studies to gain an understanding of potential impact of long-term deformations. [ABSTRACT FROM AUTHOR]

Published
2018
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150. Rockslide deformation monitoring with fibre optic strain sensors

Author

Moore, Jeffrey R., Gischig, Valentin, Button, Edward Alan, and Loew, Simon

Abstract

With micro-strain resolution and the capability to sample at rates of 100 Hz and higher, fiber optic (FO) strain sensors offer exciting new possibilities for in-situ landslide monitoring. Here we describe a new FO monitoring system based on long-gauge fiber Bragg grating sensors installed at the Randa Rockslide Laboratory in southern Switzerland. The new FO monitoring system can detect sub-micrometer scale deformations in both triggered-dynamic and continuous measurements. Two types of sensors have been installed: (1) fully embedded borehole sensors and (2) surface extensometers. Dynamic measurements are triggered by sensor deformation and recorded at 100 Hz, while continuous data are logged every 5 min. Deformation time series for all sensors show displacements consistent with previous monitoring. Accelerated shortening following installation of the borehole sensors is likely related to long-term shrinkage of the grout. A number of transient signals have been observed, which in some cases were large enough to trigger rapid sampling. The combination of short- and long-term observation offers new insight into the deformation process. Accelerated surface crack opening in spring is shown to have a diurnal trend, which we attribute to the effect of snowmelt seeping into the crack void space and freezing at night to generate pressure on the crack walls. Controlled-source tests investigated the sensor response to dynamic inputs, which compared an independent measure of ground motion against the strain measured across a surface crack. Low frequency signals were comparable but the FO record suffered from aliasing, where undersampling of higher frequency signals generated spectral peaks not related to ground motion., Natural Hazards and Earth System Sciences, 10 (2), ISSN:1561-8633, ISSN:1684-9981

Published
2010
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