Stress Paths Research Articles

Boundary quantitative characterization of the top-coal limit equilibrium zone in fully mechanized top-coal caving stope along the strike direction of working face

In the process of fully mechanized top-coal caving mining, the top-coal is affected by mining-induced stress, and the stress varies along the strike direction of working face, so the boundary position of its entering the limit equilibrium state changes accordingly. The determination of the boundary along the strike direction of working face can provide scientific guidance for the stability control of support-surrounding rock in fully mechanized top-coal caving face. Using the research methods of theoretical analysis, physical similarity simulation experiment and numerical simulation experiment, the stress state analysis model of the boundary position of the top-coal limit equilibrium zone under macro-scale conditions was established, the stress state characterization method of the boundary of the top-coal limit equilibrium zone along the strike direction of working face was given, and the quantitative characterization of the boundary of the top-coal limit equilibrium zone along the strike direction of working face was realized by combining with the mining-induced stress path, and the distance relationship between the boundary of the top-coal limit equilibrium zone and the langwall face along the strike direction of working face was revealed. The results show that after critical mining in fully mechanized top-coal caving face, the distance between the boundary of top-coal limit equilibrium zone and the langwall face along the strike direction of working face presents a relationship of increasing from top to bottom. The distance between the top-coal upper boundary and the langwall face was 2.85 m and the distance between the top-coal lower boundary and the langwall face was 5.39 m. The boundary of top-coal limit equilibrium zone along the strike direction of working face was verified by the top-coal elastic–plastic zone boundary and the boundary of the peak position of front abutment pressure in different layers of top-coal. The results show that the quantitative characterization of the top-coal limit equilibrium zone boundary along the strike direction of working face was reasonable. In order to improve mine production efficiency, optimization measures were put forward for hard coal seam and soft coal seam respectively.

New critical state constitutive model considering particle breakage implicitly for uncemented carbonate sands

One of the main problems of carbonate sands is the fragile nature of particles and their susceptibility to breakage. Carbonate sands are affected by volumetric strain even at low stress levels, which is not the case with silicate sands. By defining a simple breakage model, the current study develops an elastoplastic critical state constitutive model that considers the impact of particle breakage on the mechanical behavior of carbonate sands. The particle breakage model depends on mean effective stress and critical breakage stress, which is assumed to correspond with the precompression pressure of soil in the oedometer test. In the proposed model, critical state line movement with the breakage parameter (α) considers the particle breakage effect. Based on the unified clay and sand model (CASM), a novel dynamic yield surface with a shape parameter affected by particle breaking has been created. Certain modifications are made to the modified Cam-Clay stress dilatancy to predict the behavior of carbonate sand. The current model has only ten parameters that simulate the carbonate sands’ behavior even at high-stress levels without any breakage test. Experimental data with different soil densities, loading stress paths, and stress levels were compared with the model, and the results demonstrated satisfactory conformance.

Photogrammetry-Based Method for Measuring Volume Changes of Soil Specimens During Critical Phases of Triaxial Testing

Triaxial tests has been routinely used to measure the stress–strain relationship for geomaterials. During triaxial testing, many sources of errors that cannot be completely avoided but are often ignored or approximated using empirical equations. This paper presents a systematic investigation of soil volume change, volume strain nonuniformity, and cross-sectional calculation along the specimen during triaxial testing using a photogrammetry-based method. Consolidated drained triaxial tests were performed in which two parallel measurements were taken: (1) relative volume using the conventional triaxial testing and (2) absolute volume using the photogrammetry-based method. The difference in the observed volume and void ratio measurements between the two methods highlighted the importance of absolute soil volume over the relative volume method. The results revealed the effect of soil volume change in the interpretation of triaxial testing findings. Various preparation steps and procedures during triaxial testing have influenced the initial measured volume and thus caused deviation of the subsequent associated measurements. This method would present a quality control approach for different aspects of triaxial testing to improve the test simulations to accurately measure the soil behavior. The proposed method is important for more refined simulations and identification of setup-induced errors. Additional applications of the current research would allow correct determination of the stress path followed during triaxial testing, the critical-state soil mechanics parameters, and the stress–strain relationship, including deformation and strength parameters.

Undrained behavior of marine clay under one-way cyclic loading with variable confining pressure

Settlement of roads or railway caused by traffic loading has a serious effect on the safety and service performance of transport infrastructures constructed on soft marine clay. While simple cyclic triaxial test with constant confining pressure (CCP) were used in most of the previous studies, a series of cyclic triaxial tests with variable confining pressure (VCP) were carried out on normally consolidated (NC) and overconsolidated (OC) reconstituted Wenzhou soft clay in this paper to study the undrained behavior of soft marine clay due to surcharge preloading as well as cyclic traffic loading. VCP test is able to approximate the complicated stress path than CCP test caused by traffic loading. The test results indicate that NC specimens show significantly different pore water pressure (u) evolution compared with OC specimens. However, a change in the overconsolidation ratio (OCR) of OC specimens does not have a significant influence on variation of u with identical cyclic stress ratio (CSR) and total stress path (η). The slope of the effective stress path is dependent on the total stress path under which the specimen was loaded cyclically, regardless of the OCR and whether variable confining pressure was applied. The slope of the effective stress path is broadly in line with the slope of corresponding total stress path. In addition, the development of permanent axial strain (εpa) due to one-way cyclic loading was shown to depend significantly on the values of η and OCR. Both the variable confining pressure and OCR limited the strain accumulation of saturated marine clay under undrained one-way cyclic loading. Finally, the effects of η and OCR on the magnitude of εpa after 1000 loading cycles are quantified and incorporated in a power law function for the permanent deformation prediction of soft marine clay due to traffic loading.

Thermodynamic constitutive model for calcareous sand considering specimen preparation

Specimen preparation methods significantly affect indoor sand soil’s properties. A theoretical model was developed based on granular thermodynamics to describe the undrained shear characteristics of calcareous sand. We validated the model using specimens prepared through four different methods under various relative densities, stress paths, and confining pressures. The hardening index and granular temperature of this model effectively reflect the impact of specimen preparation methods on the strength characteristics of calcareous sand. The model successfully captured these differences in shear behavior resulting from four preparation methods by adjusting the hardening parameters. Furthermore, this model can accurately describe the influence of stress path changes on the mechanical responses of the calcareous sand granular system by incorporating the third elastic strain invariant.

In Situ Stress Paths Applied in Rock Strength Characterisation Result in a More Correct and Sustainable Design

In Situ Stress Paths Applied in Rock Strength Characterisation Result in a More Correct and Sustainable Design

On characteristics of K0 value and shear behaviour of loess using triaxial test

Compared with conventional soils, such as sand and clay, little knowledge on the coefficient of lateral earth pressure at-rest (K0) has been established for loess in the current literature. This paper presents an experimental investigation on K0 of compacted loess and the associated impacts on undrained shear behaviour. By adopting a K0 consolidation module in the triaxial system, the K0 stress state for loess samples was achieved through a unique feedback control. During the K0 consolidation, the deviatoric stress (q) increases progressively with the premise that the volumetric strain (εv) of the sample equals to the axial strain (εa). The results show that the K0 value of compacted loess is in a range of 0.28 to 0.53, which is dependent on the packing density and the clay content. A distinguishable decrease of K0 was found in the course of K0 consolidation for the loosely compacted loess sample, whereas a similar trend was not observed in the dense sample. In the undrained shear stage, all loess specimens revealed contractive response in the stress path (q-p’) diagram, which can be quantified by a modified collapsibility index (Ic). The index is consistently higher for the K0 consolidated loess samples than for the isotropic ones. The experimental results indicate a strong impact of the initial stress state on the shear behaviour of compacted loess.

Carbon Dioxide Capture and Storage (CCS) in Saline Aquifers versus Depleted Gas Fields

Saline aquifers have been used for CO2 storage as a dedicated greenhouse gas mitigation strategy since 1996. Depleted gas fields are now being planned for large-scale CCS projects. Although basalt host reservoirs are also going to be used, saline aquifers and depleted gas fields will make up most of the global geological repositories for CO2. At present, depleted gas fields and saline aquifers seem to be treated as if they are a single entity, but they have distinct differences that are examined here. Depleted gas fields have far more pre-existing information about the reservoir, top-seal caprock, internal architecture of the site, and about fluid flow properties than saline aquifers due to the long history of hydrocarbon project development and fluid production. The fluid pressure evolution paths for saline aquifers and depleted gas fields are distinctly different because, unlike saline aquifers, depleted gas fields are likely to be below hydrostatic pressure before CO2 injection commences. Depressurised depleted gas fields may require an initial injection of gas-phase CO2 instead of dense-phase CO2 typical of saline aquifers, but the greater pressure difference may allow higher initial injection rates in depleted gas fields than saline aquifers. Depressurised depleted gas fields may lead to CO2-injection-related stress paths that are distinct from saline aquifers depending on the geomechanical properties of the reservoir. CO2 trapping in saline aquifers will be dominated by buoyancy processes with residual CO2 and dissolved CO2 developing over time whereas depleted gas fields will be dominated by a sinking body of CO2 that forms a cushion below the remaining methane. Saline aquifers tend to have a relatively limited ability to fill pores with CO2 (i.e., low storage efficiency factors between 2 and 20%) as the injected CO2 is controlled by buoyancy and viscosity differences with the saline brine. In contrast, depleted gas fields may have storage efficiency factors up to 80% as the reservoir will contain sub-hydrostatic pressure methane that is easy to displace. Saline aquifers have a greater risk of halite-scale and minor dissolution of reservoir minerals than depleted gas fields as the former contain vastly more of the aqueous medium needed for such processes compared to the latter. Depleted gas fields have some different leakage risks than saline aquifers mostly related to the different fluid pressure histories, depressurisation-related alteration of geomechanical properties, and the greater number of wells typical of depleted gas fields than saline aquifers. Depleted gas fields and saline aquifers also have some different monitoring opportunities. The high-density, electrically conductive brine replaced by CO2 in saline aquifers permits seismic and resistivity imaging, but these forms of imaging are less feasible in depleted gas fields. Monitoring boreholes are less likely to be used in saline aquifers than depleted gas fields as the latter typically have numerous pre-existing exploration and production well penetrations. The significance of this analysis is that saline aquifers and depleted gas fields must be treated differently although the ultimate objective is the same: to permanently store CO2 to mitigate greenhouse gas emissions and minimise global heating.

A shear‐transformation‐zone model for time‐dependent behaviours of clay

AbstractA shear‐transformation‐zone (STZ) model is proposed for time‐dependent behaviours of clay, in which the viscoplastic deformation is described by the evolution of a temperature‐like state variable. The model is featured by rate‐dependent dilatancy and a unique critical state stress ratio. In its present form, it has nine parameters, most of which can be handily calibrated, and can predict the rate‐dependent undrained shear strength, primary, and secondary creep, as well as stress relaxation for normally consolidated clay under complex stress paths using a single set of parameters. The capabilities of the model have been verified by available experimental results on five different clays.

Finite element modelling of pipe piles driven in low-to-medium density chalk under monotonic axial loading

Percussive driving in low-to-medium density chalk creates a thin annulus of fully de-structured ‘putty’ chalk around pile shafts and an outer annular zone where fracturing is more intense than in the natural chalk. This damage and the related generation, and subsequent equalisation, of excess pore pressures impacts the piles’ time-dependent axial loading behaviour, as do other ageing processes. This paper presents finite element analyses of open steel tubular piles driven for the recent ALPACA and ALPACA Plus research projects under monotonic axial loading to failure after extended ageing periods. A nonlinear elastic stiffness model with a nonlocal deviatoric strain-based Mohr-Coulomb failure criterion was employed, with different sets of properties to represent the de-structured, fractured chalk and intact chalk. It is shown that the piles’ axial responses are mainly controlled by the puttified chalk annuli. A simplified but efficient means is adopted to impose pre-loading chalk effective stress conditions that explicitly capture the effects of installation damage and subsequent ageing. The potential for strain-softening in the brittle chalk is examined and the effective stress paths developed in representative chalk elements throughout the loading are considered in conjunction with the mobilisation of accumulated deviatoric strains. The simulations indicate 264% higher shaft capacity in compression than in tension, which is mainly attributed to the internal chalk plug.

The effect of overlying rock fracture and stress path evolution in steeply dipping and large mining height stope

To study the fracture instability characteristics and fracture mechanism of overlying strata in steeply dipping and large mining height stope, through the combination of physical simulation experiment, numerical calculation, theoretical analysis and field measurement, the correlation between the evolution of mining stress field and the fracture of overlying strata in large mining height stope under the effect of dip angle is analyzed, and the stress transfer path and the fracture mechanism of overlying strata are revealed. Finally, the influence mechanism of key strata on the evolution of the mining stress field of overlying strata is explained. The research shows that under the influence of gravity dip angle effect and mining height increase, the mining stress is non-equilibrium transferred and evolved in space, and the principal stress field presents the characteristics of partition evolution. The fracture trajectory of rock strata is multi-step and “八” shaped, and the inclined masonry structure and multi-step key strata form a cooperative bearing structure. The key layer of the ladder is the stress transmission structure between the caving zone and the stress arch, and the high stress in the overhanging area under the main roof is transmitted to the advanced roof. With the increase of dip angle, the height of the caving zone decreases, and the fracture range of ladder and principal stress arch decreases. The cracks in the middle and upper broken rock blocks increase, and the instability of the “high ladder rock layer” is easy to induce the simultaneous fracture of the “inclined masonry structure”, resulting in regional unbalanced instability and impact. The research results can provide some reference and guiding significance for the stability control of surrounding rock in longwall stope with steeply dipping.

Rockburst simulation tests on structural plane of deep high-stress circular tunnels: A true triaxial test study on several different hard rocks

After deep rock excavation, rockbursts can easily occur. The structural plane and rock properties play important roles in controlling rockbursts. To study the role of structural planes and rock properties in controlling rockbursts in the peripheral rocks of tunnels, five different brittle and hard rocks (Red sandstone, Gray sandstone, Granite,Marble, and Limestone) were tested, and cubic specimens with round holes (100 mm*100 mm*100 mm, Φ = 50 mm) and prefabricated fissures were used to simulate the structural plane. A high-speed camera system and an acoustic emission system were used to monitor the damage and microfracture processes of the surrounding rock. The damage process, acoustic emission characteristics, and damage patterns of five brittle rocks with and without specimens with structural planes during loading were comparatively analyzed. The main conclusions were as follows. (1) The presence of structural planes in the rock mass changed the stress and energy propagation paths in the rock mass, resulting in stress concentration; the structural planes in the five brittle rocks could not store energy, hence increasing the propensity for rockbursts. Structural planes in the Marble had the most obvious effect on the promotion of rockbursts. However, structural planes in the Gray sandstone had the weakest effect on the promotion of rockbursts. (2) The presence of structural planes significantly increased the rockburst intensity. In the Marble, regardless of the presence of structural planes, the damage mode of the surrounding rock was mainly shear flexural damage. However, the fine particle contents of specimens with structural planes were significantly higher than those without structural planes. Thus, the rockburst intensities of specimens with structural planes were greater than those without structural planes. For the Red sandstone, Gray sandstone, Granite and Limestone, the surrounding rock specimens with structural planes experienced mainly violent shear ejection damage. (3) The distribution of RA and AF and the number of ruptures in the specimens with and without structural planes were obviously changed, and the numbers of rupture events in the Red sandstone, Gray sandstone, and Granite specimens obviously increased because of the presence of structural planes. The numbers of rupture events for Marble and Limestone specimens significantly decreased due to the presence of structural planes. Gray sandstone and Granite had the most rupture events. (4) Different brittle rock specimens with structural planes had different sizes. The degrees of damage were observed to significantly increase for a period. In addition, the rock body experienced additional internal shear rupture, which intensified due to the structural planes and the tunnels between the rock columns. The ruptures were caused by the destruction of the structural planes during rockburst, and they served as predictions and early warnings, thus providing a certain reference basis. (5) In the Marble during rockbursts, there were structural planes that underwent flexural strain and slip strain. In the Red sandstone, Gray sandstone, and Granite during rockbursts, there were structural planes that underwent compression shear strain and slip strain. The disaster chain was introduced to understand and discuss the mechanism of slip strain for the development of rockburst in structural planes, which should be the final damage form. The disaster chain was a series of rockbursts caused by compression shear and slip of specimens with structural planes. The development of rockbursts in specimens with structural planes occurred sequentially. The final damage pattern was the end result of the development of a series of rockbursts in specimens with structural planes.

An extended graphical solution for undrained cylindrical cavity expansion in K0‐consolidated Mohr–Coulomb soil

AbstractThis paper develops a general and complete solution for the undrained cylindrical cavity expansion problem in nonassociated Mohr‐Coulomb soil under nonhydrostatic initial stress field (i.e., arbitrary values of the earth pressure coefficient), by expanding a unique and efficient graphical solution procedure recently proposed by Chen and Wang in 2022 for the special in situ stress case with . It is interesting to find that the cavity expansion deviatoric stress path is always composed of a series of piecewise straight lines, for all different case scenarios of K0 being involved. When the cavity is sufficiently expanded, the stress path will eventually end, exclusively, in a major sextant with Lode angle θ in between and or on the specific line of . The salient advantage/feature of the present general graphical approach lies in that it can deduce the cavity expansion responses in full closed form, nevertheless being free of the limitation of the intermediacy assumption for the vertical stress and of the difficulty existing in the traditional zoning method that involves cumbersome, sequential determination of distinct Mohr–Coulomb plastic regions. Some typical results for the desired cavity expansion curves and the limit cavity pressure are presented, to investigate the impacts of soil plasticity parameters and the earth pressure coefficient on the cavity responses. The proposed graphical method/solution will be of great value for the interpretation of pressuremeter tests in cohesive‐frictional soils.

Undrained cavity expansion-contraction analysis in CASM and its application for pressuremeter tests

Many geotechnical scenarios involve cavity unloading from a loaded state, particularly in pressuremeter tests, and the unloading data of pressuremeter tests has exceptional attraction as it is less disturbed by the insertion process. However, the analyses for continuous cavity loading and unloading (i.e., cavity initially experiences expansion and then contracts) in critical state soils are rarely studied. To this end, a novel semi-analytical solution based on the unified state parameter model for clay and sand (CASM) is proposed for the whole expansion-contraction of spherical and cylindrical cavities under undrained condition. The problem assumes that the cavity is unloaded after a monotonic loading stage, leading to plastic regions during both loading and unloading periods. The cavity response for the whole expansion-contraction process is investigated, with the total pressure and stress paths at the cavity wall presented and validated against numerical simulation. The developed solution is successfully implemented to interpret both loading and unloading data of pressuremeter tests. The undrained shear strength, in-situ effective horizontal stress and initial overconsolidation ratio are back analyzed by using a curve fitting method based on the proposed solution.

Investigation of foam concrete's mechanical properties and multi-scale damage evolution characteristics under uniaxial loading

The macroscopic mechanical behavior of foam concrete damages its mesoscopic pore structure. To study the macroscopic mechanical properties of foam concrete, a series of uniaxial compression tests were carried out. AE-DIC and SEM characterization were used to reveal the multi-dimensional damage evolution characteristics of foam concrete under load. Furthermore, the porosity response law of the foam concrete damage characteristics was analyzed using the finite element method. The results show a significant linear relationship between the residual and peak stress of foam concrete under uniaxial load (σr = kσp), and the coefficient k decreases with an increase in porosity. During uniaxial compression, tensile and shear damages were dominant on both sides of the yield point of foam concrete. The macroscopic load retention characteristics in the failure stage typically result from microscopic fracture structure reorganization and stress path optimization. The damage-dissipation energy during this period accounts for approximately 100% of absorbed energy. An increase in the porosity led to frequent adjustments in the stress path of the foam concrete, and the tensile damage tended to be significant and diffuse.

Influence of acoustic emission sequence length on intelligent identification accuracy of 3-D loaded rock’s fracture stage

Accurate prediction of impending disasters in underground projects is crucial and requires the identification of rock fracture stages. Currently, rock fractures are commonly analyzed using microseismic parameter statistics or individual waveform features for engineering disaster early warning systems. However, rock fracturing is a continuous process, and waveform sequences contain a wealth of information on fractures, which is often overlooked by existing research that generally neglects the information within continuous waveforms. In this study, we leverage acoustic emission (AE) data and employ a transfer learning approach with a convolutional neural network (CNN) to identify rock fracture stages under three-dimensional (3-D) stress paths induced by true triaxial compression tests. Failure experiments were performed on seven sandstone specimens under various 3-D stress paths. To fully utilize the characteristics of the crack rupture sequence, we introduce the concept of AE waveform sequence length. This concept integrates the discrete features of AE time–frequency images, thereby improving the CNN model’s performance. Utilizing waveforms of six different lengths (1, 2, 3, 5, 10, and 15) to train the neural networks, our findings reveal that a sequence length of 10 enables the CNN to effectively identify rock fracture stages under 3-D stresses with an accuracy rate of up to 90.4%. This demonstrates that appropriately increasing the sequence length to process the discrete features of AE waveforms structurally is a viable strategy to enhance CNN identification accuracy. Our results underscore that rock fracturing is a sequential process with significant inter-sequence correlations, which critically influence the CNN model’s ability to accurately identify rock fracture stages. These insights offer valuable theoretical contributions to the automatic monitoring of rock fracture stages in deep engineering projects.

Improvement of a hypoplastic model for sand under undrained loading conditions

An important drawback of the hypoplastic model is the inaccurate prediction of the sand behavior under undrained monotonic loading conditions. The model is not able to reproduce the limited liquefaction type response widely observed in undrained tests on loose sand, and it often underestimates the initial stiffness and hardening rate of sand during the shearing. To address these issues, three novel modifications are introduced into a basic hypoplastic model to enhance its undrained predictive capability. Firstly, a new factor is added to the nonlinear term of the model, allowing the simulation of a purely elastic response at the beginning of loading. By doing so, the model can accurately capture the initial stiffness and undrained effective stress path of sand. Secondly, the characterized void ratios are related to an evolving state variable, enabling the model to reasonably reproduce the limited flow response and quasi-steady state. Furthermore, a new term is incorporated into the deviatoric part of the strain rate to adjust the hardening rate of the model. The model performance for undrained loading is significantly improved through the above modifications, as evidenced by the good agreement between simulation results and experimental data for tests with varying densities and confining pressures.

The Relationship Between Stress, Disability Acceptance, and Quality of Life of People With Physical Disabilities in South Korea: Focused on the Psychosocial Adaptation Model

Psychosocial adaptation is important for individuals with disabilities as they strive for optimal quality of life (QOL). However, studies applying the psychosocial model in South Korea are limited. Our study especially focused on identifying the relationship between stress, disability acceptance, and QOL among people with physical disabilities. Stress by disability can act as an antecedent event and a risk factor, while disability acceptance serves as a process that promotes positive QOL within the framework of the psychosocial adaptation. So, this study investigates whether disability acceptance mediates the relationship between stress and QOL in South Korea. The study used the 2021 Panel Survey of Employment for the Disabled in Korea data. A total of 1,872 participants (68.9% male;M= 50.02 years) were included. Results from the regression model through PROCESS Macro in SPSS revealed that disability stress is partially negatively related to QOL. The indirect path of stress, disability acceptance, and QOL was also significant, indicating that disability acceptance is a significant mediator of the relationship between stress and QOL. Especially, individuals with physical disabilities who experience high levels of stress on their disabilities are more likely to have a lower QOL. This association is partially due to decreased disability acceptance. This study is significant because it applied the psychosocial adaptation model to individuals with physical disabilities in South Korea, providing empirical validation of its effectiveness. The results suggest that disability acceptance plays a crucial role in partially lowering the negative impact of stress on QOL for individuals with physical disabilities.

The influence of mining stress paths on rock damage and permeability

The influence of mining stress paths on rock damage and permeability

Rock strength weakening subject to principal stress rotation: Experimental and numerical investigations

During the construction and operation of gas storage reservoirs, changes in the principal stress direction can induce fracture propagation under conditions of lower differential stress, potentially leading to failure in the surrounding rock. However, the weakening of strength due to pure stress rotation has not yet been investigated. Based on fracture mechanics, an enhanced Mohr-Coulomb strength criterion considering stress rotation is proposed and verified with experimental and numerical simulations. The micro-damage state and the evolution of the rock under the pure stress-rotation condition are analyzed. The findings indicate that differential stress exceeding the crack initiation stress is a prerequisite for stress rotation to promote the development of rock damage. As the differential stress increases, stress rotation is more likely to induce rock damage, leading to a transition from brittle to plastic failure, characterized by wider fractures and a more complex fracture network. Overall, a negative exponential relationship exists between the stress rotation angle required for rock failure and the differential stress. The feasibility of applying the enhanced criterion to practical engineering is discussed using monitoring data obtained from a mine-by tunnel. This study introduces new concepts for understanding the damage evolution of the surrounding rock under complex stress paths and offers a new theoretical basis for predicting the damage of gas storage reservoirs.