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Pore Pressure (pore + pressure)
Kinds of Pore Pressure Selected AbstractsIndentation of a free-falling lance penetrometer into a poroelastic seabedINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2005Derek Elsworth Abstract A solution is developed for the build-up, steady and post-arrest dissipative pore fluid pressure fields that develop around a blunt penetrometer that self-embeds from freefall into the seabed. Arrest from freefall considers deceleration under undrained conditions in a purely cohesive soil, with constant shear strength with depth. The resulting decelerating velocity field is controlled by soil strength, geometric bearing capacity factors, and inertial components. At low impact velocities the embedment process is controlled by soil strength, and at high velocities by inertia. With the deceleration defined, a solution is evaluated for a point normal dislocation penetrating in a poroelastic medium with a prescribed decelerating velocity. Dynamic steady pressures, PD, develop relative to the penetrating tip geometry with their distribution conditioned by the non-dimensional penetration rate, UD, incorporating impacting penetration rate, consolidation coefficient and penetrometer radius, and the non-dimensional strength, ND, additionally incorporating undrained shear strength of the sediment. Pore pressures develop to a steady peak magnitude at the penetrometer tip, and drop as PD=1/xD with distance xD behind the tip and along the shaft. Peak induced pressure magnitudes may be correlated with sediment permeabilities, post-arrest dissipation rates may be correlated with consolidation coefficients, and depths of penetration may be correlated with shear strengths. Together, these records enable strength and transport parameters to be recovered from lance penetrometer data. Penetrometer data recorded off La Palma in the Canary Islands (J. Volcanol. Geotherm. Res. 2000; 101:253) are used to recover permeabilities and consolidation coefficients from peak pressure and dissipation response, respectively. Copyright © 2004 John Wiley & Sons, Ltd. [source] Modelling the hysteresis in the velocity pattern of slow-moving earth flows: the role of excess pore pressureEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2005T. W. J. van Asch Abstract This paper describes the velocity pattern of a slow-moving earth flow containing a viscous shear band and a more or less rigid landslide body on top. In the case of small groundwater fluctuations, Bingham's law may describe the velocity of these slow-moving landslides, with velocity as a linear function of excess shear stress. Many authors have stated that in most cases a non-linear version of Bingham's law best describes the moving pattern of these earth flows. However, such an exponential relationship fails to describe the hysteresis loop of the velocity, which was found by some authors. These authors showed that the velocity of the investigated earth flows proved to be higher during the rising limb of the groundwater than during the falling limb. To explain the hysteris loop in the velocity pattern, this paper considers the role of excess pore pressure in the rheological behaviour of earth flows by means of a mechanistic model. It describes changes in lateral internal stresses due to a change in the velocity of the earth flow, which generates excess pore pressure followed by pore pressure dissipation. Model results are compared with a hysteresis in the velocity pattern, which was measured on the Valette landslide complex (French Alps). Copyright © 2005 John Wiley & Sons, Ltd. [source] Near-lithostatic pore pressure at seismogenic depths: a thermoporoelastic modelGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2006Francesca Zencher SUMMARY A model is presented for pore pressure migration through a transition layer separating a meteoric aquifer at hydrostatic pressure from a deeper reservoir at lithostatic pressure. This configuration is thought to be pertinent to the South Iceland seismic zone (SISZ) and to other tectonically active regions of recent volcanism, where volatiles are continuously released by ascending magma below the brittle,ductile transition. Poroelastic parameters are computed for basaltic rock. The model is 1-D, the fluid viscosity is temperature dependent and rock permeability is assumed to be pressure dependent according to a dislocation model of a fractured medium. Environment conditions are considered, pertinent to basalt saturated with water at shallow depth (case I) and at mid-crustal depth (case II). If the intrinsic permeability of the rock is high, no significant effects are observed in the pressure field but advective heat transfer shifts the brittle,ductile transition to shallower depths. If the intrinsic permeability is low, the pressure-dependent permeability can propagate near-lithostatic pore pressures throughout most of the transition layer, while the temperature is practically unaffected by advective contributions so that the rock in the transition layer remains in brittle condition. Geometrical parameters characterizing the fracture distribution are important in determining the effective permeability: in particular, if an interconnected system of fractures develops within the transition layer, the effective permeability may increase by several orders of magnitude and near-lithostatic pore pressure propagates upwards. These modelling results have important bearings on our understanding of seismogenic processes in geothermal areas and are consistent with several geophysical observations in the SISZ, in connection with the two 2000 June M= 6.5 earthquakes, including: (i) fluid pressure pulses in deep wells, (ii) low resistivity at the base of the seismogenic layer, (iii) low VP/VS ratio and time-dependent seismic tomography, (iv) heterogeneity of focal mechanisms, (v) shear wave splitting, (vi) high b -value of deep foreshocks, (vii) triggered seismicity and (viii) Radon anomalies. [source] Seismic anisotropy of shalesGEOPHYSICAL PROSPECTING, Issue 5 2005C.M. Sayers ABSTRACT Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology and pore pressure from seismic data, and to understand time-to-depth conversion errors and non-hyperbolic moveout. A single anisotropy parameter, Thomsen's , parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to interpret the small-offset AVO response. The sign of this parameter is poorly understood, with both positive and negative values having been reported in the literature. , is sensitive to the compliance of the contact regions between clay particles and to the degree of disorder in the orientation of clay particles. If the ratio of the normal to shear compliance of the contact regions exceeds a critical value, the presence of these regions acts to increase ,, and a change in the sign of ,, from the negative values characteristic of clay minerals to the positive values commonly reported for shales, may occur. Misalignment of the clay particles can also lead to a positive value of ,. For transverse isotropy, the elastic anisotropy parameters can be written in terms of the coefficients W200 and W400 in an expansion of the clay-particle orientation distribution function in generalized Legendre functions. For a given value of W200, decreasing W400 leads to an increase in ,, while for fixed W400, , increases with increasing W200. Perfect alignment of clay particles with normals along the symmetry axis corresponds to the maximum values of W200 and W400, given by and . A comparison of the predictions of the theory with laboratory measurements shows that most shales lie in a region of the (W200, W400)-plane defined by W400/W200,Wmax400/Wmax200. [source] The seismic response to overpressure: a modelling study based on laboratory, well and seismic dataGEOPHYSICAL PROSPECTING, Issue 5 2001José M. Carcione We investigate the seismic detectability of an overpressured reservoir in the North Sea by computing synthetic seismograms for different pore-pressure conditions. The modelling procedure requires the construction of a geological model from seismic, well and laboratory data. Seismic inversion and AVO techniques are used to obtain the P-wave velocity with higher reliability than conventional velocity analysis. From laboratory experiments, we obtain the wave velocities of the reservoir units versus confining and pore pressures. Laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions. These measurements provide the basis for calibrating the pressure model. Overpressures are caused by different mechanisms. We do not consider processes such as gas generation and diagenesis, which imply changes in phase composition, but focus on the effects of pure pore-pressure variations. The results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep North Sea reservoirs. [source] Mechanisms and pathways of lateral flow on aspen-forested, Luvisolic soils, Western Boreal Plains, Alberta, CanadaHYDROLOGICAL PROCESSES, Issue 21 2010Todd Redding Abstract Rainfall simulation experiments by Redding and Devito (2008, Hydrological Processes 23: 4287,4300) on two adjacent plots of contrasting antecedent soil moisture storage on an aspen-forested hillslope on the Boreal Plain showed that lateral flow generation occurred only once large soil storage capacity was saturated combined with a minimum event precipitation of 15,20 mm. This paper extends the results of Redding and Devito (2008, Hydrological Processes 23: 4287,4300) with detailed analysis of pore pressure, soil moisture and tracer data from the rainfall simulation experiments, which is used to identify lateral flow generation mechanisms and flow pathways. Lateral flow was not generated until soils were wet into the fine textured C horizon. Lateral flow occurred dominantly through the clay-rich Bt horizon by way of root channels. Lateral flow during the largest event was dominated by event water, and precipitation intensity was critical in lateral flow generation. Lateral flow was initiated as preferential flow near the soil surface into root channels, followed by development of a perched water table at depth, which also interacted with preferential flow pathways to move water laterally by the transmissivity feedback mechanism. The results indicate that lateral flow generated by rainfall on these hillslopes is uncommon because of the generally high available soil moisture storage capacity and the low probability of rainfall events of sufficient magnitude and intensity. Copyright © 2010 John Wiley & Sons, Ltd. [source] A three-dimensional integral equation model for calculating poro- and thermoelastic stresses induced by cold water injection into a geothermal reservoirINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2009X. X. Zhou Abstract Poro-mechanical and thermo-mechanical processes change the fracture aperture and thus affect the water flow pattern in the fracture during the cold water injection into enhanced geothermal systems (EGS). In addition, the stresses generated by these processes contribute to the phenomenon of reservoir seismicity. In this paper, we present a three-dimensional (3D) partially coupled poro-thermoelastic model to investigate the poroelastic and thermoelastic effects of cold water injection in EGS. In the model, the lubrication fluid flow and the convective heat transfer in the fracture are modeled by the finite element method, while the pore fluid diffusion and heat conductive transfer in the reservoir matrix are assumed to be 3D and modeled by the boundary integral equation method without the need to discretize the reservoir. The stresses at the fracture surface and in the reservoir matrix are obtained from the numerical model and can be used to assess the variation of in situ stress and induced seismicty with injection/extraction. Application of the model shows that rock cooling induces large tensile stresses and increases fracture conductivity, whereas the rock dilation caused by fluid leakoff decreases fracture aperture and increases compressive total stresses around the injection zone. However, increases in pore pressure reduce the effective stresses and can contribute to rock failure, fracture slip, and microseismic activity. Copyright © 2009 John Wiley & Sons, Ltd. [source] Consolidation around stone columns.INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 7 2009Influence of column deformation Abstract A solution is presented for the radial consolidation around stone columns under constant surcharge load. The solution considers the influence of vertical and radial deformation of the column, either in elastic and elastoplastic regimes. The solution is in terms of the average excess pore pressure in the soil. It is based on previous solutions, initially developed for rigid column, or including only vertical deformation. For elastic column, the solution gives the variation of strains and stresses between the undrained and final states, for which it coincides with the existing elastic solutions. All the results are given in closed form, and both the elastic and plastic deformations of the column lead to an equivalent coefficient of consolidation for the radial flow, which enables the application of the existing methods of integration of the consolidation equation. A parametric study is presented, showing the influence of the main problem features. A design example is used to illustrate the application to practical cases. Copyright © 2008 John Wiley & Sons, Ltd. [source] Porothermoelastic analyses of anisotropic hollow cylinders with applicationsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2005Mazen Kanj Abstract It has been known that material anisotropy and thermal stresses affect borehole stability significantly. Aiming at the experimental studies associated with borehole stability in anisotropic (transversely isotropic) poroelastic materials subject to non-isothermal conditions, this paper details and applies an anisotropic porothermoelastic solution to an unjacketed hollow cylinder in a triaxial set-up. Numerical analyses are presented to demonstrate thermal and material anisotropy effects on the pore pressure and the stress concentrations in and around the geometry of a hollow cylinder subjected to thermal and stress perturbations. Copyright © 2004 John Wiley & Sons, Ltd. [source] Modelling poroelastic hollow cylinder experiments with realistic boundary conditionsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2004S. Jourine Abstract A general poroelastic solution for axisymmetrical plane strain problems with time dependent boundary conditions is developed in Laplace domain. Time-domain results are obtained using numerical inversion of the Laplace transform. Previously published solutions can be considered as special cases of the proposed solution. In particular, we could reproduce numerical results for solid and hollow poroelastic cylinders with suddenly applied load/pressure (Rice and Cleary, Rev. Geophys. Space Phys. 1976; 14:227; Schmitt, Tait and Spann, Int. J. Rock Mech. Min. Sci. 1993; 30:1057; Cui and Abousleiman, ASCE J. Eng. Mech. 2001; 127:391). The new solution is used to model laboratory tests on thick-walled hollow cylinders of Berea sandstone subjected to intensive pressure drawdown. In the experiments, pressure at the inner boundary of the hollow cylinder is observed to decline exponentially with a decay constant of 3,5 1/s. It is found that solutions with idealized step-function type inner boundary conditions overestimate the induced tensile radial stresses considerably. Although basic poroelastic phenomena can be modelled properly at long time following a stepwise change in pressure, realistic time varying boundary conditions predict actual rock behaviour better at early time. Experimentally observed axial stresses can be matched but appear to require different values for , and , than are measured at long time. The proposed solution can be used to calculate the stress and pore pressure distributions around boreholes under infinite/finite boundary conditions. Prospective applications include investigating the effect of gradually changing pore pressure, modelling open-hole cavity completions, and describing the phenomenon of wellbore collapse (bridging) during oil or gas blowouts. Copyright © 2004 John Wiley & Sons, Ltd. [source] An analytical solution of one-dimensional consolidation for soft sensitive soil groundINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 9 2004Yun-Min Chen Abstract An analytical solution of one-dimensional consolidation for soft sensitive soil ground is presented. The moving boundary is introduced to indicate the notable change of consolidation behaviour of sensitive soil with the increase of stress level. It is assumed that the soil structure of the upper subsoil gradually destroys downwards with the dissipation of pore pressure, and the coefficient of consolidation as well as the coefficient of permeability of the upper subsoil become small, which hinders the dissipation of pore pressure of the lower subsoil. The consolidation degree curve obtained from the present solution is found to lie between the two curves obtained from Terzaghi one-dimensional consolidation solution with the parameters of the undisturbed soil and the remolded soil. The calculated results provide a new explanation for a general phenomenon in the consolidation of soft sensitive soil ground, as that for high loadings the consolidation is longer than for small ones. It should be pointed out from this study that both the deep mixing method and the long vertical drains methods are effective techniques for improving deep sensitive soil ground. Copyright © 2004 John Wiley & Sons, Ltd. [source] Extension of the Griffith's fracture criteria to saturated claysINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2003K.M. Dégué Abstract Inglis [1] has solved the problem of distribution of stress in an elastic plate around an elliptical hole. His works clarify the role of cracks in the failure of an elastic material. However, his solution cannot be applied to saturated clay because he considers only total stresses, while, in saturated clay, the criterion of rupture should be expressed in terms of effective and not total stresses. The solution of Atkinson and Craster [2] using Biot's poroelasticity theory, shows that there is no high pore pressure in the vicinity of the crack tips for saturated clay. The major difference between this approach and the Biot's theory of is that, in saturated clay, strain is a function of the variation of the effective stress [3], while, in poroelastic media, strain is only a function of the variation of the total stress [4, Equation 2.2]. Also in their solution there is continuity between the pore fluid and the inner fluid in the crack. Their solution is valid for poroelastic media involving a movement of the pore fluid. In our solution there is no movement of the pore fluid (Undrained condition). In this paper we have solved the same problem as Inglis [1], but for the particular case of saturated clay obeying elastic law. By solving this problem we obtained the expressions for pore pressure, effective stress, total stress and displacements. The results show that not only the total stress but also the pore pressure and the effective stress are also high in the vicinity of the crack tips. A new failure criterion, based on Griffith's strain energy principle [5] and maximum tensile stress [6], valid for saturated clay is developed in this paper. Copyright © 2003 John Wiley & Sons, Ltd. [source] Performance of Jacobi preconditioning in Krylov subspace solution of finite element equationsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2002F.-H. Lee Abstract This paper examines the performance of the Jacobi preconditioner when used with two Krylov subspace iterative methods. The number of iterations needed for convergence was shown to be different for drained, undrained and consolidation problems, even for similar condition number. The differences were due to differences in the eigenvalue distribution, which cannot be completely described by the condition number alone. For drained problems involving large stiffness ratios between different material zones, ill-conditioning is caused by these large stiffness ratios. Since Jacobi preconditioning operates on degrees-of-freedom, it effectively homogenizes the different spatial sub-domains. The undrained problem, modelled as a nearly incompressible problem, is much more resistant to Jacobi preconditioning, because its ill-conditioning arises from the large stiffness ratios between volumetric and distortional deformational modes, many of which involve the similar spatial domains or sub-domains. The consolidation problem has two sets of degrees-of-freedom, namely displacement and pore pressure. Some of the eigenvalues are displacement dominated whereas others are excess pore pressure dominated. Jacobi preconditioning compresses the displacement-dominated eigenvalues in a similar manner as the drained problem, but pore-pressure-dominated eigenvalues are often over-scaled. Convergence can be accelerated if this over-scaling is recognized and corrected for. Copyright © 2002 John Wiley & Sons, Ltd. [source] Explicit solutions for the instantaneous undrained contraction of hollow cylinders and spheres in porous elastoplastic mediumINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 3 2002A. Giraud Abstract In this article we present closed-form solutions for the undrained variations in stress, pore pressure, deformation and displacement inside hollow cylinders and hollow spheres subjected to uniform mechanical pressure instantaneously applied to their external and internal boundary surfaces. The material is assumed to be a saturated porous medium obeying a Mohr,Coulomb model failure criterion, exhibiting dilatant plastic deformation according to a non-associated flow rule which accounts for isotropically strain hardening or softening. The instantaneous response of a porous medium submitted to an instantaneous loading is undrained, i.e. without any fluid mass exchange. The short-term equilibrium problem to be solved is now formally identical to a problem of elastoplasticity where the constitutive equations involve the undrained elastic moduli and particular equivalent plastic parameters. The response of the model is presented (i) for extension and compression undrained triaxial tests, and (ii) for unloading problems of hollow cylinders and spheres through the use of appropriately developed closed-form solutions. Numerical results are presented for a plastic clay stone with strain hardening and an argilite with strain softening. The effects of plastic dilation, of the strain softening law and also of geometry of the cavity on the behaviour of the porous medium have been underlined. Analytical solutions provide valuable benchmarks enabling various numerical methods in undrained conditions with a finite boundary to be verified. Copyright © 2002 John Wiley & Sons, Ltd. [source] Analysis of undrained cavity expansion in elasto-plastic soils with non-linear elasticityINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 1 2002Lai Fa Cao Abstract A large strain analysis of undrained expansion of a spherical/cylindrical cavity in a soil modelled as non-linear elastic modified Cam clay material is presented. The stress,strain response of the soil is assumed to obey non-linear elasticity until yielding. A power-law characteristic or a hyperbolic stress,strain curve is used to describe the gradual reduction of soil stiffness with shear strain. It is assumed that, after yielding, the elasto-plastic behaviour of the soil can be described by the modified Cam clay model. Based on a closed-form stress,strain response in undrained condition, a numerical solution is obtained with the aid of simple numerical integration technique. The results show that the stresses and the pore pressure in the soil around an expanded cavity are significantly affected by the non-linear elasticity, especially if the soil is overconsolidated. The difference between large strain and small strain solutions in the elastic zone is not significant. The stresses and the pore pressure at the cavity wall can be expressed as an approximate closed-form solution. Copyright © 2001 John Wiley & Sons, Ltd. [source] Solutions of pore pressure build up due to progressive wavesINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 9 2001L. Cheng Abstract The analytical solution of soil pore pressure accumulations due to a progressive wave is examined in detail. First of all, the errors contained in a published analytical solution for wave-induced pore pressure accumulation are addressed, and the correct solution is presented in a more general form. The behaviour of the solution under different soil conditions is then investigated. It is found that the solution for deep soil conditions is sensitive to the soil shear stress in the top thin layer of the soil. However the solution is significantly influenced by the shear stress in the thin layer of soil near the impermeable base, for shallow and finite depth soil conditions. It is also found that a small error in the soil shear stress can lead to a large error in the accumulated pore pressure. An error analysis reveals the relationships between the accuracy of the pore pressure accumulation and the accuracy of the soil shear stress. A numerical solution to the simplified Biot consolidation equation is also developed. It is shown that the error analysis is of significant value for the numerical modelling of pore pressure buildup in marine soils. Both analytical and numerical examples are given to validate the error estimation method proposed in the present paper. Copyright © 2001 John Wiley & Sons, Ltd. [source] Wave-induced seepage flux into anisotropic seabedsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 8 2001D. S. Jeng Abstract Considerable effort has been devoted to quantifying the wave-induced soil response in a porous seabed in the last few decades. Most previous investigations have focused on the analysis of pore pressure and effective stresses within isotropic sediments, despite strong evidence of anisotropic soil behaviour reported in the literature. Furthermore, the seepage flux, which is important in the context of contaminant transport, has not been examined. In this paper, we focus on water wave-driven seepage in anisotropic marine sediments of finite thickness. The numerical results predict that the effects of hydraulic anisotropy and anisotropic soil behaviour on the wave-driven seepage in marine sediment are significant. Copyright © 2001 John Wiley & Sons, Ltd. [source] A finite element algorithm for parameter identification of material models for fluid saturated porous mediaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 5 2001R. Mahnken Abstract In this contribution an algorithm for parameter identification of geometrically linear Terzaghi,Biot-type fluid-saturated porous media is proposed, in which non-uniform distributions of the state variables such as stresses, strains and fluid pore pressure are taken into account. To this end a least-squares functional consisting of experimental data and simulated data is minimized, whereby the latter are obtained with the finite element method. This strategy allows parameter identification based on in situ experiments. In order to improve the efficiency of the minimization process, a gradient-based optimization algorithm is applied, and therefore the corresponding sensitivity analysis for the coupled two-phase problem is described in a systematic manner. For illustrative purpose, the performance of the algorithm is demonstrated for a slope stability problem, in which a quadratic Drucker,Prager plasticity model for the solid and a linear Darcy law for the fluid are combined. Copyright © 2001 John Wiley & Sons, Ltd. [source] Finite element analysis of land subsidence above depleted reservoirs with pore pressure gradient and total stress formulationsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2001Giuseppe Gambolati Abstract The solution of the poroelastic equations for predicting land subsidence above productive gas/oil fields may be addressed by the principle of virtual works using either the effective intergranular stress, with the pore pressure gradient regarded as a distributed body force, or the total stress incorporating the pore pressure. In the finite element (FE) method both approaches prove equivalent at the global assembled level. However, at the element level apparently the equivalence does not hold, and the strength source related to the pore pressure seems to generate different local forces on the element nodes. The two formulations are briefly reviewed and discussed for triangular and tetrahedral finite elements. They are shown to yield different results at the global level as well in a three-dimensional axisymmetric porous medium if the FE integration is performed using the average element-wise radius. A modification to both formulations is suggested which allows to correctly solve the problem of a finite reservoir with an infinite pressure gradient, i.e. with a pore pressure discontinuity on its boundary. Copyright © 2001 John Wiley & Sons, Ltd. [source] Fault rock analysis of the northern part of the Chelungpu Fault and its relation to earthquake faulting of the 1999 Chi-Chi earthquake, TaiwanISLAND ARC, Issue 1 2005Kohtaro UjiieArticle first published online: 3 MAR 200 Abstract The 1999 Chi-Chi earthquake in Taiwan (Mw = 7.6) produced a surface rupture along the north,south-striking Chelungpu thrust fault with pure dip-slip (east side up) and left lateral strike-slip displacements. Near-field strong-motion data for the northern part of the fault illustrate a distinct lack of the high-frequency seismic radiation associated with a large slip (10,15 m) and a rapid slip velocity (2,4 m/s), suggesting a smooth seismic slip associated with low dynamic frictional resistance on the fault. A drillhole was constructed at shallow depths in the possible fault zones of the northern part of the Chelungpu Fault, which may have slipped during the 1999 earthquake. One of the zones consists of a 20-cm-thick, unconsolidated fault breccia with a chaotic texture lacking both discrete slip surfaces (e.g. Riedel shears) and grain crushing. Other possible fault zones are marked by the narrow (less than a few centimeters) gouge zone in which clayey material intrudes into the damaged zone outside of the gouge zone. These characteristic fault rock textures suggest that the slip mechanisms at shallow levels during the earthquake involved either granular flow of initially unconsolidated material or slip localization under elevated pore pressure along the narrow clayey gouge zone. Because both mechanisms lead to low dynamic frictional resistance on the fault, the rapid seismic slip in the deep portions of the fault (i.e. the source region of strong-motion radiation) could have been accommodated by frictionless slip on the shallow portions of the fault. The combination of strong-motion data and fault rock analysis suggests that smooth slip associated with low dynamic friction occurred on both the deep and shallow portions of the fault, resulting in a large slip between the source region and the surface in the northern region. [source] THE APPLICATION OF ELECTRON SPIN RESONANCE AS A GUIDE TO THE MATURATION AND TYPING OF ORGANIC MATTER IN THE NORTH SEAJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2003R. A. McTavish In early electron spin resonance (ESR) analysis of North Sea wells, maturation of organic matter (OM) was expressed in terms of maximum palaeotemperature (MPT) based on North American calibrations that did not consider the influences of kerogen composition or overpressure. In the North Sea, the MPTs were anomalous in overpressured sequences and relative to other indices of OM maturation such as vitrinite reflectance, so the ESR method was abandoned there in geochemical studies. However, early empirical study of North Sea ESR data indicated that, in relation to functions that linked temperature and pore pressure, some ESR parameters were predictable without reference to MPTs. In order to re-evaluate ESR parameters as indices of OM maturation, the physical factors (temperature and pressure) which affect OM maturation are related in the present paper to the ESR parameters "g" (spectral position) and Ng (spin density) at six well locations in the northern North Sea. A third ESR parameter, W (line width), is not an effective guide to maturation levels due to its complex relationship to the physical factors and kerogen types. However, cross-plots of W versus "g" and Ng appear to be as effective as pyrolysis for kerogen typing. Levels of maturation investigated in the North Sea wells range through the equivalent vitrinite reflectance values of about 0.50,1.50%. The values of "g" and Ng have been differentiated for kerogen type, but undifferentiated values of "g" have also been studied. Regression analysis has shown that there are linear relationships between the ESR parameters "g" and Ng, and the physical factors present-day temperature (To), "effective" temperature (Te), and differential pressure (Pd). Correlation coefficients for both "g" (undifferentiated and differentiated) and Ng (differentiated) relative to the physical factors are high; the highest values are for "g" and Ng relative to Te and Pd (r =,0.950 for "g" differentiated or undifferentiated, r = 0.944,0.976 for Ng differentiated, respectively). However, correlation coefficients were lower for "g" and Ng relative to To. More frequent high correlation coefficients and larger sample populations suggest that "g" (undifferentiated) is a more reliable index of OM maturation than Ng(differentiated). However, the estimation of levels of OM maturation is improved if both indices are used together. The ESR method appears to be effective both for estimating levels of OM maturation and for kerogen typing. It has a number of potential advantages over other geochemical methods: firstly, it is more sensitive for estimating OM maturation than most other methods; secondly, it can be used to analyze organic matter which is as old as Proterozoic; thirdly, it does not destroy the samples analyzed. [source] Solifluction processes in an area of seasonal ground freezing, Dovrefjell, NorwayPERMAFROST AND PERIGLACIAL PROCESSES, Issue 1 2008Charles Harris Abstract Continuous monitoring of soil temperatures, frost heave, thaw consolidation, pore water pressures and downslope soil movements are reported from a turf-banked solifluction lobe at Steinhøi, Dovrefjell, Norway from August 2002 to August 2006. Mean annual air temperatures over the monitored period were slightly below 0°C, but mean annual ground surface temperatures were around 2°C warmer, due to the insulating effects of snow cover. Seasonal frost penetration was highly dependent on snow thickness, and at the monitoring location varied from 30,38,cm over the four years. The shallow annual frost penetration suggests that the site may be close to the limit of active solifluction in this area. Surface solifluction rates over the period 2002,06 ranged from 0.5,cm yr,1 at the rear of the lobe tread to 1.6,cm yr,1 just behind the lobe front, with corresponding soil transport rates of 6,cm3,cm,1 yr,1 and 46,cm3,cm,1 yr,1. Pore water pressure measurements indicated seepage of snowmelt beneath seasonally frozen soil in spring with artesian pressures beneath the confining frozen layer. Soil thawing was associated with surface settlement and downslope soil displacements, but following clearance of the frozen ground, later soil surface settlement was accompanied by retrograde movement. Summer rainfall events caused brief increases in pore pressure, but no further soil movement. Surface displacements exceeded maximum potential frost creep values and it is concluded that gelifluction was an important component of slow near-surface mass movements at this site. Temporal and spatial variations in solifluction rates across the area are likely to be considerable and strongly influenced by snow distribution. Copyright © 2008 John Wiley & Sons, Ltd. [source] Poroelastodynamic Boundary Element Method in Time Domain: Numerical AspectsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2005Martin Schanz Based on Biot's theory the governing equations for a poroelastic continuum are given as a coupled set of partial differential equations (PDEs) for the unknowns solid displacements and pore pressure. Using the Convolution Quadrature Method (CQM) proposed by Lubich a boundary time stepping procedure is established based only on the fundamental solutions in Laplace domain. To improve the numerical behavior of the CQM-based Boundary Element Method (BEM) dimensionless variables are introduced and different choices studied. This will be performed as a numerical study at the example of a poroelastic column. Summarizing the results, the normalization to time and spatial variable as well as on Young's modulus yields the best numerical behavior. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Material Modelling of Porous Media for Wave Propagation ProblemsPROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003M. Schanz PD Dr.-Ing. Under the assumption of a linear geometry description and linear constitutive equations, the governing equations are derived for two poroelastic theories, Biot's theory and Theory of Porous Media (TPM), using solid displacements and pore pressure as unknowns. In both theories, this is only possible in the Laplace domain. Comparing the sets of differential equations of Biot's theory and of TPM, they show different constant coefficients but the same structure of coupled differential equations. Identifying these coefficients with the material data and correlating them leads to the known problem with Biot's ,apparent mass density'. Further, in trying to find a correlation between Biot's stress coefficient to parameters used in TPM yet unsolved inconsistencies are found. For studying the numerical effect of these differences, wave propagation results of a one-dimensional poroelastic column are analysed. Differences between both theories are resolved only for compressible constituents. [source] Experimental study of rill bank collapseEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2007Jovan R. Stefanovic Abstract Rill bank collapse is an important component in the adjustment of channel morphology to changes in discharge and sediment flux. Sediment inputs from bank collapse cause abrupt changes in flow resistance, flow patterns and downstream sediment concentrations. Generally, bank retreat involves gradual lateral erosion, caused by flow shear stress, and sudden bank collapse, triggered by complex interactions between channel flow and bank and soil water conditions. Collapse occurs when bank height exceeds the critical height where gravitational forces overcome soil shear strength. An experimental study examined conditions for collapse in eroding rill channels. Experiments with and without a deep water table were carried out on a meandering rill channel in a loamy sand and sandy loam in a laboratory flume under simulated rainfall and controlled runon. Different discharges were used to initiate knickpoint and rill incision. Soil water dynamics were monitored using microstandpipes, tensiometers and time domain reflectometer probes (TDR probes). Bank collapse occurred with newly developed or rising pre-existing water tables near rill banks, associated with knickpoint migration. Knickpoint scour increased effective bank height, caused positive pore water pressure in the bank toe and reduced negative pore pressures in the unsaturated zone to near zero. Matric tension in unsaturated parts of the bank and a surface seal on the ,interrill' zone behind the bank enhanced stability, while increased effective bank height and positive pore water pressure at the bank toe caused instability. With soil water contents >35 per cent (sandy loam) and >23 per cent (loamy sand), critical bank heights were 0·11,0·12 m and 0·06,0·07 m, respectively. Bank toe undercutting at the outside of the rill bends also triggered instability. Bank displacement was quite different on the two soils. On the loamy sand, the failed block slid to the channel bed, revealing only the upper half of the failure plane, while on the sandy loam the failed block toppled forwards, exposing the failure plane for the complete bank height. This study has shown that it is possible to predict location, frequency and magnitude of the rill bank collapse, providing a basis for incorporation into predictive models for hillslope soil loss or rill network development. Copyright © 2006 John Wiley & Sons, Ltd. [source] Near-lithostatic pore pressure at seismogenic depths: a thermoporoelastic modelGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2006Francesca Zencher SUMMARY A model is presented for pore pressure migration through a transition layer separating a meteoric aquifer at hydrostatic pressure from a deeper reservoir at lithostatic pressure. This configuration is thought to be pertinent to the South Iceland seismic zone (SISZ) and to other tectonically active regions of recent volcanism, where volatiles are continuously released by ascending magma below the brittle,ductile transition. Poroelastic parameters are computed for basaltic rock. The model is 1-D, the fluid viscosity is temperature dependent and rock permeability is assumed to be pressure dependent according to a dislocation model of a fractured medium. Environment conditions are considered, pertinent to basalt saturated with water at shallow depth (case I) and at mid-crustal depth (case II). If the intrinsic permeability of the rock is high, no significant effects are observed in the pressure field but advective heat transfer shifts the brittle,ductile transition to shallower depths. If the intrinsic permeability is low, the pressure-dependent permeability can propagate near-lithostatic pore pressures throughout most of the transition layer, while the temperature is practically unaffected by advective contributions so that the rock in the transition layer remains in brittle condition. Geometrical parameters characterizing the fracture distribution are important in determining the effective permeability: in particular, if an interconnected system of fractures develops within the transition layer, the effective permeability may increase by several orders of magnitude and near-lithostatic pore pressure propagates upwards. These modelling results have important bearings on our understanding of seismogenic processes in geothermal areas and are consistent with several geophysical observations in the SISZ, in connection with the two 2000 June M= 6.5 earthquakes, including: (i) fluid pressure pulses in deep wells, (ii) low resistivity at the base of the seismogenic layer, (iii) low VP/VS ratio and time-dependent seismic tomography, (iv) heterogeneity of focal mechanisms, (v) shear wave splitting, (vi) high b -value of deep foreshocks, (vii) triggered seismicity and (viii) Radon anomalies. [source] The seismic response to overpressure: a modelling study based on laboratory, well and seismic dataGEOPHYSICAL PROSPECTING, Issue 5 2001José M. Carcione We investigate the seismic detectability of an overpressured reservoir in the North Sea by computing synthetic seismograms for different pore-pressure conditions. The modelling procedure requires the construction of a geological model from seismic, well and laboratory data. Seismic inversion and AVO techniques are used to obtain the P-wave velocity with higher reliability than conventional velocity analysis. From laboratory experiments, we obtain the wave velocities of the reservoir units versus confining and pore pressures. Laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions. These measurements provide the basis for calibrating the pressure model. Overpressures are caused by different mechanisms. We do not consider processes such as gas generation and diagenesis, which imply changes in phase composition, but focus on the effects of pure pore-pressure variations. The results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep North Sea reservoirs. [source] A depth-integrated, coupled SPH model for flow-like landslides and related phenomenaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2009M. Pastor Abstract In the past decades, flow-like catastrophic landslides caused many victims and important economic damage around the world. It is therefore important to predict their path, velocity and depth in order to provide adequate mitigation and protection measures. This paper presents a model that incorporates coupling between pore pressures and the solid skeleton inside the avalanching mass. A depth-integrated, coupled, mathematical model is derived from the velocity,pressure version of the Biot,Zienkiewicz model, which is used in soil dynamics. The equations are complemented with simple rheological equations describing soil behaviour and are discretized using the SPH method. The accuracy of the model is assessed using a series of benchmarks, and then it is applied to back-analyse the propagation stage of some catastrophic flow-like slope movements for which field data are available. Copyright © 2008 John Wiley & Sons, Ltd. [source] Analytical study of mine closure behaviour in a poro-visco-elastic mediumINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2008H. Wong Abstract This paper is interested in the hydro-mechanical behaviour of an underground cavity abandoned at the end of its service life. It is an extension of a previous study that accounted for a poro-elastic behaviour of the rock mass (Int. J. Comput. Geomech. 2007; DOI: 10.1016/j.compgeo.2007.11.003). Deterioration of the lining support with time leads to the transfer of the loading from the exterior massif to the interior backfill. The in situ material has a poro-visco-elastic constitutive behaviour while the backfill is poro-elastic, both saturated with water. This loading transfer is accompanied by an inward cavity convergence, thereby compressing the backfill, and induces an outward water flow. This leads to a complex space,time evolution of pore pressures, displacements and stresses, which is not always intuitive. In its general setting, a semi-explicit solution to this problem is developed, using Laplace transform, the inversion being performed numerically. Analytical inversion leading to a quasi-explicit solution in the time domain is possible by identifying the characteristic creep and relaxation times of volumetric strains with those of the deviatoric strains, on the basis of a parametric study. A few numerical examples are given to illustrate the hydro-mechanical behaviour of the cavity and highlight the influence of key parameters (e.g. stiffness of backfill, lining deterioration rate, etc.). Further studies accounting for more general material behaviours for the backfill and external ground are ongoing. Copyright © 2008 John Wiley & Sons, Ltd. [source] A numerical procedure for predicting rainfall-induced movements of active landslides along pre-existing slip surfacesINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 4 2008Michele Calvello Abstract A numerical model to predict landslide movements along pre-existing slip surfaces from rainfall data is presented. The model comprises: a transient seepage finite-element analysis to compute the variations of pore water pressures due to rainfall; a limit equilibrium stability analysis to compute the factors of safety along the slip surface associated with transient pore pressure conditions; an empirical relationship between the factor of safety and the rate of displacement of the slide along the slip surface; an optimization algorithm for the calibration of analyses and relationships based on available monitoring data. The model is validated with reference to a well-monitored active slide in central Italy, characterized by very slow movements occurring within a narrow band of weathered bedrock overlaid by a clayey silt colluvial cover. The model is conveniently divided and presented in two parts: a groundwater model and a kinematic model. In the first part, monthly recorded rainfall data are used as time-dependent flow boundary conditions of the transient seepage analysis, while piezometric levels are used to calibrate the analysis by minimizing the errors between monitoring data and computed pore pressures. In the second part, measured inclinometric movements are used to calibrate the empirical relationship between the rate of displacement along the slip surface and the factor of safety, whose variation with time is computed by a time-dependent stability analysis. Copyright © 2007 John Wiley & Sons, Ltd. [source] | |||||||||||||