Simon Loew

Site Effects in Unstable Rock Slopes: Dynamic Behavior of the Randa Instability (Switzerland)

We evaluate the dynamic response of the large (around five million cubic meters) unstable rock slope at Randa, Switzerland, based on ground-motion recordings during small earthquakes. Seismic measurements revealed strong polarization of the wavefield in the direction of maximum slope displacement with spectral amplification factors ∼5 within the unstable area. Results further highlighted resonant frequencies of both the large unstable rock mass and individual blocks within it. Block vibration was confirmed by phase analysis of in situ displacement measurements from two bounding tension fractures during a small earthquake. Numerical simulation using a discontinuous elastic model showed that the presence of steeply-dipping compliant fractures is crucial for recreating the observed site response. Such fractures are common in rock slope instabilities and are here shown to contribute to polarization and amplification of seismic shaking, site effects that are rarely considered for hard rock slopes and which may influence the potential for earthquake-triggered failure.

Beyond debuttressing: Mechanics of paraglacial rock slope damage during repeat glacial cycles

Cycles of glaciation impose mechanical stresses on underlying bedrock as glaciers advance, erode, and retreat. Fracture initiation and propagation constitute rock mass damage and act as preparatory factors for slope failures; however, the mechanics of paraglacial rock slope damage remain poorly characterized. Using conceptual numerical models closely based on the Aletsch Glacier region of Switzerland, we explore how in situ stress changes associated with fluctuating ice thickness can drive progressive rock mass failure preparing future slope instabilities. Our simulations reveal that glacial cycles as purely mechanical loading and unloading phenomena produce relatively limited new damage. However, ice fluctuations can increase the criticality of fractures in adjacent slopes, which may in turn increase the efficacy of fatigue processes. Bedrock erosion during glaciation promotes significant new damage during first deglaciation. An already weakened rock slope is more susceptible to damage from glacier loading and unloading and may fail completely. We find that damage kinematics are controlled by discontinuity geometry and the relative position of the glacier; ice advance and retreat both generate damage. We correlate model results with mapped landslides around the Great Aletsch Glacier. Our result that most damage occurs during first deglaciation agrees with the relative age of the majority of identified landslides. The kinematics and dimensions of a slope failure produced in our models are also in good agreement with characteristics of instabilities observed in the field. Our results extend simplified assumptions of glacial debuttressing, demonstrating in detail how cycles of ice loading, erosion, and unloading drive paraglacial rock slope damage.

Automatic detection of alpine rockslides in continuous seismic data using hidden Markov models

Data from continuously recording permanent seismic networks can contain information about rockslide occurrence and timing complementary to eyewitness observations and thus aid in construction of robust event catalogs. However, detecting infrequent rockslide signals within large volumes of continuous seismic waveform data remains challenging and often requires demanding manual intervention. We adapted an automatic classification method using hidden Markov models to detect rockslide signals in seismic data from two stations in central Switzerland. We first processed 21 known rockslides, with event volumes spanning 3 orders of magnitude and station event distances varying by 1 order of magnitude, which resulted in 13 and 19 successfully classified events at the two stations. Retraining the models to incorporate seismic noise from the day of the event improved the respective results to 16 and 19 successful classifications. The missed events generally had low signal-to-noise ratio and small to medium volumes. We then processed nearly 14 years of continuous seismic data from the same two stations to detect previously unknown events. After postprocessing, we classified 30 new events as rockslides, of which we could verify three through independent observation. In particular, the largest new event, with estimated volume of 500,000 m3, was not generally known within the Swiss landslide community, highlighting the importance of regional seismic data analysis even in densely populated mountainous regions. Our method can be easily implemented as part of existing earthquake monitoring systems, and with an average event detection rate of about two per month, manual verification would not significantly increase operational workload.

Hazard and Risk Classification for Large Unstable Rock Slopes in Norway

We present a classification system for hazard and risk that is posed by unstable rock slopes in Norway that might undergo catastrophic failure in future and can cause loss of life. The system is scenario-based as the intensity and rate of displacement, as well as the geological structures activated by the sliding rock mass vary significantly on the slopes. In addition, for each scenario the secondary effects, such as genera- tion of displacement waves or landslide damming of valleys with the potential of later outburst floods, are evaluated. The hazard analysis is based on two types of criteria: 1) Structural site investigations including analysis of the development of a back-scarp, lateral boundaries and basal sliding surface. This includes a kinematic analysis for sliding and toppling based on slope orientation, persistence of main structures and morphologic expressions of the sliding surface. 2) Analysis of slope activity primarily based on slide velocity, change of deformation rates, observation of rockfall activity, and historic or prehistoric events. The analysis of consequences focuses on the potential fatalities to the rock slide scenarios and secondary ef- fects. Based on the hazard and consequence analysis each scenario is classified in a risk matrix into cat - egory low, medium or high risk.

Geomechanical behaviour of Opalinus Clay at multiple scales: results from Mont Terri rock laboratory (Switzerland)

The paper represents a summary about our research projects conducted between 2003 and 2015 related to the mechanical behaviour of Opalinus Clay at Mont Terri. The research summarized covers a series of laboratory and field tests that address the brittle failure behaviour of Opalinus Clay, its undrained and effective strength, the dependency of petro-physical and mechanical properties on total suction, hydro-mechanically coupled phenomena and the development of a damage zone around excavations. On the laboratory scale, even simple laboratory tests are difficult to interpret and uncertainties remain regarding the representativeness of the results. We show that suction may develop rapidly after core extraction and substantially modifies the strength, stiffness, and petro-physical properties of Opalinus Clay. Consolidated undrained tests performed on fully saturated specimens revealed a relatively small true cohesion and confirmed the strong hydro-mechanically coupled behaviour of this material. Strong hydro-mechanically coupled processes may explain the stability of cores and tunnel excavations in the short term. Pore-pressure effects may cause effective stress states that favour stability in the short term but may cause longer-term deformations and damage as the pore-pressure dissipates. In-situ observations show that macroscopic fracturing is strongly influenced by bedding planes and faults planes. In tunnel sections where opening or shearing along bedding planes or faults planes is kinematically free, the induced fracture type is strongly dependent on the fault plane frequency and orientation. A transition from extensional macroscopic failure to shearing can be observed with increasing fault plane frequency. In zones around the excavation where bedding plane shearing/shearing along tectonic fault planes is kinematically restrained, primary extensional type fractures develop. In addition, heterogeneities such as single tectonic fault planes or fault zones substantially modify the stress redistribution and thus control zones around the excavation where new fractures may form.

Detailed hydrogeological analysis of a deep-seated rockslide at the Gepatsch reservoir (Klasgarten, Austria)

The hydrogeology of the deep-seated, slowly creeping Klasgarten rockslide in Austria is investigated in this study based on detailed surface and subsurface field data, laboratory analyses, and analytical and numerical simulations. Field data are derived from several deep exploration and monitoring boreholes, an exploration drift located within the rockslide, and geological and geomorphological mapping. Particular attention is given to the pore pressure measurements and their temporal and spatial variability. These pore pressure variations are controlled by a thin layer of clayey fault gouge (representing the basal shear zone of the rockslide), a high-permeability rockslide mass, and moderately fractured paragneissic bedrock. Variably saturated equivalent-continuum hydraulic conductivities and storage properties are derived from packer tests, laboratory tests and optical televiewer images. These data sets are used for two-dimensional numerical groundwater models to study the flow-field and pore-pressure variations caused by the reservoir water-level fluctuations, the transient groundwater infiltration from snowmelt and precipitation along the slope, and the exploration drift. The strongest pressure transients in the rockslide are caused by reservoir level fluctuations and not the natural groundwater recharge, even at substantial distances from the reservoir. The response times are very short and only a minor distance-dependent attenuation is observed. The results of this study are essential to analyse the hydromechanical control of the deformation behaviour of rockslides adjacent to hydropower reservoirs. Further, it helps to understand how the formation of a rockslide can change the original bedrock aquifer.RésuméL’hydrogéologie du glissement de terrain de Klasgarten, enraciné en profondeur et à dynamique lente en Autriche est étudiée sur la base de données détaillées de surface et de subsurface, d’analyses en laboratoire, et de simulations analytiques et numériques. Les données de terrain sont tirées de plusieurs forages profonds d’exploration et de forages de surveillance, une galerie d’exploration située au sein du glissement de terrain, et de cartographie géologique et géomorphologique. Une attention particulière est accordée aux mesures de pression interstitielle et de leur variabilité spatiale et temporelle. Ces variations de pression interstitielle sont contrôlées par une mince couche de brêches argileuses de faille (représentant la zone de cisaillement à la base du glissement de terrain), une masse de terrain glissé à haute perméabilité, et le substratum rocheux de paragneiss modérément fracturé. Les conductivités hydrauliques à saturation variable d’un milieu équivalent continu et les propriétés de coefficient d’emmagasinement sont dérivées des essais entre obturateurs, des tests en laboratoire et des images optiques par caméra en forage. Ces ensembles de données sont utilisées dans des modèles numériques d’écoulements d’eaux souterraines en 2D afin d’étudier le champ d’écoulement et les variations de la pression interstitielle qui sont à l’origine des fluctuations du niveau piézométrique dans le réservoir, de l’infiltration transitoire vers les eaux souterraines et des eaux de fonte de neige et des précipitations le long de la pente et au niveau de la galerie. Les plus fort transferts de pression dans le glissement de terrain sont causés par des fluctuations des niveaux d’eau au sein du réservoir et non pas par la recharge naturelle des eaux souterraines, même à des distances importantes du réservoir. Les temps de réponse sont très courts et seule une atténuation dépendant de la distance est observée. Les résultats de cette étude sont essentiels pour analyser le contrôle hydromécanique du comportement de la déformation de glissement de terrain adjacent à des réservoirs hydroélectriques. En outre, il aide à comprendre comment la formation d’un glissement de terrain peut changer l’aquifère d’origine de socle.ResumenEn este estudio se investiga la hidrogeología del deslizamiento de rocas del emplazamiento profundo y de lenta reptación en Klasgarten, Austria, basado en detallados datos de campo de la superficie y el subsuelo, en análisis de laboratorio y en simulaciones analíticas y numéricas. Los datos de campo provienen de varias exploraciones profundas y pozos de monitoreo, de un socavón ubicado dentro del deslizamiento de las rocas, y de mapeos geológicos y geomorfológicos. Se presta especial atención a las medidas de presión poral y su variabilidad temporal y espacial. Estas variaciones en la presión poral son controladas por una fina capa arcillosa de jaboncillo de falla (que representa la zona de corte basal del deslizamiento de las rocas), por la alta permeabilidad de la masa de deslizamiento de rocas, y por una roca de base paragneisica moderadamente fracturada. Las conductividades hidráulicas del equivalente del continuo variablemente saturado y las propiedades de almacenamiento se derivan de las pruebas de packer, de las pruebas de laboratorio e imágenes ópticos de televisión. Estos conjuntos de datos se utilizan para los modelos numéricos bidimensionales de agua subterránea a fin de estudiar las variaciones del flujo de campo y de la presión poral causadas por las fluctuaciones nivel de agua en el embalse, por la infiltración transitoria de agua subterránea a partir del derretimiento de nieve y las precipitaciones a lo largo de la pendiente y del socavón. Las fuertes presiones transitorias en del deslizamiento de las rocas son causadas por las fluctuaciones del nivel del embalse y no por la recarga natural de agua subterránea, incluso a distancias considerables desde el embalse. Los tiempos de respuesta son muy cortos y sólo se observa una pequeña atenuación dependiente de la distancia. Los resultados de este estudio son esenciales para analizar el control hidromecánico del comportamiento de la deformación de los deslizamientos de rocas adyacentes a embalse hidroeléctricos. Además, ayuda a entender cómo la formación de un deslizamiento de rocas puede cambiar la base original en el acuífero.摘要根据详细的地表及地表以下野外资料、实验室分析以及解析和数值模拟结果,本项研究调查了奥地利Klasgarten地区深层、缓慢移动的崩落岩石的水文地质状况。野外资料来源于几个深层钻探监测井、崩落岩石区域内的一次探险漂移和地质和地貌填图。对孔隙压力测量及其时空变化给予特别关注。这些孔隙压力变化受薄层的粘土质断层泥(代表崩落岩石的基部剪切带)、渗透性高的崩落岩石体和中度断裂的共生基岩的控制。变化不定的饱和等效连续体导水率和储存特性来自于压水试验、实验室试验和光电视收视者图片。这些资料集用于二维数值地下水模型,用于研究水库水位变化、沿边坡的雪融水和降水瞬时地下水入渗以及探险漂流引起的流场和孔隙压力变化。崩落岩石内的最强压力瞬变是由水库水位波动引起的,不是由天然地下水补给引起的,即使离水库距离很远也是如此。响应时间非常短,仅仅观测到很小的依赖距离的衰减。本项研究的结果对于分析水力发电水库附近崩落岩石变形状况流体力学控制因素必不可少。此外,有助于使人了解崩落岩石的变形能够怎样改变原始的基岩含水层。ResumoAs características hidrogeológicas dos colapsos profundos em Klasgarten, na Áustria, foram investigados por meio de estudos de superfície e subsuperfície em detalhe, análises em laboratório e modelagem analítica e numérica. Os dados de campo foram obtidos utilizando-se diversas sondagens exploratórias e poços de monitoramento, caracterização dos condutos colapsados e mapeamento geológico e morfológico. Um dos principais fatores avaliados foi a pressão de poro e sua variação temporal e espacial. Estas variações na pressão de poro são controladas por uma fina camada de argila sob pressão que preenche as falhas (representando a zona de cisalhamento basal do deslizamento), condutividade hidráulica elevada na matriz da rocha e fraturamento moderado da rocha, que é composta por paragnaisse. Valores de condutividade hidráulica e armazenamento para meios equivalentes contínuos com saturação variada foram obtidos a partir de ensaios com obturadores, ensaios em laboratório e perfilagem ótica (optical televiewer images). Estes dados foram utilizados na elaboração de modelos numéricos bidimensionais para estudar as variações no fluxo e na pressão de poro causados pela variação no nível d’água do reservatório e recarga do aquífero em regime transiente, decorrente do degelo e precipitação nas áreas de recarga e em locais de escavações das minas. As maiores variações na pressão de poro são causadas pela flutuação do nível do reservatório até mesmo em locais distantes, havendo pouca influencia da recarga natural do aquífero. Os tempos de resposta observados são muito curtos, com pequeno efeito de atenuação registrado em função da distância. Os resultados deste estudo são essenciais para a análise hidromecânica que controla o comportamento de deformação nos processos de colapso de condutos próximos aos reservatórios de hidroelétricas. Além disso, melhora o entendimento de como a ocorrência dos colapsos podem alterar as características naturais do aquífero cárstico.

Analysis of Fracture Mechanics Tests on Opalinus Clay

Many studies have recently been conducted to evaluate various mechanical characteristics of the Opalinus Clay (OPA) formation in view of its potential use as the hosting rock for the Swiss nuclear waste repositories. Its sedimentary bedding makes OPA a transversely isotropic rock and its directional mechanical properties need to be measured. This paper reports on an experimental and computational approach that was adopted to define the parallel-to-bedding fracture mechanics (FM) parameters of OPA in Mode-I. OPA cores from Mont Terri Underground Research Laboratory (URL) were submitted to laboratory tests on notched semi-circular specimens under three-point bending (SCB). In these tests, crack propagation is forced along the notch direction. However, the 45° bedding inclination of the specimen axis frequently deviated the crack from the expected direction. An analysis of the SCB tests was performed by means of non-linear FM techniques and the pertinent Mode-I parameters along the bedding were estimated.

Factors controlling the permeability distribution in fault vein zones surrounding granitic intrusions (Ore Mountains/Germany)

An outstanding legacy data set has been compiled from underground excavations mostly prospected and mined by the former Soviet (German) Stock Company Wismut describing the hydrology of faulted basement rocks in the Ore Mountains (SE Germany). It consists of more than 5000 detailed descriptions of groundwater inflows to about 660 km of tunnels and 57 km of drillings measured during or shortly after excavation. Inflow measurements (recorded between 1E−8 and 4E−2 m3/s) have been converted to fracture transmissivities using a simplified analytical solution. Discarding site specific effects, the median log transmissivity decreases from 1E−7 to 1E−10 m2/s within the studied depth interval of 0–2000 meters below ground surface (mbgs), and the spacing of conductive fracture increases from 0.1 to 2500 m. This general trend is overprinted at three mining sites by a clear reversal of fracture transmissivity which correlates with contact metamorphic aureoles around Variscan granite intrusions (327–295 Ma). We hypothesize that this transmissivity increase is caused by processes accompanying granite intrusion and contact metamorphism. The thickness of these hydraulically active aureoles is greater in lower-grade metamorphic schist than in higher-grade metamorphic gneisses. Rock mass equivalent continuum conductivities have been estimated by arithmetic averaging of fracture and matrix transmissivities over 100 m intervals and have been converted to permeabilities. The median equivalent continuum permeability decreases with depth according to log(k) = − 1.7 * log(z) − 17.3 (k in m2 and increasing depth z in kilometer being positive). Matrix conductivity controls the bulk conductivity below about 1000 mbgs and is less sensitive to the occurrence of contact metamorphic aureoles.

Pore Pressure Transients in Brittle Translational Rockslides

In a recent paper (Loew and Strauhal, Pore pressure distributions in brittle translational rockslides. In: Italian Journal of Engineering Geology and Environment, Book Series (6): International Conference on Vajont 1963-2013 – thoughts and analyses after 50 years since the catastrophic landslide, pp 181–191, 2013) we showed how variable extents, depths and hydraulic properties of slope parallel sliding zones and fractured rockslide masses can explain most of the observed steady-state hydrogeologic features of translational rockslides, such as seepage faces or pore pressure distributions. In this paper we extend this analysis to transient conditions, controlled both by annual recharge variations from snowmelt and summer rainstorms and hydropower reservoir level fluctuations. We apply a geometrically simple 2D numerical model to investigate and explain characteristic transient pressure variations, as observed in highly instrumented rockslides at the Mica Reservoir in British Columbia. We characterize pore pressure transients and seepage forces above and below major sliding zones at different elevations above the reservoir level. Sliding plane geometry or toe constraints caused by thick alluvial deposits at the valley bottom explain why reservoir level fluctuations significantly impact slope movements and stability, even though the induced pressure variations are relatively small and only impact smaller parts of the rockslide rupture planes.

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

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 warning by using the ESA Sentinels, and show that in critical scenarios they are often not sufficient to reliably interpret the evolution of surface deformation. The insights obtained from this case study are relevant for similar scenarios in the Alps and elsewhere.