A method for detecting the thickness of frozen soil curtain based on current focusing principle under seepage conditions

Abstract

The present invention discloses a method for detecting the thickness of a frozen soil curtain under seepage conditions based on the principle of current focusing, and relates to the technical field of frozen soil curtain detection. The method comprises: using indoor tests combined with numerical simulation methods to establish a correlation function between the focusing resistivity and the thickness of the frozen soil curtain, taking into account the influence of four factors, namely, groundwater flow velocity, flow direction, freezing temperature, and the spacing between frozen pipes, before and after the freezing cycle, and during excavation; applying an insulating coating to the frozen pipes, and laying two sets of orthogonal focusing electrode systems on the insulating coatings. Based on the three sets of correlation functions, the specific values ​​of the four factors measured before freezing and the focusing resistivity during freezing are used to obtain real-time information on the thickness of the frozen soil curtain between the pipes.

Description

Technical Field

[0001] The present application relates to the technical field of frozen soil curtain detection, and in particular to a method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions. Background Art

[0002] The thickness of the permafrost curtain is the key to controlling its mechanical stability. However, due to abnormal changes in the local hydrogeological conditions of the freezing site and insufficient precision in the initial engineering survey, the permafrost curtain often exhibits abnormal development quality, causing the permafrost curtain thickness to deviate from the design value and show an irregular development trend. If this abnormal development of the permafrost curtain thickness cannot be captured in time during the freezing process, it will pose a safety hazard to subsequent excavation.

[0003] Existing technology uses focusing electrode system parameters corresponding to optimal focusing to establish a correlation function between the freezing front position and the focused resistivity between freezing pipes. This function, validated using typical freezing projects, has resulted in a method for detecting the thickness of a frozen soil curtain in large-scale urban underground freezing projects. However, this correlation function fails to capture abnormal changes in the frozen soil curtain during the freezing process, leading to inaccuracies in the thickness detection and impacting frozen soil construction decisions. Summary of the Invention

[0004] Based on this, it is necessary to provide a permafrost curtain thickness detection method based on the current focusing principle under seepage conditions to address the above technical problems, so as to solve the problem of being unable to capture abnormal changes in the permafrost curtain during the freezing process, thereby causing deviations in the detection of the permafrost curtain thickness.

[0005] This manual adopts the following technical solutions:

[0006] This manual provides a method for detecting the thickness of frozen soil curtain under seepage conditions based on the current focusing principle, including:

[0007] Considering four factors, including freezing pipe spacing, ground freezing temperature, groundwater velocity, and groundwater flow direction, an orthogonal test table was established. Based on the orthogonal test table, indoor tests and numerical simulation tests were carried out on typical soil types to obtain the focused resistivity of the freezing pipes at three stages: before freezing, after freezing, and during excavation.

[0008] Based on the focused resistivity of the three stages, a first correlation function between the normalized focused resistivity and the normalized frozen soil curtain thickness before the freezing cycle, a second correlation function between the normalized focused resistivity and the normalized frozen soil curtain thickness after the freezing cycle, and a third correlation function between the normalized focused resistivity and the normalized frozen soil curtain thickness during the excavation process are constructed; and correlation equations between the coefficients of the first correlation function, the coefficients of the second correlation function, and the coefficients of the third correlation function and the four factors are established by using a multivariate nonlinear regression method;

[0009] During the actual detection process, before freezing, the surfaces of two adjacent freezing pipes are coated with insulation to form a thermally conductive insulating coating. The membrane-type transmitting electrode and the membrane-type loop electrode are arranged on the thermally conductive insulating coating on the surface of the same freezing pipe. Two sets of orthogonal focusing electrode systems are arranged in parallel and perpendicular directions to the connecting line of the two freezing pipes.

[0010] Through the focusing electrode system, the focusing resistivity around the frozen pipe is obtained in real time in three stages, including before the freezing circle, after the freezing circle and during the excavation process. Through the first correlation function parallel to the direction of the connection between the two frozen pipes and the second and third correlation functions perpendicular to the direction of the connection between the two frozen pipes, combined with the four measured factors, the coefficients of the first correlation function, the coefficients of the second correlation function and the coefficients of the third correlation function and the correlation equations between the four factors are jointly established with the first correlation function, the second correlation function and the third correlation function to determine the thickness information of the frozen soil curtain.

[0011] Preferably, obtaining the focused resistivity of the frozen pipe at three stages, before freezing, after freezing, and during excavation, specifically includes:

[0012] According to the four factors, and considering multiple values ​​for each factor, an orthogonal test table is established;

[0013] According to the orthogonal test table, typical soil types were selected for indoor testing to obtain the freezing point and temperature-resistivity relationship function; and according to the ground freezing temperature, the freezing temperature field was obtained through numerical simulation experiments;

[0014] Obtain the resistivity field according to the temperature-resistivity relationship function and the freezing temperature field;

[0015] According to the resistivity field, the focused resistivity of the frozen pipe was obtained in three stages: before freezing, after freezing and during excavation.

[0016] Preferably, the first correlation function is established based on the detection results of the focusing electrode system parallel to the direction of the line connecting the two freezing tubes.

[0017] Preferably, the second correlation function and the third correlation function are established by detecting results of a focusing electrode system perpendicular to the direction of the line connecting the two freezing tubes.

[0018] Preferably, the freezing circle closing moment is the moment when the sum of the thicknesses of the left and right parts of the frozen soil curtain between the freezing pipes is equal to the distance between the freezing pipes.

[0019] Preferably,

[0020] The first correlation function is ;

[0021] The second correlation function is ;

[0022] The third correlation function is ;

[0023] Where, is the initial resistivity of the formation, is the freezing pipe spacing, 、 and are the thickness of the frozen soil curtain before, after and during the excavation process, respectively. 、 and are the focused resistivity before, after and during the excavation process;

[0024] The normalized focused resistivity is the ratio between the focused resistivity obtained by real-time detection and the initial resistivity of the formation;

[0025] The normalized frozen soil curtain thickness is the ratio of the frozen soil curtain thickness to the freezing pipe spacing.

[0026] Preferably, the correlation equations between the coefficients of the first correlation function, the coefficients of the second correlation function, the coefficients of the third correlation function and the four factors are:

[0027] ;

[0028] ;

[0029] ;

[0030] Where, 、 and are the coefficients of the first, second and third correlation functions, respectively. 、 、 、 , 、 、 and All are nonlinear regression coefficients, obtained by fitting numerical simulation data using MATLAB software. 、 、 、 These are the freezing pipe spacing, ground freezing temperature, groundwater flow velocity and groundwater flow direction measured before freezing.

[0031] At least one of the above technical solutions adopted in this specification can achieve the following beneficial effects:

[0032] Compared with the existing inventions, this specification can not only realize the detection before the freezing circle is completed, but also realize the detection of the thickness of the frozen soil curtain after the freezing circle is completed and during the excavation process; compared with the existing inventions during the detection process, this specification prepares two sets of orthogonal focusing electrode systems on the surface of the freezing pipe, and based on the strategy of arranging the transmitting electrode and the loop electrode on the same pipe, realizes the detection of the thickness of the frozen soil curtain in the direction parallel to the freezing pipe connection (before the freezing circle is completed) and in the direction perpendicular to the freezing pipe connection (after the freezing circle is completed and during the excavation process).

[0033] This manual takes the influence of seepage into consideration. By focusing resistivity between pipes, correlation functions are established before and after the freezing circle and during excavation to detect the thickness of the frozen soil curtain in the directions parallel and perpendicular to the freezing pipe connection. This allows detection of the process from the freezing front intersection to the stabilization of the frozen soil curtain thickness, the degradation of the frozen soil curtain due to internal temperature rise and external seepage erosion, and even the "windowing" process. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0035] Figure 1 A flow chart of a method for detecting the thickness of a frozen soil curtain based on the current focusing principle under seepage conditions provided in this embodiment. DETAILED DESCRIPTION

[0036] To make the purpose, technical solutions, and advantages of this specification more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the specific embodiments of this specification and the corresponding drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in this specification, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0037] The following describes in detail the technical solutions provided by various embodiments of the present application in conjunction with the accompanying drawings.

[0038] Figure 1 This is a flow chart of a method for detecting the thickness of a frozen soil curtain based on the current focusing principle under seepage conditions in this embodiment, which specifically includes the following steps:

[0039] S101: Considering four factors including freezing pipe spacing, ground freezing temperature, groundwater flow velocity and groundwater flow direction, an orthogonal test table is established. Based on the orthogonal test table, typical soil types are selected to conduct indoor tests and numerical simulation tests in sequence to obtain the focused resistivity of the freezing pipes in three stages: before freezing, after freezing, and during excavation.

[0040] Optionally, the focused resistivity of the frozen pipe is obtained at three stages: before freezing, after freezing, and during excavation, including:

[0041] An orthogonal test table was established based on four factors, with five levels for each factor. Based on the orthogonal test table, typical soil types were selected for indoor tests to obtain the freezing point and temperature-resistivity relationship function. Based on the freezing temperature of the stratum, the freezing temperature field was obtained through numerical simulation experiments. Based on the temperature-resistivity relationship function and the freezing temperature field, the resistivity field was obtained. Based on the resistivity field, the focused resistivity of the freezing pipe was obtained in three stages: before the freezing cycle, after the freezing cycle, and during the excavation process.

[0042] Specifically, the freezing pipe spacing can be selected from five levels: 0.6m, 0.8m, 1.0m, 1.2m, and 1.4m. The ground freezing temperature can be selected from five levels: -5°C, -10°C, -15°C, -20°C, and -25°C. The groundwater flow rate can be selected from five levels: 0m / d, 2m / d, 4m / d, 8m / d, and 16m / d. The groundwater flow direction can be selected from five levels: 0°, 15°, 30°, 60°, and 90°, based on the angle between the groundwater flow and the normal to the excavation surface. Measured data from common water-rich sand layers in municipal applications or published results from the published paper "Freezing Front Location Method Based on Focused DC Resistivity" were selected. A numerical simulation orthogonal test table was then established based on the above four factors and five levels. Through numerical simulation experiments, the temperature field between the two pipes, the resistivity field, and the focused resistivity around the pipe were sequentially obtained.

[0043] S102: Through the focused resistivity of the three stages, construct a first correlation function between the normalized focused resistivity and the normalized frozen soil curtain thickness before the freezing cycle, a second correlation function between the normalized focused resistivity and the normalized frozen soil curtain thickness after the freezing cycle, and a third correlation function between the normalized focused resistivity and the normalized frozen soil curtain thickness during the excavation process; by adopting the multivariate nonlinear regression method, establish the correlation equations between the coefficients of the first correlation function, the coefficients of the second correlation function, the coefficients of the third correlation function and the four factors.

[0044] Optionally, the first correlation function is established based on the detection results of the focusing electrode system in the direction parallel to the connection line of the two freezing tubes; the second correlation function and the third correlation function are established based on the detection results of the focusing electrode system in the direction perpendicular to the connection line of the two freezing tubes.

[0045] Optionally, the freezing circle closing moment is the moment when the sum of the thicknesses of the left and right parts of the frozen soil curtain between the freezing pipes is equal to the spacing between the freezing pipes.

[0046] Optionally, the first correlation function is ; The second correlation function is ; The third correlation function is ;in, is the initial resistivity of the formation, is the freezing pipe spacing, 、 and are the thickness of the frozen soil curtain before, after and during the excavation process, respectively. 、 and They are the focused resistivity before the circle is crossed, after the circle is crossed and during the excavation process.

[0047] Optionally, a correlation equation between the coefficients of the first correlation function, the coefficients of the second correlation function, the coefficients of the third correlation function and the four factors is established by adopting a multivariate nonlinear regression method, and the formula is:

[0048] ;

[0049] ;

[0050] ;

[0051] Where, 、 and are the coefficients of the first, second and third correlation functions, respectively. 、 、 、 , 、 、 and are nonlinear regression coefficients, 、 、 、 These are the freezing pipe spacing, ground freezing temperature, groundwater flow velocity and groundwater flow direction measured before freezing.

[0052] S103: During the actual detection process, before freezing construction, the surfaces of the two adjacent freezing pipes are coated with insulation to form a thermally conductive insulating coating, and the membrane-type transmitting electrode and the membrane-type loop electrode are arranged on the thermally conductive insulating coating on the surface of the same freezing pipe, and two groups of orthogonal focusing electrode systems are arranged in a direction parallel to and perpendicular to the direction connecting the two freezing pipes.

[0053] Specifically, the insulating coating is primarily made of alumina particle-based polymers with thermal insulation properties. The coating is applied on-site to a thickness of 100 to 200 μm. The frozen pipe surface must be derusted before application. The coating height range for the frozen pipe at a certain test depth is 3 m, meaning 1.5 m above and below the detection depth. The focusing electrode system is a conductive film no thicker than 100 μm. It consists of an emitting electrode, two pairs of symmetrically arranged shielding electrodes, two pairs of supervisory electrodes for potential monitoring, and a loop electrode arranged on the same pipe. The focusing electrode system is connected to external focusing electrical measuring equipment via wires. The focusing electrodes have a 60° circumferential angle, and their height and spacing are selected based on the published results of the paper "Freezing Front Positioning Method Based on Focused DC Resistivity." One set of parallel pipe connections and one set of perpendicular pipe connections are used.

[0054] S104: Through the focusing electrode system, the focusing resistivity around the frozen pipe is obtained in real time in three stages, including before the freezing circle, after the freezing circle and during the excavation process. Through the first correlation function parallel to the direction of the connection between the two frozen pipes and the second correlation function and the third correlation function perpendicular to the direction of the connection between the two frozen pipes, combined with the four measured factors, the coefficients of the first correlation function, the coefficients of the second correlation function and the coefficients of the third correlation function and the correlation equations between the four factors are jointly established with the first correlation function, the second correlation function and the third correlation function to determine the thickness information of the frozen soil curtain.

[0055] Specifically, the freezing pipe spacing and freezing temperature are freezing design parameters, which are directly obtained according to the freezing design plan. The groundwater flow rate and flow direction are directly obtained according to the geological survey report. The three correlation functions are obtained according to step S102.

[0056] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for detecting the thickness of frozen soil curtain under seepage conditions based on the current focusing principle is characterized by: include: Considering four factors, including freezing pipe spacing, ground freezing temperature, groundwater velocity, and groundwater flow direction, an orthogonal test table was established. Based on the orthogonal test table, indoor tests and numerical simulation tests were carried out on typical soil types to obtain the focused resistivity of the freezing pipes at three stages: before freezing, after freezing, and during excavation. Based on the focused resistivity of the three stages, a first correlation function, a second correlation function, and a third correlation function are constructed between the normalized focused resistivity of the three stages and the normalized frozen soil curtain thickness; and a correlation equation is established between the coefficients of the first correlation function, the coefficients of the second correlation function, and the coefficients of the third correlation function and four factors by using a multivariate nonlinear regression method. By using the first correlation function parallel to the direction connecting the two freezing pipes and the second and third correlation functions perpendicular to the direction connecting the two freezing pipes, combined with the four measured factors, the correlation equations between the coefficients of the first correlation function, the coefficients of the second correlation function, and the coefficients of the third correlation function and the four factors are jointly established with the first, second, and third correlation functions to determine the thickness information of the frozen soil curtain.

2. The method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions according to claim 1 is characterized in that: Before determining the thickness information of the frozen soil curtain, the method further includes: During the actual detection process, before freezing construction, the surfaces of two adjacent freezing pipes are coated with insulation to form a thermally conductive insulating coating, and the membrane transmitting electrode and the membrane loop electrode are arranged on the thermally conductive insulating coating on the surface of the same freezing pipe. Two groups of orthogonal focusing electrode systems are arranged in a direction parallel to and perpendicular to the direction connecting the two freezing pipes. Through the focusing electrode system, the focusing resistivity around the freezing pipe is obtained in real time in three stages, including before the freezing circle, after the freezing circle, and during the excavation process.

3. The method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions according to claim 1, characterized in that: The first correlation function is established based on the detection results of the focusing electrode system parallel to the direction of the line connecting the two freezing tubes.

4. The method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions according to claim 1, characterized in that: The second correlation function and the third correlation function are established by detecting results of a focusing electrode system perpendicular to the direction of the line connecting the two freezing tubes.

5. The method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions according to claim 1, characterized in that: The freezing circle closing moment is when the sum of the thickness of the two parts of the frozen soil curtain between the freezing pipes is equal to the distance between the freezing pipes.

6. The method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions according to claim 1, characterized in that: The first correlation function is ; The second correlation function is ; The third correlation function is ; Where, is the initial resistivity of the formation, is the freezing pipe spacing, 、 and are the thickness of the frozen soil curtain before, after and during the excavation process, respectively. 、 and are the focused resistivity before, after and during the excavation process; The normalized focused resistivity is the ratio between the focused resistivity obtained by real-time detection and the initial resistivity of the formation; The normalized frozen soil curtain thickness is the ratio of the frozen soil curtain thickness to the freezing pipe spacing.

7. The method for detecting the thickness of frozen soil curtain based on the current focusing principle under seepage conditions according to claim 1, characterized in that: The correlation equations between the coefficients of the first correlation function, the coefficients of the second correlation function, and the coefficients of the third correlation function and the four factors are: ； ； ； Where, 、 and are the coefficients of the first, second and third correlation functions, respectively. 、 、 、 , 、 、 and are nonlinear regression coefficients, obtained by fitting numerical simulation data. 、 、 、 These are the freezing pipe spacing, ground freezing temperature, groundwater flow velocity and groundwater flow direction measured before freezing.

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