Variation Of Effective Stress Research Articles

Forecasting the long-term deterioration of a cut slope in high-plasticity clay using a numerical model

This paper details development of a numerical modelling approach that has been employed to forecast the long-term performance of a cut slope formed in high plasticity clay. It links hydrological and mechanical behaviour in a coupled saturated and unsaturated model. This is used to investigate the influence of combined dissipation of excavation-generated excess pore water pressures and seasonal weather-driven near-surface cyclic pore water pressures. Deterioration of slope performance is defined in terms of both slope deformations (i.e. service) and factor of safety against shear failure (i.e. safety). Uniquely, the modelling approach has been validated using 16 years of measured pore water pressure data from multiple locations in a London Clay cut slope. Slope deterioration was shown to be a function of both construction-induced pore water pressure dissipation and seasonal weather-driven pore water pressure cycles. These lead to both transient and permanent changes in factor of safety due to effective stress variation and mobilisation of post-peak strength reduction over time, respectively, ultimately causing shallow first-time progressive failure. It is demonstrated that this long-term (90 year) deterioration in slope performance is governed by the hydrological processes in the weathered near surface soil zone that forms following slope excavation.

Experimental Research on Dynamic Variation of Permeability and Porosity of Low-Rank Inert-Rich Coal Under Stresses.

The effective stress variation can cause permeability change during coalbed methane (CBM) production. At the same time, macerals have an important influence on the stress sensitivity of coal reservoirs. To investigate the low-rank inert-rich coal permeability dynamic response to effective stress during the production of CBM, eight low-rank coal samples were collected from the Huanglong coalfield, China, and the dynamic changes in helium permeability and porosity were tested under conditions in which the effective stress changed from 0, 15, to 0 MPa. Then, the permeability dynamic change, stress sensitivity control mechanism, and their influence on the development of low-rank CBM are discussed. The results show that with the increase in effective stress, the permeability and porosity of coal samples decrease in the form of negative exponents. During the process of effective stress increase, the permeability variation of low-rank coal includes three stages: rapid loss stage (0–3 MPa), slow loss stage (3–9 MPa), and stable loss stage (>9 MPa). In the process of effective stress reduction, the irreversible damage rate linearly increases. Fracture permeability is easier to recover than pore permeability during pressure relief. Coal macerals have an important influence on the initial permeability and stress sensitivity of low-rank coal. The ratio of vitrinite to inertinite of low-rank coal is positively correlated with initial permeability, but negatively correlated with the permeability loss rate and the fracture compression coefficient (Cf). Cf shows a decreasing trend of the negative index with an increase in effective stress and is highly positively correlated with the average permeability loss rate. The porosity–permeability index model and the permeability–effective stress-sensitive model of low-rank coal under effective stress are established. Research shows that inertinite is helpful to improve the stress sensitivity of low-rank coal. Accordingly, to reduce the damage of coal reservoir permeability, the pressure should steadily change either in the process of fracturing or drainage.

The Effect of Mineral Dissolution on the Effective Stress Law for Permeability in a Tight Sandstone

AbstractWe present flow‐through experiments to delineate the processes involved in permeability changes driven by effective stress variations and mineral cement dissolution in porous rocks. CO2‐enriched brine is injected continuously into a tight sandstone under in situ reservoir conditions for 455 hrs. Due to the dolomite cement dissolution, the bulk permeability of the sandstone specimen significantly increases, and two dissolution passages are identified near the fluid inlet by X‐ray CT imaging. Prereaction and postreaction examinations of the effective stress law for permeability suggest that after reaction, the bulk permeability is more sensitive to pore pressure changes and less sensitive to effective stress changes. These observations are corroborated by Scanning Electron Microscopy and X‐ray CT observations. This study deepens our understanding of the effect of mineral dissolution on the effective stress law for permeability, with implications for characterizing subsurface mass and energy transport, particularly during fluid injection/production into/from geologic reservoirs.

Thermal Behavior of a Rod during Hot Shape Rolling and Its Comparison with a Plate during Flat Rolling

The thermal behavior of a rod during the hot shape rolling process was investigated using the off-line hot rolling simulator and numerical simulation. Additionally, it was compared with a plate during the flat rolling process to understand the thermal behavior of the rod during the hot rolling process in more detail. The temperature of the rod and plate during the hot rolling process was measured at several points with thermocouples using the rolling simulator, and then the measured temperature of each region of a workpiece was analyzed with numerical simulation. During hot rolling process, the temperature distribution of the rod was very different from the plate. The temperature deviation of the rod with area was much higher than that of the plate. The variation in effective stress of the rod along the circumferential direction can induce the temperature difference with area of the rod, whereas the plate had a relatively lower temperature deviation with area due to the uniform effective stress on the surface area. The heat generation by plastic deformation during the forming process also increased the temperature deviation of the rod with area, whereas strain distribution of the plate during flat rolling contributed to the uniformity of temperature of the plate with area. The higher temperature deviation of the rod along the circumferential and radial directions during the shape rolling process can increase the possibility of occurrence in surface defects compared to the plate during flat rolling.

Proučavanje rotirajućeg diska od funkcionalno gradiranog materijala sa metalnom matricom bazirano na prenosnom odnosu napona i deformacije primenom helpin-cajovog i MROM modela za analizu granice elastičnosti

Limit elastic analysis of a functionally graded (FG) rotating disk with material grading based on modified rule of mixture (MROM) is reported in the present study. In MROM, stress to strain transfer ratio (q) is an unknown parameter which restricts the application of MROM as yield strength estimation of a material depends on this ratio. Till now the determination of stress to strain transfer ratio, which varies with size, shape, manufacturing processes and composition, is reported by means of experiments. In current work, the effective Young's modulus is calculated by two means i.e. MROM and Halpin-Tsai. Later stress to strain transfer ratio is quantified using MROM using inverse approach and then effective yield stress variation of FGM has been calculated. Different combinations of metal matrix ceramic reinforced FG material were selected. Metals having high strength to weight ratio were combined with non-oxide ceramics of low density. Variational formulation method has been employed to solve the elasticity problem of the rotating disk taking radial displacement field as unknown variable and best material combination is proposed on the basis of maximum limit elastic speed obtained.

SPH/FEM modeling for laser-assisted machining of fused silica

Laser-assisted machining (LAM) is a promising technology for the machining of difficult-to-cut materials with reduced cutting forces, improved surface finishing, and enhanced tool life. Smoothed particle hydrodynamics (SPH) method is used to study the cutting mechanism during the laser-assisted machining of fused silica. In the present study, a 3-D transient thermal model is initially proposed to predict the temperature field distribution in the cutting zone of the fused silica. A Fortran subroutine is developed to simulate the moving laser with the Gaussian distribution. Both simulation and experiment results indicate that the thermal model produces an accurate prediction of the temperature in the workpiece. Then, temperature distributions are taken as the initial condition to establish the thermo-mechanical coupled cutting model. The dynamic cutting process is visualized in the cutting model. Moreover, discontinuous and continuous chips are found in both simulation and experiment. It implies that the cutting mechanism works under a hybrid mode of brittle fracture and plastic deformation in the LAM process. The variation of effective stress and plastic strain with temperature is analyzed in the cutting process. It is found that the application of the laser heating remarkably reduces fluctuations of the cutting force. Then, simulated cutting forces are evaluated experimentally. Comparisons show that simulated forces have excellent agreement with those from the experiment. It is concluded that localized high temperature originating from the laser significantly improves the machined surface quality.

Dynamic response characteristics of cylindrical baffled liquid storage tank to the baffle number

The goal of current study is to numerically investigate the dynamic response characteristics of cylindrical baffled liquid storage tank, which is subjected to a vertical acceleration at boosting, with respect to the number of baffles. Both the storage tank and baffles are modeled as flexible elastic structures, and the suppression of sloshing-induced dynamic responses by baffles are evaluated in terms of the hydrodynamic pressure, the dynamic displacement and stress. Through the numerical experiments, it has been observed that the introduction of baffles to the partially filled liquid tank remarkably decreases the maximum values of displacement and effective stress at both the bottom plate and baffles. And, the variation of displacement and effective stress is stabilized much faster when baffles are installed and the baffle number increases. Thus, it has been verified that the baffled liquid storage tank can be stably and safely applied to the liquid fuel storage tank for various transport vehicles.

Characterization of laboratory-scale hydraulic fracturing for EGS

To investigate the impact of reservoir stimulation by hydraulic fracturing, a series of advanced lab-scale block tests were conducted with multiple measurement methods to characterize the fracture and to assess the operation and the performance of an enhanced or engineered geothermal system (EGS). The stimulation test samples were 330-mm cubic blocks of Sierra White granite with one injection well and four producers drilled into it. Acoustic emissions generated during the fracture initiation and propagation were monitored with twenty AE sensors mounted on all sides of the block to help characterize the fracture initiation and propagation process. The recorded AE signals were analyzed to obtain the event location and the failure mechanism. The localized AE events shows that due to the low viscosity of the fracturing fluid (water), the fracture propagated rapidly and took only about 1.4 s to reach near the block surface. During the fracturing phase, about 47 percent of the events were determined to be shear/mixed mode failure events, 28 percent tensile failure events, and 25 percent compressional failure events. Just after the simulation, a step-rate (constant flowrate) injection test was carried out to investigate the reservoir injectivity enhancement. During this phase additional compressional failure events were observed and the percentages of tensile failure and shear/mixed mode failure decreased. The dramatic increase of the injection rate with small increments of the injection pressure demonstrates the nonlinear response of the fracture to effective normal stress variation with much higher fracture opening increase during the low effective normal stress interval. Hydraulic fracturing tests under different stress levels show that more AE hits/events are recorded when the rock block is under higher applied stress. The reconstructed 3D fracture geometry shows good agreement with the AE monitoring results. After fracturing, a tracer test was carried out and the fracture aperture was estimated to be around 139 μm. The tracer recovery curve shows two linear portions of the tracer tail on a semi-log plot similar to that observed in some field data and predicted by numerical modeling. Laser scanning of the fracture surfaces indicates that the induced fracture has a rough surface with a joint roughness coefficient (JRC) of 12.98 and tortuosity of 1.10, both of which are slightly larger than that from the Brazilian test fracture on the same rock. The thin section and SEM observations show that the fracture follows the weak boundaries between the Quartz and Albite grains or the weak beddings in the grains.

Premelting increases the rate of regelation by an order of magnitude

ABSTRACTGlacier sliding over small obstacles relies on melting on their upstream sides and refreezing downstream. Previous treatments have appealed to ‘pressure melting’ as the cause of the spatial variations in melting temperature that drive thisregelationprocess. However, we show that typical liquid pressure variations across small obstacles are negligible and therefore variations in ice pressure closely approximate variations in effective stress. For a given change in effective stress, the equilibrium melting temperature changes by an order of magnitude more than when the pressure of ice and liquid both change by an equal amount. In consequence, the temperature gradients that drive heat flow across small obstacles are larger than previously recognized and the rate of regelation is faster. Under typical conditions, the transition wavelength at which ice deformation and regelation contribute equally is of m-scale, several times longer than previous predictions, which have been reported to underestimate field inferences.

A semi-empirical relationship for the small-strain shear modulus of soft clays

The small-strain shear modulus (Gmax) is a soil property that has many practical applications. The authors compiled a database of Gmaxmeasurements for 40 normally consolidated to slightly overconsolidated low to high plasticity clays. Using these data, the authors propose a semi-empirical relationship between Gmax, effective stress (σ'v or σ'c), preconsolidation stress (σ'p) and in-situ void ratio (e0) for four ranges of plasticity index (Ip): Ip < 30%, 30% ≤ Ip < 50%, 50% ≤ Ip < 80% and 80% ≤ Ip < 120%. With results from bender element tests on a Gulf of Mexico clay subjected to multiple load-unload consolidation loops, the authors were able to validate the proposed relationships for 30% ≤ Ip < 50% and 50% ≤ Ip < 80%. The proposed relationship for 30% ≤ Ip < 50% and 50% ≤ Ip < 80% captures changes in laboratory Gmaxresulting from variations in effective stress (σ'c), maximum past stress (σ'v,max), and void ratio. The proposed relationships are a simple and efficient tool that can provide independent insight on Gmaxif the stress history of a clay is known, or on stress history if Gmaxis known.

Limit angular speed of L-FGM rotating disk for both temperature dependent and temperature independent mechanical properties

Abstract The necessity of weight saving, and high strength at elevated temperatures in a rotating disk motivates researchers to study FGM, to overcome such restrains effectively metal-ceramic graded disk is found superior among other classifications. The present study was carried out for uniform thickness FG rotating disk by employing variational formulation method considering radial displacement field as unknown variable. Temperature variation was considered along radius, Halpin–Tsai method was used to calculate effective Young’s modulus for two cases namely; temperature dependent (TD) and temperature independent (TID). Effective yield stress variation was calculated to obtain accurate limit angular speed for the above mentioned cases. Yield stress variation was plotted along with other stresses to identify yield location. From results it was found that for L-FGM disk, limit speed decreases as aspect ratio increase, a comparison of limit speed with TD and TID properties is also reported.

Effect of Thermal Memory in Acoustic Emission in Fossil Coal after Pre-Disintegration by Cryogenic Treatment

Acoustic emission response of fossil coals being at different stages of metamorphism to cyclic variation of effective thermal stresses is experimentally investigated. The equipment and procedure used in the experiments are described. The features of the response are revealed and analyzed in the samples of anthracite, lignite and bituminous coal with different damage extent governed by the preliminary cyclic freezing and thawing, as well as by water saturation. It is shown that the signature of such features is a thermal analog of the Felicity effect which appears in each cycle of temperature action. The regularities of this effect are found, and their physical explanation is given based on the analysis of defect formation in coals at different stages of thermal treatment. The methodical approaches are proposed and substantiated, which allow structural damage, thermal resistance, oxidation and proneness to frost weathering of coal to be estimated by the Felicity effect in the acoustic emission response of coal to cyclic thermal forces. Possibility of using the found features to predict structural changes in coal products which are in long-term storage under specific climatic conditions, as well as for forecasting risk of self-heating and spontaneous combustion of coal products is discussed.

A Coupled Fluid Flow—Geomechanical Approach for Subsidence Numerical Simulation

This paper investigates the effect on compaction and subsidence induced by gas production using two techniques for coupling fluid-flow and geomechanics. To this end, a synthetic case representing a typical, shallow, weakly compacted, multi-layered, off-shore gas reservoir in the Adriatic Sea was set up and its dynamic and mechanic behavior during gas production was analyzed. Three numerical models (i.e., geological; fluid-flow and geomechanical) were built using high quality data set from an existing gas-bearing formation (off-shore Croatia). The laboratory analyses for deformation and strength parameters determination were conducted together with tests to define the coupling law required by the adopted coupling technique. Experimental data showed strong permeability stress-dependent behavior of core samples retrieved from gas bearing layers. Nevertheless, the results showed that the system stress-strain evolution always remains in the elastic domain and the deformation magnitude is extremely narrow (10−4 m/m) due to the limited net effective stress variation induced by the stressed production scenarios. The difference between the coupling techniques is negligible in terms of subsidence evolution at ground level but not in terms of compaction at reservoir level. Furthermore, the two-way coupled technique could be used for better development planning by integrating reservoir, drilling and completion management.

Segmentation of Slow Slip Events in South Central Alaska Possibly Controlled by a Subducted Oceanic Plateau

AbstractRecent GPS observations show that slow slip events in south central Alaska are segmented along strike. Here we review several mechanisms that might contribute to this segmentation and focus on two: along‐strike variation of slab geometry and effective normal stress. We then test them by running numerical simulations in the framework of rate‐and‐state friction with a nonplanar fault geometry. Results show that the segmentation is most likely related to the along‐strike variation of the effective normal stress on the fault plane caused by the Yakutat Plateau. The Yakutat Plateau could affect the effective normal stress by either lowering the pore pressure in Upper Cook Inlet due to less fluids release or increasing the normal stress due to the extra buoyancy caused by the subducted Yakutat Plateau. We prefer the latter explanation because it is consistent with the relative amplitudes of the effective normal stress in Upper and Lower Cook Inlet and there is very little along‐strike variation in Vp/Vs ratio in the fault zone from receiver function analysis. However, we cannot exclude the possibility that the difference in effective normal stress results from along‐strike variation of pore pressure due to the uncertainties in the Vp/Vs estimates. Our work implies that a structural anomaly can have a long‐lived effect on the subduction zone slip behavior and might be a driving factor on along‐strike segmentation of slow slip events.

Fully coupled two-phase flow and poromechanics modeling of coalbed methane recovery: Impact of geomechanics on production rate

This study presents the development and application of a fully coupled two-phase (methane and water) flow, transport, and poromechanics numerical model for the analysis of geomechanical impacts on coalbed methane (CBM) production. The model considers changes in two-phase fluid flow properties, i.e., coal porosity, permeability, water retention, and relative permeability curves through changes in cleat fractures induced by effective stress variations and desorption-induced shrinkage. The coupled simulator is first verified for poromechanics coupling, and simulation parameters of a CBM reservoir model are calibrated by history matching against one year of CBM production field data from Shanxi Province, China. Then, the verified simulator and the calibrated CBM reservoir model are used for predicting the impact of geomechanics on the production rate for twenty years of continuous CBM production. The simulation results show that desorption-induced shrinkage is the dominant process in increasing permeability in the near wellbore region. Away from the wellbore, desorption-induced shrinkage is weaker, and permeability is reduced by pressure depletion and increased effective stress. A sensitivity analysis shows that for coal with a higher sorption strain, a larger initial Young's modulus and a smaller Poisson's ratio promote the enhancement of permeability as well as an increased production rate. Moreover, the conceptual model of the cleat system, whether dominated by vertical cleats with permeability correlated to horizontal stress or with permeability correlated to mean stress, can have a significant impact on the predicted production rate. Overall, the study clearly demonstrates and confirms the critical importance of considering geomechanics for an accurate prediction of CBM production.

Non-linear effects of pore pressure increase on seismic event generation in a multi-degree-of-freedom rate-and-state model of tectonic fault sliding

Abstract. The influence of fluid injection on tectonic fault sliding and seismic event generations was studied by a multi-degree-of-freedom rate-and-state friction model with a two-parametric friction law. A system of blocks (up to 25 blocks) elastically connected to each other and connected by elastic springs to a constant-velocity moving driver was considered. Variation of the pore pressure due to fluid injection led to variation of effective stress between the first block and the substrate. Initially the block system was in a steady-sliding state; then, its state was changed by the pore pressure increase. The influence of the model parameters (number of blocks, spring stiffness, velocity weakening parameter) on the seismicity variations was considered. Various slip patterns were obtained and analysed.

Laboratory Investigation of Coal Deformation Behavior and Its Influence on Permeability Evolution During Methane Displacement by CO2

Geological sequestration of CO2 in coal seams is of significant interest to both academia and industry. A thorough laboratory investigation of mechanical and flow behaviors is crucial for understanding the complex response of coalbeds to CO2 injection-enhanced coalbed methane recovery (CO2-ECBM) operation. In this work, systematic experiments were carried out on cylindrical coal core specimens under different uniform confining stresses. The coal deformation caused by variations in effective stress as well as the sorption-induced matrix swelling/shrinkage was monitored. The competitive gas sorption characteristics and permeability evolution during the process of methane displacement by CO2 were also investigated. The measured volumetric strain results indicate that sorption-induced strain is the dominant factor in the coal deformation. The relationship between the volumetric strain and the adsorbed gas volume has been revealed to be a linear function. Experimental results obtained under different stress conditions suggest that higher confining stress suppresses the increase in both volumetric strain and the adsorbed gas volume. Furthermore, both methane displacement and CO2 injection are reduced when applying higher confining stresses. In addition, the permeability enhancement is heavily suppressed at higher confining stress. At a certain confining stress, a characteristic “U-shaped” trend of permeability is presented as a function of decreasing pore pressure. This study contributes to the understanding of coal deformation and its impact on permeability evolution under uniformly stressed condition, which has practical significance for CO2 sequestration and CO2-ECBM operation in the Qinshui basin.

Lateral force–displacement prediction for buried pipe under different effective stress condition

The aim of the present study is to elucidate the soil–pipe interaction in saturated sand with different effective stress. A force–displacement curve that took into account the variation of the effective stress was formulated based on the results of lateral loading experiments on a model pipe. To reproduce liquefaction experimentally, the effective stress of the soil bed was controlled by the upward seepage induced by a hydraulic gradient. The model pipe was pulled laterally under either displacement or load control. The experimental results indicate that the normalised force–displacement relationships can be well approximated by hyperbolic curves under each hydraulic gradient. When the variation in effective stress is reflected in the submerged unit weight of the soil, the two coefficients of the hyperbolic curves also show a hyperbolic dependence on the unit weight, and the ultimate resistive force increases in proportion to the unit weight. The predicted force–displacement curve fits relatively well to the experimental results.

Measurement of Suction-Stress Characteristic Curve Under Drying and Wetting Conditions

Suction-stress characteristic curve (SSCC) is an important constitutive relation defining variation of effective stress because of changes in soil water content. It can be intrinsically related to soil–water-retention curve. SSCC reflects the amount of mechanical work done to a unit volume of soil by its pore water at a particular state of matric potential or water content. A new testing system, based on a drying cake method was established to measure SSCC under both drying and wetting conditions. A previously developed theory of incremental linear elasticity allows the determination of suction stress by monitoring elastic modulus and deformation of a soil cake under varying water contents. Changes in suction stress and elastic modulus result in radial displacement field of a soil cake. A twin-cake testing procedure was established to independently measure: the elastic modulus change with water content on one cake by using a miniature-loading system, and the displacement field on the other cake by using a digital still camera throughout the drying and wetting processes. A particle image velocimetry technique was used to analyze a series of sequential images to calculate the evolution of radial displacement field and its center. The test results on four different soils, covering from sand to silt and clay, were compared and validated with the results obtained independently by a transient water release and imbibition method. It is demonstrated that this new testing system provides a simple, fast, and non-destructive way to measure SSCC under varying drying and wetting conditions.

Seismic velocity changes associated with aseismic deformations of a fault stimulated by fluid injection

AbstractFluid pressure plays an important role in the stability of tectonic faults. However, the in situ mechanical response of faults to fluid pressure variations is still poorly known. To address this question, we performed a fluid injection experiment in a fault zone in shales while monitoring fault movements at the injection source and seismic velocity variations from a near‐distance (<10 m) monitoring network. We measured and located the P and S wave velocity perturbations in and around the fault using repetitive active sources. We observed that seismic velocity perturbations dramatically increase above 1.5 MPa of injection pressure. This is consistent with an increase of fluid flow associated with an aseismic dilatant shearing of the fault as shown by numerical modeling. We find that seismic velocity changes are sensitive to both fault opening by fluid invasion and effective stress variations and can be an efficient measurement for monitoring fluid‐driven aseismic deformations of faults.