Method for determining critical anchoring length of anchor rod based on numerical simulation

Abstract

The invention relates to the technical field of anchoring structure design, in particular to a method for determining the critical anchoring length of an anchor rod based on numerical simulation. Firstly, the critical anchoring length is determined, a short anchor rod pull-out test is conducted on site, a pull-out force and displacement curve of the anchor rod is obtained through the field test, the pull-out displacement curve is converted into the relation between shear stress and shear displacement through a formula, then numerical simulation is conducted, FLAC3D is selected for numerical simulation of the pull-out test, and the tensile strength of the anchor rod is improved. By increasing the nodes of the pile structure units and continuously increasing the drawing length of the anchor rod, a large number of tests do not need to be carried out on site, the influence of site environment and human factors on the determination of the anchoring length of the anchor rod does not need to be considered, manpower and material resources are saved, and the mechanical property of site surrounding rock is not destructed; and engineering can be well adapted.

Description

Technical Field

[0001] The present invention relates to the technical field of anchoring structure design, and particularly relates to a method for determining the critical anchoring length of an anchor bolt based on numerical simulation. Background Technique

[0002] At present, anchor bolts are widely used in civil engineering, water conservancy, mining and other projects in China, and have become one of the key supporting means in underground engineering. Anchor bolt support has significant advantages such as high economic efficiency and good support effect. The anchor bolt support changes the mechanical state of the surrounding rock itself through the anchor bolts inside the surrounding rock, forms an integral and stable rock belt around the roadway, and utilizes the combined action of the anchor bolts and the surrounding rock to achieve the purpose of maintaining the stability of the building. The anchoring section length of the anchor bolt has a critical length. When the anchoring section length does not reach the critical length, with the increase of the anchoring length, the anchoring performance of the anchor bolt has a certain improvement. When the anchoring section length exceeds the critical anchoring length, the anchoring performance of the anchor bolt no longer has an obvious improvement. Accurately determining the critical anchoring length can avoid waste of materials and save engineering costs in actual projects.

[0003] At present, the determination of the critical anchoring length of anchor bolts is mainly through on-site tests, and there are generally two methods for on-site tests. One is to conduct pull-out tests on multiple anchor bolts with different anchoring lengths. This method requires multiple repeated pull-out tests, which causes great disturbance to the surrounding rock around the engineering site, and also requires a large amount of manpower and material resources. The second method is to arrange strain gauges on the treated anchor bolt rod body. As the pull-out force increases, the change of strain at different positions on the rod body is observed to determine the critical anchoring length. The disadvantage of this method is that there are too many wires connecting the strain gauges, which are easy to block the slurry, resulting in insufficient grouting and affecting the test results. At the same time, the strain gauges are easy to be damaged during the test, resulting in the invalidation of the test, so it cannot be well applied to the project and is not convenient enough. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for determining the critical anchoring length of an anchor bolt based on numerical simulation, aiming to solve the technical problems in the prior art that the pull-out test of the anchor bolt needs to consider the influence of the on-site environment and human factors on the determination of the anchoring length of the anchor bolt, consumes a lot of manpower and material resources, has a destructive effect on the mechanical properties of the surrounding rock on-site, cannot be well applied to the project, and is not convenient enough.

[0005] To achieve the above purpose, a method for determining the critical anchoring length of an anchor bolt based on numerical simulation adopted by the present invention includes the following steps:

[0006] Determine the critical anchoring length. Conduct a short anchor bolt pull-out test on-site, obtain the pull-out displacement curve of the anchor bolt through the on-site test, and convert the pull-out displacement curve into the relationship between shear stress and shear displacement using the formula;

[0007] Perform numerical simulation. Select FLAC3D for the numerical simulation of the pull-out test, choose the pile structural element for modeling, and define the relationship between shear stress and shear displacement for each section of the bolt element.

[0008] Add nodes to the pile structural element, continuously increase the pull-out length of the bolt, observe the displacement of the last node of the pile structural element, that is, input the bond-slip relationship.

[0009] When the displacement of the last node of the pile structural element is less than 1×10 -8 m, the anchorage length is the critical anchorage length.

[0010] Among them, in the steps of determining the critical anchorage length, conducting a short bolt pull-out test on site, obtaining the pull-out force-displacement curve of the bolt through the on-site test, and converting the pull-out displacement curve into the relationship between shear stress and shear displacement using the formula:

[0011]

[0012] d represents the bolt diameter, τ represents the shear stress along the bolt, F s represents the shear force on the bolt during the pull-out process, and L represents the anchorage section length.

[0013] However, in another case, the anchorage length of the bolt changes during the test because when the bolt is pulled out, the contact area between the interface of the bolt and the resin decreases. Therefore, in this case, it can be modified to:

[0014]

[0015] where s represents the axial displacement of the bolt during the pull-out process.

[0016] Among them, in the steps of performing numerical simulation and selecting FLAC3D for the numerical simulation of the pull-out test: Select the pile element in FLAC3D for the modeling of the pull-out test.

[0017] Among them, in the steps of performing numerical simulation and selecting FLAC3D for the numerical simulation of the pull-out test: Assign attributes to each section of the pile node element respectively.

[0018] A new method is proposed in this paper to simulate the transfer mechanism of the shear force at the bolt interface. In this study, the shear force along the nodes of the Pile structural element and the grid is regarded as a function of CS_scoh and is independent of the friction angle property when CS_sfric is set to zero. According to the logic of the Pile structural element in FLAC, only one set of properties can be defined for one bolt element. Therefore, in order to obtain the relationship of the local interface bonding stress of the bolt, the bolt can be defined as multiple separated and connected bolt elements. The FISH function in FLAC is used to achieve the connection between bolt elements.

[0019] For the conversion of the bond-slip relationship, by using CS_sctable to define the relationship between the shear stress and the relative displacement per unit length of the contributing element, the bond-slip relationship can be input into the bolt element through conversion. According to the definition in FLAC, CS_scoh [N / m] is the cohesive strength of the shear coupling spring, and the characteristics of the shear bolt coupling spring can be defined as:

[0020] cs scoh = πd b τ

[0021] Where:

[0022] d b is the bolt diameter

[0023] τ represents the shear stress along the bolt.

[0024] Therefore, the mathematical relationship of the bond-slip model can be transformed into the relationship between the shear stress per unit length and the shear displacement through an equation.

[0025] Where, in the step of increasing the nodes of the pile structural element and continuously increasing the bolt pulling length:

[0026] Taking the displacement of the bolt pulling end node of 2 mm as the standard, continuously pulling, when the displacement is less than 1×10 -8 m, the anchorage length is the critical anchorage length.

[0027] A method for determining the critical anchorage length of a bolt based on numerical simulation according to the present invention first determines the critical anchorage length, conducts a short bolt pulling test on site, obtains the bolt pulling force and displacement curve through the on-site test, converts the pulling force displacement curve into the relationship between the shear stress and the shear displacement using a formula, and then conducts a numerical simulation. Select FLAC3D for the numerical simulation of the pulling test. For the modeling of the pulling test, select the pile element in FLAC3D. By increasing the nodes of the pile structural element, continuously increase the bolt pulling length. Taking the displacement of the bolt pulling end node of 2 mm as the standard, continuously pull. When the displacement of the pulling end is less than 1×10 -8When it is m, the anchorage length is the critical anchorage length. Through the above method, it is realized that a large number of on-site tests are not required, the influence of on-site environment and human factors on the determination of the bolt anchorage length does not need to be considered, manpower and material resources are saved, and the mechanical properties of the surrounding rock on site are not damaged, which can well adapt to the project. Brief Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0029] Figure 1 It is a step flowchart of the method for determining the critical anchorage length of a bolt based on numerical simulation of the present invention.

[0030] Figure 2 It is a schematic diagram of the Pile structural unit of the shear coupling spring.

[0031] Figure 3 It is a graph of shear stress and shear displacement of the short bolt pull-out of the present invention.

[0032] Figure 4 It is a three-dimensional numerical simulation model diagram of the pull-out test of the present invention.

[0033] Figure 5 It is a comparison diagram of the shear stress and shear displacement curves and the input curve at two points on the pile structural unit of the present invention.

[0034] Figure 6 It is a schematic diagram of the displacement of the last node unit of the pile node with the increase of the anchorage length of the present invention. Detailed Embodiments

[0035] Please refer to Figures 1 to 6 , the present invention provides a method for determining the critical anchorage length of a bolt based on numerical simulation, including the following steps:

[0036] S100: Determine the critical anchorage length, conduct a short bolt pull-out test on site, obtain the pull-out force and displacement curve of the bolt through on-site tests, and use the formula to convert the pull-out force displacement curve into the relationship between shear stress and shear displacement.

[0037] In this embodiment, first, the critical anchorage length is determined, a short bolt pull-out test is conducted on site, the pull-out force and displacement curve of the bolt are obtained through on-site tests, and the pull-out force displacement curve is converted into the relationship between shear stress and shear displacement using the formula.

[0038] S200: Conduct numerical simulation, and select FLAC3D to conduct numerical simulation of the pull-out test.

[0039] In this embodiment, FLAC3D numerical simulation software is used for simulation. First, modeling is carried out. For the modeling of the pull-out test, the pile element in FLAC3D is selected to simulate the mechanical behavior of the anchor bolt, omitting the steps of establishing the anchor bolt model. At the same time, there is no need to develop a new constitutive relationship, greatly improving the simplicity. After the modeling is completed, it is necessary to simulate the mechanical behavior of the anchor bolt. In FLAC3D, when simulating the mechanical behavior of the anchor bolt, the pile element is selected. When using the cable element to simulate the anchor bolt, the relationship between the axial force and axial strain of the anchor bolt can only be defined as a roughly linearly increasing relationship. When using the pile element to simulate the mechanical behavior of the anchor bolt, when the pull-out force on the anchor bolt reaches the limit, it can still continue to bear the load. A three-dimensional numerical model is established in FLAC3D, and the model size is X×Y×Z = 0.4m×0.4m×0.05m. By substituting the load-displacement curve and shear stress-shear displacement curve obtained from the test into FLAC3D, the length of the anchor bolt is selected to be the same as that in the short anchor bolt pull-out test, which is 50mm. The pile element is used to simulate the anchor bolt, and the anchor bolt is divided into 5 segments, with each part having a length of 10mm. The shear stress-shear displacement curve of each segment of the anchor bolt follows the curve value obtained from the short anchor bolt pull-out test. The parameters of the anchor bolt and the anchoring agent are calibrated respectively through indoor tensile tests and uniaxial tests. Among them, the establishment of the three-dimensional numerical model is as Figure 3 shown;

[0040] To simulate the pull-out force applied to the anchor bolt in the pull-out test, a constant pull-out speed of 1×10 -6 m / s is applied to the top node 1 of the pile element here. Through numerical simulation, the shear stress-shear displacement curves of each node are finally obtained. To verify the accuracy of the numerical simulation, two nodes in the pile structural element are selected, and the shear stress-shear displacement curves of these two nodes are output, and compared with the shear stress and shear displacement curves input into FLAC3D, that is, the shear stress and shear displacement curves obtained from the field test, as shown in Figure 3 . It can be seen from Figure 3 that the curve fitting is very good, indicating that the result of the numerical simulation is very accurate.

[0041] S300: Increase the nodes of the pile structural element and continuously increase the pull-out length of the anchor bolt.

[0042] In this embodiment, in the pull-out test of the bolt, when the anchorage length of the bolt does not reach the critical anchorage length, the shear force of the bolt has an obvious increase as the anchorage length increases. However, when the anchorage length of the bolt exceeds the critical anchorage length, the shear force of the bolt no longer has an obvious increase. This is because in the process of pulling out the bolt, the part exceeding the critical anchorage length basically does not provide shear force. Based on this, in the numerical simulation of the bolt pull-out test process, since the pile structural unit is selected here, because the part where the anchorage length exceeds the critical anchorage length can still provide a very small shear force, the displacement of the pile node is never zero. When the displacement of the bottommost node in the pile structural unit approaches zero, it is defaulted that this node no longer provides shear stress, and at this time, the length of the anchorage section is the critical anchorage length. Here, it is stipulated that when the displacement of the bottommost node of the pile structural unit is less than 1×10 -8 m, the anchorage length is the critical anchorage length;

[0043] In order to determine the critical anchorage length of the pile unit, with 50 mm as a section, by increasing the nodes of the pile structural unit, the bolt pull-out length is continuously increased, and with the displacement of the bolt pull-out end node being 2 mm as the standard, the pull-out is continuously carried out. Here, the pull-out tests with anchorage lengths of 0.15 m, 0.20 m, 0.25 m, 0.30 m, 0.35 m, 0.40 m, and 0.45 m are simulated. As the anchorage length increases, the displacement of the last pile node unit becomes smaller and smaller. When the displacement is less than 1×10 -8 m, the anchorage length is the critical anchorage length. The specific simulation values are shown in Figure 4 .

[0044] In the present invention, first, the critical anchorage length is determined. A short bolt pull-out test is carried out on site. The pull-out force-displacement curve of the bolt is obtained through the on-site test. The pull-out force-displacement curve is converted into the relationship between shear stress and shear displacement by using a formula. Then, numerical simulation is carried out. FLAC3D is selected for the numerical simulation of the pull-out test. The pile unit in FLAC3D is selected for the modeling of the pull-out test. By increasing the nodes of the pile structural unit, the bolt pull-out length is continuously increased, and with the displacement of the bolt pull-out end node being 2 mm as the standard, the pull-out is continuously carried out. When the displacement of the last node unit of the bolt pull-out end, that is, the pile node unit, is less than 1×10 -8 m, the anchorage length is the critical anchorage length. Through the above method, it is realized that a large number of tests do not need to be carried out on site, and the influence of the on-site environment and human factors on the determination of the bolt anchorage length does not need to be considered, saving manpower and material resources, and having no destructive effect on the mechanical properties of the surrounding rock on site, and being able to well adapt to the project.

[0045] The above-disclosed is only a preferred embodiment of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. Those of ordinary skill in the art can understand all or part of the processes of implementing the above embodiments, and the equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A method for determining the critical anchoring length of an anchor rod based on numerical simulation, characterized in that: The steps include: Determine the critical anchoring length, conduct a short anchor pull-out test on site, obtain the anchor pull-out displacement curve through the field test, and use the formula to convert the pull-out displacement curve into the relationship between shear stress and shear displacement; Carry out numerical simulation, and select FLAC3D to carry out numerical simulation of drawing test; Increase the number of nodes in the pile structure unit and continuously increase the anchor rod pulling length.

2. The method for determining the critical anchoring length of an anchor rod based on numerical simulation according to claim 1, characterized in that: In the step of determining the critical anchoring length, conducting a short anchor pull-out test on site, obtaining the anchor pull-out displacement curve through the field test, and converting the pull-out displacement curve into the relationship between shear stress and shear displacement using the formula: Where d is the diameter of the anchor rod, τ is the shear stress along the anchor rod, and F s It represents the shear force on the anchor rod during the pulling process, and L represents the length of the anchoring section; However, in another case, the anchor length of the anchor rod changed during the test because the contact area of ​​the interface between the anchor rod and the resin decreased when the anchor rod was pulled out; therefore, in this case it can be modified to: Where s represents the axial displacement of the anchor rod during the pulling process.

3. The method for determining the critical anchoring length of an anchor rod based on numerical simulation according to claim 1, characterized in that: In the numerical simulation, FLAC3D is used to perform the numerical simulation of the drawing test: The pile element in FLAC3D is selected for modeling of the pulling test.

4. The method for determining the critical anchoring length of an anchor rod based on numerical simulation according to claim 3, characterized in that: In the numerical simulation, FLAC3D is used to perform the numerical simulation of the drawing test: Assign attributes to each pile node unit respectively.

5. The method for determining the critical anchoring length of an anchor rod based on numerical simulation according to claim 1, characterized in that: By increasing the number of nodes in the pile structure unit, the pulling length of the anchor rod is continuously increased: Taking the displacement of the anchor rod pulling end node as 2mm as the standard, the pulling is continued. When the displacement is less than 1×10 -8 m, the anchorage length is the critical anchorage length.

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