Underground cavern support strength design method based on excavation unloading strength reduction and confining pressure compensation

Abstract

The invention belongs to the technical field of underground engineering construction, discloses an underground cavern support strength design method based on excavation unloading strength reduction and confining pressure compensation, and aims to determine the underground cavern support strength in an accurate, scientific and efficient manner and give consideration to engineering safety and economy. On the basis of the characteristic that the strength of free face rock mass is reduced due to unloading in the excavation process of an underground cavern, the principle that the rock mass strength can be recovered through confining pressure compensation is utilized, and the quantitative function relation between the confining pressure unloading proportion and the peak strength reduction proportion is established through an indoor rock triaxial compression test and a triaxial confining pressure unloading test; according to the requirement of engineering for the residual strength of surrounding rock, a required confining pressure compensation value is reversely deduced, and then the anchor cable density is calculated according to the equivalent relation between the confining pressure and the supporting force, so that accurate and scientific determination of the supporting strength of the underground cavern is achieved, and engineering safety and economical efficiency are both considered.

Description

Technical Field

[0001] This invention belongs to the field of underground engineering construction technology, specifically relating to a method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation. Background Technology

[0002] Hydropower projects contain numerous underground caverns of various types. These caverns form the core foundation for the safe construction and operation of hydropower stations, and their safety and stability directly affect the overall safety of the hydropower station. During the excavation of underground caverns, the stress in the surrounding rock undergoes significant adjustments. The rock mass near the open surface is prone to deformation or even failure due to stress release (i.e., unloading), posing a serious threat to the stability of the cavern. Support structures can effectively suppress deformation and failure of the surrounding rock, enhance its bearing capacity, and form a joint force-bearing system with the surrounding rock to maintain the stability of the underground cavern. This is crucial for ensuring the safe construction and long-term operation of the cavern.

[0003] Determining the support strength of underground caverns (mainly reflected in parameters such as support depth and density) is a core aspect of support design, requiring a balance between safety and economy: excessive support strength increases project investment, delays construction progress, and wastes resources; insufficient strength may lead to rock instability and safety accidents. Therefore, scientifically and rationally determining the support strength is a key issue in the design and construction of underground caverns.

[0004] Currently, there are three main methods for determining the support strength of underground caverns: the standard method, the engineering analogy method, and the numerical simulation method. However, all of these methods have certain limitations. (1) Standard method: Based on the type of underground cavern, the support parameters are determined according to relevant standards. For example, the "NB / T 35090—2016 Design Code for Underground Powerhouse of Hydropower Station" specifies the anchor spacing (2.0m~3.0m for Class I surrounding rock, 1.5m~2.5m for Class II, and 1.2m~2.0m for Class III). However, the standard does not clearly specify key parameters such as anchor spacing, and the overall range of values ​​is relatively general. In practical applications, the accuracy is insufficient and it is difficult to adapt to complex engineering scenarios.

[0005] (2) Engineering analogy method: Refer to existing projects with similar functions, scale and geological conditions to determine the support strength. However, the geological conditions of underground caverns are often complex and variable (such as deep-buried high-stress environments), and the specific differences between different projects are significant, which reduces the reference value. Blindly making analogies may lead to insufficient or excessive support, increasing the cost of later reinforcement.

[0006] (3) Numerical simulation method: The stability of the surrounding rock of the support scheme is analyzed by numerical calculation, which is suitable for scheme comparison. However, due to the simplification and assumptions in the calculation process (such as the poor simulation effect of anchor bolts in some software), the results may deviate from the actual project, and it is difficult to use it as the sole basis for determining the support strength. In addition, the calculation volume is large and the efficiency is low. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation, so as to determine the support strength of underground caverns in a precise, scientific and efficient manner, while taking into account both engineering safety and economy.

[0008] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: A method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation includes the following steps: S1. Based on triaxial compression tests and triaxial unloading confining pressure tests of rocks, determine the functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio: Where c is the confining pressure unloading ratio. P represents the percentage decrease in peak intensity. , This is the amount of confining pressure unloading. For the initial confining pressure, This represents the peak intensity under triaxial compression. This represents the peak strength under unloaded confining pressure. S2. The proportion of remaining strength of the surrounding rock according to the support design requirements of the underground cavern. ,in, Based on the functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio... The required confining pressure unloading amount is calculated. ; S3. Based on the calculated required confining pressure unloading amount The required confining pressure compensation value is determined, and the required anchor cable density, i.e. the support strength of the underground cavern, is calculated based on the relationship between confining pressure, support force and anchor cables.

[0009] Furthermore, in step S1, the process of the rock triaxial compression test includes: a. After preparing the rock sample, an initial confining pressure is applied to the sample using a triaxial rock mechanics testing system. ; b. Maintain a constant confining pressure and apply axial pressure to the specimen at a preset rate until the specimen fails; c. Record the axial stress at the time of specimen failure, which is the peak strength corresponding to the initial confining pressure. .

[0010] Furthermore, in step S1, the triaxial unloading confining pressure test process includes: a. Use the same specimens as those used in the triaxial compression test of rocks; b. First, apply the initial confining pressure corresponding to the triaxial compression test of the rock to the specimen. ; c. Next, apply axial pressure to the specimen to a preset value (e.g., 80% of the peak strength of the triaxial compression test under confining pressure). d. Maintain a constant axial pressure, slowly unload the confining pressure to different target values, and record the confining pressure values ​​after unloading. ; e. Continue applying axial pressure until the specimen fails, and record the axial stress at this point, which is the peak strength under the unloaded confining pressure state. ; f. Calculate the confining pressure unloading amount and confining pressure unloading ratio And calculate the percentage reduction in peak intensity. , This represents the peak strength obtained from a triaxial compression test under the same initial confining pressure.

[0011] Furthermore, in step S1, the method for determining the functional relationship between the confining pressure unloading ratio and the peak intensity reduction ratio includes: By summarizing multiple sets of confining pressure unloading ratios (c) and corresponding peak strength reduction ratios (p) from triaxial unloading confining pressure tests, the least squares method was used for fitting to establish a quantitative relationship between p and c, resulting in a functional expression. .

[0012] Furthermore, in step S2, the proportion of remaining strength of the surrounding rock required by the underground cavern support design... The design is based on the actual needs of the project to meet the support design requirements under different safety levels.

[0013] Furthermore, in step S3, the required confining pressure unloading amount is calculated. Determine the required confining pressure compensation value, including: Required confining pressure compensation value = Initial confining pressure - Confining pressure unloading volume .

[0014] Furthermore, in step S3, calculating the required anchor cable density based on the relationship between confining pressure, support force, and anchor cables includes: Based on the formula: Confining pressure × Area = Support force = Number of anchor cables × Load per single anchor cable And, the relationship between the number of anchor cables = anchor cable density × area; By substituting the confining pressure compensation value and the load of a single anchor cable, the anchor cable density can be calculated, which is the support strength of the underground cavern.

[0015] The beneficial effects of this invention are: (1) Improve the accuracy and practicality of the design: This invention establishes a quantitative relationship between confining pressure unloading and rock mass strength reduction through triaxial rock tests, which can directly calculate the precise value of anchor cable density, rather than a broad range of values, thus solving the problem of the standard method having general values ​​and being inconvenient for practical application.

[0016] Engineers can directly design based on the calculation results without subjective selection within the scope, greatly improving the operability of the method. At the same time, the anchor cable density can be flexibly adjusted by adjusting the confining pressure compensation value according to the dynamic requirements of the project for support strength, adapting to the design requirements under different working conditions, making it more practical.

[0017] (2) Enhance applicability under complex geological conditions: This invention fully considers the influence of in-situ stress on the strength of surrounding rock, quantifies support requirements through the principle of confining pressure compensation, and overcomes the shortcomings of engineering analogy methods in complex geological environments (such as deep-buried caverns in the Qinghai-Tibet Plateau) by not relying on similar engineering experience. Its theoretical basis and data are all derived from indoor rock tests, making it less sensitive to geological conditions and applicable to various complex underground cavern scenarios.

[0018] (3) Ensuring the reliability and scientific validity of the theory: The core principle of this invention (the reduction in rock mass strength due to confining pressure unloading and the recoverable strength due to confining pressure compensation) has been verified by rock mechanics tests, and the parameters required for calculation are all based on experimental data. This avoids problems such as "relying on experience-based classification" in the standard method and "simplification of assumptions leading to deviation" in the numerical simulation method. The theoretical foundation is solid, which reduces the blindness of support design and improves the scientificity and credibility of the results.

[0019] (4) Improve design efficiency and economy: Compared to the massive computational load and complex process of numerical simulation methods, this invention directly derives support strength through formulaic calculations. The steps are simple and the calculations are quick, allowing for rapid response to engineering design needs. Furthermore, because it can accurately determine support strength, it avoids cost waste caused by over-support and prevents subsequent reinforcement costs due to insufficient support, achieving optimal resource allocation while ensuring safety and economic efficiency. Attached Figure Description

[0020] Figure 1 This is a flowchart of the underground cavern support strength design method in an embodiment of the present invention. Detailed Implementation

[0021] This invention aims to provide a method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation. This method achieves accurate, scientific, and efficient determination of underground cavern support strength, balancing engineering safety and economy. The core idea is as follows: Based on the characteristic of strength reduction in the rock mass at the free face during underground cavern excavation due to unloading, and utilizing the principle that confining pressure compensation can restore rock mass strength, a quantitative functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio is established through indoor triaxial compression tests and triaxial unloading confining pressure tests. Then, based on the engineering requirements for the remaining strength of the surrounding rock, the required confining pressure compensation value is derived. Finally, the anchor cable density (support strength) is calculated through the equivalent relationship between confining pressure and support force, thereby achieving accurate and scientific determination of the underground cavern support strength, balancing engineering safety and economy.

[0022] According to relevant research, in indoor triaxial compression tests of rocks, the peak strength of the rock decreases when the confining pressure is unloaded compared to the triaxial compression state. Taking basalt as an example, when the initial confining pressure is 10~40MPa, the peak strength of basalt under unloaded confining pressure decreases by 1.8%~15.9% compared to the triaxial compression state. The peak strength statistics are shown in Table 1. Table 1. Statistics of peak strength of basalt under unloaded confining pressure. Confining pressure (MPa) Measurement value (MPa) Peak strength under triaxial compression (MPa) Compared to the triaxial compression state, the reduction (%) 10 195.16 198.74 1.8 20 225.76 268.43 15.9 30 271.44 281.04 3.4 40 282.39 303.4 6.9 This invention extends this principle to an engineering scenario. During the excavation of underground caverns, the stress in the free-face rock mass is relieved, i.e., the confining pressure is relieved, which is equivalent to the unloaded confining pressure state in the aforementioned rock test. Therefore, the strength of the free-face rock mass also decreases accordingly. For example, when the initial confining pressure of the uncracked rock mass is 10 MPa, without considering the difference between the rock and the rock mass, the strength of the surrounding rock after excavation will decrease by 1.8%. In other words, if we want to increase the rock mass strength to the initial value before excavation by providing confining pressure through support, we need to add 10 MPa of confining pressure. Based on the required additional confining pressure, the required anchor cable density, i.e., the support strength, can be calculated. However, if we want to increase the rock mass strength to a portion of the initial value, such as 80%, we need to determine this through laboratory rock tests.

[0023] Therefore, this invention provides a method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation. The implementation process is described in [link to implementation details]. Figure 1 This includes the following steps: S1. Determine the functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio through experiments: In this step, in order to determine the functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio, a triaxial compression test and a triaxial unloading confining pressure test will be carried out in the rock laboratory. The triaxial compression test determines the peak strength of the rock, and the triaxial unloading confining pressure test determines the functional relationship between the confining pressure reduction value and the peak strength increase value.

[0024] In practice, the steps of the triaxial compression test in the rock chamber are as follows: a. After preparing the rock sample, an initial confining pressure is applied to the sample using a triaxial rock mechanics testing system. ; b. Maintain a constant confining pressure and apply axial pressure to the specimen at a preset rate until the specimen fails; c. Record the axial stress at the time of specimen failure, which is the peak strength corresponding to the initial confining pressure. .

[0025] The process of triaxial unloading confining pressure test includes: a. Use the same specimens as those used in the triaxial compression test of rocks; b. First, apply the initial confining pressure corresponding to the triaxial compression test of the rock to the specimen. ; c. Next, apply axial pressure to the specimen to a preset value (e.g., 80% of the peak strength of the triaxial compression test under confining pressure). d. Maintain a constant axial pressure, slowly unload the confining pressure to different target values, and record the confining pressure values ​​after unloading. ; e. Continue applying axial pressure until the specimen fails, and record the axial stress at this point, which is the peak strength under the unloaded confining pressure state. ; f. Calculate the confining pressure unloading amount and confining pressure unloading ratio And calculate the percentage reduction in peak intensity. , This represents the peak strength obtained from a triaxial compression test under the same initial confining pressure.

[0026] Finally, by summarizing the data of multiple sets of confining pressure unloading ratios c and corresponding peak strength reduction ratios p from triaxial unloading confining pressure tests, the least squares method was used for fitting to establish a quantitative relationship between p and c, resulting in a functional expression. Where c is the confining pressure unloading ratio. P represents the percentage decrease in peak intensity. , This is the amount of confining pressure unloading. For the initial confining pressure, This represents the peak intensity under triaxial compression. This represents the peak strength under unloaded confining pressure.

[0027] For this step, an exemplary implementation is as follows: Set the initial confining pressure. The peak strength under triaxial compression was recorded at a pressure of 10 MPa. The peak strength of rock under four working conditions: unloading at 2.5 MPa, 5 MPa, 7.5 MPa, and complete removal of confining pressure. Then based on and Calculate the peak intensity reduction ratio P and record its corresponding confining pressure unloading ratio. Finally, the functional relationship between the two was established by fitting using the least squares method. This function intuitively reflects the impact of confining pressure unloading on rock strength attenuation, providing a mathematical model for subsequently inferring support strength through confining pressure compensation.

[0028] S2. Calculate the confining pressure unloading amount based on the design requirements of underground cavern support and the established functional relationship; In this step, the proportion of remaining strength of the surrounding rock is determined according to the requirements of the underground cavern support design. This ratio is set according to the actual needs of the project (such as 80%, 85%, 90%, etc.) to meet the support design requirements under different safety levels.

[0029] because The peak strength reduction ratio P can then be calculated based on the previously established functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio. The confining pressure unloading amount can be calculated. .

[0030] S3. Calculate the confining pressure compensation value based on the confining pressure unloading amount, and determine the support strength based on the relationship between confining pressure, support force, and anchor cables: In this step, the required confining pressure compensation value = initial confining pressure - Confining pressure unloading volume Next, based on the formulas: confining pressure × area = support force = number of anchor cables × load of a single anchor cable, and number of anchor cables = anchor cable density × area, the anchor cable density can be calculated by substituting the confining pressure compensation value and the load of a single anchor cable, which is the support strength of the underground cavern.

[0031] For example, the support design for a certain underground cavern requires that the strength of the surrounding rock after support should be at least 80% of the initial strength, i.e. =80%, given an initial confining pressure of 10 MPa, assuming the calculated... The confining pressure is 4 MPa, meaning that when the confining pressure drops to 6 MPa, the strength of the surrounding rock decreases to 80% of its initial strength. Therefore, if the strength of the surrounding rock is required to be no less than 80% of its initial strength, the confining pressure should not be less than 6 MPa. The required confining pressure compensation is 6 MPa. Since confining pressure × area = support force = number of anchor cables × load per anchor cable, and the number of anchor cables = anchor cable density × area, the anchor cable density can be calculated.

[0032] It should be noted that this invention does not precisely account for the difference between rock mass strength and rock strength. Because there is a certain difference in the magnitude of rock mass strength and rock strength (due to the relatively poor integrity of the rock mass and the influence of structural planes causing the rock mass strength to be lower than the rock strength), this invention uses a ratio (strength reduction ratio) to equate the two, thus circumventing this difference in magnitude. The difference between rock mass strength and rock strength could be further precisely considered, such as by adding a reduction factor to the formula to characterize the reduction in rock mass strength relative to rock strength, but the core idea remains essentially the same as the solution in this invention.

[0033] Therefore, although embodiments of the present invention have been described above, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and all such changes and variations shall not depart from the protection scope of the present invention.

Claims

1. A method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation, characterized in that, Includes the following steps: S1. Based on triaxial compression tests and triaxial unloading confining pressure tests of rocks, determine the functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio: Where c is the confining pressure unloading ratio. P represents the percentage decrease in peak intensity. , This is the amount of confining pressure unloading. The initial confining pressure, This represents the peak intensity under triaxial compression. This represents the peak strength under unloaded confining pressure. S2. The proportion of remaining strength of the surrounding rock according to the support design requirements of the underground cavern. ,in, Based on the functional relationship between the confining pressure unloading ratio and the peak strength reduction ratio... The required confining pressure unloading amount is calculated. ; S3. Based on the calculated required confining pressure unloading amount The required confining pressure compensation value is determined, and the required anchor cable density, i.e. the support strength of the underground cavern, is calculated based on the relationship between confining pressure, support force and anchor cables.

2. The method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation as described in claim 1, characterized in that, In step S1, the process of the triaxial compression test of the rock includes: a. After preparing the rock sample, an initial confining pressure is applied to the sample using a triaxial rock mechanics testing system. ; b. Maintain a constant confining pressure and apply axial pressure to the specimen at a preset rate until the specimen fails; c. Record the axial stress at the time of specimen failure, which is the peak strength corresponding to the initial confining pressure. .

3. The method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation as described in claim 2, characterized in that, In step S1, the triaxial unloading confining pressure test process includes: a. Use the same specimens as those used in the triaxial compression test of rocks; b. First, apply the initial confining pressure corresponding to the triaxial compression test of the rock to the specimen. ; c. Next, apply axial pressure to the sample to the preset value; d. Maintain a constant axial pressure, slowly unload the confining pressure to different target values, and record the confining pressure values ​​after unloading. ; e. Continue applying axial pressure until the specimen fails, and record the axial stress at this point, which is the peak strength under the unloaded confining pressure state. ; f. Calculate the confining pressure unloading amount and confining pressure unloading ratio And calculate the percentage reduction in peak intensity. , This represents the peak strength obtained from a triaxial compression test under the same initial confining pressure.

4. The method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation as described in claim 3, characterized in that, In step S1, the method for determining the functional relationship between the confining pressure unloading ratio and the peak intensity reduction ratio includes: By summarizing multiple sets of confining pressure unloading ratios (c) and corresponding peak strength reduction ratios (p) from triaxial unloading confining pressure tests, the least squares method was used for fitting to establish a quantitative relationship between p and c, resulting in a functional expression. .

5. The method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation as described in claim 1, characterized in that, In step S2, the proportion of remaining strength of the surrounding rock required by the underground cavern support design is... The design is based on the actual needs of the project to meet the support design requirements under different safety levels.

6. The method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation as described in claim 1, characterized in that, In step S3, the required confining pressure unloading amount is calculated. Determine the required confining pressure compensation value, including: Required confining pressure compensation value = Initial confining pressure - Confining pressure unloading volume .

7. A method for designing the support strength of underground caverns based on excavation unloading strength reduction and confining pressure compensation as described in any one of claims 1-6, characterized in that, In step S3, calculating the required anchor cable density based on the relationship between confining pressure, support force, and anchor cables includes: Based on the formula: Confining pressure × Area = Support force = Number of anchor cables × Load per single anchor cable And, the relationship between the number of anchor cables = anchor cable density × area; By substituting the confining pressure compensation value and the load of a single anchor cable, the anchor cable density can be calculated, which is the support strength of the underground cavern.

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