Pore Pressure Fluctuations Research Articles

Experimental study of debris-flow entrainment over stepped-gradient beds incorporating bed sediment porosity

The incorporation of bed sediment by a natural debris flow can cause significant growth in its size and destructive potential. In this study, the erosion features of debris flows in channels with stepped-gradient beds were determined based on field observations and flume experiments. Field investigations revealed that channel sections with abrupt bed slope variations were eroded more intensely than others. A series of small-scale flume tests were conducted to study the pore pressure variations in erodible bed sediment and the basal entrainment behaviour of debris flows in channels with stepped-gradients. Reference tests on a uniform bed slope demonstrate that inflection points in the bed longitudinal profile greatly alter the debris mixture flow pattern and accelerate entrainment. Pore pressure generation in the bed sediment supports debris flow entrainment by reducing basal friction. Peak pore pressures at concave sites were approximately two times higher than those observed at convex sites, likely due to large changes in debris flow dynamics and intense interactions between the flow mixture and bed sediment. Low bed sediment porosities will increase the interlocking of grain particles, promote the formation of clusters and enhance the strength of the grain skeleton, suppressing basal entrainment. In the flume tests, lower peak pore pressures were observed as bed porosity decreased, possibly due to fewer connecting channels for pore fluid pressure to transmit in the compacted bed sediment. Dimensionless analysis indicates that pore pressure fluctuations in the bed sediment were mainly generated by the overriding debris flow. A new entrainment formula of debris flow over stepped-gradient beds is proposed based on flume tests and theoretical analyses, which may aid future debris flow modelling and hazard assessments.

Deterioration of an infrastructure cutting subjected to climate change

Observations show that many soils in linear geotechnical infrastructure including embankments and cuttings undergo seasonal volume changes, and different studies confirm that this is due to cycles in climatic and hydrological conditions. These cycles can give rise to progressive failure of the soil mass, which in turn may lead to deterioration of performance and ultimately slope failure. It is expected that the magnitude of the seasonal cycles of pore pressure will be increased by more extreme and more frequent events of wet and dry periods predicted by future climate scenarios. In this paper, numerical modelling has been undertaken to simulate a continuous time series pore water pressure within a representative cutting in London Clay. The approach uses synthetic control and future climate scenarios from a weather generator to investigate the potential impacts of climate change on cutting stability. Surface pore water pressures are obtained by a hydrological model, which are then applied to a coupled fluid-mechanical model. These models are able to capture the significant soil–vegetation–atmospheric interaction processes allowing the induced unsaturated hydro-mechanical response to be investigated. The chosen hydraulic conductivity variables in the model are shown to affect the total magnitude of pore pressure fluctuation and hence the rate of progressive failure. The results demonstrate for the first time that higher total magnitude of annual variation in pore pressures caused by future climate scenarios can have a significant effect on deformations in cuttings. This in turn leads to increased rates of deterioration and reduces time to failure.

Quantifying the uncertainties in a fault stability analysis of the Val d’Agri oilfield

Induced seismicity as an effect of injection of fluids in a geological basin is a widely observed phenomenon which is nowadays under public scrutiny. In most cases, the numerical simulation and interpretation of these phenomena requires models with a high degree of realism. In turn, this requires accounting for complex interactions between the fluid and solid components, as well as a detailed description of geometry and a high degree of heterogeneity in the material properties. Also, issues related to the uncertainty of parameters, to an incorrect modeling of sub-scale or multiscale effects and to a limited knowledge of initial conditions are unavoidable and can be detrimental to the reliability of the results. In this work, a statistical analysis including uncertainty quantification and sensitivity analysis on variances has been applied as a post-processing to data coming from a set of numerical simulations of a real world setting (the Val D’Agri oilfield), with the aim of studying the stability of a fault that is known to have experienced a good amount of microseismicity during the modeled period. The uncertainty quantification targeted the effects of fault surface local orientation and pore pressure fluctuations, as well as variability in the friction coefficient of the fault. The analysis of variances focused on the effects of varying the permeability of the fault damage zones (the area enveloping the faults) and the geometrical orientation of the fault as well. The model shows that the zone where the microseismicity has been measured is included in a wider region of moderate instability, which is higher the lower the permeability of the fault damage zone. From the model results the fault seems to be far from a critical state, but the analysis offers, nevertheless, some useful information on the relationship of slip tendency with geometrical and flow quantities in the system, and suggests some improvements in the dynamical model assumptions and settings.

Multi-year, three-dimensional landslide surface deformation from repeat lidar and response to precipitation: Mill Gulch earthflow, California

Slow-moving landslides are often the dominant process that shapes hillslopes and delivers sediment to channels in weak lithologies. Understanding what controls their velocities is therefore essential for deciphering their role in landscape evolution and estimating hazards. In this study, we used four sequential airborne lidar data sets to derive the three-dimensional (3D), 1-m spatial resolution surface velocity field of the Mill Gulch earthflow, northern California, for three periods of time spanning a decade. Phase correlation, an image processing technique, applied to the precisely aligned lidar digital elevation models confidently resolved horizontal velocities from 0.2 to 5 m year−1. The velocity field defined three distinct kinematic zones of the landslide with different sensitivities to precipitation, such that the head moved slowly at 0.3–0.5 m year−1, the transport zone moved fastest at 2–5 m year−1, and a forked toe moved intermittently at 0–4 m year−1. Inverting the 3D surface velocity field to infer landslide thickness suggested that the head was underlain by a 6-m-deep, concave-up slip surface, while the transport zone likely had a 1–2.5-m-deep translational slip surface. We hypothesized that velocity in the head was least sensitive to changing precipitation because of its greater inferred depth, which likely dampened the amplitude of pore pressure fluctuations. A lack of major head scarp retrogressions also may have contributed to the relatively steady velocity in the head, while buttressing and debuttressing interactions between the landslide’s toes and the creek running in Mill Gulch may have contributed to the more dramatic changes in velocity in the transport zone and toes. Although predicting landslide velocity from precipitation data alone may therefore be challenging, producing detailed 3D deformation fields from repeat topographic data can be an important tool for deciphering interactions between internal controls, such as changes to landslide geometry, and external controls, such as climate, on landslide motion.

A comprehensive framework of TDR landslide monitoring and early warning substantiated by field examples

Due to the climate change and hazardous geological environment in Taiwan, real-time landslide monitoring for safety of adjacent tribes or infrastructures is much conspicuous. Time domain reflectometry (TDR) is considered as valuable technology for continuously capturing dynamics and locations of shear deformations with high temporal and spatial coverages. In this study, a comprehensive framework was proposed by integrating up-to-date procedure of installation guide, data acquisition, sliding deformation identification, and early warning processing. The advantages of the proposed framework were then examined and substantiated via field cases. The first case is at a large colluvium slope in Lushan, central Taiwan. The colluvium is still active after heavy rainfall events, but limited evidence of sliding characteristic was collected previously by manual inclinometer logging. Therefore, the TDR framework was deployed and multiple landslide surfaces at great depths were timely observed. Movement of colluvium mass was thereby interpreted as a buckling fold. Additionally, creep occurred among the colluvium layer during the monsoon season because of strength weakening influenced by pore pressure fluctuations. Monitoring results of the second landslide case in Jufan, northern Taiwan, reported the dynamics of a thin sliding plane captured with TDR high temporal resolution. This unique behavior was further applied for devising of a novel early-warning strategy in terms of time-variation of TDR response, validating that the comprehensive framework of TDR landslide can effectively contribute to early warning.

Determination of the Thermal Stress Produced by a Thermo-poro-inclusion of Spheroid-shaped Near a Free Surface

Eshelby’s inclusion with thermo-porous eigenstrain is a classical micromechanics problem, and may find wide applications in various engineering fields including mechanical transmission system, composite materials and geophysical structures. In this paper, the elastic field produced by thermo-porous spheroidal inclusion placed vertically near a free boundary surface is investigated. The current work complements our previous publication on the horizontally aligned spheroidal inclusion [Computers & Geosciences. 2019;122:15-24], and hence concludes that a complete elasticity solution with respect to the displacement, strain and stress can be obtained in closed-form. The elastic field of spheroidal inclusion due to thermal expansion is provided, and comparative study of inclusion due to fluid withdrawal leading to a decrease in pore pressure is discussed. The present half-space inclusion model can be used to determine the elastic fields of porous materials caused by produced by pore pressure fluctuation and temperature alteration.

Influences of the Three Gorges Project on seismic activities in the reservoir area

Reservoir-induced earthquakes related with the construction of the Three Gorges Project have attracted great concerns of the public. Since the first water impoundment on May 25, 2003, a number of earthquakes have occurred during the water storage stages, in which the largest was the Badong M5.1 earthquake on December 16, 2013. In this paper, the relationships between seismic activities, b value, seismic parameters, and reservoir water level fluctuations are studied. In addition, based on the digital seismic waveform data obtained since 2000, the focal depth changes and focal mechanism characteristics before and after the water impoundment are studied as well. These provide us important information to understand the earthquake mechanisms. The results show that these earthquakes are typical reservoir-induced earthquakes, which are closely related to water infiltration, pore pressure, and water level fluctuations. The majority of the micro and small earthquakes are caused by karst collapse, mine collapse, bank reformation, superficial unloading, and so on. The larger earthquakes are related to the fault structures to some extent. Due to the persistent effects of water impoundment on the seismic and geological environments around the reservoir and water infiltration into the rocks, the influences on the crustal deformation field, gravity field, seepage field, and fault medium-softening action may vary gradually from a higher strength to a weaker one. Therefore, it is possible that small earthquakes and few medium earthquakes (M≤5.5) will occur in the reservoir area in the future.

Scattering of plane P1 waves and dynamic stress concentration by a lined tunnel in a fluid-saturated poroelastic half-space

The scattering of plane fast compressional waves (P1 waves) by a lined tunnel shallowly embedded in a poroelastic half-space is solved by indirect boundary integration equation method for a poroelastic half space. The displacement amplification on ground surface, dynamic stress concentration within the tunnel, and pore pressure fluctuation on the interface are investigated in the frequency domain, and are also compared with the results of the corresponding unlined cavity. Numerical results illustrate that the scattering of plane P1 waves around the tunnel strongly depends on the porosity of the surrounding medium, frequency and angle of the incident wave, tunnel depth. Significant amplification effect on the surface ground motion and the hoop stress in the tunnel is observed in the case of high-porosity surrounding-rock and undrained interface.

Spatio-temporal evolution of frequency-magnitude distribution and seismogenic index during initiation of induced seismicity at Guy-Greenbrier, Arkansas

In this study, we carry out detailed analyses of the spatio-temporal variations of seismic b-value during the onset of the potentially induced-earthquake sequence between Guy and Greenbrier, Arkansas, to investigate correlations with pore-pressure change. The range of b-values suggests that the seismicity in the Guy-Greenbrier area is mostly a result of the activation of pre-existing faults. The spatial distribution of b-value correlates with modeled pore-pressure changes. In the northern segment of the fault, b-value increases with depth due to large pore-pressure changes and opening of new fractures in the deeper part and stress relaxation in the shallower parts. Whereas, in the southern segment, the shallower part shows higher b-values due to higher pore-pressure fluctuations but the deeper part in the crystalline basement has low b-value due to higher confining stress. The correlation between the temporal variation of b-value and hypocentral depth explains a previously observed temporal drop of b-value. This suggests that temporal variations should be interpreted along with the spatial variations. Estimation of the seismogenic indices for the Guy-Greenbrier fault is provided. Our analysis suggests that monitoring changes in b-value and seismogenic indices during an injection period might be used to avoid the occurrence of significant events if injection volume is reduced at critical times.

Comparative analysis on spatiotemporal pore pressure evolution under surface–wave perturbations

The near surface spatiotemporal pore pressure fluctuations in a porous half space induced by seismic surface–wave perturbations in different poroelastic regimes are analytically compared. The results for the pore pressure in the space–time domain are derived by inverse transforms of the fundamental solutions of dynamic poroelasticity in the frequency–wavenumber domain. Different poroelastic regimes depend on the different low frequency approximation for Biot's multi wave mode physical behaviors. In low seismic frequency band, whereas the fast wave mode inference can generate surface wave along the interface, fluid pressure is also significantly perturbated as in solid phase, but behaves as a slow P2 mode reflected by the R surface wave. It is also shown that the amount of induced pore pressure fluctuations are different by P2 diffusivity (dispersion–difusivity regime, DD and non–dispersion–diffusivity regime, NDD) and P2 wave (full–wave regime, FW). The strong solid and fluid coupling on the interface generates the transferred RP2 mode. Therefore, the decoupled viscoleastic approximation (VA) does not represent these near surface fluid characteristics. The results can be applied to predict near surface pore pressure responses under seismic perturbations, or as a benchmark for other numerical approaches for pressure solutions in a dynamic poroelastic half space, and also favor a potential interpretation for near surface soil liquefaction failure.

Pore pressure embrittlement in a volcanic edifice

The failure mode of porous rock in compression—dilatant or compactant—is largely governed by the overlying lithostatic pressure and the pressure of pore fluids within the rock (Wong, Solid Earth 102:3009–3025, 1997), both of which are subject to change in space and time within a volcanic edifice. While lithostatic pressure will tend to increase monotonously with depth due to the progressive accumulation of erupted products, pore pressures are prone to fluctuations (during periods of volcanic unrest, for example). An increase in pore fluid pressure can result in rock fracture, even at depths where the lithostatic pressure would otherwise preclude such dilatant behaviour—a process termed pore fluid-induced embrittlement. We explore this phenomenon through a series of targeted triaxial experiments on typical edifice-forming andesites (from Volcan de Colima, Mexico). We first show that increasing pore pressure over a range of timescales (on the order of 1 min to 1 day) can culminate in brittle failure of otherwise intact rock. Irrespective of the pore pressure increase rate, we record comparable accelerations in acoustic emission and strain prior to macroscopic failure. We further show that oscillating pore fluid pressures can cause iterative and cumulative damage, ultimately resulting in brittle failure under relatively low effective mean stress conditions. We find that macroscopic failure occurs once a critical threshold of damage is surpassed, suggesting that only small increases in pore pressure may be necessary to trigger failure in previously damaged rocks. Finally, we observe that inelastic compaction of volcanic rock (as we may expect in much of the deep edifice) can be overprinted by shear fractures due to this mechanism of embrittlement. Pore fluid-induced embrittlement of edifice rock during volcanic unrest is anticipated to be highest closer to the conduit and, as a result, may assist in the development of a fractured halo zone surrounding the conduit, potentially explaining commonly observed near-conduit outgassing at many active volcanoes. Further, rock embrittlement at depth may create transient outgassing pathways by linking fracture networks near the edifice to larger-scale regional fault systems. Our experimental results affirm that pore pressure fluctuations associated with volcanic unrest may play a crucial role in dictating the evolution of a volcanic system.

Along-fault pore-pressure evolution during a slow-slip event in Guerrero, Mexico

Slow earthquakes are important constituents of the seismic cycle and are involved in the stress transfer between the viscously slipping portion of the plate interface and the seismogenic zone. Their occurrence is likely associated with the near-lithostatic pore pressure in the slow earthquake source region, where fluids might modify fault friction and whose presence is indicated by high ratios of compressional (P)–wave velocity to shear (S)–wave velocity observed at the interface between the subducting plate and the overlying crust. Here we compare two slow earthquake phenomena observed in the Guerrero region of the Mexican subduction zone: low-frequency earthquakes (LFEs) and a slow-slip event (SSE) recorded by GPS. We observe variations of the LFE occurrence rates over month-long time scales during a large SSE that we interpret as a manifestation of transient changes in the fault shear strength. We argue that these transient changes are caused by a pore pressure fluctuation that migrates updip along the subduction interface. This mechanism suggests that fluids do not only passively weaken the plate interface but also play an active role in slow earthquake source regions.

The permeability of active subduction plate boundary faults

AbstractAt subduction zones, continuous influx of fluids drives a dynamic system in which fault slip, fluid flow, and advective transport are tightly coupled. Field and numerical modeling studies have provided insight into the nature and rates of flow in these systems and illustrate that active subduction faults, including the master décollement and splay faults cutting the upper plate, are important conduits. Observations of in situ fracture dilation, modeling studies, and direct measurements documenting strong pressure dependence of fault permeability collectively suggest that permeability varies in time, perhaps due to pore pressure cycling. However, mechanical and fluid budget considerations dictate that increased fault permeability cannot be sustained, nor can it be present across the entire fault surface at a given time. The emerging conceptual model is that permeable patches or channels occupy only a fraction of the fault surface and shift transiently. Fault zone permeabilities obtained by several approaches are consistent between margins, with time‐averaged values of approximately 10−15 to 10−14 m2, several orders of magnitude higher than for the sediment matrix. Higher, transiently increased values of approximately 10−13 to 10−11 m2 are required to explain geochemical and thermal signals and observed focused flow rates. Although faults accommodate significant fluid fluxes from dewatering of the surrounding sediment, they have little effect on pore pressures within the wall rock, where drainage is limited by low matrix permeability. However, fault permeability is a key control on the transport and preservation of localized geochemical and thermal anomalies from depths where temperatures are higher and low‐temperature metamorphic reactions are underway. Despite significant recent progress, several key aspects of hydrologic behavior in these active faults remain incompletely understood, including the nature and timescale of transience, the causes of permeability enhancement and its relationship to fault slip and pore pressure fluctuations, and the depths and distances from which deeply sourced fluids are captured, mixed, and transported up‐dip.

Investigation of a pore pressure driven slope failure using a coupled hydro-mechanical model

The Belvoir Park Forest landslip is situated in Northern Ireland. It was monitored for more than five years with inclinometers, standpipe piezometers, and slip indicators providing detailed information on the long term behaviour and rate of movement of a large low angle failure in Glacial Till. A detailed geological model was constructed on the basis of the geomorphology and geology of the area and ground investigation data. A detailed geotechnical model was prepared with estimates of stiffness, permeability, shear strength and unit weight data. The geometry of the failure plane, residual strengths, movement rates, pore pressure fluctuations and rainfall data was used to complete a model of the failure with limit equilibrium methods. This model was able to demonstrate that pore water pressure was the most influential factor when considering the stability of the slope, but could not determine the effect of rainfall fluctuations on the observed rate of movement of the slip. A more sophisticated coupled hydro-mechanical model based on Shetran and Flac-tp flow was used to determine the effect of fluctuations of pore pressures, rainfall and mechanical strains on slip movement. This paper illustrates some of the advantages of using more sophisticated slope modelling approaches alongside more ‘traditional’ modelling of residual slope failures to understand their future evolution.

Sediment entrainment by debris flows: In situ measurements from the headwaters of a steep catchment

Debris flows can dramatically increase their volume, and hence their destructive potential, by entraining sediment. Yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited. Using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris‐flow events. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time‐averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry. Bed sediment was entrained from the sediment‐surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water‐rich, inter‐surge flow.En massefailure of bed sediment along the sediment‐bedrock interface was never observed. Large‐magnitude, high‐frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment. Within this near surface layer, concomitant fluctuations in Coulomb frictional resistance are expected, irrespective of the influence of pore fluid pressure or fluctuations in shear stress. If the near‐surface sediment was wet as it was overridden by a flow, additional large‐magnitude, high‐frequency pore pressure fluctuations were measured in the near‐surface bed sediment. These pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner. The depth to which large excess pore pressures propagated was typically less than 10 cm, but scaled as (D/fi)0.5, in which D is the hydraulic diffusivity and fiis the frequency of a particular pore pressure fluctuation. Shallow penetration depths of granular‐normal‐stress fluctuations and excess pore pressures demonstrate that only near‐surface bed sediment experiences the full dynamic range of effective‐stress fluctuations, and as a result, can be more easily entrained than deeper sediment. These data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment.

Interpreting pore-water pressure changes induced by water table fluctuations and mechanical loading due to soil moisture changes

Pore pressures within saturated subsurface formations respond to stress changes due to loading as well as to changes in pore pressure at the boundaries of the formation. The pore-pressure dynamics within a thick aquitard in response to water table fluctuations and mechanical loading due to soil moisture changes have been simulated using a coupled stress–strain and groundwater flow finite element formulation. This modelling approach isolates the component of pore-pressure response of soil moisture loading from that caused by water table fluctuations, by using a method of superposition. In this manner, the contributions to pore-pressure fluctuations that occur as a result of surface moisture loading (e.g., precipitation, evapotranspiration) can be isolated from the pore-pressure record. The required elastic stress–strain properties of the aquitard were obtained from the measured pore-pressure response to barometric pressure changes. Subsequently, the numerical simulations could be calibrated to the measured response by adjusting only the hydraulic conductivity. This paper highlights the significance of moisture loading effects in pore-pressure observations and describes an efficient technique for obtaining in situ stress–strain and hydraulic properties of near-surface aquitards.

Submarine landslide potential near the megasplay fault at the Nankai subduction zone

We investigate the mechanics of slope failures on the Nankai accretionary complex offshore Japan in the vicinity of a major out-of-sequence thrust fault (termed the “megasplay”). Incorporating laboratory-measured shear strength of slope sediments sampled during Integrated Ocean Drilling Project (IODP) Expeditions 315 and 316 with local seafloor slope angles from bathymetric data and constraints on in-situ effective stress conditions from drilling, we find that slopes in the study area are stable and submarine landslides are not expected to occur under static conditions. In order to assess the possibility of slope failure triggered by coseismic rupture of the megasplay fault, we use empirical relations for strong ground motion attenuation from earthquakes with M w 6–9. We find that the slope sediments should be stable based on computations from one model, developed from a catalog of worldwide subduction zone earthquakes ( Youngs et al., 1997). However, using a different model developed primarily from a catalog of crustal earthquakes in Japan ( Kanno et al., 2006), we find that slopes should be unstable for earthquakes 8 ≤ M w ≤ 9, and possibly unstable for events with 6 ≤ M w < 8, depending on the proximity of rupture to the seafloor. Considering limitations of the models and geologic observations of slope failure recurrence, the true slope stability is likely to be in between the predictions of the two models, and we suggest that it may be modulated by long-term pore pressure fluctuations.

Upper crustal structure, seismicity and pore pressure variations in an extensional seismic belt through 3-D and 4-D VPand VP/VSmodels: The example of the Val d'Agri area (southern Italy)

[1] We use local earthquake tomography and background seismicity to investigate static and transient features of the crustal velocity structure in the Val d'Agri (southern Apennines, Italy), one of the regions in central Mediterranean with the highest seismogenic potential. The upper crust is dominated by two broad high-velocity anticlines of the buried Apulia Carbonate Platform ramping on two parallel high-angle thrusts interpreted as preexisting inverted normal faults. The deep core of the anticlines consists of very high VP (up to 6.9 km/s) and low VP/VS rocks, suggesting the involvement of the Apulian crystalline basement in the Apennine belt. These results provide valuable constraints on the Apennine belt tectonic evolution, supporting a thick-skinned interpretation for the Pliocene terminal phase of the compressional tectonics. The geometry of the Val d'Agri Quaternary basin is controlled by these inherited compressive features, whereas the presently active extensional tectonics barely reworked the structure. We find inconsistency between the structure of the Apulia Carbonate Platform and the location and geometry of the Quaternary normal faults mapped at the surface. This suggests either the immaturity of the normal faults or their secondary role in accommodating the extension. We observe spatiotemporal (4-D) changes of VP and VP/VS models defining transient variations of pore fluid pressure in the upper crust. A strong change in the VP/VS ratio heralds a raise in the seismicity rate that can be related to large water level changes in a nearby artificial lake. This evidence is consistent with a mechanism of reservoir-induced seismicity by fluid pressure increase and pore pressure diffusion. The 4-D velocity variations are confined in the shallow portion of the upper crust (3–6 km depth) where fluids are stored in a highly fractured medium. Pore pressure fluctuations can affect the strength of fault segments, favoring seismicity rate changes along the active faults and possibly promoting large future earthquakes.

Geomechanical response to seasonal gas storage in depleted reservoirs: A case study in the Po River basin, Italy

Underground gas storage (UGS) in depleted hydrocarbon reservoirs is a strategic practice to cope with the growing energy demand and occurs in many places in Europe and North America. In response to summer gas injection and winter gas withdrawal the reservoir expands and contracts essentially elastically as a major consequence of the fluid (gas and water) pore pressure fluctuations. Depending on a number of factors, including the reservoir burial depth, the difference between the largest and the smallest gas pore pressure, and the geomechanical properties of the injected formation and the overburden, the porous medium overlying the reservoir is subject to three‐dimensional deformation with the related cyclic motion of the land surface being both vertical and horizontal. We present a methodology to evaluate the environmental impact of underground gas storage and sequestration from the geomechanical perspective, particularly in relation to the ground surface displacements. Long‐term records of injected and removed gas volume and fluid pore pressure in the “Lombardia” gas field, northern Italy, are available together with multiyear detection of vertical and horizontal west‐east displacement of the land surface above the reservoir by an advanced permanent scatterer interferometric synthetic aperture radar (PSInSAR) analysis. These data have been used to calibrate a 3‐D fluid‐dynamic model and develop a 3‐D transversally isotropic geomechanical model. The latter has been successfully implemented and used to reproduce the vertical and horizontal cyclic displacements, on the range of 8–10 mm and 6–8 mm, respectively, measured between 2003 and 2007 above the reservoir where a UGS program has been underway by Stogit‐Eni S.p.A. since 1986 following a 5 year field production life. Because of the great economical interest to increase the working gas volume as much as possible, the model addresses two UGS scenarios where the gas pore overpressure is pushed from the current 103%pi, where pi is the gas pore pressure prior to the field development, to 107%pi and 120%pi. Results of both scenarios show that there is a negligible impact on the ground surface, with deformation gradients that remain well below the most restrictive admissible limits for the civil structures and infrastructures.

Hydrogeology and its effect on slope stability along the coastal aquifer of Nice, France

Along the Ligurian continental margin off Nice (Western Mediterranean) a considerable portion of the sediments is affected by pore pressure transients resulting from groundwater charging by the nearby river Var, and also from leaching clay-mineral bearing deposits, which results in sensitive, weak clays. In 1979 the area suffered a tsunamigenic landslide that generated several casualties and material damage to the coastal zone. The seafloor presently shows evidence for soft sediment deformation and pore water freshening. In this paper, we present the first comprehensive in situ pore pressure data set collected over space and time between 2006 and 2009 in a landslide-prone area. Using piezometers (IFREMER Brest, France) and dynamic cone penetrometers (FF-CPTu, MARUM Bremen, Germany) as well as a simple modelling approach, we illuminate what governing factors control strength and pore pressure in marine slope sediments affected by the Var aquifer. Our results attest (i) a strong link between pore pressure transients and local precipitation over a period of 11months; (ii) overpressure at levels where groundwater flow occurs (coarse intervals containing sand and gravel); and (iii) fluidised fractures and channels that indicate high pore pressures at least temporarily. In essence, seepage and repeated pore pressure fluctuations weaken the sediment effectively, so that it cannot be ruled out that the currently stable portion adjacent to Nice Airport may fail in the near future.