Effective Stress Reduction Research Articles

Behavior of saturated micaceous silty sand subjected to a combination of static and cyclic load using simple shear tests

The dynamic behavior of saturated sands has been sufficiently studied from different points of view, but in laboratory tests only the stress conditions, prior to dynamic stress, are reproduced at the level of the surface; that is, for conditions of zero static shear stresses (τ o). For this reason, the main objective of this research is to reveal the influence that static shear stresses (τ o) have on the response of the dynamically saturated sands. The experimental phase included: identification and classification tests and six monotonic simple shear tests and 17 cyclic simple shear tests with different combinations of monotonic (τ o) and cyclic shear stresses (τ c) and in undrained conditions. The tested samples were extracted from the ‘La Bocana Norte’ sector of the Port of Barcelona–Spain. The samples were taken at depths between 31 m and 55 m, with a corresponding effective vertical stress (σ’ov) between 290 kPa and 421 kPa, respectively. Based on the results of the tests, a detailed analysis was carried out on the influence of the combination of monotonic (τ o) and cyclic (τ c) shear stresses on the variation of the dynamic shear modulus (G). Specifically: in the stress–strain behavior, the development of both permanent (γ p) and cyclic (γ c) shear strain and the reduction of the effective stresses (σ′v) as a consequence of the generation of pore water pressure (u). The results of the laboratory tests generally show that: (a) the combination of τ o and τ c controls the stress–strain behavior, the degradation of the cyclic shear modulus, the development of permanent and cyclic shear strain, the generation of pore water pressures and the liquefaction risk, (b) in the monotonic case, an empirical function was established to evaluate the internal friction angle (ϕ') as a function of the shear strain (γ m) (mobilized friction angle), (c) in the combined case of monotonic and cyclical stress, it was possible to establish an empirical function among the G/σ′ov ratio, the cyclic strain (γ c) and the number of cycles (N) and among the σ′v/σ′ov ratio, the cyclic strain (γ c) and the number of cycles (N), (d) based on empirical functions, a simple methodology can be proposed to assess liquefaction risk and (e) it is not always true that the liquefaction phenomenon occurs when the effective vertical stress (σ’v) is reduced to zero. Depending on the cyclic stress ratio (τ c/σ’ov) liquefaction can occur in the first cycle with a reduction in vertical effective stress of just 70%.

Particle-void fabric and effective stress reduction in cyclic liquefaction of granular soils

During undrained cyclic loading, granular soils undergo the reduction of effective stress in the constant volume condition. Revealing the evolution of particle-void fabric in accompany with effective stress reduction provides a significant insight into the fundamental mechanism of cyclic liquefaction. In this study, numerical tests were conducted in DEM simulations to explore the cyclic liquefaction of granular packings with different particle size distributions and void ratios. With the decrease of mean effective stress p' during cyclic liquefaction process, the decrease of particle-void descriptor Ed can be observed for all packings. From micromechanical perspective, large size voids are redistributed and local void distribution around particle becomes relative uniform. The change of particle-void fabric from consolidated state to initial liquefaction state is irrelevant to density of the packing. A power function is further adopted to describe the positive correlation between Ed and p'.

Quantification of the Rupture Potential of Patient-Specific Intracranial Aneurysms under Contact Constraints.

Intracranial aneurysms (IAs) are localized enlargements of cerebral blood vessels that cause substantial rates of mortality and morbidity in humans. The rupture possibility of these aneurysms is a critical medical challenge for physicians during treatment planning. This treatment planning while assessing the rupture potential of aneurysms becomes more complicated when they are constrained by an adjacent structure such as optic nerve tissues or bones, which is not widely studied yet. In this work, we considered and studied a constitutive model to investigate the bio-mechanical response of image-based patient-specific IA data using cardiovascular structural mechanics equations. We performed biomechanical modeling and simulations of four different patient-specific aneurysms’ data (three middle cerebral arteries and one internal carotid artery) to assess the rupture potential of those aneurysms under a plane contact constraint. Our results suggest that aneurysms with plane contact constraints produce less or almost similar maximum wall effective stress compared to aneurysms with no contact constraints. In our research findings, we observed that a plane contact constraint on top of an internal carotid artery might work as a protective wall due to the 16.6% reduction in maximum wall effective stress than that for the case where there is no contact on top of the aneurysm.

Stress Reduction to Decrease Hypertension for Black Women: A Scoping Review of Trials and Interventions.

Chronic stress is a potential root cause of racial/ethnic disparities in cardiovascular disease. This review assesses literature surrounding effective stressreduction interventions to reduce hypertension (HTN)-a cardiovascular disease (CVD) risk factor-among an understudied population, non-Hispanic black (NHB) women. We conducted an electronic search of PubMed and PsycINFO literature published between January 1, 2000 and February 1, 2020, employing the keywords: "blood pressure", "hypertension", and "women", "black", "African-American", "stress", "meditation", "stress-coping", "stress-management", and "faith-based". We manually searched the bibliographies for additional articles. Studies were excluded if they: were published before 2000; were not intervention-based; did not study Black women in the US; did not target stress reduction; or did not measure blood pressure as an outcome. Independent reviewers screened the articles, which were selected based on consensus. Effect sizes and statistical p values were reported as provided in the included articles. We identified 109 articles in total. Of those, six articles met inclusion criteria. Stronger evidence presented by a randomized control trial supported the efficacy of transcendental meditation with reductions in systolic and diastolic blood pressure up to 7mmHg. Relaxation exercises, support groups, and therapeutic massage emerged as potentially beneficial in non-randomized pilot trials with reductions in systolic BP up to 9mmHg and diastolic BP up to 5mmHg varying by type and duration of the intervention. This scoping review found that faith-based strategies and meditation can be effective stress reduction techniques to reduce BP among NHB women. However, much remains to be known about how these strategies may be leveraged to reduce blood pressure within this highly vulnerable population.

Development of soil–pile interactions and failure mechanisms in a pile-reinforced landslide

The reinforcement effect of stabilizing piles is significant for ensuring the stability of major landslide mitigation projects. To investigate the work performance of piles under different boundary conditions, we introduce two physical modelings of a pile-reinforced landslide with the same pile parameters and soil conditions but different load conditions and analyze the consequent differences in the responses of the soil and piles and their interactions in these two physical modelings. Results show that piles in the same landslide/pile systems may have different reinforcement effects; this phenomenon has been explained by quantitatively describing the role of piles and analyzing the processes of soil–pile interactions in the physical modelings. Further, the dynamic responses between the excess pore pressure and the soil–pile interaction are investigated and the excess pore pressure-dependent mechanisms of soil–pile interactions are revealed. The generation of excess pore pressure and its accumulation around piles leads internally to a mechanism in which the arching effect is weakened and externally to a phenomenon in which the progressive failure of pile-reinforced landslides is accelerated. Eventually, we calculate the effective stress in the soil around piles, and the results reveal that a sharp reduction in effective stress in the soil around piles causes failure of the pile reinforcement. In addition, for a pile-reinforced landslide with a strong soil–pile interaction, an imbalance of soil arch footholds is the main failure macro-mechanism. Based on the analysis results, we present some feasible suggestions for the installation of rowed pile and their monitoring to ensure the long-term stability of a pile-reinforced landslide.

Instability of municipal solid waste along the constant deviatoric stress path and its engineering significance

To investigate the instability behaviours of municipal solid waste under the reduction of effective means stress, consolidated undrained (CU), constant shear drained (CSD) and constant shear undrained (CSU) tests were conducted on artificial waste specimens. In CU tests, the effective stress paths were modified by considering the particle compressibility, and the waste specimens did not exhibit pre-failure softening behaviours. Instability of the waste was observed in CSD tests, but was not observed in CSU tests. When the waste specimens became unstable, the axial strains increased rapidly and the volume was severely dilated. Softening of the waste specimens, manifested in the inability to maintain constant deviatoric stresses, was also observed. For waste specimens with similar void ratios, the instability points were located around an instability line (IL), and the slope of the IL decreased with an increase in the void ratio. Some factors, including the stress path and deformation characteristic of waste, may lead to the instability of waste specimens in CSD tests by influencing the effects of reinforcements. In engineered landfills, high pore pressure is often generated in waste mass and then leads to the obvious reduction of the effective stress of waste; thus, waste instability needs to be considered in the safety evaluation of waste slopes.

Effects of clay content on the shear behaviors of sliding zone soil originating from muddy interlayers in the Three Gorges Reservoir, China

The evolution of sliding zone soil originating from a muddy interlayer in the Three Gorges Reservoir (TGR) in China is accompanied by an increase in clay content, and the subsequent changes in its mechanical properties control the initiation or reactivation of large-scale bedrock landslides. In this paper, using sliding zone soil samples from the Xingfusi landslide in the Wanzhou area, two groups of remolded saturated specimens were prepared with clay contents of 25.0% and 12.5%. Direct shear creep tests and a series of ring shear tests were carried out to investigate the shear behaviors of the specimens with different clay contents. On this basis, the effect of clay content on the evolution of landslides was discussed. The results show that (1) during the long-term creep process before landslide initiation, the shear creep modulus of sliding zone soil in steady creep state significantly decreases with an increase in clay content, which results in the increase in cumulative creep displacement. (2) The increase in clay content in sliding zone soil will lead to a reduction in peak shear strength and residual frictional resistance. In the drained shearing on saturated sliding zone soil with a high clay content, the excess pore-water pressure on the sliding surface cannot dissipate as rapidly due to the small permeability of sliding zone soil with a high clay content, which may favor postfailure movement. (3) With an increase in shear displacement rate, the residual strength first shows a weak “negative shear rate effect” and then shows a significant “positive shear rate effect”. However, with an increase in clay content, the initial shear rate for the residual strength showing a “positive rate effect” will decrease. (4) Sliding zone soil with a high clay content generally does not exhibit strength recovery, while sliding zone soil with a low clay content clearly recovers its peak residual strength due to volume change of the shear zone caused by secondary compression after a long period of reconsolidation. In this case, a greater external load or reduction in effective normal stress needs to be applied to reactivate the landslide.

Cyclic shearing behavior and dynamic characteristics of a fibrous peat

Cyclic shearing behavior, dynamic characteristics, and post-cyclic volume change of a peat sublayer from the Port Lands area of Toronto (Ontario, Canada) are investigated in this study. Laboratory specimens are trimmed from block samples collected from a depth of about 4.0 to 4.5 m. Constant-volume cyclic direct simple shear tests indicate an initial reduction of effective stress with number of stress cycles. However, the corresponding excess pore pressure ratios do not exceed 60%, indicating a cyclic mobility behavior in the peat specimens. Maximum shear moduli of the peat samples are also determined from shear wave velocity measurements. Post-cyclic volumetric strain, as well as the variations of secant modulus, modulus reduction, and damping ratio of the peat, are presented in terms of cyclic shear strain and compared with other studies. Empirical relationships are proposed for characterizing the shear modulus and damping ratio of Toronto peat.

Evaluation of instability of quasi-saturated sand with entrapped gas due to decreasing total confining stress

The collapse of quasi-saturated sand with entrapped air due to decreasing total confining stress under undrained conditions has been widely reported in engineering practice. This material instability can fundamentally be related to dual effect of occluded gas bubbles: (1) altering undrained shear characteristics and (2) changing the relative compressibility between pore fluid and solid skeleton. Existing studies, however, have mainly treated these two effects independently. This work presents a theoretical framework to interpret the aforementioned instability that allows accounting for the interactions between the dual effect of gas. For this purpose, a state-dependent constitutive relation for gassy sand is employed that can well represent the gas effects of enhancing shear resistance and transferring unloading to soil skeleton. This soil model further enables evaluating the stability of quasi-saturated soils during constant deviatoric stress undrained shear with decreasing total mean stress, based on second-order work, the singularity of the symmetric part of the constitutive matrix, and test controllability. The experimentally observed collapse of loose gassy sand during the aforementioned stress path is analyzed by combining the constitutive model and stability criteria. This analysis finds that the reduction in effective stresses is initially driven by transferring unloading to soil skeleton, but becomes dominated by shear-induced contraction of soil matrix as failure is approached. Observed soil collapse can be well anticipated by probing the loss of controllability, yet lagging vanishing second-order work. The latter condition corresponds to the reversal in pore pressure and soil volume rate and represents a precursor for instability. Lastly, this work shows that the competition between the dual effect of the gas is key to understanding the non-monotonic dependence of instability potential on the properties of pore fluid and its compressibility.

Comparison of liquefaction behavior of granular material under SH- and Love-wave strain conditions by 3D DEM

In the study of liquefaction behavior associated with seismic loading conditions, it is often assumed that liquefaction occurs owing to the upward propagation of shear waves, despite evidence that liquefaction damage may result from or be aggravated by horizontally propagating surface waves. In this study, a series of numerical tests, based on the three-dimensional discrete element method, is performed to examine the liquefaction behavior of granular materials under Love-wave strain conditions. The response of granular packings under horizontally polarized shear- (SH-) and Love-wave strain conditions is discussed at both macro- and microscales. The simulation results indicate that, at the macroscale, the effective stress reduction ratio increases more rapidly under Love-wave strain conditions than under SH-wave strain conditions. Based on the concept of energy, the granular materials under Love-wave strain conditions can be considered more vulnerable to liquefaction than those under SH-wave strain conditions. Microscale analysis indicates the spatial rotation of the dominant direction of backbone force-chains under Love-wave strain conditions. In addition, focus here is placed on the coordination number, which represents the average contact number per particle. The difference in the degradation speed of the skeleton structure of the granular packings between the SH- and Love-wave strain conditions may not appear until the coordination number has decreased to a critical value of around 4. After the coordination number has approached approximately 3, the granular packings become unstable and soon liquefy. The minimum mean effective stress is discussed herein.

Effect of soil permeability on soil–structure and structure–soil–structure interaction of low-rise structures

Earthquake-induced soil liquefaction can generate excessive damage to building structures due to significant reduction in soil effective stress. With increasing urbanisation and population growth, the performance of closely spaced buildings, such as those in towns and cities, is of greater concern where structure–soil–structure interaction (SSSI) may occur in conjunction with full or partial liquefaction. This study investigates the seismic performance of isolated and adjacent structures built on shallow foundations on soils of different permeability (i.e. with different amounts of drainage and therefore generating different amounts of excess pore water pressure) using a combination of dynamic centrifuge modelling and finite-element modelling. The results demonstrate that the reduction in free-field ground surface intensity measure (specifically, Housner intensity) due to increasing liquefaction can be correlated to an index which is the normalised integral of excess pore pressure ratio (ru) with depth. This index can express the amount of liquefaction uniquely, even when full liquefaction occurs to only partial depth, or where excess pore water pressures are increased but below the level of full liquefaction at all depths. This underlying correlation means that structural demand reduction (e.g. reduction in inter-storey drift ratio) with liquefaction (SSI effect) can be linked to either: (a) the ru–depth index; (b) the depth of the liquefaction front, which can be estimated from a liquefaction triggering analysis; or (c) the ground surface intensity amplification/attenuation in the free field from the results of one-dimensional free-field soil column analyses. Where there are adjacent structures, a strongly beneficial SSSI effect on co-seismic settlement and a detrimental effect on drift in non-liquefied soil, as observed in this and previous studies, reduced towards a null effect in both cases with increased liquefaction, with liquefaction appearing to isolate the structures from each other (at least for the configuration considered herein). This has also been linked to the amount of amplification/attenuation of surface ground motion in the free field (or amount of liquefaction) as a simple indicator of the likely importance of SSSI effects in liquefiable soil.

Cyclic Water Injection Potentially Mitigates Seismic Risks by Promoting Slow and Stable Slip of a Natural Fracture in Granite

Induced seismicity associated with fluid injection has raised serious concerns for the safety and efficiency of geo-energy systems. Cyclic injection has recently been proposed as an alternative injection scheme to reduce the large magnitude injection-induced seismicity. However, the influence of cyclic injection on the activation of natural fractures in granite and the resulting seismic risk is not yet clear. This study investigates the injection-induced activation of a critically stressed natural fracture in a granite core sample, particularly focusing on the comparison between monotonic and cyclic water injection under pressure-controlled and volume-controlled conditions. Experimental results show that the acceleration and deceleration of fracture slip are modulated by the shear stress imbalance between the fixed shear stress and the evolving frictional strength of the fracture. Fracture slip affects the fluid pressure distribution on the fracture, which in turn regulates the frictional strength of the fracture. At a small total shear displacement (i.e., ~ 0.9 mm in this study), cyclic injection with a restricted peak injection pressure results in aseismic fracture slip at much smaller peak slip rates compared to that during the monotonic injection. On the one hand, the more uniform reduction in effective normal stress caused by cyclic injection encourages slow and stable fracture slip, characterized by the smaller peak slip rates. On the other hand, the flowback of injected fluid or suspension of injection could prevent the occurrence of fast-accelerated fracture slip during cyclic injection. However, the fracture can become unstable when it has experienced a considerable amount of total shear displacement (larger than ~ 0.9 mm in this study), and likely gained a significantly enhanced permeability. Continued injection after the unstable shut-in stage, signified by an unusual increase in slip rate and an accelerated drop in injection pressure, could result in rapid and unstable fracture slip.

Numerical simulation of liquefaction phenomenon considering infinite boundary

The response of liquefaction behaviour can be analyzed by incorporating the effects of the transient flow of the pore-fluid through the voids, which requires a two-phase continuum formulation for saturated porous media named ‘mixture theory’. The saturated soil system is modelled as a two-phase material based on the Biot's theory for porous media. Infinite elements have been incorporated in the solution algorithms to simulate infinite boundary. A computer code is developed in FORTRAN 90 considering fully coupled dynamic analysis to determine the responses of pore fluid and soil skeleton of an unbounded saturated sandy layer. The unconditionally stable generalized Newmark-Beta method is employed in the solution algorithm for evaluation of response at each time step. A generalized non-associative plasticity-bounding surface model Pastor–Zienkiewicz Mark III is considered to describe the inelastic behaviour of soils under earthquake loading. A parametric study is conducted to consider the effect of the material nonlinearity of the soil grain on liquefaction behaviour. A significant reduction in displacement and excess pore pressure is observed while considering unbounded domain. The salient feature of the solution algorithm is simulation of build up pore pressure and consequent reduction in mean effective stress. It was also observed that key parameters like permeability, earthquake intensity and shear modulus had significant effect on liquefaction response.

Seismic swarms produced by rapid fluid injection into a low permeability laboratory fault

Fluid injection, from activities such wastewater disposal, hydraulic stimulation, or enhanced geothermal systems, decreases effective normal stress on faults and promotes slip. Earthquake nucleation models suggest the slip at low effective normal stress will be stable and aseismic—contrary to observed increases in seismicity that are often attributed to fluid injection. We conducted laboratory experiments using a biaxial loading apparatus that demonstrate how an increase in fluid pressure can induce “stick-slip” events along a preexisting saw-cut fault in a poly(methyl methacrylate) (PMMA) sample. We compared slip events generated by externally squeezing the sample (shear-triggered) to those due to direct fluid injection (fluid-triggered) and studied the effects of injection rate and stress levels. Shear-triggered slip events began on a localized nucleation patch and slip smoothly accelerated from slow and aseismic to fast and seismic. Fluid-triggered slip events initiated far more abruptly and were associated with swarms of tiny foreshocks. These foreshocks were able to bypass the smooth nucleation process and jump-start a mainshock resulting in an abrupt initiation. Analysis of these foreshocks indicates that the rapid injection of fluid into a low permeability fault promotes heterogeneous stress and strength which can cause many events to initiate—some of which grow large. We conclude that while a reduction in effective normal stress stabilizes fault slip, rapid fluid injection into a low permeability fault increases multi-scale stress/strength heterogeneities which can initiate small seismic events that have the potential to grow rapidly, even into low stress regions.

Development of smoothed particle hydrodynamics (SPH) method to model the interaction of sand and water during liquefaction with bingham fluid model adaptation

Smoothed Particle Hydrodynamics (SPH) is a mesh-free numerical method that models the movement of particles in the Langrangian method. SPH method treats the domain as discrete particles instead of continuous entities, making it more accurate to model phenomenons that have a big deformation. The purpose of this research is to analyze the interaction between soil and water particles, based on the particle movements, density, pressure, internal forces, and external forces. This research continues the previous research of interaction between fluid-fluid particles in SPH by applying the Bingham fluid model to one type of fluid to represent coarse-grained soil particles. In the future, we hope this research can be further developed into a liquefaction prediction model by quantifying the main factors of liquefaction, such as the amplitude of shear strain, effective stress reduction, and excess pore water pressure.

Stress Reduction Techniques for Health Care Providers Dealing With Severe Coronavirus Infections (SARS, MERS, and COVID-19): A Rapid Review.

ObjectiveA rapid review was conducted to identify the most effective stress reduction techniques for health care providers dealing with patients infected with severe coronavirus (SARS, MERS, and COVID-19).MethodsPubMed, PsychInfo, Embase, and CINAHL databases were searched to identify relevant studies. Searches were restricted by date (2000 until present). All empirical quantitative and qualitative studies in which relaxation techniques of various types implemented on health care providers caring for patients during severe coronavirus pandemics and articles that consider the implementation of mental health care services considered to be pertinent, such as commentaries, were included.ResultsFourteen studies met the selection criteria, most of which were recommendations. Only one study described a digital intervention, and user satisfaction was measured. In the recommendations, both organizational and individual self-care interventions were suggested.ConclusionsFurther research is necessary to establish tailor-made effective stress reduction interventions for this population, during these challenging and particular times.

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 Lightweight Expanded Clay Aggregate on the Mitigation of Liquefaction in Shaking Table

Non-conventional materials are often applied to dissipate the energy of dynamic loads and mitigate liquefaction. In this study, the effect of Lightweight Expanded Clay Aggregate (LECA) on the mitigation of liquefaction was examined using shaking table tests. LECA, a common lightweight material, was added to saturated sand in a range of 0–20% to study the variation of excess pore water pressure (EPWP) generation, as well as the acceleration response of the soil. Based on the obtained results, the addition of 5% and 10% LECA to saturated sand resulted in a significant decrease in the generation of EPWP, while higher LECA content led to an increase in the generation of EPWP. The mechanisms behind such behaviors contradict the effects of increasing the damping of the soil mixture and reducing the effective initial stress as a function of overall mixture density. Adding 5 and 10% LECA reduces EPWP because aggregates accelerate the dissipation of EPWP during and after vibration. As the percentage of LECA increases by 15% and 20%, excess pore pressure ratio increases. The cause of this may be the reduction of effective initial stress leading to reduced soil density in higher-LECA contents. 10% LECA is presented as the threshold for changing the liquefaction behavior of sand-LECA mixtures. This study concludes that adding LECA to sand samples increases the rate of dissipation of EPWP, but also leads to a significant decrease in initial effective stress.

Wellbore injectivity response to step-rate CO2 injection: Coupled thermo-poro-elastic analysis in a vertically heterogeneous formation

Safe and permanent carbon geological storages require an elaborate analysis of geomechanical stability. Subsurface injection of CO2 changes the local temperature and pore pressure and, further, alters the stress due to thermo-poro-elastic responses.A permanent CO2 storage testing was conducted in the water leg of Cranfield reservoir in Mississippi, USA, where hosted CO2 enhanced-oil recovery activities. During CO2 injection, the injection rate was ramped up twice but the bottom-hole pressure did not increase with the imposed injection rates as expected. This unexpected field observation suggests the possibility of an open-mode fracture development at the injector. However, the injector response and potential fracture development have not been rigorously interpreted with detailed near-wellbore temperature and pore pressure change upon the CO2 injection.In this study, we performed history matching of CO2 injection using coupled thermo-poro-elastic reservoir simulation to examine the possibility of fracturing. We built a reservoir model including vertical heterogeneity in both petrophysical and geomechanical properties estimated from well-logging analysis and laboratory experiments.The simulation results show that CO2 injection changes stresses and support the hypothesis of development of an open-mode fracture at the injector during the Cranfield test. The near-injector region exhibits a large temperature reduction up to 55 °C with ensuing effective horizontal stress reduction up to 9.1 MPa. However, a caprock integrity issue is unlikely because of the horizontal stress contrast within layers and low hydraulic communication with the injection zone.This study indicates that the injectant temperature should be considered in the design of high-rate CO2 injector.

Earth pressure and deformation of cut-and-cover and trenchless flexible pipes under different soil properties and groundwater conditions

A field case was first studied to verify the reliability of PLAXIS 2D and the accuracy of the modeling and calculation methods, in terms of calculating the radial earth pressure and vertical deformation on the cut-and-cover and trenchless pipes. Then, four types of classical soils were considered to compare the effects of cut-and-cover and trenchless construction methods on the installed flexible pipes. Besides, groundwater conditions and main physical properties of soils were also considered. Results demonstrate that deviations among the simulation calculated and the field measured value on earth pressure and deformation are all within 5%, showing a high accuracy and reliability. The maximum earth pressure and deformation on the cut-and-cover flexible pipe are 2.66–11.65 times and 3.96–11.95 times larger than that on the trenchless pipe, which is mainly due to the passive earth pressure caused by the deformation of the trenchless pipe. As the groundwater level increases from the bottom to 5.5 m above the top of pipes, the maximum earth pressure and deformation of the cut-and-cover pipe increase by 18.74% and 13.38%, while that on the trenchless pipe increased by 6 times and 3 times, because of the difference of reduction in effective stress. Density, cohesion, and friction angle are positively related to the earth pressure and deformation on the cut-and-cover pipe, and Poisson’s ratio and density are positively related to that on the trenchless pipe, while the others are the opposite. Density and cohesion are respectively the most significant factors affecting the cut-and-cover pipe and trenchless pipes.