Method for evaluating relaxation thickness of surrounding rock of cavern with same lithology by borehole resistivity

Abstract

The application discloses a method for evaluating the relaxation thickness of the same lithology surrounding rock of a cavern by using borehole resistivity, which comprises the following steps: making a gradient electrode system, making a table of the variation of the measured resistivity of a single hole with the hole depth, counting the average value of the measured resistivity of the single hole, and drawing a curve of the variation of the measured resistivity with the hole depth; calculating a resistivity normalization coefficient, making a table of the variation of the resistivity normalization coefficient with the hole depth, and drawing a curve of the variation of the resistivity normalization coefficient with the hole depth; preliminarily determining the boundary between the relaxed rock mass and the original rock according to the hole depth of the first point corresponding to the resistivity value equal to the average value of the measured resistivity of the single hole, i.e. the first point corresponding to the resistivity normalization coefficient of 1, and then comprehensively determining the relaxation thickness of the surrounding rock of the cavern by combining the trend of the resistivity curve and the normalization curve with the geological condition. The method has the advantages of rapidness, accuracy and relative quantification, and can be popularized to the evaluation of the relaxation thickness of the surrounding rock of a cavern, the blasting influence depth of a foundation rock mass, the weathering unloading depth of a rock mass and other aspects.

Description

Technical Field

[0001] This invention relates to a method for evaluating and supporting the stability of the surrounding rock in a water conservancy and hydropower engineering tunnel. Specifically, it is a method for accurately obtaining the relaxation thickness of the surrounding rock of the same type in the tunnel by measuring the resistivity of the borehole. Background Technology

[0002] Typically, the characteristics of loosened surrounding rock in caverns include a loose rock structure, stress concentration outside the loosened layer which becomes the bearing layer of the cavern, and a stress transition zone outside the bearing layer that eventually transitions back to the original stress state. Different stress zones often exhibit significant differences in their physical properties, thus providing a fundamental premise for geophysical exploration methods to determine the thickness of loosened surrounding rock in caverns.

[0003] Currently, elastic wave testing is commonly used to evaluate relaxation loops. However, when elastic wave testing is inconvenient to implement, there is a lack of effective evaluation methods. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for evaluating the relaxation thickness of the surrounding rock of the same type in a tunnel using borehole resistivity, so as to quickly, accurately and quantitatively determine the relaxation thickness of the surrounding rock of the tunnel and objectively evaluate the stability of the surrounding rock of the tunnel in water conservancy and hydropower projects.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for evaluating the relaxation thickness of the surrounding rock of the same lithology in a cavern using borehole resistivity, comprising the following steps:

[0006] A. Fabricating a gradient electrode system for testing borehole resistivity;

[0007] B. Create a table showing the variation of the measured resistivity of a single hole with hole depth, calculate the average value of the measured resistivity of a single hole, and plot the curve of the measured resistivity as a function of hole depth;

[0008] C. Calculate the resistivity normalization coefficient, create a table showing the variation of the resistivity normalization coefficient with hole depth, and plot the curve showing the variation of the resistivity normalization coefficient with hole depth.

[0009] D. Based on the hole depth of the first point corresponding to the resistivity value equal to the average resistivity measured in a single hole (i.e., the resistivity normalization coefficient is 1), the boundary between the relaxed rock mass and the original rock is initially determined. Then, by combining the geological conditions and analyzing the trends of the resistivity curve and the normalization curve, the relaxation thickness of the surrounding rock of the cavern can be comprehensively determined.

[0010] In section B, the depth of the borehole into the original rock should be ≥1m, and the resistivity values ​​of a single borehole should be no less than 5 measuring points.

[0011] The beneficial effects of this invention are: a novel method for evaluating the relaxation thickness of surrounding rock of the same lithology in a cavern using borehole resistivity, which has the advantages of being rapid, accurate, and relatively quantitative, and can be extended to evaluate the relaxation thickness of caverns, the depth of impact of blasting in foundation rock masses, and the unloading depth of rock masses in different fields. The combined application of this invention with elastic wave testing can leverage multi-parameter joint evaluation of the relaxation thickness of the surrounding rock in caverns, further improving the accuracy of the evaluation, and also achieving multiple uses for a single borehole, increasing the utilization rate of the borehole. Attached Figure Description

[0012] Figure 1 This is the gradient electrode system prepared according to the present invention.

[0013] Figure 2 This is a curve showing the ZK1 borehole depth versus resistivity and resistivity normalization coefficient according to an embodiment of the present invention.

[0014] Figure 3 These are the borehole depth-resistivity and resistivity normalization coefficient curves for ZK1 to ZK4 in embodiments of the present invention.

[0015] Figure 4 These are the borehole depth-resistivity and resistivity normalization coefficient curves for ZK5 to ZK10 boreholes according to embodiments of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0017] The method of evaluating the relaxation thickness of the surrounding rock of the same lithology in a cavern using borehole resistivity according to the present invention includes the following steps:

[0018] A. Fabricating a gradient electrode system for testing borehole resistivity;

[0019] B. Create a table showing the variation of the measured resistivity of a single hole with hole depth, calculate the average value of the measured resistivity of a single hole, and plot the curve of the measured resistivity as a function of hole depth;

[0020] C. Calculate the resistivity normalization coefficient, create a table showing the variation of the resistivity normalization coefficient with hole depth, and plot the curve showing the variation of the resistivity normalization coefficient with hole depth.

[0021] D. Based on the hole depth of the first point corresponding to the resistivity value equal to the average resistivity measured in a single hole (i.e., the resistivity normalization coefficient is 1), the boundary between the relaxed rock mass and the original rock is initially determined. Then, by combining the geological conditions and analyzing the trends of the resistivity curve and the normalization curve, the relaxation thickness of the surrounding rock of the cavern can be comprehensively determined.

[0022] In section B, the depth of the borehole into the original rock should be ≥1m, and the resistivity values ​​of a single borehole should be no less than 5 measuring points.

[0023] like Figure 1 As shown, step A includes: fabricating a gradient electrode system consisting of three electrodes: a power supply electrode A, two measuring electrodes M and N (abbreviated as A1M0.2N, indicating that the distance from the power supply electrode A to the measuring electrode M is 1m and the distance from the measuring electrode M to MN is 0.2m, with an electrode spacing L = 1.1m); and a fourth electrode B placed on the ground near the borehole opening. The electrodes are made of lead or copper, with a diameter of 1–3cm and a length of 2–4cm.

[0024] Average resistivity of a single hole

[0025] ρ=(ρ1+ρ2+ρ3+…+ρ i +…+ρ n ) / n…………(1)

[0026] ρ: Average measured resistivity of a single orifice, in Ω·m;

[0027] ρ i : Measured resistivity at the i-th measuring point of a single orifice, in Ω·m;

[0028] n: Total number of measurement points.

[0029] Resistivity normalization coefficient

[0030] K i =ρ i / ρ…………(2)

[0031] K i : The resistivity normalization coefficient at the i-th measurement point.

[0032] The hole depth h corresponding to the first point where the resistivity value equals the average measured resistivity of a single hole (i.e., the resistivity normalization coefficient is 1) is calculated. s This was determined to be the boundary between the relaxed rock mass and the original rock, i.e., the thickness of the relaxed rock mass was h. s .

[0033] This invention introduces the borehole resistivity method. For testing the relaxation thickness of surrounding rock using this method, it is based on the fact that in a cavern filled with water, the relaxation layer exhibits a low resistivity zone, the bearing layer a high resistivity zone, and the initial stress zone has a normal resistivity. By testing and analyzing the resistivity variation patterns of rock masses at a certain depth in areas such as the sidewalls and the cave floor, the relaxation thickness of the surrounding rock can be determined. This invention has the advantages of being rapid, accurate, and relatively quantitative, and can be extended to various fields such as determining the relaxation of surrounding rock in caverns, evaluating the depth of impact of blasting on foundation rock masses, and assessing the depth of weathering and unloading of rock masses. The combined application of this invention with the elastic wave method allows for multi-parameter evaluation of the relaxation thickness of surrounding rock in caverns, further improving the accuracy of the evaluation, and also enables multiple uses for a single borehole, increasing the utilization rate of the borehole.

[0034] The invention will be further explained below with reference to the loosening thickness test of the surrounding rock in a certain water conservancy and hydropower project: (1) Project Overview

[0035] The resistivity of 10 boreholes with two sections (A and B) were drilled in the underground powerhouse cavern of a certain water conservancy and hydropower project to determine the relaxation thickness of the surrounding rock of the cavern. The test depth of each borehole was 2.1 to 2.3 m.

[0036] (2) Results Analysis

[0037] ① Figure 2 The curves for borehole depth, resistivity, and resistivity normalization coefficient of borehole ZK1 in section A of the underground powerhouse are shown. The measured borehole depth is 0.1-2.1m, the measured resistivity is 2573-2944Ω·m, and the average measured resistivity is 2707Ω·m. The first point where the resistivity normalization coefficient equals 1 corresponds to a borehole depth of 1.2m. This determines that the boundary between the loose rock mass and the original rock is 1.2m, that is, the thickness of the loose rock mass is 1.2m.

[0038] ② Figure 3 The curves show the borehole depth, resistivity, and resistivity normalization coefficient of four boreholes (ZK1 to ZK4) of the same lithology in section A of the underground powerhouse. The measured borehole depths range from 0.1 to 2.1 m, and the measured resistivity ranges from 2577 to 2743 Ω·m. The average measured resistivity is 2654 Ω·m. The first point where the resistivity normalization coefficient equals 1 corresponds to a borehole depth of 1.2 m. Therefore, the boundary between the loose rock mass and the original rock in section A is determined to be 1.2 m, i.e., the loose rock thickness is 1.2 m.

[0039] ③ Figure 4 The data consists of six boreholes (ZK5 to ZK10) of the same lithology in section B of the underground powerhouse, showing their depth, resistivity, and resistivity normalization coefficient curves. The measured borehole depths ranged from 0.1 to 2.3 m, and the measured resistivity ranged from 3543 to 3866 Ω·m. The average measured resistivity was 3727 Ω·m. The first point where the resistivity normalization coefficient equals 1 corresponds to a borehole depth of 1.0 m. This establishes the boundary between the loose rock mass and the original rock in section B as 1.0 m, meaning the loose rock thickness in section B is 1.0 m.

[0040] In summary, the ZK1 borehole of the underground powerhouse revealed a loose rock mass thickness of 1.2m, and the comprehensive evaluation of the loose rock thickness of sections A and B of the underground powerhouse cavern showed that the thicknesses were 1.2m and 1.0m, respectively.

[0041] When applying this method, the drilling depth should be determined in conjunction with the geological conditions. Generally, the hole depth should be able to reflect the resistivity values ​​of the original rock at a depth of ≥1m and at no less than 5 measuring points.

[0042] The embodiments described above are only used to illustrate the technical ideas and features of the present invention. Their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. The patent scope of the present invention should not be limited by these embodiments. That is, all equivalent changes or modifications made in accordance with the spirit disclosed in the present invention still fall within the patent scope of the present invention.

Claims

1. A method for evaluating the relaxation thickness of surrounding rock of the same lithology in a cavern using borehole resistivity, characterized in that, Includes the following steps: A. Fabricating a gradient electrode system for testing borehole resistivity; B. Create a table showing the variation of measured resistivity of a single borehole with borehole depth, calculate the average value of measured resistivity of a single borehole, and plot a curve showing the variation of measured resistivity with borehole depth; the depth of the borehole into the original rock should be ≥1m, and the resistivity values ​​of a single borehole should be no less than 5 points. C. Calculate the resistivity normalization coefficient, create a table showing the variation of the resistivity normalization coefficient with hole depth, and plot the curve showing the variation of the resistivity normalization coefficient with hole depth. D. Based on the hole depth of the first point corresponding to the resistivity value equal to the average resistivity measured in a single hole (i.e., the resistivity normalization coefficient is 1), the boundary between the relaxed rock mass and the original rock is initially determined. Then, by combining the geological conditions and analyzing the trends of the resistivity curve and the normalization curve, the relaxation thickness of the surrounding rock of the cavern can be comprehensively determined.

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