Pore Pressure Fluctuations Research Articles

Seasonal slow slip in landslides as a window into the frictional rheology of creeping shear zones.

Whether Earth materials exhibit frictional creep or catastrophic failure is a crucial but unresolved problem in predicting landslide and earthquake hazards. Here, we show that field-scale observations of sliding velocity and pore water pressure at two creeping landslides are explained by velocity-strengthening friction, in close agreement with laboratory measurements on similar materials. This suggests that the rate-strengthening friction commonly measured in clay-rich materials may govern episodic slow slip in landslides, in addition to tectonic faults. Further, our results show more generally that transient slow slip can arise in velocity-strengthening materials from modulation of effective normal stress through pore pressure fluctuations. This challenges the idea that episodic slow slip requires a narrow range of transitional frictional properties near the stability threshold, or pore pressure feedbacks operating on initially unstable frictional slip.

Characterization of Seismic Signal Patterns and Dynamic Pore Pressure Fluctuations Due to Wave-Induced Erosion on Non-Cohesive Slopes

Wave erosion of slopes can easily trigger landslides into marine environments and pose severe threats to both the ecological environment and human activities. Therefore, near-shore slope monitoring becomes crucial for preventing and alerting people to these potential disasters. To achieve a comprehensive understanding, it is imperative to conduct a detailed investigation into the dynamics of wave erosion processes acting on slopes. This research is conducted through flume tests, using a wave maker to create waves of various heights and frequencies to erode the slope models. During the tests, seismic signals, acoustic signals, and pore pressure generated by wave erosion and slope failure are recorded. Seismic and acoustic signals are analyzed, and time-frequency spectra are calculated using the Hilbert–Huang Transform to identify the erosion events and signal frequency ranges. Arias Intensity is used to assess seismic energy and explore the relationship between the amount of erosion and energy. The results show that wave height has a more decisive influence on erosion behavior and retreat than wave frequency. Rapid drawdown may potentially cause the slope to slide during cyclic swash and backwash wave action. As wave erosion changes from swash to impact, there is a significant increase in the spectral magnitude and Power Spectral Density (PSD) of both seismic and acoustic signals. An increase in pore pressure is observed due to the rise in the run-up height of waves. The amplitude of pore pressure will increase as the slope undergoes further erosion. Understanding the results of this study can aid in predicting erosion and in planning effective management strategies for slopes subject to wave action.

Dynamic evolution of reservoir permeability and deformation in geothermal battery energy storage using abandoned mines

Retasking existing subsurface abandoned mines as infrastructure for solar energy storage could be a feasible approach in overcoming the low thermal gradient present in shallow formations. In this work, the potential for thermal storage in the high permeability goaf of abandoned mines through diurnal cyclic injection-then-extraction using coupled thermo-hydro-mechanical modeling was explored by coupling FLAC3D with TOUGH2. The temperature sensibility of reservoir during 30 days of cyclic injection-then-production was examined at various injection temperatures (ranging from 50°C to 250°C) and rates (ranging from 1 kg/s to 10 kg/s) and for representative reservoir physical and thermal properties, including variable thermal expansion coefficients. The simulation results reveal that: The principal mechanisms driving reservoir deformation result from the combined influence of thermal poroelastic and thermal effects. With the change of reservoir temperature, the reservoir is perturbed by pressure and thermal stresses causing permeability evolution. Permeability reduces ∼10% for a maximum injection temperature of 250°C – although effects are reduced the lower injection temperatures. The pore pressure fluctuations for an injection rate of 10 kg/s is ∼6.5 times that for a rate of 1 kg/s. The pressure perturbation of the reservoir during the injection process decreases with the injection rate, and the reservoir is relatively more stable. When the thermal stress becomes predominant, the reservoir volume expands. Uplift displacements 220 m above the hot injection well are trivial an of the order of ∼1.5 mm at a mean temperature of 163°C.

Shaking table tests and numerical analysis of monopile-supported offshore wind turbines under combined wind, wave and seismic loads

Currently, large-diameter monopile-supported offshore wind turbines (OWTs) are widely used in offshore areas of China. However, considerable numbers of wind farms are located in seismically active areas with liquefiable soil layers. Therefore, shaking table tests and numerical simulation based on a scaled model of Siemens SWT-4.0-130 OWTs were carried out to investigate the failure mechanism of the OWT system in liquefiable sand under combined wind, wave and seismic loads. The scaling law focusing on the governing physical mechanism of the OWT system considering multi-field coupling are presented and verified. The dynamic responses of the OWT and the excess pore pressure fluctuation around the soil-monopile domain in different operating states are discussed and compared. The results show that the proposed scaling laws can well restore the dynamic characteristics of the prototype. The seismic excitation is the dominant load for inducing the structural response under combined wind, wave and seismic loads. Due to the coupling effect, the dynamic responses under combined wind and wave loads were slightly less than the superposition of structural responses under separated wind or wave loads. Liquefaction of the shallow soil layer within the range of four times the pile diameter was detected when the PGA reached 0.4 g (three-dimensional input) or 0.64 g (one-dimensional input), which induced obvious tilt and settlement of the OWT model after the test. The sudden increase in the pore pressure resulted in the liquefaction of soil, which further led to a significant decrease in the natural frequencies and decay of the acceleration responses at the tower top. The above findings are expected to provide valuable experience for the 1 g underwater shaking table tests on OWT systems and insights for the practical design of OWTs in seismically active areas.

Allowable Pillar Width for Salt Cavern Gas Storage Based on Triangular Well Layout: A Case Study in China

Salt rock, renowned for its remarkable energy storage capabilities, exists in deep underground environments characterized by high temperature and pressure. It possesses advantageous properties such as high deformability, low permeability, and self-healing from damage. When establishing a cluster of salt cavern gas storage facilities, the careful selection of ore column widths between these reservoirs is crucial for minimizing the risk of structural failure, optimizing salt rock resource utilization, and enhancing the construction and operation of gas storage reservoirs. In current practices, square triangular arrangements are commonly used in designing well layouts for reservoir groups to balance stability and economic considerations. This study, conducted in the context of the Jintan salt cavern gas storage project in Jiangsu Province, employed FLAC3D to create a finite element model for proposed gas storage configurations. A comprehensive analysis of the long-term operational safety of salt cavern gas storage with triangular well layouts was carried out. Various indices were examined, covering aspects such as cavern wall displacement, characteristics of the plastic zone, volume shrinkage, safety coefficients, seepage range, pore pressure fluctuations, and seepage volume. The study also considered the mechanical behavior of hexagonal columns within the surrounding rock during extended storage operations, leading to the optimization of allowable widths for these columns. The results indicate that, at operating pressures ranging from 6.5 to 17 MPa, the permissible column width should exceed 1.67 times the maximum cavern diameter to ensure compliance with criteria for long-term stability and containment within a square triangular layout. These findings provide valuable insights into determining the optimal allowable widths of salt cavern columns for positive triangular layouts.

The Porothermoelastic Stress Evolution of Inhomogeneous Solid due to Near-Surface Reservoir

Anisotropic and heterogeneous solids, comprising a minimum of two or more elements with different properties, appear pervasively in rock materials including the pore structure and mineral composition of quartz and granite and usually have extensive applications in construction aggregates, dimension stone, geotechnical engineering, and petrology/geology. The elastic stress fields of inhomogeneous materials or composites inevitably change due to the presence of heterogeneous microstructure under applied external conditions, and hence the total mechanical and physical properties are affected by the temperature and pore pressure fluctuations beneath the surface. In this paper, the thermoporoelastic (TPE) approach on the basis of eigenstrain concept is introduced to predict the stress fields in fluid-saturated porous geologic materials like hydrocarbon reservoirs or aquifers by accounting for the coupling between thermal, poroelastic, and mechanical effects. It is an extension of the micromechanical theory that also incorporates thermal effects like pore pressure changes and temperature alternations. In addition, the TPE approach provides an important multiphysical modeling tool for understanding subsurface fluid-rock interactions and stress states in applications like unconventional oil/gas and geothermal energy.

Bedrock Fractures Control Groundwater‐Driven Mountain Slope Deformations

AbstractSeasonal deformation of mountain rock slopes can be driven by groundwater infiltration and depletion. Such processes could explain our field observation in the Aletsch Valley, Switzerland, where GNSS‐derived 3D annual displacement amplitudes reach 3.4 cm. However, the physical mechanisms behind such groundwater‐driven surface displacements are not well understood. Here, we develop a fully coupled hydromechanical model to simulate the relevant processes in a valley slope embedded with numerous fractures of variable sizes. The magnitude and orientation of transient annual slope surface displacement obtained from our model are in overall agreement with the field observations. The key geological factors controlling the type and magnitude of reversible mountain slope deformations are fracture network geometry, fracture aperture, and regional stress field. We show that the heterogeneity and anisotropy of bedrock hydromechanical responses, originating from depth‐dependent variations of fracture properties, play a critical role in groundwater recharge and valley slope deformation. During recharge events, pore pressure perturbations migrate downward from the groundwater table and toward the receiving stream and the deep subsurface. This process driven by pressure diffusion and poroelastic stressing develops in the subsurface with a great reach of up to a few kilometers, called critical hydromechanical response zone, and controls surface deformation patterns. During groundwater recession, this hydromechanical response zone expands downward and ground surface displacement vectors rotate upwards. Our results suggest that slope surface deformation can inform about subsurface permeability structures and pore pressure fluctuations, which have important implications for understanding groundwater flow in fractured bedrock slopes.

Three-dimensional hydrological thresholds to predict shallow landslides

Past studies have focused on the importance of hydrological variables in analyzing landslide initiation condition. Even though precipitation is the main driver of shallow landslides and debris flows, use of only rainfall-based parameters has shown some limitations. Soil moisture has been used widely to improve threshold detection capabilities. Since soil moisture directly reflects the wetness status of the ground, it can be used to identify pore pressure fluctuations more effectively. This study used rainfall and soil moisture simultaneously to capture landslide initiation conditions in detail. Results showed that continued rainfall on the day of landslide leaded to a sudden increase in soil moisture, and that soil moisture increments (∆SM) were positive in 155 out of 170 landslide cases (91%). Two simple thresholds (daily precipitation over 40 mm, ∆SM over 0) and daily precipitation (P), Antecedent Precipitation Index (API), ∆SM-based three-dimensional threshold planes having 5%, 20% probability levels were applied and compared. With respect to false alarms (FA), P-based threshold was most effective among the single thresholds (FA ranging from 24 to 28 from September 2016 to December 2019 at five validation locations). Combining P- and ∆SM-based thresholds, FA reduced without compromising the detection accuracy (2 to 3 reduction in FA). Additionally combining three-dimensional threshold with 20% probability level, FA reduced significantly (ranging from 12 to 16), at the cost of two detection failures. These findings demonstrate the need for combining precipitation and soil moisture to determine landslide thresholds.

Shake table testing and computational framework for seismic response of utility-scale bucket foundation offshore wind turbines

Shake table testing was conducted to document the seismic response of a bucket foundation offshore wind turbine (OWT) system. Salient response of the system's soil-structure interaction effects is presented and discussed. Among the observed response characteristics, excess pore pressure fluctuation within and around the soil-bucket domain is thoroughly addressed, including the strong tendency for the soil dilation excursions driven by the induced cyclic strains. The experimental data is used to calibrate a numerical model with dynamic soil response simulated by a coupled solid-fluid formulation. The calibrated model is extended to investigate seismic response of a prototype utility-scale OWT, with and without added wind loading effects. Overall, the research outcomes indicate that: i) excess pore pressure fluctuations in the vicinity of the bucket play an important role in dictating the extent of potential permanent base rotation, ii) consideration should be given to wind loading that might further exacerbate this base rotation, and iii) it is of importance to model the turbine tower as a system of discrete masses rather than the simplified proposed for practice equivalent top mass idealization.

Creep-dilatancy development at a transform plate boundary

How tectonic plates slip slowly and episodically along their boundaries, is a major, open question in earthquake science. Here, we use offshore in-situ sediment pore-pressure acquired in the proximity of the active offshore Main Marmara Fault and onshore geodetic time-series data set from a single GPS station to demonstrate the pore-pressure/deformation coupling during a 10-month slow-slip event. We show that pore pressure fluctuations are the expression of hydro-mechanical process affecting the deep seismogenic zone and indicate that small disturbances in geodetic data may have important meaning in terms of transient deformations. These results have major implications in understanding the spatial impact of slow-slip processes and their role in earthquake cycles. We demonstrate that piezometers measuring along a transform fault can help define the time scale regulating the coupling between slow-slip events and earthquake nucleation process.

Effects of fracture network distribution on excavation-induced coupled responses of pore pressure perturbation and rock mass deformation

We perform a numerical study of the spatio-temporal evolution of rock damages and fracture displacements around underground excavation in fractured rocks. We model fracture networks based on a power law model and explore different values of the length exponent and fracture intensity. An elasto-brittle constitutive relation is employed to characterise the deformation/failure of intact rocks, while a non-linear stress–displacement relation is used to mimic the normal/shear displacement of pre-existing fractures. Fluid flow through fractured porous media is simulated based on Darcy’s law. Both direct coupling (via poroelasticity) and indirect coupling (via changes of rock mass properties) are considered, with excavation-induced stress alteration, pore pressure variation, damage evolution, and fracture deformation realistically captured. It is observed that, fracture systems dominated by large fractures in general accommodate significant sliding along pre-existing fractures but minor damage in intact rocks, while fracture systems governed by small fractures can have profound damage and slippage simultaneously. Excavation-induced stress variation near the tunnel can instantly cause pore pressure fluctuations in low-permeability rock matrix, promoting fracture shear and rock damage. In addition, we investigate the microseismicity generated during and after the excavation as well as analyse the mechanisms of excavation and drainage-induced disturbances. Our results have important implications for underground excavation activities involved in many geoengineering applications.

Technology Focus: Formation Evaluation (February 2022)

This marks the first of two Formation Evaluation features for 2022. The COVID-19 pandemic naturally has affected SPE meetings, causing many to be rescheduled or postponed indefinitely, but SPE papers continue to be a crucial source of technical knowledge. The selected papers explore simple and complex innovative approaches toward reservoir characterization to work around the absence of certain data. Paper SPE 203903 addresses issues related to safe and optimal operation of geoenergy applications related to induced seismicity as a result of pore-pressure fluctuations. The authors developed a finite-volume method with a multipoint approximation of fluxes for geomechanics and poromechanics that will cope with discontinuities in displacements, as occur in faults, on the level of discretization. The next paper demonstrates local calibration of a simple rock physics model that can provide valuable insights into the elastic properties, which then can be combined with seismic data for quantitative reservoir characterization away from the wellbore. The key, according to the author of paper OTC 30359, is the use of the Hashin-Shtrikman effective medium model with absolute upper and lower bounds for compressional and shear velocities of media that are composed of multiple constituents. The third paper explores the challenges of identifying and quantifying hydrocarbon sections because of little resistivity contrast between reservoirs and water zones in the absence of radioactive tools. The authors of paper SPE 202420 develop a work flow that integrates nuclear magnetic resonance with azimuthal resistivity to overcome the uncertainty of fluid typing and optimal wellbore placement. Despite the challenges our industry continues to face, authors of SPE conference papers continue to provide an important source of knowledge and experience. I hope you enjoy these papers and encourage you to visit OnePetro to find similar works that may deepen your interests and enhance your skills. Recommended additional reading at OnePetro: www.onepetro.org. SPE 203130 - Managing Sanding Risk in Sandstone Reservoir Through a New Constitutive Model by Surej Kumar Subbiah, Universiti Teknologi Malaysia and Schlumberger, et al. SPE 201672 - Estimating Reservoir and Fracture Properties From Stage-by-Stage Pressure-Decline Analysis in Horizontal Wells by HanYi Wang, The University of Texas at Austin, et al. SPE 202248 A Digital Core Laboratory Characterizing Oil/Water Transition Zones by Graeme Morrison, Woodside Energy

Finite-Volume Method Enables Simulation of Induced Seismicity

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 203903, “A Collocated Finite-Volume Scheme for High-Performance Simulation of Induced Seismicity in Geoenergy Applications,” by A. Novikov, D.V. Voskov, SPE, and M. Khait, SPE, Delft University of Technology, et al. The paper has not been peer reviewed. In the complete paper, the authors develop a collocated finite-volume method (FVM) to study induced seismicity as a result of pore-pressure fluctuations. A discrete system is obtained based on a fully implicit coupled description of flow, elastic deformation, and contact mechanics at fault surfaces on a fully unstructured mesh. The cell-centered collocated scheme leads to convenient integration of the different physical equations because the unknowns share the same discrete locations on the mesh. Additionally, a multipoint flux approximation is formulated in a general procedure to treat heterogeneity, anisotropy, and cross-derivative terms for both flow and mechanics equations. Introduction The FVM has become an essential tool for flow and transport simulation because of its local conservation property. For mechanical deformation, however, the conservation property does not have the same importance. Nonetheless, the FVM is still an attractive choice because it represents conservation laws in integral form more naturally. Some authors in the literature use fixed-stress splitting algorithms to decouple mechanics and flow equations. These are a form of sequential implicit solution schemes and often lead to more-efficient simulations than fully implicit (FI) simulation. However, sequential schemes introduce certain restrictions on timestep sizes. On the other hand, FI schemes provide unconditionally convergent solutions and are more-robust and -convenient approaches for investigation of complex multiphysical problems. Although the FI approach does not imply any restriction on timestep size, it requires an efficient linear equation solution strategy for high-resolution models. In this study, the authors develop a collocated FI multipoint FVM scheme for poromechanics simulation of faulted reservoirs. The scheme can be used to solve poromechanics problems on unstructured polyhedral grids with minimal degrees of freedom per cell. It is also capable of considering material heterogeneity while preserving mass and momentum balances. This scheme is extended to consider discontinuities in displacements at faults. The developed algorithms have been embedded into the open-source Delft Advanced Research Terra Simulator (DARTS).

Laboratory characterization of cyclic hydraulic fracturing for deep shale application in Southwest China

As one of the most promising shale gas reservoirs in China, the Lower Silurian Longmaxi formation in the Sichuan Basin has produced a large amount of shale gas during the last few years. However, two significant concerns have been raised during the contemporary shale gas development in this area: dramatic increases in well depths and potential induced seismicity . The extreme depths have resulted in higher in-situ stress and reservoir breakdown pressures . The other concern is whether recent earthquakes are related to hydraulic fracturing operations since they are close in space and time. An advanced fluid injection protocol-cyclic fluid injection has been proposed and used to lower the breakdown pressure of reservoirs and mitigate the potential seismic risks caused by hydraulic stimulations. In this study, we combine a series of laboratory experiments and a numerical simulation model to investigate the breakdown mechanism, seismic risks, permeability enhancement performance of cyclic fluid injection. The results indicate that the cyclic injection method decreases the shale breakdown pressure by ∼25% compared with the conventional monotonic rate injection , which is the result of fatigue failure near the fluid injection borehole induced by pore pressure fluctuations lagging behind the periodical injection pressure changes. Even though more acoustic emission events are observed during and post-the cyclic injection, the maximum acoustic emission amplitude decreases by 26%, which is equivalent to transforming AE events with large amplitudes to more AE events with small amplitudes. Moreover, fracture morphology and permeability measurement results indicate that cyclic injection creates hydraulic fractures with higher fracture tortuosity but smaller aperture and lower permeability, which contributes to the fatigue mechanism of rock breakdown. Our experimental results and theoretical analysis initially validate the potential of using cyclic injection methods to perform hydraulic fracturing stimulation in shale reservoirs to lower breakdown pressure and mitigate seismic risks.

Rock Slope Temperature Evolution and Micrometer‐Scale Deformation at a Retreating Glacier Margin

AbstractIn deglaciating environments, rock mass weakening and potential formation of rock slope instabilities is driven by long‐term and seasonal changes in thermal‐ and hydraulic‐ boundary conditions, combined with unloading due to ice melting. However, in‐situ observations are rare. In this study, we present new monitoring data from three highly instrumented boreholes, and numerical simulations to investigate rock slope temperature evolution and micrometer‐scale deformation during deglaciation. Our results show that the subsurface temperatures are adjusting to a new, warmer surface temperature following ice retreat. Heat conduction is identified as the dominant heat transfer process at sites with intact rock. Observed non‐conductive processes are related to groundwater exchange with cold subglacial water, snowmelt infiltration, or creek water infiltration. Our strain data shows that annual surface temperature cycles cause thermoelastic deformation that dominate the strain signals in the shallow thermally active layer at our stable rock slope locations. At deeper sensors, reversible strain signals correlating with pore pressure fluctuations dominate. Irreversible deformation, which we relate with progressive rock mass damage, occurs as short‐term (hours to weeks) strain events and as slower, continuous strain trends. The majority of the short‐term irreversible strain events coincides with precipitation events or pore pressure changes. Longer‐term trends in the strain time series and a minority of short‐term strain events cannot directly be related to any of the investigated drivers. We propose that the observed increased damage accumulation close to the glacier margin can significantly contribute to the long‐term formation of paraglacial rock slope instabilities during multiple glacial cycles.

A Computational Scheme for Evaluating the Stress Field of Thermally and Pressure Induced Unconventional Reservoir

AbstractThe fluid flow connecting the hydraulic fracture and associated unconventional gas or oil reservoir is of great importance to explore such unconventional resource. The deformation of unconventional reservoir caused by heat transport and pore pressure fluctuation may change the stress field of surrounding layer. In this paper, the stress distribution around a penny-shaped reservoir, whose shape is more versatile to cover a wide variety of special case, is investigated via the numerical equivalent inclusion method. Fluid production or hydraulic injection in a subsurface resource caused by the change of pore pressure and temperature within the reservoir may be simulated with the help of the Eshelby inclusion model. By employing the approach of classical eigenstrain, a computational scheme for solving the disturbance produced by the thermally and pressure induced unconventional reservoir is coded to study the effect of Biot coefficient and some other important factors. Moreover, thermo-poro transformation strain and arbitrarily orientated reservoir existing within the surrounding layer are also considered.

Role of hydro-mechanical coupling in excavation-induced damage propagation, fracture deformation and microseismicity evolution in naturally fractured rocks

We present a numerical study of spatio-temporal damage evolution and fracture displacements around underground excavation in fractured rock masses. We conduct a comparative analysis of this problem based on a mechanical (M) model by assuming an invariant pore pressure and a hydro-mechanical (HM) model by solving the coupling between the fluid and solid fields. In both models, an elasto-brittle constitutive law is employed to mimic the deformational and failure behavior of intact rocks, while a non-linear stress-displacement relationship is used to account for the normal compression and shear dislocation of natural fractures. In the HM model, we simulate excavation-induced transient groundwater flow in fractured porous rocks based on Darcy's law. The hydraulic and mechanical fields are linked based on the coupling mechanisms of poroelasticity (direct HM coupling) and stress-dependent material properties (indirect HM coupling). Many important HM phenomena such as stress perturbation, pore pressure fluctuation, damage evolution and fracture deformation are realistically captured in our simulation. We find that both direct and indirect couplings play important roles, and the excavation-induced disturbance in rock varies with the coupling degree characterized by the Biot coefficient. One interesting observation is that excavation-induced stress redistribution around the tunnel instantly causes perturbed pore pressure in low-permeability matrix, which gradually becomes dissipated during the post-excavation drainage. We also highlight the role of natural fractures in the tunnel inflow process and elucidate the consequences of spatial and temporal pressure variations. In addition, we analyze the microseismicity occurrence during and after the excavation to gain insights into excavation and drainage-induced responses in the fractured rock. The results of our simulation and analysis have important implications for underground excavation involved in many geoengineering applications such as civil infrastructure, nuclear waste repository and oil/gas/geothermal wellbore systems.

The Impact of Excess Pore Pressure on the Support Stability of a Shield When Tunnelling in Sand Stratum

When the large-diameter shield tunneling through the soft soil stratum, the pore pressure fluctuation would influence the tunnel stability, yet how it affected was still hard to figure out. Based on field tests data collected from a tunnel project in China, the distribution characteristics of excess pore pressure in sand stratum were obtained. By combining the limit equilibrium theory and variational calculus method, a modified formula of the limit support pressure was proposed, considering the effects of excess pore pressure and slope angle. The results showed that the excess pore water pressure had a big influence on the fracture angel of the sliding surface and the support pressure of the shield. If the effect of the pore pressure were taken into account, the support pressure should be set larger than the normal condition when the shield entered sand layer. The limit support pressure would decrease with the increase of the cohesive force and friction angle. Thus, effective reinforcement on the stratum should be adopted before the shield passing through.

Natural pore pressure fluctuations as an indicator of shallow aquitard continuity

The natural pore pressure response of a shallow aquifer/aquitard system can be used to assess aquitard continuity. This is the first paper to examine aquitard continuity at the field scale based on the barometric head response function (BHRF) obtained from multiple-regression analysis. The pore pressure measurements made during winter provided more representative BHRFs for the bounding aquifers and the aquitard. Distinct BHRFs were found for the shallow unconfined aquifer, clay aquitard and confined aquifer. This implies how the clay aquitard isolates the underlying confined aquifer from the shallow unconfined aquifer. It is found that the BHRFs for intact clay formation are delayed and differ from those for the fractured zone and confined aquifer. For the fractured zone, the observed BHRF quickly reached equilibrium. The BHRFs for intact clay and fractured zone equilibrated at high BHRF values. For the confined aquifer, the BHRF was low and constant. Numerical simulations provided a hydraulic conductivity below 1 × 10−8 m/s for the observed BHRF. Fast Fourier transform showed that the pore pressure components with a period smaller than one cycle per day were fully explained by barometric pressure fluctuations. Furthermore, the response of a deep piezometer to the moisture loadings provided an opportunity to estimate that the annual snowfall, for the study area, lost 25–35% of its weight.

Transient Groundwater Flow Through a Coastal Confined Aquifer and Its Impact on Nearshore Submarine Slope Instability

AbstractRare in situ pore pressure and temperature data sets collected over more than one decade in a submarine landslide‐prone area off Nice (Southeast France) have been used to characterize a transient groundwater exchange between a coastal confined aquifer and the nearshore shallow‐water submarine shelf. The near‐coast study zone is highly exposed to geohazards and remains sadly famous for the 1979 catastrophic tsunamigenic submarine landslide with several casualties and substantial material damage. Measured pore pressure buildup does not appear to be synchronized with either the discharge of the nearby Var River or with precipitation in the Nice area. Unexpectedly, pore pressure fluctuations synchronizes well with seabed temperature trends indicating that the nearshore submarine shelf is mainly impacted by long smooth seasonal variations. We used the pore pressure and temperature data to describe the general transport mechanisms of pore waters and show that pore pressure diffusion is predominantly responsible for the near‐seabed pore pressure buildup. Based on our results and previously published geotechnical data, slope stability calculations reveal that groundwater exchange has a major impact on slope instability and shear zone formation in a prone‐to‐failure area. Minor earthquake‐induced ground accelerations seem to contribute to the nearshore slope instability by decreasing its factor of safety.