Rock unloading gradient failure test method based on non-uniform stress distribution

Abstract

The application discloses a rock unloading gradient damage test method based on non-uniform stress distribution and relates to the technical field of geotechnical engineering and rock mechanics test, and comprises the following steps: in a true triaxial servo loading device, a rock sample is divided into multiple layers along the thickness direction, and different loads are applied to each layer, so that a non-uniform stress gradient which continuously changes from the surface layer to the deep layer is formed in the rock sample. The application forms a non-uniform stress gradient which continuously changes from the surface layer to the deep layer and implements a step-by-step unloading from outside to inside, so that the whole test process is in a controllable spatial stress evolution environment, and the stress release and redistribution process under the surrounding rock excavation condition is truly simulated. Meanwhile, multi-source response information is synchronously obtained under the stable and continuous multi-layer stress state, and spatial reconstruction is carried out, so that the evolution characteristics of rock mass gradient damage are completely revealed, and a reliable test means for studying the rock mass damage mechanism under the non-uniform stress unloading condition is provided.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering and rock mechanics testing technology, specifically to a rock unloading gradient failure test method based on non-uniform stress distribution. Background Technology

[0002] Rock unloading gradient failure test based on non-uniform stress distribution refers to a rock mechanics test method that artificially constructs a non-uniform stress distribution state that gradually changes from the surface to the depth of the surrounding rock during the excavation or tunneling of deep underground engineering under laboratory conditions, and then implements differentiated unloading processes according to spatial levels. This test simulates the process of the original rock stress transforming from triaxial compressive stress to a multi-directional non-uniform unloading state under excavation disturbance, creating stress gradient zones with different stress release degrees and energy accumulation states at different depths within the sample. This reproduces the multi-level failure evolution behavior of the surrounding rock under stress redistribution conditions. In this way, the gradient development mechanism of rock mass from local damage to overall failure under the combined action of non-uniform stress field and layered unloading can be revealed experimentally, more closely reflecting the actual stress and failure process of surrounding rock in deep tunnels.

[0003] Existing technologies have the following shortcomings: Due to limitations in the experimental setup and loading control methods, it is difficult to construct and maintain a stable, long-term non-uniform stress distribution that gradually changes from the surface to the depth within the sample. During loading and unloading, stress continuously evolves over time, but existing technologies lack closed-loop adjustment mechanisms for spatial stress gradients. This makes it impossible to synchronously feedback and dynamically correct stress changes at different depths, easily leading to localized stress shifts or distortion of the overall stress distribution. Consequently, the gradual stress release and redistribution process experienced by the surrounding rock under actual engineering excavation conditions cannot be realistically, continuously, and repeatably reconstructed in a laboratory environment.

[0004] The information disclosed in the background section is only intended to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a rock unloading gradient failure test method based on non-uniform stress distribution, so as to solve the problems in the background art mentioned above.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a rock unloading gradient failure test method based on non-uniform stress distribution, comprising the following steps: In the true triaxial servo loading device, the rock sample is divided into multiple layers along the thickness direction, and different loads are applied to each layer to form a non-uniform stress gradient that continuously changes from the surface to the depth inside the rock sample, which serves as the initial stress distribution state for the unloading process. Based on the formation of non-uniform stress gradient, the load of each layer is reduced sequentially from the outside to the inside in a spatial order, and the rate of change of the load of the outer layer is controlled to be greater than the rate of change of the load of the inner layer, so that the non-uniform stress gradient evolves in an orderly manner over time, forming a graded unloading state. During the staged unloading process, stress change information is collected at the corresponding positions of each layer and fed back to the true triaxial servo loading device in real time. The load output of each layer is adjusted synchronously according to the preset non-uniform stress gradient pattern, so that the multi-layer stress state formed during the staged unloading process remains stable and continuous. Under a stable and continuous multi-layered stress state, strain acquisition structures, acoustic acquisition structures, and temperature acquisition structures are arranged at different depths. Combined with the sample surface deformation acquisition method, the correspondence between the multi-layered stress change process and the internal energy accumulation process and crack evolution process is established. Based on the correspondence between the multi-layer stress change process, energy accumulation process, and crack evolution process, the stress distribution, energy distribution, and crack evolution at different stages are spatially reconstructed in the data processing platform to obtain the evolution process of rock mass gradient failure under non-uniform stress unloading conditions.

[0007] Preferably, the steps for constructing a non-uniform stress gradient that continuously varies from the surface to the depth are as follows: A rock sample is placed in the loading space of a true triaxial servo loading device, with the thickness direction aligned with the layering direction, and a multi-layer clamping structure arranged layer by layer along the thickness direction is set around the outer surface of the rock sample. The rock sample is divided into multiple stress layers along the thickness direction, so that each stress layer has a clear boundary and a continuous stress relationship is formed through the transition contact surface; Each stress layer corresponds to an independent load application channel, and loads of different magnitudes and different rates of change are applied to each stress layer through independent hydraulic cylinders, so that the internal stress forms a continuously varying structure along the thickness direction, and a controllable gradient is formed by using any one of linear gradient, power function gradient or piecewise gradient. This ensures that each stress layer maintains its established load output state, and maintains a continuous and progressive relationship between layers through structural connection and output coordination, so that the formed non-uniform stress gradient serves as the initial stress distribution state during the unloading process.

[0008] Preferably, when the independent hydraulic cylinder applies differentiated loads to different stress layers, the load transfer component and the multi-layer clamping structure are fixedly connected in a surface contact manner, so that the load is evenly diffused within the corresponding stress layer range, and the force change relationship between adjacent stress layers is maintained through output coordination during the load holding phase, so that the internal stress forms a continuous transition structure in the thickness direction.

[0009] Preferably, the steps for forming the staged unloading state are as follows: Before the unloading stage begins, the load state of each stress layer formed during the non-uniform stress gradient construction stage remains unchanged, and the spatial order of the outer stress layer and the inner stress layer is determined in the thickness direction, so that the outer stress layer is the unloading starting layer. The loads on the outer and inner stress layers are reduced sequentially from the outside to the inside, so that the stress in the outer region is released first and the unloading process is advanced inward along the thickness direction. The load change rate of the outer stress layer is made greater than that of the inner stress layer, and a time relationship is formed between the release of outer stress and the delayed release of inner stress during the load reduction process, so that the non-uniform stress gradient continues to evolve over time. This process creates multiple stress release layers, including the outer stress layer, the intermediate stress layer, and the deep stress layer, during the unloading process, resulting in a graded unloading state inside the rock sample.

[0010] Preferably, the steps for maintaining a stable and continuous multi-layered stress state during the staged unloading process are as follows: Before the graded unloading begins, stress change information acquisition positions are set at the corresponding spatial positions of each stress layer according to the stress layer division results. The acquisition positions correspond to each stress layer from the outside to the inside along the thickness direction and continuously acquire stress change information of each stress layer. The stress change information of each stress layer that is continuously acquired is transmitted to the true triaxial servo loading device in real time according to the stress layer number sequence, so that the true triaxial servo loading device can maintain real-time monitoring of the stress state of multiple layers throughout the entire process of graded unloading. The true triaxial servo loading device synchronously adjusts the load output of each stress layer according to the preset non-uniform stress gradient pattern, so that the stress level of the outer layer, middle layer and deep layer during unloading and propulsion maintains a gradient relationship that matches the spatial position. This ensures that the multi-layered stress state after synchronous adjustment remains continuous in both spatial and temporal dimensions, enabling the outer, middle, and deep layers to form a stable multi-layered stress evolution structure.

[0011] Preferably, the steps for establishing the correspondence between the multi-layer stress change process, the internal energy accumulation process, and the crack evolution process under a stable and continuous multi-layer stress state are as follows: Strain acquisition structures are set at different depths along the thickness direction of the rock sample, so that the strain acquisition structures are in contact with the internal medium of the rock sample and continuously acquire strain change information at each depth. An acoustic acquisition structure is set up at the same depth as the strain acquisition structure, so that the acoustic acquisition structure continuously acquires acoustic change information at each depth position, and the acoustic change information corresponds to the strain change information in time. A temperature acquisition structure is set at the same depth as the strain acquisition structure and the acoustic acquisition structure, so that the temperature acquisition structure continuously acquires temperature change information at each depth position, and establishes a corresponding relationship between the temperature change information and the strain change information and the acoustic change information. A surface deformation acquisition method is set on the outer surface of the rock sample to establish an internal-external correspondence between the external surface deformation changes and the strain, acoustic and temperature changes at various depths, thereby establishing a correspondence between the multi-layer stress change process and the internal energy accumulation and crack evolution process.

[0012] Preferably, when establishing the correspondence between the multi-layer stress change process and the internal energy accumulation process and crack evolution process, the strain acquisition structure, acoustic acquisition structure and temperature acquisition structure are kept in fixed contact at each depth position, and the outer surface deformation acquisition method continuously acquires the outer surface deformation change, so that the outer surface deformation change and the strain change information, acoustic change information and temperature change information at each depth position form a continuous correspondence.

[0013] Preferably, the spatial reconstruction steps for stress distribution, energy distribution, and fracture evolution at different stages in the data processing platform are as follows: Throughout the entire process of graded unloading, the stress change information, strain change information, acoustic change information, temperature change information and surface deformation change information at each depth are organized in chronological order and stage calibration is completed, so that each stage corresponds to a different unloading level. After completing the stage calibration, the stress change information at different depths within each stage is mapped to the spatial framework of the data processing platform according to spatial coordinates, so that the outer region, the middle region and the deep region form a continuously changing spatial stress distribution. In a spatial framework, the energy accumulation characterization and the fracture evolution characterization are superimposed at the corresponding depth position, so that the energy distribution, fracture evolution and stress distribution are spatially aligned and form an isotopic expression of multiple physical quantities. The results of each stage of spatial representation are spliced ​​and output in chronological order, so that the outer layer release stage, the intermediate transition stage and the deep release stage form a continuous evolution sequence, thereby obtaining the evolution process of rock mass gradient failure under non-uniform stress unloading conditions.

[0014] The technical effects and advantages provided by the present invention in the above technical solution are as follows: This invention constructs a non-uniform stress gradient that continuously varies from the surface to the depth under true triaxial servo loading conditions. During unloading, it incorporates a staged unloading method from the outside in, ensuring that the rock sample remains within a controllable spatial stress gradient environment throughout the entire test. This allows for a realistic reflection of the gradual stress release and redistribution process experienced by the surrounding rock in deep underground engineering under excavation disturbance conditions. Real-time adjustment and stable maintenance of multi-layered stress states ensure the continuity and consistency of the stress evolution path formed during the test, effectively improving the simulation capability of the test process for the actual stress state of the engineering project, and making the test results more representative and repeatable.

[0015] This invention, under a stable and continuous multi-layered stress state, simultaneously acquires strain, acoustic, and temperature response information at different depths. Combined with changes in sample surface deformation, it establishes a clear correspondence between stress variation and internal energy accumulation and crack evolution. Furthermore, through spatial reconstruction, it presents a holistic view of stress distribution, energy distribution, and crack evolution at different stages, thus providing a direct description of the entire process of rock mass gradient failure. This method allows the evolutionary characteristics of rock mass from localized damage to gradual expansion and then to overall failure to be fully displayed in both spatial and temporal dimensions, providing reliable experimental evidence for revealing the failure mechanism of rock mass under non-uniform stress unloading conditions. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0017] Figure 1 This is a flowchart of the rock unloading gradient failure test method based on non-uniform stress distribution according to the present invention. Detailed Implementation

[0018] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided so that the description of this disclosure will be more complete and fully convey the concept of the exemplary embodiments to those skilled in the art.

[0019] This invention provides, for example Figure 1 The rock unloading gradient failure test method shown includes the following steps: In the true triaxial servo loading device, the rock sample is divided into multiple layers along the thickness direction, and different loads are applied to each layer to form a non-uniform stress gradient that continuously changes from the surface to the depth inside the rock sample, which serves as the initial stress distribution state for the unloading process. To create a non-uniform stress gradient that continuously varies from the surface to the depth within the rock sample, serving as the initial stress distribution during the unloading process, layered division and zoned loading are performed in a true triaxial servo loading device to form a controllable spatial stress transition structure. This is implemented specifically according to the following steps: A rock sample with a standard geometric shape is selected within the loading space of a true triaxial servo loading device. The thickness direction of the rock sample is defined as the direction from the surface to the depth, with the two ends of the thickness direction corresponding to the surface end face and the deep end face, respectively. The rock sample is placed at the center of the loading space, ensuring that the thickness direction is consistent with the layering direction of the partitioned loading structure, and that the layering direction is spatially consistent with the subsequent stress gradient direction. A multi-layered clamping structure is arranged around the outer surface of the rock sample, with each layer arranged sequentially along the thickness direction. The clamping structure is in close contact with the outer surface of the rock sample, and each layer of the clamping structure forms an independent stress-bearing zone.

[0020] Subsequently, based on the thickness of the rock sample, the number of stress layers and the thickness of each layer were determined. A hierarchical arrangement from the surface to the depth was adopted, dividing the rock sample into multiple stress layers along the thickness direction. These stress layers were arranged sequentially adjacent to each other in space, with the surface layer corresponding to the first stress layer and the deeper layers corresponding to the final stress layers, ensuring that each stress layer has a clear boundary and a defined location of action. To avoid abrupt changes in stress between layers, a transition contact surface was provided at the boundary between adjacent stress layers in the multi-layer clamping structure, ensuring a continuous connection in the stress transmission between adjacent stress layers. This provides a structural basis for the subsequent formation of a continuously varying non-uniform stress gradient.

[0021] After layering, each stress layer is assigned an independent load application channel, which is connected to an independent hydraulic cylinder. This allows different stress layers to be subjected to loads of varying magnitudes and rates of change. To ensure stable load transfer, the output end of the independent hydraulic cylinder is fixedly connected to the corresponding stress layer's clamping structure. The output end distributes the load evenly across the corresponding stress layer region via a force transfer component. The force transfer component and the clamping structure maintain surface contact, ensuring uniform load diffusion within the stress layer. During load application, a true triaxial loading device is first established to create a consistent confining pressure environment, placing the rock sample under triaxial stress constraint. Then, while maintaining this overall confining pressure, differentiated loads are sequentially applied to different stress layers, creating progressively varying internal stress levels from the surface to deeper layers.

[0022] In the linear gradient form, the load difference between adjacent stress layers remains consistent; in the power function gradient form, the load increase of the deep stress layer is higher than that of the surface stress layer; in the piecewise gradient form, the thickness direction is divided into multiple gradient segments, each gradient segment maintains a fixed difference change, and different difference changes are used between different gradient segments, thereby matching the differentiated needs of hard rock, soft rock, and bedding rock mass in the deep stress field.

[0023] After the differential load is applied, each stress layer is kept in its current load output state, and the interlayer continuous gradient relationship is maintained to ensure that the internal stress of the rock sample changes continuously along the thickness direction, without abrupt changes or local shifts between layers. The maintenance process is carried out simultaneously from both structural and control aspects: structurally, the transition contact surface of the multi-layer clamping structure is used to ensure that the force transmission path of adjacent stress layers is continuously connected, avoiding sudden changes in stress between layers due to stiffness differences; in terms of control, the output of independent hydraulic cylinders is synchronously coordinated according to the layer sequence to ensure that the load output of the surface stress layer and the deep stress layer is stable and consistent during the maintenance phase, preventing the overall gradient morphology from being destroyed due to output fluctuations in a certain stress layer.

[0024] To ensure the requirement of continuous stress gradient from the surface to the depth, the load difference and load change rate difference between adjacent stress layers are fixed during the maintenance stage. This maintains the load difference as the gradient distribution result determined in the construction stage and the load change rate as the layer-by-layer rate result determined in the construction stage, thereby stabilizing the internal stress gradient over time. At this point, a clear spatial stress distribution structure is formed inside the rock sample: the surface region is at a lower stress level, gradually rising to a higher stress level towards the depth region, with the stress continuously varying along the thickness direction, forming an initial non-uniform stress distribution that can be used to simulate the state of the surrounding rock before excavation.

[0025] After forming and maintaining a continuously varying non-uniform stress gradient, the current gradient state is defined as the initial stress distribution state of the unloading process. The layered division and load distribution relationships serve as the direct basis for the unloading stage, enabling subsequent unloading processes to proceed along predetermined spatial layers. To prepare for this, firstly, the stress layer numbers and spatial order are fixed, with the surface stress layer serving as the outer starting layer and the deep stress layer as the inner terminating layer. The spatial order remains consistent from the outside in, ensuring that subsequent unloading paths can directly utilize the same layered structure in the spatial hierarchy. Secondly, the load magnitude and load change rate of each stress layer are used as initial parameters for the unloading stage, allowing for graded unloading from the existing gradient state without altering the layered structure. Finally, while maintaining the three-dimensional constraints of the true triaxial servo loading device, the partitioned loading structure remains in a controllable output state. This provides direct input conditions for subsequent graded unloading, where the load of each layer is reduced sequentially from the outside in according to a preset spatial order, and the outer layer load changes faster than the inner layer. This completes the establishment and preparation of the initial stress distribution state for the unloading process.

[0026] Based on the formation of non-uniform stress gradient, the load of each layer is reduced sequentially from the outside to the inside in a spatial order, and the rate of change of the load of the outer layer is controlled to be greater than the rate of change of the load of the inner layer, so that the non-uniform stress gradient evolves in an orderly manner over time, forming a graded unloading state. A non-uniform stress gradient, continuously varying from the surface to the depth, has already formed inside the rock sample. Based on this non-uniform stress gradient being determined as the initial stress distribution state during the unloading process, the rock sample undergoes a graded unloading process to allow the non-uniform stress gradient to evolve in an orderly manner over time. The specific implementation process includes the following steps: Before entering the unloading stage, the load state of each stress layer formed during the non-uniform stress gradient construction stage is kept unchanged, and this load state is used as the starting condition for the unloading stage. The spatial arrangement order of each stress layer is clearly defined, with the stress layer closest to the outer surface of the rock sample being identified as the outer stress layer, and the stress layers closest to the interior of the rock sample being identified as the inner stress layers, arranged sequentially from the outside to the inside along the thickness direction. Based on this, the load application channels corresponding to each stress layer are numbered, so that the load channels corresponding to the outer stress layers are located at the beginning of the unloading sequence, and the load channels corresponding to the inner stress layers are located at the subsequent positions in the unloading sequence. In this way, the non-uniform stress gradient is clearly decomposed into multiple unloading levels with a sequential relationship in space, so that the subsequent unloading process can strictly proceed in a spatial order from the outside to the inside, and ensure that the stress distribution at the start of unloading is completely consistent with the non-uniform stress gradient formed in the previous stage.

[0027] Based on a clear spatial unloading sequence and maintaining the initial load state, the loads of each stress layer are reduced sequentially from the outside in. Specifically, the load corresponding to the outermost stress layer is first controlled by a channel, causing the load output of this outermost stress layer to decrease, thus releasing the stress in the outer region first. During the load reduction of the outer stress layer, the inner stress layer maintains its original load output state, keeping the inner region at a relatively high stress level during this stage. Once the load of the outer stress layer decreases to a preset level, the load reduction operation begins on the adjacent inner stress layer, advancing the unloading process inward along the thickness direction.

[0028] During the process of gradually reducing the load from the outside in, the load change rates of different stress layers are clearly distinguished and controlled, ensuring that the load change rate of the outer stress layer is always greater than that of the inner stress layer. Specifically, after the outer stress layer begins to unload, the output of the load channel is adjusted to maintain a high load reduction rate per unit time, thus allowing the stress in the outer region to be released quickly. When unloading progresses to the inner stress layer, the rate of load reduction is limited, ensuring that the load reduction rate per unit time is significantly less than that of the outer stress layer, thus delaying the stress release process in the inner region. This method ensures that the stress in the outer region is released first, followed by the stress in the inner region, creating a temporal misalignment.

[0029] Under the condition of progressively reducing the load layer by layer from the outside to the inside, while maintaining a higher rate of change for the outer layer load than for the inner layer load, the original non-uniform stress gradient inside the rock sample undergoes an orderly evolution over time, ultimately forming a graded unloading state. This graded unloading state is characterized by: the outer stress layer being at a low stress level and having completed its main stress release; the middle stress layer being in a transitional state of continuous release; and the deep stress layer maintaining a higher stress level and gradually entering the release stage. By maintaining the correspondence between the spatial order of unloading and the rate of change of load, the phenomenon of parallel evolution of multiple stress states exists within the rock sample throughout the unloading process, thus forming a clear stress release hierarchy structure within the sample. This structure is consistent with the actual process of gradual unloading and stress redistribution of surrounding rock from the outside to the inside under excavation disturbance conditions in deep underground engineering, providing a stable and continuous stress evolution basis for subsequent stress change information acquisition, stress state maintenance, and failure evolution observation around the graded unloading state.

[0030] During the staged unloading process, stress change information is collected at the corresponding positions of each layer and fed back to the true triaxial servo loading device in real time. The load output of each layer is adjusted synchronously according to the preset non-uniform stress gradient pattern, so that the multi-layer stress state formed during the staged unloading process remains stable and continuous. Based on the rock sample being in a staged unloading state progressing layer by layer from the outside in and maintaining a non-uniform stress gradient evolution path, the stress state of each layer is continuously acquired during the staged unloading process. The stress change information is transmitted to the true triaxial servo loading device in real time, and the load output of each layer is synchronously adjusted according to the preset non-uniform stress gradient shape, so that the multi-layer stress state formed during the staged unloading process remains stable and continuous. Specifically, the following steps are included: Before the staged unloading begins, based on the established stress layer division, stress change information acquisition points are set at spatial locations corresponding to each stress layer. These acquisition points correspond one-to-one with the stress layers and are arranged sequentially from the outside to the inside along the thickness direction of the rock sample. The arrangement of the acquisition points follows a fixed relative relationship principle, ensuring that the acquisition points are spatially consistent with the zoned loading areas. This means that the acquisition points for the outer stress layer correspond to the outer load application area, and the acquisition points for the inner stress layer correspond to the inner load application area, thus guaranteeing that the acquired stress change information reflects the stress changes of the corresponding stress layer during the staged unloading process. During the staged unloading process, the acquisition points continuously acquire stress change information for the corresponding stress layer. The acquisition process covers the rapid unloading stage of the outer stress layer, the transitional unloading stage of the intermediate stress layer, and the slow unloading stage of the deep stress layer. This ensures that the stress change information for each stress layer has temporal continuity, allowing for a continuous description of the stress release degree at different unloading moments and synchronously presenting the stress differences between different stress layers. This provides direct input information for subsequent real-time feedback and synchronous adjustment.

[0031] Based on the continuous acquisition of stress change information at corresponding locations in each layer, the stress change information of each layer is transmitted in real time to the true triaxial servo loading device in the same order as the stress layer number. This ensures that the true triaxial servo loading device holds the current stress state information of the outer stress layer, intermediate stress layer, and deep stress layer at every moment of staged unloading. The transmission process is synchronized with the staged unloading process, so that the stress change information generated when the load of the outer stress layer decreases rapidly can be immediately transmitted to the true triaxial servo loading device, and the stress change information generated when the load of the inner stress layer decreases slowly can be continuously transmitted to the true triaxial servo loading device. This ensures that the true triaxial servo loading device is always in a state of real-time monitoring of the stress state of multiple layers.

[0032] After stress change information is fed back to the true triaxial servo loading device in real time, the preset non-uniform stress gradient pattern determined before the start of staged unloading is called as the target pattern. The adjustment direction of the load output of each layer revolves around this target pattern, ensuring that the outer stress layer, intermediate stress layer, and deep stress layer always maintain a gradient relationship that matches their spatial positions. In specific implementation, when the stress decrease of the outer stress layer deviates from the preset gradient relationship during rapid unloading, the true triaxial servo loading device adjusts the amplitude or rhythm of the corresponding load output of the outer stress layer to bring the stress level of the outer stress layer back to the gradient range that matches its spatial position. When the deep stress layer shows a tendency to release stress prematurely during slow unloading, the true triaxial servo loading device maintains or slows down the load output of the deep stress layer to keep the stress level of the deep stress layer within the gradient range that matches its spatial position. When the intermediate stress layer shows a tendency to become unbalanced during unloading, the true triaxial servo loading device synchronously coordinates the corresponding load output of the intermediate stress layer to maintain a continuous and gradual transition state between the outer and deep layers.

[0033] Under the conditions of continuous acquisition, real-time feedback, and continuous synchronous adjustment of stress change information at each layer, the multi-layered stress state formed during the staged unloading process remains stable and continuous. The specific meaning of stability and continuity includes two aspects: First, in the spatial dimension, the outer stress layer, intermediate stress layer, and deep stress layer maintain a stress distribution relationship that changes progressively from the outside to the inside at any given time, ensuring that the non-uniform stress gradient always exists and continuously evolves during the unloading process; second, in the temporal dimension, the change process of the stress state at each layer is continuous, allowing the stress release process of the outer stress layer to proceed smoothly, the stress transition process of the intermediate stress layer to proceed smoothly, and the stress release process of the deep stress layer to proceed smoothly, thus ensuring the consistency and coherence of the stress evolution path throughout the entire unloading process. By maintaining the aforementioned stable and continuous multi-layered stress state, different levels of stress release and energy accumulation conditions are always present at different depths within the rock sample. This provides a consistent stress basis for subsequent strain acquisition, acoustic acquisition, temperature acquisition, and surface deformation acquisition under a stable and continuous multi-layered stress state. This allows subsequent multi-physical quantity acquisition to be correlated within the same stress evolution framework and further supports the data processing platform in spatially reconstructing stress distribution, energy distribution, and fracture evolution at different stages, thus fully presenting the evolution process of rock mass gradient failure under non-uniform stress unloading conditions.

[0034] Under a stable and continuous multi-layered stress state, strain acquisition structures, acoustic acquisition structures, and temperature acquisition structures are arranged at different depths. Combined with the sample surface deformation acquisition method, the correspondence between the multi-layered stress change process and the internal energy accumulation process and crack evolution process is established. Based on the multi-layered stress state that has been formed and maintained stably during the graded unloading process, strain acquisition structures, acoustic acquisition structures, and temperature acquisition structures are arranged at different depths of the rock sample. Combined with the sample surface deformation acquisition method, the stress change process at different depths can be correlated with the internal energy accumulation process and crack evolution process. The specific implementation process includes the following steps: Under the condition that a stable and continuous multi-layered stress state has been established and maintained, multiple depth positions are determined along the thickness direction of the rock sample according to the aforementioned stress layer division results. Each depth position corresponds to a stress layer center region, ensuring a one-to-one spatial correspondence between strain acquisition positions and stress layers. Subsequently, a strain acquisition structure is set up at each depth position, ensuring close contact between the strain acquisition structure and the internal medium of the rock sample and maintaining a fixed relative position, allowing the strain acquisition structure to reflect the minute deformation process at that depth position. To ensure the comparability of deformation responses of different stress layers, the strain acquisition structures of the outer stress layer, the middle stress layer, and the deep stress layer are all arranged in the same orientation along the thickness direction, ensuring that the obtained strain change information is consistent in the spatial direction.

[0035] During the staged unloading propulsion process, the strain acquisition structure continuously records the strain change information at the corresponding depth position, so that the deformation response of the outer stress layer under a faster unloading rhythm, the deformation response of the intermediate stress layer under a transitional unloading rhythm, and the deformation response of the deep stress layer under a slower unloading rhythm all have temporal continuity. This forms a layered deformation evolution sequence indexed by depth position, providing a depth positioning basis for the subsequent corresponding arrangement of acoustic and temperature responses.

[0036] Based on the deep positioning and continuous acquisition of strain change information by the strain acquisition structure, an acoustic acquisition structure is deployed at the same depth as the strain acquisition structure, ensuring a correspondence between the acoustic and strain acquisition structures within the same stress layer. The acoustic acquisition structures are positioned according to the principles of consistent depth and orientation, with the acoustic acquisition structure for the outer stress layer located within its range and close to its center, the acoustic acquisition structure for the middle stress layer located within its range and close to its center, and the acoustic acquisition structure for the deep stress layer located within its range and close to its center. This allows the acoustic response to exhibit stratified characteristics. During the graded unloading process, the acoustic acquisition structure continuously acquires acoustic signal changes within the rock sample caused by stress adjustment, deformation development, and crack propagation. These acoustic signal changes are correlated with strain changes at the same depth along a time axis, ensuring that the deformation development process within the same stress layer is synchronously presented with the acoustic response process.

[0037] Based on the existing depth-corresponding arrangement of strain and acoustic acquisition structures, which continuously acquire strain and acoustic change information, a temperature acquisition structure is placed at the same depth, establishing a three-way correspondence between the temperature, strain, and acoustic acquisition structures within the same stress layer. The temperature acquisition structure is arranged according to the principle of fixed contact and fixed position, ensuring long-term thermal contact with the internal medium of the rock sample. This allows the acquired temperature change information to reflect the thermal response changes at that depth during unloading. During staged unloading, the temperature acquisition structure continuously acquires temperature change information at different depths, enabling layered recording of temperature changes in the outer stress layer during rapid unloading, the intermediate stress layer during transitional unloading, and the deep stress layer during slow unloading. By establishing a temporal correspondence between temperature change information and strain and acoustic change information at the same depth, the deformation development, acoustic response, and temperature response of the same stress layer during stress changes can be jointly characterized. This provides a correlated multidimensional response characteristic for the internal energy accumulation process and supplements the layered description of crack evolution.

[0038] Based on the existing simultaneous acquisition of strain, acoustic, and temperature change information at different depths within the rock sample, a surface deformation acquisition method is deployed on the outer surface of the sample. This method covers the outer region and extends along the thickness direction to the observation range aligned with the internal stress layer, thus establishing an internal-external correspondence between the outer surface deformation evolution and the response information at different depths. The surface deformation acquisition method continuously acquires the outer surface deformation evolution throughout the staged unloading process. This ensures that the surface deformation changes caused by stress release in the outer stress layer correspond to the strain changes at the outer depth, the deformation expansion trend caused by stress adjustment in the intermediate stress layer corresponds to the acoustic changes at the intermediate depth, and the overall deformation accumulation caused by the lag in stress release in the deep stress layer corresponds to the temperature changes at the deep depth. By acquiring information synchronously inside and outside the time, the multi-layer stress change process can be correlated with the internal energy accumulation process and crack evolution process. Specifically, when the strain change information of a certain stress layer shows a continuous accumulation trend, the corresponding acoustic change information shows an enhanced activity trend, accompanied by a local change trend in temperature change information. At the same time, the outer surface deformation acquisition results show deformation development characteristics consistent with the spatial position of the layer, thus forming a corresponding link of stress change - deformation response - acoustic response - temperature response - surface deformation indexed by depth position.

[0039] Based on the correspondence between the multi-layer stress change process, energy accumulation process and crack evolution process, the stress distribution, energy distribution and crack evolution at different stages are spatially reconstructed in the data processing platform to obtain the evolution process of rock mass gradient failure under non-uniform stress unloading conditions. Based on the established correspondence between the multi-layered stress change process, the internal energy accumulation process, and the crack evolution process during the aforementioned implementation process, the stress distribution, energy distribution, and crack evolution at different stages of the entire staged unloading process are spatially reconstructed using a data processing platform. This allows the evolution process of rock mass gradient failure under non-uniform stress unloading conditions to be presented in the form of a continuous spatial evolution sequence. The specific implementation process includes the following steps: Based on the established correspondence, the collected data from the entire staged unloading process are organized chronologically. This ensures that the data at each sampling moment includes stress change, strain change, acoustic change, temperature change, and surface deformation change information at the same depth. During the organization process, the consistency between depth location and stress layer numbering is maintained, ensuring that the outer layer depth, intermediate depth, and deep depth always correspond to fixed numbers in the data. Subsequently, the staged unloading process is calibrated in stages, ensuring that the stage calibration is consistent with the staged unloading sequence. This allows the outer stress layer to begin reducing load until it completes its main stress release, forming the outer layer release stage; the intermediate stress layer to enter the transitional release stage and continue to evolve, forming the intermediate transitional stage; and the deep stress layer to begin releasing and gradually evolve, forming the deep release stage. This ensures that different stages have clear boundaries on the time axis. After the stage calibration is completed, the actual geometric dimensions of the rock sample and the results of the layer division are imported into the spatial frame of the data processing platform. This ensures that the thickness direction is consistent with the depth direction, and that the outer surface position, the position of each stress layer, and the deep position have fixed coordinate expressions in the spatial frame. This allows the data of all subsequent stages to be mapped under the same spatial coordinates, forming a data foundation after the staged data units and spatial coordinates are unified.

[0040] Based on the unification of staged data units and spatial coordinates, for each stage, stress change information at each depth is input into the data processing platform. The stress values ​​are then mapped to their corresponding spatial locations according to the depth coordinates, mapping the stress values ​​of the outer stress layer to the outer coordinate region, the intermediate stress layer to the intermediate coordinate region, and the deep stress layer to the deep coordinate region. To ensure spatial continuity, a continuous transition expression is established between adjacent stress layers, ensuring a continuous spatial progression in stress distribution between the outer and intermediate layers, and between the intermediate and deep layers. This allows the stress distribution at each stage to exhibit a gradually changing spatial morphology from the outside in. By repeatedly performing the stress mapping process for each stage, the stress distribution at different stages of the graded unloading process is spatially expressed as a stage sequence, thus completing the spatial reconstruction of the stress distribution at different stages.

[0041] Based on the spatial representation of stress distribution at different stages, information related to energy accumulation and crack evolution is introduced into the same spatial framework, enabling spatial superposition of energy distribution and crack evolution with stress distribution. Specifically, strain, acoustic, and temperature changes at the same depth are converted into energy accumulation characteristics at that depth, aligning the energy accumulation characteristics with the stress values ​​on the same coordinate point. Simultaneously, crack evolution characteristics corresponding to acoustic activity changes and surface deformation changes at the same depth are mapped to the same coordinate point, giving crack evolution a layered spatial hierarchy. During spatial superposition, the principle of corresponding spatial coordinates at the same depth within the same stage is maintained, ensuring strict spatial alignment of stress distribution, energy distribution, and crack evolution. This results in the isotopic representation of multiple physical quantities at each stage, allowing the outer region to reflect the corresponding states of outer stress release, energy change, and crack development; the middle region to reflect the corresponding states of intermediate stress transition, energy accumulation, and crack propagation; and the deep region to reflect the corresponding states of deep stress maintenance, energy accumulation, and crack initiation.

[0042] After spatial reconstruction of stress distribution, energy distribution, and fracture evolution at each stage, resulting in a staged spatial representation, the results of each stage are spliced ​​together according to the time sequence of graded unloading. This ensures continuous spatial connection between the outer release stage, intermediate transition stage, and deep release stage, maintaining the same spatial coordinate framework. This allows for direct comparison of spatial changes across different stages, forming a continuous evolution sequence. This continuous evolution sequence reveals the spatial changes in the outer region from an initial loaded state to a rapid release state, the intermediate region from a loaded transition state to a gradual release state, and the deep region from a high-stress maintenance state to a delayed release state. This allows the orderly evolution of the non-uniform stress gradient over time to be fully represented in the spatial representation. Based on the output of the evolution sequence, the spatial coupling relationship between stress distribution, energy distribution, and fracture evolution is utilized to obtain the gradient failure evolution process of the rock mass under non-uniform stress unloading conditions, from localized damage in the outer layer to gradual expansion into the deeper layers, ultimately leading to overall failure. This completes the evolution process of gradient failure of the rock mass under non-uniform stress unloading conditions as defined in the independent claim.

[0043] This invention constructs a non-uniform stress gradient that continuously varies from the surface to the depth under true triaxial servo loading conditions. During unloading, it incorporates a staged unloading method from the outside in, ensuring that the rock sample remains within a controllable spatial stress gradient environment throughout the entire test. This allows for a realistic reflection of the gradual stress release and redistribution process experienced by the surrounding rock in deep underground engineering under excavation disturbance conditions. Real-time adjustment and stable maintenance of multi-layered stress states ensure the continuity and consistency of the stress evolution path formed during the test, effectively improving the simulation capability of the test process for the actual stress state of the engineering project, and making the test results more representative and repeatable.

[0044] This invention, under a stable and continuous multi-layered stress state, simultaneously acquires strain, acoustic, and temperature response information at different depths. Combined with changes in sample surface deformation, it establishes a clear correspondence between stress variation and internal energy accumulation and crack evolution. Furthermore, through spatial reconstruction, it presents a holistic view of stress distribution, energy distribution, and crack evolution at different stages, thus providing a direct description of the entire process of rock mass gradient failure. This method allows the evolutionary characteristics of rock mass from localized damage to gradual expansion and then to overall failure to be fully displayed in both spatial and temporal dimensions, providing reliable experimental evidence for revealing the failure mechanism of rock mass under non-uniform stress unloading conditions.

[0045] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A rock unloading gradient failure test method based on non-uniform stress distribution, characterized in that, Includes the following steps: In the true triaxial servo loading device, the rock sample is divided into multiple layers along the thickness direction, and different loads are applied to each layer to form a non-uniform stress gradient that continuously changes from the surface to the depth inside the rock sample. Based on the formation of non-uniform stress gradient, the load of each layer is reduced sequentially from the outside to the inside in a spatial order, and the rate of change of the load of the outer layer is controlled to be greater than the rate of change of the load of the inner layer, so that the non-uniform stress gradient evolves in an orderly manner over time, forming a graded unloading state. During the staged unloading process, stress change information is collected at the corresponding positions of each layer and fed back to the true triaxial servo loading device in real time. The load output of each layer is adjusted synchronously according to the preset non-uniform stress gradient pattern, so that the multi-layer stress state formed during the staged unloading process remains stable and continuous. Under a stable and continuous multi-layered stress state, strain acquisition structures, acoustic acquisition structures, and temperature acquisition structures are arranged at different depths. Combined with the sample surface deformation acquisition method, the correspondence between the multi-layered stress change process and the internal energy accumulation process and crack evolution process is established. Based on the correspondence between the multi-layer stress change process, energy accumulation process, and crack evolution process, the stress distribution, energy distribution, and crack evolution at different stages are spatially reconstructed in the data processing platform to obtain the evolution process of rock mass gradient failure under non-uniform stress unloading conditions.

2. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 1, characterized in that, The steps to construct a non-uniform stress gradient that continuously varies from the surface to the depth are as follows: A rock sample is placed in the loading space of a true triaxial servo loading device, with the thickness direction aligned with the layering direction, and a multi-layer clamping structure arranged layer by layer along the thickness direction is set around the outer surface of the rock sample. The rock sample is divided into multiple stress layers along the thickness direction, so that each stress layer has a clear boundary and a continuous stress relationship is formed through the transition contact surface; Each stress layer corresponds to an independent load application channel, and loads of different magnitudes and different rates of change are applied to each stress layer through independent hydraulic cylinders, so that the internal stress forms a continuously varying structure along the thickness direction, and a controllable gradient is formed by using any one of linear gradient, power function gradient or piecewise gradient. This ensures that each stress layer maintains its established load output state, and maintains a continuous and progressive relationship between layers through structural connection and output coordination, so that the formed non-uniform stress gradient serves as the initial stress distribution state during the unloading process.

3. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 2, characterized in that, When the independent hydraulic cylinder applies differentiated loads to different stress layers, the load transfer component and the multi-layer clamping structure are fixedly connected in a surface contact manner, so that the load is evenly diffused within the corresponding stress layer range. During the load holding phase, the force change relationship between adjacent stress layers is maintained through output coordination, so that the internal stress forms a continuous transition structure in the thickness direction.

4. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 2, characterized in that, The steps for forming a staged unloading state are as follows: Before the unloading stage begins, the load state of each stress layer formed during the non-uniform stress gradient construction stage remains unchanged, and the spatial order of the outer stress layer and the inner stress layer is determined in the thickness direction, so that the outer stress layer is the unloading starting layer. The loads on the outer and inner stress layers are reduced sequentially from the outside to the inside, so that the stress in the outer region is released first and the unloading process is advanced inward along the thickness direction. The load change rate of the outer stress layer is made greater than that of the inner stress layer, and a time relationship is formed between the release of outer stress and the delayed release of inner stress during the load reduction process, so that the non-uniform stress gradient continues to evolve over time. This process creates multiple stress release layers, including the outer stress layer, the intermediate stress layer, and the deep stress layer, during the unloading process, resulting in a graded unloading state inside the rock sample.

5. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 4, characterized in that, The steps to maintain a stable and continuous multi-layered stress state during the staged unloading process are as follows: Before the graded unloading begins, stress change information acquisition positions are set at the corresponding spatial positions of each stress layer according to the stress layer division results. The acquisition positions correspond to each stress layer from the outside to the inside along the thickness direction and continuously acquire stress change information of each stress layer. The stress change information of each stress layer that is continuously acquired is transmitted to the true triaxial servo loading device in real time according to the stress layer number sequence, so that the true triaxial servo loading device can maintain real-time monitoring of the stress state of multiple layers throughout the entire process of graded unloading. The true triaxial servo loading device synchronously adjusts the load output of each stress layer according to the preset non-uniform stress gradient pattern, so that the stress level of the outer layer, middle layer and deep layer during unloading and propulsion maintains a gradient relationship that matches the spatial position. This ensures that the multi-layered stress state after synchronous adjustment remains continuous in both spatial and temporal dimensions, enabling the outer, middle, and deep layers to form a stable multi-layered stress evolution structure.

6. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 5, characterized in that, The steps to establish the correspondence between the multi-layer stress change process, the internal energy accumulation process, and the crack evolution process under a stable and continuous multi-layer stress state are as follows: Strain acquisition structures are set at different depths along the thickness direction of the rock sample, so that the strain acquisition structures are in contact with the internal medium of the rock sample and continuously acquire strain change information at each depth. An acoustic acquisition structure is set up at the same depth as the strain acquisition structure, so that the acoustic acquisition structure continuously acquires acoustic change information at each depth position, and the acoustic change information corresponds to the strain change information in time. A temperature acquisition structure is set at the same depth as the strain acquisition structure and the acoustic acquisition structure, so that the temperature acquisition structure continuously acquires temperature change information at each depth position, and establishes a corresponding relationship between the temperature change information and the strain change information and the acoustic change information. A surface deformation acquisition method is set on the outer surface of the rock sample to establish an internal-external correspondence between the external surface deformation changes and the strain, acoustic and temperature changes at various depths, thereby establishing a correspondence between the multi-layer stress change process and the internal energy accumulation and crack evolution process.

7. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 6, characterized in that, When establishing the correspondence between the multi-layer stress change process and the internal energy accumulation process and crack evolution process, the strain acquisition structure, acoustic acquisition structure and temperature acquisition structure are kept in fixed contact at each depth position, and the outer surface deformation acquisition method continuously acquires the outer surface deformation change, so that the outer surface deformation change and the strain change information, acoustic change information and temperature change information at each depth position form a continuous correspondence.

8. The rock unloading gradient failure test method based on non-uniform stress distribution according to claim 6, characterized in that, The steps for spatial reconstruction of stress distribution, energy distribution, and fracture evolution at different stages in the data processing platform are as follows: Throughout the entire process of graded unloading, the stress change information, strain change information, acoustic change information, temperature change information and surface deformation change information at each depth are organized in chronological order and stage calibration is completed, so that each stage corresponds to a different unloading level. After completing the stage calibration, the stress change information at different depths within each stage is mapped to the spatial framework of the data processing platform according to spatial coordinates, so that the outer region, the middle region and the deep region form a continuously changing spatial stress distribution. In a spatial framework, the energy accumulation characterization and the fracture evolution characterization are superimposed at the corresponding depth position, so that the energy distribution, fracture evolution and stress distribution are spatially aligned and form an isotopic expression of multiple physical quantities. The results of each stage of spatial representation are spliced ​​and output in chronological order, so that the outer layer release stage, the intermediate transition stage and the deep release stage form a continuous evolution sequence, thereby obtaining the evolution process of rock mass gradient failure under non-uniform stress unloading conditions.

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