Pore Pressure Rise Research Articles

Multi-scale simulation of thermal pressurization of fault fluid under CO2 injection for storage and utilization purposes

We investigated the impact of thermal pressurization (TP) on temperature and pore pressure changes within the shear zone of a slipping fault under supercritical CO2 injection. As a weakening mechanism, TP of fault fluid during slip tends to reduce frictional resistance by increasing pore pressures along the fault shear zone. We employed a multi-scale modeling approach in which the CO2 injection was simulated in a large-scale model of reservoir and the results were then exported into a micro-scale model of the fault shear zone which incorporates the TP effect. A poroelasticity finite element model was created in the large-scale model to determine the stress and pressure changes in the reservoir and within the fault zone during CO2 injection. The fault slip, pressure, and stresses obtained from the large-scale model were fed into the small-scale model to define hydraulic and mechanical boundary conditions of the shear zone of the fault. Two small-scale models were developed to compare the response of the fault shear zone with and without considering the TP. For the model with the TP effect, a simple constitutive law was implemented to couple pore pressure changes with temperature rise due to frictional heating. The results indicated that, in general, TP can lead to a significant increase of pore pressure build up due to CO2 injection. The effects of hydraulic diffusivity, slip velocity, shear zone thickness as well as injection rate on pore pressure, temperature and fracture energy prediction were evaluated. It was found that temperature and pore pressure rise decrease significantly by decreasing the slip velocity and increasing the shear zone thickness. For the values which were examined, hydraulic diffusivity was found to have insignificant effect on pore pressure and temperature changes which can be attributed to high compressibility of CO2. Findings of this study can be used and further expanded in future studies for more accurate assessment of induced seismicity potential due to CO2 injection.

Ground-Penetrating Radar monitoring of concrete at high temperature

Water front in concrete exposed to rapid heating is the layer where water vaporization and the subsequent pore pressure rise take place. Pore pressure is one of the main triggering factors in heat-induced explosive spalling (relevant for structures, such as tunnels, exposed to fire), while moisture migration influences concrete radiation shielding capability (important in containment shells of nuclear power plants and radioactive waste repositories). Hence, the experimental monitoring of water front in concrete at high temperature is a very interesting – though challenging – task. In a recent experimental campaign carried out at Politecnico di Milano, promising results have been obtained by coupling pore pressure-temperature measurements and water front monitoring through Ground-Penetrating Radar. This technique was implemented in a fire test performed on a concrete slab heated at the bottom face and proved to be effective in detecting the position of the water front during heating. The combination with pressure measurement allowed to confirm that pressure peaks are achieved in correspondence of the water front.

Disasters – Special Focus on Earthquake Induced Soil Liquefaction

Disasters disrupt progress and destroy the developmental efforts, often pushing nations back by several decades. Disaster due to soil liquefaction is also one of them. A parametric study is taken up for disaster caused by soil liquefaction in present work. Tests had been conducted on saturated sand in the shake table at different accelerations varying from 0.1 g to 0.3 g. Uniaxial geogrid had been used to reinforce the fine sand. The rise of excess pore pressure is responsible for liquefaction and experiments have been done with reinforced sand. The liquefaction resistance of sand increases due to addition of geogrid layers. For 0.1 g at 35% density the average increase in liquefaction resistance is about 17.87%. Further by increasing acceleration to 0.3 g the liquefaction resistance further increase to 20.67%. In present paper efforts had been made to see the effect of reinforcement against liquefaction. There was decrease in rise of excess pore pressure with use of geogrid in shake table. Thus punching shear foundation failure of structures can be prevented. Thus geogrids can be used as anti soil liquefaction measure. Keywords: Disaster, Earthquakes, Goegrids, Liquefaction, Soil

Effect of Stratum Properties on Wellbore Stability

The influence of stratum properties on wellbore stability was studied by numerical simulation. The results showed that the safe mud density window (SMDW) for both permeable and impermeable strata becomes narrower with increase in the ground stress non-uniformity coefficient. The probability of wellbore wall instability increases where the strata are permeable. If the ground stress non-uniformity coefficient is constant in this case, then there is a linear increase in the pressure at which stability loss and wellbore-wall collapse occur and a linear increase in stratum fracture pressure (SFP). Here, fracture pressure increases more rapidly than collapse pressure. The SMDW narrows with rise in pore pressure and widens with an increase in stratum tensile strength and the cohesive force between molecules. The SMDW is wider for permeable strata than for impermeable strata in this case. The width of the SMDW also increases with an increase in the internal friction angle, but it becomes narrower with an increase in the effective stress. The SMDW is not affected by increases in the Poisson’s ratio or porosity if there is no permeation but becomes smaller if there is permeation. In the absence of a supporting force, wellbore stability decreases drastically and the formation near the wellbore also becomes unstable.

Dynamic weakening of smectite‐bearing faults at intermediate velocities: Implications for subduction zone earthquakes

AbstractThe hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 µm/s to 1.3 m/s for water‐saturated smectite‐quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite‐bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 µm/s, the friction coefficients decreased gradually from 0.5–0.6 to 0.1 with an increase in smectite content from 20 to 50 wt %. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1–0.05, even for low smectite contents (roughly <30 wt %). Drastic slip weakening occurred at smectite contents of 10–30 wt % at slip rates of ~10 mm/s, which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types. The intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear‐enhanced compaction and microscopic thermal pressurization of pore fluids. This process could weaken the fault even below seismic slip rates, leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench.

Investigation of Cyclic Liquefaction with Discrete Element Simulations

A discrete-element method (DEM) assembly of virtual particles is calibrated to approximate the behavior of a natural sand in undrained loading. The particles are octahedral, bumpy clusters of spheres that are compacted into assemblies of different densities. The contact model is a Jäger generalization of the Hertz contact, which yields a small-strain shear modulus that is proportional to the square root of confining stress. Simulations made of triaxial extension and compression loading conditions and of simple shear produce behaviors that are similar to sand. Undrained cyclic shearing simulations are performed with nonuniform amplitudes of shearing pulses and with 24 irregular seismic shearing sequences. A methodology is proposed for quantifying the severities of such irregular shearing records, allowing the 24 sequences to be ranked in severity. The relative severities of the 24 seismic sequences show an anomalous dependence on sampling density. Four scalar measures are proposed for predicting the severity of a particular loading sequence. A stress-based scalar measure shows superior efficiency in predicting initial liquefaction and pore pressure rise.

Transient pore pressure response to confining stress excursions in Berea sandstone flooded with an aqueous solution of CO2

AbstractWe measured the pore pressure response due to carbon dioxide (CO2) gas bubble nucleation and growth in a Berea sandstone core flooded with an initially subsaturated aqueous solution of CO2, in response to a rapid drop in confining stress, under conditions representative of a confined aquifer. A portion of the CO2 in the Earth's crust, derived from volcanic, magmatic, and biogenic sources, dissolves in groundwater. Sudden reductions in confining stress in the Earth's crust occur due to dilational strain generated by the propagation of seismic Rayleigh and P waves, or aseismic slip in the near field of earthquakes. A drop in confining stress produces a proportional drop in pore fluid pressure. When the pore fluid contains dissolved CO2, the pore pressure responds to a drop in confining stress like it does in the dissolved gas‐free case, until the pore pressure falls below the bubble pressure. Gas bubble nucleation and diffusive growth in the pore space trigger spontaneous, transient buildup of the pore fluid pressure, and reduction of effective stress. We measured the rate of pore fluid pressure buildup in the 100 s immediately following the confining stress drop, as a function of the saturation with respect to CO2 at the lowest pore pressure realized during the confining stress drop, using five different CO2 partial pressures. The rate scales with the saturation with respect to dissolved CO2, from 10 kPa/min at 1.25 to 166 kPa/min at 1.8. The net pore pressure rise was as large as 0.7 MPa (100 psi) over 5 h.

Uplift mechanism for a shallow-buried structure in liquefiable sand subjected to seismic load: centrifuge model test and DEM modeling

Based on a centrifuge model test and distinct element method (DEM), this study provides new insights into the uplift response of a shallow-buried structure and the liquefaction mechanism for saturated sand around the structure under seismic action. In the centrifuge test, a high-speed microscopic camera was installed in the structure model, by which the movements of particles around the structure were monitored. Then, a two-dimensional digital image processing technology was used to analyze the microstructure of saturated sand during the shaking event. Herein, a numerical simulation of the centrifuge experiment was conducted using a two-phase (solid and fluid) fully coupled distinct element code. This code incorporates a particle-fluid coupling model by means of a “fixed coarse-grid” fluid scheme in PFC3D (Particle Flow Code in Three Dimensions), with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate the uplifts of the shallow-buried structure and the sharp rise in excess pore pressure. The corresponding micro-scale responses and explanations are provided. Overall, the uplift response of an underground structure and the occurrence of liquefaction in saturated sand are predicted successfully by DEM modeling. However, the dynamic responses during the shaking cannot be modeled accurately due to the restricted computer power.

Numerical study of heat and moisture transport through concrete at elevated temperatures

This research article focuses on numerical investigation of heat and moisture transport through concrete exposed to high temperatures such as fire. The conservation equations for moisture and energy transport through concrete have been represented in terms of temperature, pore pressure and vapor content as field variables. As the resulting governing equations are coupled and non-linear, the equations were solved numerically using Galerkin’s weighted residual finite element method and an iterative solution technique. After validating the model, a detailed simulation study has been carried out to understand the role of gradients of temperature, pore pressure and vapor content on heat and moisture transport through concrete exposed to ISO 834 fire curve. Results obtained at the end of 30 minutes exposure of concrete show that the temperature gradients become very steep after 12 minutes of exposure of concrete, which in turn results in increased vapor generation and 93% of vapor generation is completed at the end of 20 minutes. Due to steep temperature gradient along the length of concrete, condensation of vapor takes place which is followed by blockage of pores giving rise to sudden peak pore pressure rise and 97% of peak pore pressure is attained at the end of 18 minutes itself. It is observed that for the initial 18 minutes, the peak pore pressure front and peak vapor content front follow the same path and after 18 minutes the peak vapor content front moves slightly ahead of the peak pore pressure front.

Shear Deformation Development and the Increase of Pore Pressure due to Rainfall Infiltration in Sandy Model Slope under Different Inclination

Monitoring of surface displacement of slope has been widely adopted as time prediction method of shallow landslide due to rainfall. Modeling of shear deformation according to rainfall infiltration is necessary for establishment of the method. In order to examine the constitutive relation for the model, surface displacement, pore pressure at the base, volumetric water content (hereafter, V.W.C.) and suction, shear strain in the sandy model slope are monitored during artificial rainfall. Analysis of the monitored data shows that surface displacement and shear strain in the slope proceed at small rate under unsaturated condition first, then increase remarkably with the rise of pore pressure. Unsaturated component of surface displacement of steeper slope is larger than that of gentler slope. It is likely to be due to that shear strain with the increase of suction is larger in steeper slope while shear strain according to the increase of pore pressure is larger in gentler slope.

The long runout of the Heart Mountain landslide: Heating, pressurization, and carbonate decomposition

The Heart Mountain landslide of northwestern Wyoming is the largest subaerial landslide known. This Eocene age slide slid ∼50 km along a shallow 2° slope, posing a long‐standing enigma regarding its emplacement mechanism. We suggest here a mechanism for the catastrophic emplacement of the Heart Mountain landslide that is independent of slide triggering. The mechanism is a feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates at the slide shear zone. Such a feedback arises when a porous, fluid‐filled shear zone heats up because of frictional sliding. If the shear zone is confined, the generated heat leads to pore pressure rise, which in turn reduces frictional resistance to sliding, leading to acceleration. Temperatures at the shear zone quickly reach the decomposition temperature of carbonates. Since the shear zone of the Heart Mountain slide is located within a dolomite layer, it is expected that thermal decomposition of dolomite occurred within the Heart Mountain shear zone. This prediction is supported by ample field evidence for carbonate decomposition during the emplacement. Simulation of the sliding dynamics of the Heart Mountain block, accounting for feedback between shear heating, thermal pressurization, and thermal decomposition of carbonates, successfully reproduce the travel distance of the Heart Mountain block. The simulation results also predict that the maximum sliding velocity ranged between tens of meters per second to more than 100 m s−1 (depending on model assumptions) and that the duration of sliding was of the order of a few tens of minutes, in agreement with previous assessments.

Thermochemical pressurization of faults during coseismic slip

During earthquakes, frictional heating on the fault plane induces a temperature rise and thus a pore pressure rise, which is known as thermal pressurization (TP). Coseismic mineral dehydrations may occur because of this temperature increase and are included within the TP framework. Dehydrations are modeled as a source term for pore pressure because of the total volume change and as a sink term for temperature because they are endothermic. The reaction occurs within the slipping zone when a threshold temperature Ts is reached. Dehydration reaction kinetic is modeled using a first‐order reaction rate. Using energy and fluid mass conservation, we derive analytically the equations of evolution of pore pressure, temperature, and reaction extent in the undrained, adiabatic case using a constant reaction rate. We investigate the values of the kinetic rate constant required to produce a significant effect, which are much higher than laboratory data reported in the literature on clay, serpentine, and phyllosilicate dehydration. We show, however, that such high values can be reached if the temperature dependency of the rate constant is taken into account. Next, we include fluid and heat transport and use an Arrhenius law to calculate the rate constant as a function of temperature. The subsequent set of differential equations is then solved numerically. The main effect of dehydration reactions is an increase of pore pressure and a stabilization of the temperature during slip. We explore a wide range of parameters in order to determine in which cases dehydration can be considered as a nonnegligible process. For high‐permeability rocks (>10−18 m2) and when the amount of water that can be released is of the order of 10%, dehydration is an important mechanism as it delays the onset of melting, which would normally occur even within the TP framework. If the onset temperature is low compared to the initial temperature T0 (Ts − T0 ≲ 150°C), overpressure can occur. If the reactions are highly endothermic and if their kinetic is fast enough, frictional melting would not occur unless the dehydration reactions are completed within the slipping zone.

Landslides in the Rumphi District of Northern Malawi: characteristics and mechanisms of generation

Landslides are a common phenomenon in all the regions of Malawi. A number of historical landslides have been documented and are summarized here. This paper examines the occurrence of landslides in the Rumphi District of Northern Malawi. The study is based on a detailed study of 98 landslides that occurred in 2003 at Ntchenachena and Chiweta areas in the Rumphi District. This paper examines factors that contributed to and caused these landslides. The paper suggests that landslides were triggered by 206 mm of rain received in 2 days. The high percentage of medium to fine sand and abrupt rise in pore pressure accelerated the process of liquefaction at Ntchenachena. Cleft water pressure at the point between regolith and soil mass caused a number of slides in the Chiweta areas. High slope angles, deep weathering of the basement and high annual total rainfall contributed to the slope instability. Human activities through cultivation on steep slope, slope remodeling, and deforestation significantly altered the conditions of the slopes, thereby increasing the degree of landslide hazard present in these areas.

Behavior of a shallow water table under periodic flow conditions

A new laboratory data set on the behavior of a shallow water table in a sand column aquifer subject to simple harmonic periodic forcing at its base is presented and discussed. The data are analyzed using the dynamic effective porosity, which is defined as the ratio of the rate of change in total moisture to the rate of change in water table elevation; thus, a reduction in this parameter means that the extent of moisture exchange has been reduced relative to a given water table fluctuation. The data show a clear decrease in the dynamic effective porosity with increasing proximity of the water table to the sand surface, which is consistent with previous research under a steadily rising or falling shallow water table. The observed reduction in moisture exchange due to shallowness of the water table has implications for periodic flow scenarios such as the propagation of water table waves in coastal and beach groundwater systems. That is, as moisture exchange is reduced, less work is being done by the flow, and thus, energy dissipation rates for shallow water tables will be reduced relative to the case of a deeper water table. At present no account of the influence of water table shallowness has been included in theories describing water table wave dispersion. The present experiments, in conjunction with the dynamic effective porosity concept, provide a framework in which this gap in knowledge can be further investigated. Additional experiments were designed such that the free surface transgressed the sand surface for part of the oscillation period to investigate the influence of meniscus formation and deformation at the sand surface on periodic flow dynamics. The observed behavior is consistent with previous observations of steady infiltration above shallow water tables, namely, a rapid drop (rise) in pore pressure with the onset of meniscus formation (deformation). A simple “wetting and drying” model is derived, accounting for the variation in effective porosity caused by the free surface transgressing the sand surface, which is shown to accurately capture the observed behavior. A finite element solution of the Richards equation in which the transient upper boundary condition is easily mimicked by means of a surface element with special storage features also shows excellent agreement with the observed data.

A model for grain-crushing-induced landslides—Application to Nikawa, Kobe 1995

A novel method is developed, motivated by one of the mysteries of the Hyogoken-Nambu (Kobe): the Nikawa landslide. To this end, prompted by the hypothesis of “sliding-surface liquefaction” advocated by Sassa [Development of a new cyclic loading ring shear apparatus to study earthquake-induced-landslides. Report for grant-in-aid for development of scientific research by the Ministry on Education, Science and Culture, Japan (project no. 03556021), 1994, p. 1–106; Keynote lecture: access to the dynamics of landslides during earthquakes by a new cyclic loading high-speed ring shear apparatus. In: Proceedings of the sixth international symposium on landslides, 1992. In: Landslides, vol. 3. Balkema: Rotterdam; 1995. p. 1919–39], a dynamic analysis of the early stages of an earth slide is presented considering two mechanically coupled sub-structures: (a) the rapidly deforming shear band at the base of the slide and (b) the accelerating sliding mass modeled as a rigid body. The proposed model for sliding is based on: (i) the concept of high pore-water pressure generation by grain crushing along the sliding surface (proposed by Sassa et al. in 1995), (ii) an experimental model developed by Hardin [Crushing of soil particles. J Geotech Eng 1985;111(10):1177–92] for crushing of soil particles under compression and shear, expressed with a set of developed equations governing the mechanism of breakage, and (iii) the hysteretic stress–strain Bouc–Wen-type constitutive model coupled with the Coulomb friction law. An attempt is made to adjust the model parameters to Sassa's experimental data in ring-shear tests. The method leads to a reasonable prediction of the large displacement of the Nikawa landslide. A sensitivity analysis is also carried out for the influence of key model parameters (e.g. shape, crushing hardness, void ratio, grain size distribution, effective normal stress) on the pore-pressure rise due to particle breakage.

A model for presentation of seismic pore water pressures

A model for presentation of pore water pressures induced in sand samples during cyclic undrained testing is described. The proposed model belongs to a class of so-called ‘damage parameter models’, which correlate the pore pressure rise with a parameter based on an accumulated variable during testing. The concept of threshold strain is also incorporated in the model. The model has been verified on several series of published cyclic test data. Its parameters lie in a narrow band for a wide range of sand properties. The empirical functions that represent the common shape of individual curves for interpreted pore pressure data are also suggested. The proposed procedure has been adapted for presentation of other cyclic soil tests, and examples of interpretation of cyclic stress-controlled as well as cyclic drained tests are included.

Influence of Kinematics on Landslide Mobility and Failure Mode

Geometry of discontinuities greatly influences the kinematic behavior of slopes and landslides. Discontinuous deformation analysis (DDA) is used here to analyze two typical examples of slope failure, demonstrating that accurate representation of the geometry of the discontinuities is essential for identification of kinematically correct failure modes. Then, static stability analyses of the Vaiont, Italy, landslide of October 9, 1963 are used to show that the DDA results compare favorably with previously published limit equilibrium studies using similar geometries, and that the location and number of discontinuities have a significant effect on the predicted stability and failure velocities of the landslide. Dynamic DDA simulations of the Vaiont landslide show that the peak velocity increases up to 50% as the number of blocks increases, indicating that internal disintegration of the landslide mass results in increasing acceleration and higher peak velocity. DDA analyses simulating pore pressure rise resulting from frictional heating of the basal slide plane show that the peak velocity similarly increases up to 50% as the number of blocks increases. The magnitude of the increase in peak velocity as a result of disintegration (i.e., increasing number of blocks) suggests that as much attention should be paid to the geometry of discontinuities as is typically paid to shear strength and pore pressure.

CT-integrated treatment planning for robot-assisted, laser-guided radiofrequency ablation

The Sudbury Structure, formed by meteorite impact at 1850 Ma, consists of three major components: (1) the Sudbury Basin; (2) the Sudbury Igneous Complex, which surrounds the basin as an elliptical collar; and (3) breccia bodies in the footwall known as Sudbury Breccia. In general, the breccia consists of subrounded fragments set in a dark, fine-grained to aphanitic matrix. A comparison of the chemical composition of host rocks, clasts and matrices indicates that brecciation was essentially an in-situ process. Sudbury Breccia forms irregular-shaped bodies or dikes that range in size from mm to km scale. Contacts with the host rocks are commonly sharp. The aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls. The fractal dimension (Dr)=1.55. Although there appears to be some concentration of brecciation within concentric zones, small Sudbury Breccia bodies within and outside these zones have more or less random strikes and steep dips. Sudbury Breccia bodies near an embayment structure tend to be subparallel to the base of the Sudbury Igneous Complex. Sudbury Breccia occurs as much as 80 km from the outer margin of the Sudbury Igneous Complex. In an inner zone, 5 to 15 km wide, breccia comprises 5% of exposed bedrock with an increase in brecciation intensity in embayment structures. Sudbury Breccia may be classified into three types based on the nature of the matrix: clastic, pseudotachylite and microcrystalline. Clastic Sudbury Breccia, the dominant type in the Southern Province, is characterized by flow-surface structures. Possibly, a sudden rise in pore pressure caused explosive dilation and fragmentation, followed by fluidization and flowage into extension fractures. Pseudotachylite Sudbury Breccia, mainly confined to Archean rocks, apparently formed by comminution and frictional melting. Microcrystalline Sudbury Breccia formed as a result of the thermal metamorphism, of the North Range footwall, by the Sudbury Igneous Complex. This produced a zone, approximately 1.2 km wide, wherein the matrix of the breccia either recrystallized or, locally, melted. An overprint of regional metamorphism obliterated contact effects in the South Range footwall. The Ni–Cu–PGE magmatic sulphide deposits may be classified into four types based on structural setting: Sudbury Igneous Complex–footwall contact, footwall, offset, and sheared deposits. Sudbury Breccia is the main host for footwall deposits (e.g., McCreedy East, Victor, Lindsley). Sudbury Breccia locally hosts mineralization in radial (e.g., Parkin and Copper Cliff) and concentric (e.g., Frood–Stobie) offset dikes.

Progressive Consolidation Settlement of Naturally Deposited Clayey Soil Under Embankment Loading

ABSTRACT In a naturally deposited soil foundation subject to embankment loading, a delayed settlement can often be observed after the embankment's completion, sometimes also showing acceleration over time. Frequently in such cases the soil exhibits an increase rather than a dissipation of excess pore pressure in some of its parts, a phenomenon which cannot be explained by conventional elasto-plastic consolidation theory. In this paper the possible mechanism for this kind of delayed settlement is investigated numerically using a soil-water coupled elasto-plastic computation under plane strain conditions. It is assumed that the soil section contains a medium dense sand layer and highly structured clay layers. A Super/subloading Yield Surface Cam-clay model is used to describe the elasto-plastic behavior of both the sand and the clay with respect to soil structure, overconsolidation and anisotropy. It is found that the delayed settlement behavior occurs under a certain constant embankment load, and persists over a period of 40 years, with increases in the settlement rate accompanied by both dissipation and a rise in excess pore pressure. The cause of this is the softening that co-occurs with the plastic compression of the soil skeleton. In other words, consolidation settlement can be considered as an example of “progressive consolidation with decay of structure”. Some typical characteristics of this delayed settlement behavior are also numerically examined with reference to the height and weight of the embankment, and to soil improvements using sand drains. When the embankment is much lower, the foundation does not undergo delayed consolidation, and when it is higher, the foundation becomes subject to circular slip failure. Soil improvement with sand drains can effectively shorten the length of time up to final settlement.

Mechanical behaviour of unsaturated marine sediments: experimental and theoretical approaches

The COSTA target areas exhibit different slope failure events, which reflect different triggering mechanisms including gas hydrates dissociation. Gas hydrates stability law depends on temperature, pore pressure, gas chemistry, and pore water salinity. Any change in the equilibrium parameters may convert the hydrate to gas plus water, causing significant weakening of the sediment, and generating a rise of pore pressure. A significant consequence of the hydrate melting is the gas production, which alters significantly the behaviour and mechanical properties of the marine sediments and could be very hazardous when the sediment is unloaded in undrained conditions. The behaviour of the unsaturated marine sediment has to be accurately identified in order to quantify the geological risks associated to the hydrate dissociation [see, for instance, Sultan, N., Cochonat, P., Foucher, J.P., Mienert, J., this volume. Effect of gas hydrates melting on seafloor slope instability]. Thus, in this work, five tests were carried out on synthetic marine sediments in order to identify the gas effect on the hydro-mechanical behaviour of unsaturated marine sediments. Based on the experimental results obtained from this study, on previous constitutive models [Alonso, E.E., Gens, A., Josa A., 1990. A constitutive model for partially saturated soils. Géotechnique 40(3), 405–430; Gens, A., Alonso, E.E., 1992. A framework for the behaviour of unsaturated expansive clays. Can. Geotech. J. 29, 1013–1032] and on the relationship of Van Genuchten [Van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898], a new mathematical formulation was proposed to simulate the behaviour of the unsaturated soil. Qualitative comparison between experimental results and model predictions shows the capacity of the proposed model to reasonably reproduce the essential features of the hydro-mechanical behaviour of marine unsaturated sediment. Experimental and simulation results show that unloading (tidal cycles, erosion, natural slope instabilities, excavation) a submarine unsaturated sediment slope in shallow water is much more hazardous than unloading the same submarine slope in deep water.