Device and method for applying three-dimensional crustal stress to prestressed steel strand in simulated and graded manner

Abstract

The invention belongs to the technical field of deep ground environment mechanical testing, and discloses a device and a method for applying three-dimensional ground stress in a simulated and graded manner by a prestressed steel strand. The device comprises a test piece main body system, a prestress applying system, a positioning reinforcing system and a monitoring feedback system. The core of the method is that based on a post-tensioning prestressing technology, after steel strand rib arrangement and concrete pouring are completed at a time, the same test piece is made to sequentially bear three-dimensional crustal stress of different grades through step-by-step calculation and adjustment of tensioning control force applied to steel strands, and therefore the dynamic evolution process of a crustal stress field is simulated. The problem that it is difficult to achieve multi-stage controllable stress path simulation on the same test piece in a traditional loading mode is solved, and real and stable experimental simulation of the stress grading evolution environment in deep ground engineering is achieved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of deep geoenvironmental mechanics testing, and in particular relates to a device and method for simulating stepwise application of three-directional ground stress by prestressed steel strand. BACKGROUND

[0002] Deep three-directional ground stress is a core feature of the mechanical environment of deep rock mass. With the expansion of major projects such as energy exploitation, deep tunneling, underground storage, and the like, into deeper strata, engineering rock mass is often subjected to a complex and variable ground stress environment. Actual engineering activities (such as excavation, support, grouting) or geological processes (such as tectonic movement, seepage change) often cause dynamic adjustment and redistribution of the ground stress field, which is manifested as stepwise, gradual changes in the level of ground stress over time or construction stages. Therefore, not only is it necessary to simulate a certain fixed ground stress state in the laboratory, but it is also necessary to have the ability to simulate the process of stepwise evolution of ground stress in order to truly reflect the mechanical behavior, damage evolution, and catastrophic mechanism of rock mass under different stress paths.

[0003] At present, the techniques for simulating deep ground stress environment in the laboratory mainly include hydraulic true triaxial loading, mechanical multi-directional pressurization, conventional triaxial testing, and large-scale physical model testing, and the like. These methods generally focus on applying and maintaining a certain set static stress state, and it is difficult to achieve stepwise, controllable adjustment of stress on the same test piece. For example, although the hydraulic system can achieve three-directional independent loading, the system response is complex and the stability control is difficult when performing multi-step stress adjustment, and multiple pressure increases and decreases can introduce sealing and fatigue problems; the mechanical loading method is prone to local stress concentration or unloading rebound when adjusting stress, making it difficult to ensure the smoothness and accuracy of the stress path; the conventional triaxial apparatus cannot achieve multi-step adjustment of the true triaxial stress state due to its loading mechanism. In view of the above problems, there is an urgent need for a device and method that can accurately simulate stepwise application of three-directional ground stress environment. SUMMARY

[0004] In order to overcome the defects of the prior art, the present application provides a device and method for simulating stepwise application of three-directional ground stress by prestressed steel strand.

[0005] The method uses post-tensioning prestressing technology, uses high-strength concrete as a transmission medium, and applies prestress of steel strand to the internal target stress test piece uniformly. On the premise of keeping the configuration (number and arrangement) of the prestressed steel strand unchanged, the stress is controlled by stepwise adjustment of tensioning to apply different levels of three-directional ground stress to the same test piece at different stages, thereby simulating the process of stepwise loading of ground stress or change of stress path, and constructing an efficient and controllable deep three-directional ground stress environment.

[0006] To achieve the above purpose, the technical scheme of the present application is as follows:

[0007] 1. A device and method for simulating the step-by-step application of three-dimensional ground stress by prestressed steel strand, comprising: a test specimen body system, a prestressed force application system, a fixed reinforcement system, and a monitoring feedback system; wherein: the test specimen body system comprises, from the inside out, a target stress test specimen and a high-strength concrete outer layer. The high-strength concrete outer layer bears the pre-compression stress of the steel strand and uniformly transmits the stress to the internal target stress test specimen. The prestressed force application system is composed of a slow-bonding steel strand, a tensioning device, and an anchor. The slow-bonding steel strand is self-contained with anti-corrosion grease and a sheath, which can replace the hole forming and temporary protection functions of the corrugated pipe, and there is no need for additional buried corrugated pipes. The slow-bonding prestressed steel strand is independently arranged in the up-down, left-right, and front-back directions of the test specimen, achieving independent control of three-dimensional stress. The tensioning device is used to apply a preset prestressed force by the post-tensioning method, and the anchor is installed at both ends of the steel strand to fix the prestressed force and prevent stress decay. The fixed reinforcement system is composed of a perforated formwork and a square grid additional steel bar. The formwork is precisely perforated according to the number and arrangement spacing of the steel strand, and is used to fix the position of the slow-bonding steel strand before pouring concrete; the square grid additional steel bar is laid inside the high-strength concrete and cooperatively arranged with the steel strand, having the dual functions of preventing local concrete crushing and secondary fixing of the steel strand. The monitoring feedback system is composed of a resistance strain gauge and a data acquisition instrument. The resistance strain gauge is attached to the surface of the target stress test specimen; the data acquisition instrument is used to collect strain data of the target stress test specimen, calculate the prestressed force value, and realize data storage and analysis.

[0008] Compared with the prior art, the present application has the following beneficial effects:

[0009] 1. The prestressed steel strand is used to apply stress by the post-tensioning method, which can independently control the main stress size in the length, width, and height directions, thereby accurately simulating the deep three-dimensional ground stress environment.

[0010] 2. By step-by-step regulation of the steel strand tensioning force, multi-stage loading from low stress to high stress can be realized on the same test specimen, breaking through the limitation of traditional methods that can only simulate a single static stress state, and restoring the dynamic evolution path of the ground stress in engineering activities.

[0011] 3. After the prestressed steel strand is fixed, its stress state is long-term stable, ensuring the long-term stability of the simulated three-dimensional ground stress field and the reliability of the test results.

[0012] 4. The device structure is relatively flexible, and different sizes of simulation test specimens can be prepared according to test requirements. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 Figure 1 is a schematic diagram of the overall device for simulating the step-by-step application of three-dimensional ground stress by prestressed steel strand.

[0014] Figure 2 Figure 3 is a schematic diagram of the prestressed tendon tensioning end and fixed end node.

[0015] Figure 3 This is a schematic diagram of the overall specimen cross-section and the arrangement of resistance strain gauges.

[0016] Figure 4 This is a schematic diagram of the perforation template.

[0017] Figure 5 This is a schematic diagram of an additional steel reinforcement grid.

[0018] Figure 6 These are three views of the arrangement of prestressed steel strands.

[0019] In the figure: 1. Specimen main system, 2. Prestressing application system, 3. Fixing and reinforcement system, 4. Detection feedback system.

[0020] like Figure 1 As shown, a device for simulating the graded application of triaxial ground stress in prestressed steel strand includes: a specimen main body system 1, a prestressing application system 2, a fixing and reinforcement system 3, and a detection and feedback system 4; wherein: the prestressing application system 2 acts on the specimen main body system 1 to apply ground stress of different directions and magnitudes to the entire specimen; the fixing and reinforcement system 3 realizes the casting and molding of the specimen main body system 1; the detection and feedback system 4 is connected to the surface of the stressed specimen through 41 resistance strain gauges to receive strain signals in real time, and the data acquisition instrument realizes data storage and analysis.

[0021] The main body system 1 of the specimen includes a target stress specimen 11 (taking a cylindrical specimen as an example) and a high-strength concrete outer layer 12, wherein: the high-strength concrete outer layer 12 bears the pre-compression stress of the steel strand and uniformly transfers the stress to the internal target stress specimen.

[0022] The prestressing application system 2 includes a loosely bonded prestressed steel strand 21, a hydraulic tensioning machine 22, a fixed-end compression anchor 23, an anchor plate 24, a spiral stirrup under the anchor 25, and a tensioning-end wedge anchor 26. The loosely bonded prestressed steel strand 21 is equipped with anti-corrosion grease and a sheath, which can replace the duct forming and temporary protection functions of corrugated pipes, eliminating the need for additional corrugated pipe installation and simplifying the construction process. The hydraulic tensioning machine 22 is used to apply pre-set prestress to the loosely bonded prestressed steel strand 21 using a post-tensioning method. The tensioning-end wedge anchor 26 is used to fix the tension of the steel strand. The anchor plate 24 bears the concentrated force and achieves stress diffusion, preventing local crushing. The spiral stirrup under the anchor 25 provides circumferential restraint to counteract splitting tensile stress. The fixed-end compression anchor 23 is pre-embedded in the non-tensioning end of the concrete, providing reliable reaction force for the tensioning operation.

[0023] The fixed reinforcement system 3 includes a perforated template 31 and a grid-reinforced steel bar 32. The template is precisely perforated according to the design quantity and spacing of the steel strands to ensure that each slow-bonded prestressed steel strand can be accurately positioned and fixed in the preset direction and position before concrete pouring. The grid-reinforced steel bar 32 is laid inside the high-strength concrete and is arranged in coordination with the steel strands. When under stress, it enhances the local compressive strength of the high-strength concrete and prevents crushing damage. During the construction stage, it forms a secondary positioning and constraint for the steel strands, effectively ensuring their spatial stability during the pouring and hardening process.

[0024] The detection feedback system 4 includes a resistance strain gauge 41 and a data acquisition instrument 42. The resistance strain gauge 41 is attached to the surface of the target stress specimen. The data acquisition instrument 42 is used to collect the strain data of the target stress specimen, calculate the prestress value, and realize the data storage and analysis.

[0025] The operation method of a device for simulating the graded application of triaxial in-situ stress in prestressed steel strands is as follows:

[0026] (1) Based on the maximum geostress level σ in each direction required for the study, x , σ y , σ z (Final target stress state) Determine the total preload F required in each direction. 总预压 Calculate the total cross-sectional area A of the prestressed steel strand. 总钢绞线 The total number of strands, n, remains unchanged afterward, and the number and arrangement of subsequent steel strands will not be altered.

[0027] F 总预压 =σ j ·A i

[0028]

[0029] σ 有效 =σ con -σ 损失

[0030] Where, σ j It represents the highest stress in each direction (j = x, y, z), A i It is the area of ​​the bearing surface in each direction (i = x, y, z), σ 有效 It is the effective stress value of the steel strand, σ con It is the control stress value of the steel strand, σ 损失 It is the prestress loss value of the steel strand.

[0031] (2) Determine the target prestress value (σ) for each level. x (1) , σ y (1) , σz (1) ), (σ x (2) , σ y (2) , σ z (2) ...

[0032] (3) Calculate the tensioning parameters for each stage.

[0033]

[0034] in, It is the total preload value of the i-th stress, σ j (i) These are the target prestress values ​​in each direction under the i-th stress state (i = 1, 2, 3…). It is the effective stress value of the steel strand under the i-th stress state. It is the control stress value of the steel strand under the i-th stress state. It is the tension force required to be applied to each steel strand under the i-th stress state.

[0035] (4) Design the opening template according to the calculation results in (1) to ensure that the opening positions on each side of the template are consistent with the design coordinates. Prepare the steel reinforcement grid, anchor plate, anchor spiral stirrup, fixed end extrusion anchor and tension end wedge anchor, etc.

[0036] (5) At the fixed end of the specimen, install the extrusion anchor at the end of the prestressing tendon. Pass the prestressing tendon with the extrusion anchor through the corresponding hole in the template and use the template for end positioning. Tie a steel grid inside the template to provide auxiliary support and positioning for the middle section of the prestressing tendon, forming a preliminary spatial skeleton.

[0037] (6) At the tensioning end of the specimen, install the anchor plate tightly against the inside of the template and tie the spiral stirrup behind it. Pass the other end of the prestressing tendon through the area of ​​the anchor plate and spiral stirrup, in preparation for subsequent tensioning.

[0038] (7) Place the target stress specimen in the center of the template frame at the designed position and fix it. Select points on the surface of the target stress specimen and attach resistance strain gauges, lead out wires and connect them to the data acquisition instrument to complete the debugging of the monitoring system.

[0039] (8) Pour high-strength concrete in layers inside the formwork, vibrate to ensure compaction, and ensure that the prestressing tendons and internal specimens do not shift. After pouring, perform standard curing until the concrete reaches the design strength.

[0040] (9) Remove the lateral formwork. Install the tensioning end anchorage and hydraulic tensioning machine at the tensioning end. Tension according to the design tension value. Post-tensioning was performed on the prestressed tendons in all directions under the first-level stress state. During the tensioning process, strain data was collected in real time using a data acquisition instrument to monitor the strain response of the test specimen and verify the prestress transfer effect.

[0041] (10) After the prestressed steel strands in all directions are tensioned and anchored, the system is in a stable first-level triaxial stress state. The stress and strain data are recorded by the data acquisition instrument throughout the test.

[0042] (11) When the i-th level (i>1) loading is required, reinstall the tensioning equipment. First, perform a controlled temporary loosening of the anchorage, and then re-tension all the steel strands to the new target force value. And then re-anchored. This process allows the stress state of the specimen to smoothly transition from level 1 to level i.

Claims

1. A device for simulating the graded application of triaxial ground stress in prestressed steel strands, comprising: The system comprises a main specimen system, a prestressing application system, a fixing and reinforcement system, and a monitoring and feedback system. The main specimen system, from the inside out, consists of the target stress specimen and a high-strength concrete outer layer. The high-strength concrete outer layer bears the prestress of the steel strands and uniformly transfers the stress to the target stress specimen. The prestressing application system consists of loosely bonded steel strands, tensioning equipment, and anchors. The loosely bonded steel strands are evenly distributed along the specimen in three directions: vertical, horizontal, and front-back. The tensioning equipment applies the pre-set prestress using the post-tensioning method, and the anchors are installed at both ends of the steel strands to fix the prestress and prevent stress attenuation. The fixing and reinforcement system consists of perforated templates and a grid of additional reinforcing bars. The templates are pre-drilled according to the design to precisely position each steel strand. The grid of additional reinforcing bars is laid inside the high-strength concrete, working in conjunction with the steel strands to both prevent localized concrete crushing and provide secondary fixation for the steel strands. The monitoring and feedback system consists of resistance strain gauges and a data acquisition instrument. Resistance strain gauges are attached to the surface of the target stress specimen; a data acquisition instrument is used to collect strain data of the target stress specimen, calculate prestress values, and realize data storage and analysis. The operating method of the device used above for simulating the graded application of triaxial ground stress with prestressed steel strand is as follows: (1) Based on the maximum geostress level σ in each direction required for the study, x , σ y , σ z (Final target stress state) Determine the total preload F required in each direction. 总预压 Calculate the total cross-sectional area A of the prestressed steel strand. 总钢绞线 The total number of strands, n, remains unchanged afterward, and the number and arrangement of subsequent steel strands will not be altered. F 总预压 =s j ·A i s 有效 =s con -s 损失 Where, σ j It represents the highest stress in each direction (j = x, y, z), A i It is the area of ​​the bearing surface in each direction (i = x, y, z), σ 有效 It is the effective stress value of the steel strand, σ con It is the control stress value of the steel strand, σ 损失 It is the prestress loss value of the steel strand. (2) Determine the target prestress value (σ) for each level. x (1) , σ y (1) , σ z (1) ), (σ x (2) , σ y (2) , σ z (2) ... (3) Calculate the tensioning parameters for each stage. in, It is the total preload value of the i-th stress, σ j (i) These are the target prestress values ​​in each direction under the i-th stress state (i = 1, 2, 3…). It is the effective stress value of the steel strand under the i-th stress state. It is the control stress value of the steel strand under the i-th stress state. It is the tension force required to be applied to each steel strand under the i-th stress state. (4) Design the opening template according to the calculation results in (1) to ensure that the opening positions on each side of the template are consistent with the design coordinates. Prepare the steel reinforcement grid, anchor plate, anchor spiral stirrup, fixed end extrusion anchor and tension end wedge anchor, etc. (5) At the fixed end of the specimen, install the extrusion anchor at the end of the prestressing tendon. Pass the prestressing tendon with the extrusion anchor through the corresponding hole in the template and use the template for end positioning. Tie a steel grid inside the template to provide auxiliary support and positioning for the middle section of the prestressing tendon, forming a preliminary spatial skeleton. (6) At the tensioning end of the specimen, install the anchor plate tightly against the inside of the template and tie the spiral stirrup behind it. Pass the other end of the prestressing tendon through the area of ​​the anchor plate and spiral stirrup, in preparation for subsequent tensioning. (7) Place the target stress specimen in the center of the template frame at the designed position and fix it. Select points on the surface of the target stress specimen and attach resistance strain gauges, lead out wires and connect them to the data acquisition instrument to complete the debugging of the monitoring system. (8) Pour high-strength concrete in layers inside the formwork, vibrate to ensure compaction, and ensure that the prestressing tendons and internal specimens do not shift. After pouring, perform standard curing until the concrete reaches the design strength. (9) Remove the lateral formwork. Install the tensioning end anchorage and hydraulic tensioning machine at the tensioning end. Tension according to the design tension value. Post-tensioning was performed on the prestressed tendons in all directions under the first-level stress state. During the tensioning process, strain data was collected in real time using a data acquisition instrument to monitor the strain response of the test specimen and verify the prestress transfer effect. (10) After the prestressed steel strands in all directions are tensioned and anchored, the system is in a stable first-level triaxial stress state. The stress and strain data are recorded by the data acquisition instrument throughout the test. (11) When the i-th level (i>1) loading is required, reinstall the tensioning equipment. First, perform a controlled temporary loosening of the anchorage, and then re-tension all the steel strands to the new target force value. And then re-anchored. This process allows the stress state of the specimen to smoothly transition from level 1 to level i.

2. The apparatus and method for simulating graded application of triaxial ground stress in prestressed steel strands according to claim 1, characterized in that: In the graded loading method, the total number and arrangement of prestressed steel strands remain unchanged after the initial configuration is completed. Different levels of triaxial stress simulation are achieved only by adjusting the tension of individual steel strands in stages.

3. The apparatus and method for simulating graded application of triaxial ground stress in prestressed steel strands according to claim 1, characterized in that: The anchorages in the prestressing system include a fixed-end compression anchor, a tensioning-end wedge anchor, an anchor plate, and a spiral stirrup under the anchor. The compression anchor is embedded in the fixed end of the concrete to provide reaction force for tensioning. The wedge anchor is used to tension and lock the steel strands. The anchor plate receives and diffuses the concentrated force. The spiral stirrup provides circumferential restraint to prevent the high-strength concrete from splitting.

4. The apparatus and method for simulating graded application of triaxial ground stress in prestressed steel strands according to claim 1, characterized in that: The template is precisely perforated according to the design quantity and spacing of the steel strands to ensure that each steel strand is fixed in the preset direction and position; the additional reinforcing bars of the grid are arranged in coordination with the steel strands to perform secondary positioning and constraint of the steel strands during the construction stage.

5. The apparatus and method for simulating graded application of triaxial ground stress in prestressed steel strands according to claim 1, characterized in that: Slow-bonded prestressed steel strands come with their own anti-corrosion grease and sheath, which can replace the duct forming and temporary protection functions of corrugated pipes, eliminating the need for additional corrugated pipe installation; the slow-bonded prestressed steel strands are independently arranged in the three directions of the specimen (up, down, left, right, and front and back) to achieve independent control of three-dimensional stress.

6. The apparatus and method for simulating graded application of triaxial ground stress in prestressed steel strands according to claim 1, characterized in that: The high-strength concrete outer layer bears the pre-compression stress of the steel strands, and uniformly transfers the stress to the target stress specimen.

7. The apparatus and method for simulating graded application of triaxial ground stress in prestressed steel strands according to claim 1, characterized in that: The monitoring and feedback system includes resistance strain gauges and a data acquisition instrument to collect and store stress and strain data during the experiment.

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