Variation Of Effective Stress Research Articles

Linear Elastic Analysis of Bovine Cortical Bone under Compression Loading

Linear elastic response of the bovine cortical bone has been examined under compression load. Experimental and computational methods were used to observe and predict the response of cortical bone. In computational method, two mechanical behaviors of isotropic and orthotropic were considered to simulate the cortical bone deformation. In experimental process, the specimens were designed to show maximum stiffness and strength by specifying osteon direction along loading axis during tests. The tests were controlled by displacement rate of 0.5 mm/minute and the overall stiffness responses of the structures were recorded to extract mechanical properties and also for validation aims. Finite Element Method (FEM) was used to model the linear response of the structure by using ABAQUS6.9EF. The FE results using orthotropic definition shows a good correlation with experimental data. A discussion was given based on overall stiffness and effective stress variation for both mechanical behaviors. In order to design the optimal implant structure, the presented study was proposed for prediction of bone structure deformation that attached to the orthopedic implants.

Influence of state and compressibility on liquefied strength of sands

Critical-state soil mechanics is a useful framework to understand sand behavior. In this paper, a relationship is developed for estimating undrained critical shear strength of sands based on the critical-state framework. The application of this relationship is demonstrated by comparison with laboratory test results and sand liquefied strength from field liquefaction flow failure case histories. Using this relationship, the effects of effective stress variation and density on undrained critical shear strength are studied for different combinations of critical-state line parameters corresponding to several reference sands. The parametric study indicates that depending on sand void ratio, undrained critical shear strength may increase, remain the same or decrease as sand shearing–compressibility (represented by the slope of the critical-state line) increases. The underlying mechanisms of field failures in dense sands and reverse behavior of compressible sands are explained through this relationship. It is suggested that the critical-state parameter alone is insufficient for describing the behavior of liquefiable sands and sand shearing–compressibility should be also taken into account for estimating undrained shear strength corresponding to the changes in density and effective confining stress.

파압에 의한 해안구조물-해저지반의 침하거동에 대한 수치해석

Seabed settlement underneath a coastal structure may occur due to wave loading generated by storm surge. If the foundation seabed consists of sandy soil, the possibility of the seabed settlement may be more susceptible because of generation of residual excess pore-water pressure and cyclic mobility. However, most coastal structures, such as breakwater, quay wall, etc., are designed by considering wave load assumed to be static condition as an uniform load and the wave load only acts on the structure. In real conditions, however, the wave load is dynamically applied to seabed as well as the coastal structure. In this study, therefore, a real-time wave load is considered and which is assumed acting on both the structure and seabed. Based on a numerical analysis, it was found that there exists a significant effect of wave load on the structure and seabed. The deformation behavior of the seabed according to time was simulated, and other related factors such as the variation of effective stress and the change of effective stress path in the seabed were clearly observed.

Regional Flow and Deformation Analysis of Basin‐Fill Aquifer Systems Using Stress‐Dependent Parameters

Changes in effective stress due to water pressure variations modify the intrinsic hydrodynamic properties of aquifers and aquitards. Overexploited groundwater systems, such as basins with heavy pumping, are subject to nonrecoverable modifications. This results in loss of permeability, porosity, and specific storage due to system consolidation. This paper presents (1) the analytical development of model functions relating effective stress to hydrodynamic parameters for aquifers and aquitards constituted of unconsolidated granular sediments, and (2) a modeling approach for the analysis of aquifer systems affected by effective stress variations, taking into account the aforementioned dependency. The stress-dependent functions were fit to laboratory data, and used in the suggested modeling approach. Based on only few unknowns, this approach is computationally simple, efficiently captures the hydromechanical processes that are active in regional aquifer systems under stress, and readily provides an estimate of their consolidation.

Coupling Geomechanics and Transport in Naturally Fractured Reservoirs

Large amounts of hydrocarbon reserves are trapped in naturally fractured reservoirs which arechallenging in terms of accurate recovery prediction because of their joint fabric complexity andlithological heterogeneity. Canada, for example, has over 400 billion barrels of crude oil in fracturedcarbonates in Alberta, most of this being bitumen of viscosity greater than 106 cP in the GrosmontFormation, which has an average porosity of about 13-15%. Thermal methods are the most commonexploitation approaches in such viscous oil reservoirs which, in the case of steam injection, are associatedwith up to 275-300°C temperature changes, leading to considerable thermoelastic expansion. Thistemperature change, combined with pore pressure changes from injection and production processes, leadsto massive effective stress variations in the reservoir and surrounding rocks. The thermally-induced(thermoelastic) stress changes can easily be an order of magnitude greater than the pore pressure effectsbecause of the high intrinsic stiffness of the low porosity limestone and bounding strata. Study of thesestress-pressure-temperature effects requires a thermo-hydro-mechanical (THM) coupling approach whichconsiders the simultaneous variation of effective stress, pore pressure, and temperature and theirinteractions. For example, thermal expansion can lead to significant joint dilation, increasing themacroscopic, joint-dominated transmissivity by an order of magnitude in front of and normal to thethermal front, while reducing it in the direction tangential to the heating front. This leads to stronginduced anisotropy of transport processes, which in turn affects the spatial distribution of the heatingarising from advective heat transfer.

Finite element investigation of the poroelastic effect on the Xinfengjiang Reservoir-triggered earthquake

Coulomb failure stress changes (ΔCFS) are used in the study of reservoir-induced seismicity (RIS) generation. The threshold value of ΔCFS that can trigger earthquakes is an important issue that deserves thorough research. The Ms6.1 earthquake in the Xinfengjiang Reservoir in 1962 is well acknowledged as the largest reservoir-induced earthquake in China. Therefore, it is a logical site for quantitative calculation of ΔCFS induced by the filling of the reservoir and for investigating the magnitude of ΔCFS that can trigger reservoir seismic activities. To better understand the RIS mechanism, a three-dimensional poroelastic finite element model of the Xinfengjiang Reservoir is proposed here, taking into consideration of the precise topography and dynamic water level. We calculate the instant changes of stress and pore pressure induced by water load, and the time variation of effective stresses due to pore water diffusion. The ΔCFS on the seismogenesis faults and the accumulation of strain energy in the reservoir region are also calculated. Primary results suggest that the reservoir impoundment increases both pore pressure and ΔCFS on the fault at the focal depth. The diffusion of pore pressure was likely the main factor that triggered the main earthquake, whereas the elastic stress owing to water load was relatively small. The magnitude of ΔCFS on seismogenesis fault can reach approximately 10 kPa, and the ΔCFS values at the hypocenter can be about 0.7–3.0 kPa, depending on the fault diffusion coefficient. The calculated maximum vertical subsidence caused by the water load in the Xinfengjiang Reservoir is 17.5 mm, which is in good agreement with the observed value of 15 mm. The accumulated strain energy owing to water load was only about 7.3×1011 J, even less than 1% of the seismic wave energy released by the earthquake. The reservoir impoundment was the only factor that triggered the earthquake.

Direct volume variation measurements in fused silica specimens exposed to femtosecond laser

We introduce a new method to investigate localized volume variations resulting from laser exposure. Our method is based on the measurement of fused silica cantilevers deflection from which we calculate the effective stress and density variation in laser-affected zones. Specifically, we investigate density variations in fused silica exposed to femtosecond laser exposure in the regime where nanogratings are found. We demonstrate that a volume expansion is taking place in that particular regime.

Thermo-hydro-mechanical behaviour of tuffeau stone masonry

This paper deals with thermo-hydromechanical behaviour of a stone masonry wall assembled with mortar. Macroscopic modelling is proposed that allows an evaluation of the impact of everyday thermal and hydraulic variations on the generation of variations in effective stress. The thermo-hydromechanic parameters of the unsaturated stone have been estimated by experimental tests for tuffeau, such as that found in the Chambord monument. An inverse identification has been used for the optimisation of these parameters based on in- situ measurements, before using the model for long-term analysis. The results of this numerical study show that the contrast between the properties alone is not sufficient to generate a stress that is superior to the stone strength. Otherwise, the variations in thermal and hydraulic conditions on the surface of the stone could lead to a fatigue-like degradation of historic monuments. Cet article s’intéresse au comportement thermo-hydromécanique d’un mur maçonné en pierre liée avec un mortier. Nous proposons une modélisation macroscopique qui permet d’évaluer l’impact de la variation quotidienne hydromécanique à la génération des variations des contraintes effectives au sein de la pierre. Les paramètres utilisés dans le modèle sont ceux identifiés au cours d’essais de laboratoire sur des échantillons de pierres similaires à ceux du château de Chambord. Une identification inverse a été utilisée pour les paramètres inconnus basée sur les mesures in situ. Les résultats de nos modélisations montrent que seul le contraste des propriétés ne conduit pas à des valeurs de contraintes suffisantes pour générer la rupture de la pierre. En revanche les variations cycliques d’hygrométrie et de température conduisent à des variations cycliques de la contrainte effective et la contrainte totale susceptibles d’endommager la pierre par fatigue.

Large-scale aseismic motion identified through 4-D P-wave tomography

In 2000, a large water injection (over 23 000 m3) has been conducted in granite through a 5-km-deep borehole at Soultz-sous–Forêts, in the Upper Rhine Graben (northeastern France). The microseismicity induced by this hydraulic stimulation was monitored with a network of 14 seismic stations deployed at ground surface. Some 7215 well-located events have been used to conduct a 4-D tomography of P-wave velocities. The method combines a double-difference tomography method with an averaging post-processing that corrects for parameter dependence effects. The total set of 7 215 events has been divided into 14 subsets that explore periods defined with respect to the injection scheme. Particular attention is given to changes in injected flow rates, periods of stationary injection conditions and post-injection periods. Fast changes in VP velocities are identified in large rock mass volumes precisely when the injection flow rate varies while little velocity variation is detected during stationary injection periods. The VP anomalies observed during stationary injection conditions are interpreted as being caused by effective stress variations linked to fluid diffusion, while the fast changes observed concomitantly to changes in flow rate are considered to be caused by non-seismic motions.

Constitutive Relationship of TA15 Alloy and its Application in the Hammer Forging Process Simulation

A Kumar-type constitutive relationship of TA15 alloy was investigated by non-linear regression analysis, and the equations were established based on test data from hot process simulator. Applying this constitutive equation in commercial FEM software of SFTC/Deform, the hammer forging process of TA15 alloy beam was simulated, meanwhile the final shape of forging piece, the hammering times, the height reduction variation and the maximal effective stress variation along with forging time were predicted. The hammer forging test was carried out and the test results showed that the simulation based on Kumar-type constitutive relationship meet the practical need to a great extent.

Transient effective stress variations forced by changes in conduit pressure beneath glaciers and ice sheets

AbstractThe resistance to sliding beneath soft-bedded glaciers depends on the effective stress, N, which is controlled by the subglacial hydrological system. While large-scale water transport is primarily through conduits, the effective stress profile beneath most of the glacier base is expected to be controlled by seepage flows through the permeable sediments. Models of the response to forced changes of the near-conduit effective stress demonstrate that perturbations in N decay over a characteristic lateral distance that is typically only a few times the sediment thickness. This implies that only relatively small changes to the spatially averaged effective stress, can be produced if conduit spacing is comparatively large and the glacier and sediments remain in contact. To produce larger changes to , it is likely that flotation must be achieved beneath a significant portion of the glacier base. At higher values, spatial variations in N can produce gradients in the thickness, h, of a fringe of ice-infiltrated sediments immediately adjacent to the glacier base; this has implications for the development of glacial landforms when sliding causes sediment transport.

Soil radon concentration and effective stress variation at Mt. Etna (Sicily) in the period January 2003–April 2005

In this study we discuss the relationship between the in-soil radon concentration and the effective stress variation at Mt. Etna volcano in the period January 2003–April 2005. The acquired radon concentration trend was compared with the one obtained by the b- value variation analysis in the frequency–magnitude relationship for earthquakes that occurred at Mt. Etna volcano (Sicily) during the investigation period. The b- value calculated using both the maximum likelihood and the least square methods shows a similar trend and, in particular, an increase starting a few months before the 7th September 2004 eruption and a decrease during the following 9 months. A similar trend was recorded during 2003 when a recharging magma phase of the volcano occurred. Results obtained by studying the b- value trend and the comparison with in-soil radon concentration are also discussed.

Spontaneous and triggered aseismic deformation transients in a subduction fault model

Aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault‐sliding experiments. When that constitutive formulation is applied to model subduction earthquake sequences, transients can arise spontaneously for certain effective stress () variations with depth. We show that if interstitial fluids are present and pore pressure is near‐lithostatic around and downdip from the frictional stability transition, transients with recurrence intervals of ∼1 year are predicted on the basis of laboratory friction parameters and their temperature (hence depth) variations. The recurrence interval decreases with and reaches 14 months when is ∼2–3 MPa. Dimensional analysis and numerical studies show that the fault response primarily depends on a parameter W/h*. Here the high pore pressure zone extends distance W updip from the stability transition, and h* is the stable patch size for steady sliding. Evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed. Transient sequences can also be triggered by a modest, one‐time, step‐like interseismic stress perturbation on the subduction fault, due to nearby earthquakes, or to pore pressure changes, e.g., during episodes of metamorphic fluid release. Properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced, its relative location along the subduction fault, and its magnitude.

Effect and mechanism of stresses on rock permeability at different scales

The effect of geo-stress fields on macroscopic hydro-geological conditions or microcosmic permeability of water-bearing media should follow some laws or principles. Cases study and tests show that: (1) At macro-geologic large scale, deformed and crashed rocks which were induced by geo-stress fields changing provided space for groundwater storage and flow. Groundwater adjusts water-bearing space and dilatants fractures by flowing and press transferring. Coupling of liquid and solid can be implemented for rocks and groundwater. Although tectonic fields witness several times of change and build-up in geological time, stress fields forming regional tectonic framework are coherent with seepage fields, orientation of the maximum horizontal stress demonstrates main seepage directions. (2) At macro-geologic middle scale, zones of stresses changing sharply, quite low stresses, stress or shear concentration can be used to show locations and types of main fractures, zones of geo-stresses changing equably can be acted as normal base media zones of tri-porosity media. (3) At micro-geologic small scale, tri-porosity media include fractured rocks, porous rocks and capillary rocks. Investigations indicate that porosity or permeability is functions of effective stresses, and porosity or permeability changing rules of porous rocks with variation of effective stresses can be described as the index model, the model of power exponent functions is suitable for those of fractured rocks, the model of the second power parabola for capillary rocks. The porosity and permeability loss in fractured rocks, which are greater than that in porous rocks, are shown by calculation of effective compressive coefficient and closing pressure in cracks. The calculations can also explain the mechanism why porosity changes are always larger than permeability changes. It is proved by the thick wall cylinder theory that the second power parabola relation between porosity or permeability loss and effective stresses for capillary rocks is correct.

Simulation of heat extraction from crystalline rocks: The influence of coupled processes on differential reservoir cooling

Processes operating during the extraction of heat from fractured rocks influence dynamically their fluid flow and heat transport characteristics. The incorporation of pressure- and temperature-dependent rock parameters, coupled with geomechanical deformation, is particularly important for predictive modelling of geothermal reservoirs hosted in crystalline rock masses. Changes in flow and transport parameters of fractures caused by variations in local effective stress are computed using an experimentally validated geomechanical model [McDermott, C.I., Kolditz, O., 2006. Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads. Hydrogeol. J. 14, 487–498]. Local effective stress changes are linked to alterations in reservoir fluid pressures, and to in situ stress conditions, including the build-up of thermal stresses resulting from the cooling of the rock mass. These processes are simulated using a finite-element model in order to study the behaviour of the Spa Urach (southwestern Germany) potential geothermal reservoir. The model couples mechanical deformation and alteration of fracture parameters with pressure-, temperature- and salinity-dependent fluid parameter functions. The effects of potential reservoir damage on reservoir productivity are investigated to help identify optimal heat recovery schemes for the long-term economical exploitation of geothermal systems. Simulation results indicate that preferential fluid flow paths and shortcuts may develop, depending on the mechanical and thermal stress releases that occur during intense exploitation of these systems.

Experimental study of seismic cyclic loading effects on small strain shear modulus of saturated sands

The seismic loading on saturated soil deposits induces a decrease in effective stress and a rearrangement of the soil-particle structure, which may both lead to a degradation in undrained stiffness and strength of soils. Only the effective stress influence on small strain shear modulus Gmax is considered in seismic response analysis nowadays, and the cyclic shearing induced fabric changes of the soil-particle structure are neglected. In this paper, undrained cyclic triaxial tests were conducted on saturated sands with the shear wave velocity measured by bender element, to study the influences of seismic loading on Gmax. And Gmax of samples without cyclic loading effects was also investigated for comparison. The test results indicated that Gmax under cyclic loading effects is lower than that without such effects at the same effective stress, and also well correlated with the effective stress variation. Hence it is necessary to reinvestigate the determination of Gmax in seismic response analysis carefully to predict the ground responses during earthquake more reasonably.

The effects of drained and undrained loading on visco-elastic waves in rock-like composites

We outline a micromechanical approach to the acousto-elastic effect in rock-like composites characterized by multiple solid constituents and non-dilute concentrations of interconnected pores and cracks. Estimates of the T-matrix type (known from the theory of stochastic waves) are first transformed from the stiffness to the compliance domain, so that the (small) strain variation within a single (but interacting) inclusion can be related to the (small) applied effective stress (rather than strain) variation, via a ‘non-dilute’ K-tensor that depends upon the overall properties as well as the (arbitrary) homogeneous reference material and inclusion (particle, cavity) shape/orientation. In order to deal with large changes in the applied effective stress under dry conditions, one generally has to integrate a system of ordinary differential equations (ODE’s) for the evolution of the microstructural variables (crack densities, porosities, mineral concentrations, aspect ratios of inclusions and correlation functions) under loading. Under undrained conditions, the (total fluid mass within a representative volume element is conserved) solution to the single cavity deformation problem can be found from the same system of ODE’s (and initial conditions) as in the dry case, provided that one replaces dry with saturated (effective compliances) K-tensors depending on second-rank tensors of pore pressure build-up coefficients that can be found from the boundary conditions, in combination with a higher-order expression for the change in porosity (for each cavity type) and the constitutive relation for the fluid. Under drained conditions, the (fluid pressure is constant) dry system of ODE’s can still be used, provided that one replaces (dry with saturated effective compliances) effective with aparent stress variations that depend on the boundary conditions as well as (small) changes in the dry responses during loading. This use of a fluid inclusion-dependent aparent stress in the dry evolution law is possible since the integrated results are independent of the loading-path, in the absence of hysteresis. An interacting cavity model of wave-induced fluid flow (which is consistent with the quasi-static considerations outlined above, as well as the Brown–Korringa relations) will finally be presented, and used to estimate the effects of undrained and drained hydrostatic loading on the velocity and attenuation spectra of an isotropic reservoir (example), involving (highly compliant) grain-boundary cracks as well as relatively flat clay-related pores, and more rounded quartz-related pores.

Contribution of natural soil compaction on hardsetting behavior

Many soils in tropical countries present genetically inherited compacted layers, which are commonly referred to as hardsetting. The occurrence of hardsetting is attributable to certain soil physical, chemical and mineralogical properties. We hypothesized that natural soil compaction contributes significantly to the occurrence of a hardsetting behavior in soils from Brazil. The aim of the study was to determine the effect of natural soil compaction on the occurrence of hardsetting horizons of a clayey Yellow Cohesive Latosol. This effect was analyzed: (a) in thin sections and microclods, by determining micropedological features, (b) in soil clods, by measuring tensile strength, and (c) in undisturbed samples, by determining the relationship between penetration resistance and effective stress. The analysis of thin sections and microclods revealed a denser particle packing for the hardsetting horizon. Tensile strength of the clods was significantly higher for the hardsetting horizon, probably due to higher packing of particles. For the variation in effective stress ( σ′), the increment in penetration resistance (SS) was higher for the hardsetting horizon, which also had a higher bulk density ( D b). The natural soil compaction was the main factor that, directly or indirectly, influenced the physical and morphological properties analyzed and was responsible for the differences observed between non-hardsetting and hardsetting horizons, confirming the tested hypothesis.

Determining Hydraulic Conductivity of Soil-Bentonite Using the API Filter Press

AbstractThe American Petroleum Institute (API) filter press is commonly used to measure the hydraulic conductivity of soil-bentonite, both during mix design and as a part of construction quality assurance and quality control. However, interpretation of the test results is complicated by the fact that, during the test, the soil-bentonite specimen is consolidated by seepage forces, which produce a variation in effective consolidation stress from the top of the specimen to the bottom. This paper presents the results of a laboratory investigation undertaken to illustrate that seepage consolidation theory provides a logical way to interpret filter press test results. A description of seepage consolidation theory, which relates stress, compressibility, and hydraulic conductivity in soil consolidated by seepage forces, is presented. Following this is a description of the laboratory testing program that involved comparing the hydraulic conductivity of two soil-bentonite mixes, as measured in rigid wall consolidometer permeameter cells, a flexible wall permeameter, and an API filter press. Using seepage consolidation theory to interpret the filter press tests produces hydraulic conductivities consistent with those obtained from the other tests. The filter press test results are used to illustrate a method for estimating the hydraulic conductivity of soil-bentonite in a cutoff wall by taking backfill consolidation pressures into account.

Compressional‐ and shear‐wave velocities as a function of confining stress in dry sandstones

A laboratory study was carried out to investigate the influence of confining stress on compressional‐ and shear‐wave velocities for a set of rock samples from gas‐producing sandstone reservoirs in the Cooper Basin, South Australia. The suite of samples consists of 22 consolidated sublitharenites with helium porosity ranging from 2.6% to 16.6%. We used a pulse‐echo technique to measure compressional‐ and shear‐wave velocities on dry samples (cylindrical 4.6 × 2 cm) at room temperature and at elevated confining stress (≤ 60 MPa). Compressional‐ and shear‐wave velocities in samples increase non‐linearly with confining stress. A regression equation of the form V = A − Be−DP gives a good fit to the measured velocities with improved prediction of velocities at high confining stresses compared with equations suggested by other studies. The predicted microcrack‐closure stresses of the samples show values ranging from 70 MPa to 95 MPa and insignificant correlation with porosity, permeability or clay content. There is a positive correlation between change in velocity with core porosity and permeability, but this association is weak and diminishes with increasing confining stress. Experimental results show that pore geometry, grain‐contact type, and distribution and location of clay particles may be more significant than total porosity and clay content in describing the stress sensitivity of sandstones at in situ reservoir effective stress. The stress dependence of Cooper Basin sandstones is very large compared with data from other studies. The implication of our study for hydrocarbon exploration is that where the in situ reservoir effective stress is much less than the microcrack‐closure stress of the reservoir rocks, the variation of reservoir effective stress could cause significant changes in velocity of the reservoir rocks. The velocity changes induced by effective stress in highly stress‐sensitive rocks can be detected at sonic‐log and probably surface‐seismic frequencies.