RCPTU-ERT combined two-dimensional high-precision electrical method test evaluation method

Abstract

This invention discloses a two-dimensional high-precision electrical resistivity testing and evaluation method combining RCPTU-ERT. First, for the soil section to be tested, in-situ ERT testing technology is used to conduct two-dimensional resistivity profile testing of the soil section, obtaining a coarse resistivity value. Then, in-situ RCPTU testing technology is used to conduct continuous longitudinal tests at equal intervals on the already conducted ERT test profile. Next, the longitudinally continuous high-precision resistivity index obtained from the in-situ RCPTU is used as calibration parameters to correct the coarse resistivity value, obtaining a high-precision resistivity test profile. Based on the cone tip resistance index and sidewall friction index obtained from the RCPTU test, noise is eliminated in the analysis of resistivity indices related to soil type, water content, and density, based on the high-precision resistivity test profile. This invention improves the testing accuracy of two-dimensional resistivity parameters in a site and solves the accuracy problem of single in-situ testing equipment.

Description

Technical Field

[0001] This invention relates to a method for evaluating site engineering characteristics. Background Technology

[0002] Current methods for evaluating geotechnical parameters and site engineering properties typically involve borehole sampling and laboratory testing. However, due to soil disturbance, stress release, and other significant environmental influences, the test results often fail to accurately reflect the true condition of soft soil foundations. Therefore, there is an urgent need to develop advanced in-situ testing methods and technologies to achieve rapid and accurate on-site evaluation of site engineering properties.

[0003] Existing research indicates that electrical testing parameters can significantly characterize the basic physicochemical properties of soil, such as the distribution characteristics of its internal pore structure, moisture content, state characteristics, and particle size and shape. Generally speaking, soil is a three-phase system composed of a solid phase (soil particles), a liquid phase (pore water), and a gaseous phase. Because the resistivity of the gaseous phase is extremely high, its conductivity is negligible. The main conductivity of soil depends on the conductivity of its pore water and the conductivity of the soil particle surfaces. Therefore, the main parameters affecting the conductivity of soil are: the salinity and saturation of pore water, the adsorption capacity of soil particle surfaces, and the bonding capacity between soil particles. The main indicators characterizing the conductivity of soil are soil resistivity ρ and electrical conductivity σ (the two are reciprocals of each other): σ = 1 / ρ, where soil resistivity is the total resistance per unit length of the soil sample cross-section.

[0004]

[0005] In the formula, L is the length of the soil sample; A is the cross-sectional area of ​​the soil sample; ΔU is the voltage difference between the two electrodes; and I is the soil sample current.

[0006] Due to the non-destructive nature of electrical parameters and their characterization of engineering parameters, electrical testing techniques such as resistivity tomography (ERT) and transient electromagnetic method (TEM) are widely used in engineering site evaluation and monitoring. However, the design parameters of engineering sites are usually derived from measured data using empirical or semi-empirical model relationships. Data processing relies on inversion experience, and the results themselves are not unique and have significant uncertainties. Moreover, existing in-situ testing equipment is limited to qualitative analysis due to limitations in equipment accuracy and noise levels, requiring integration with other testing methods to ensure accurate detection.

[0007] At the same time, field testing technology is constrained by technical bottlenecks such as the lack of theoretical models for data processing, the accuracy of single testing equipment, and the impact of noise on test results. It is still in the stage of qualitative analysis, and research on the quantitative evaluation of site engineering characteristic parameters is not yet mature. Simple and efficient in-situ testing and evaluation methods are urgently needed.

[0008] The resistivity method works by applying a stable current to the soil surface to establish a stable electric field. Specialized instruments measure the distribution of this electric field within the soil, and based on this data, the resistivity distribution within the tested soil is calculated, thus inferring the soil's overall structure. The stable electric field potential distribution under a point current source satisfies Poisson's equation:

[0009]

[0010] In the formula, σ is the conductivity; U is the electric potential; I is the current intensity of the current source; and A is the coordinate of the current source.

[0011] Resistivity tomography (ERT) is a technique that observes and studies the changes and distribution patterns of potential and apparent resistivity under an artificially established stable electric field based on the resistivity differences of subsurface media. Combined with post-processing, it analyzes the distribution of subsurface physical properties and thus detects the distribution characteristics of the subsurface media. ERT technology integrates the advantages of DC electrical profiling and electrical sounding, providing information on transverse and longitudinal resistivity changes within a certain depth range, revealing the distribution of shallow surface soil and rock masses. It boasts advantages such as high precision, wide range, and ease of operation. However, as the detection depth increases, the accuracy of ERT technology is affected to some extent, increasing the difficulty of inverting deep subsurface models.

[0012] A complete ERT system mainly consists of three parts: conductivity sensors, a data acquisition system, and an image reconstruction computer. The conductivity sensors in an ERT system are composed of an electrode array arranged at equal intervals on the inner wall of the pipe or process vessel being measured. The control unit sends commands to the data acquisition unit to apply excitation signals to the excitation electrodes, establishing a sensitive field inside the process object. The system acquires the signals on the measuring electrodes and transmits the obtained measurement data to the image reconstruction unit. An appropriate algorithm is used to reconstruct the conductivity distribution inside the object, thus obtaining a medium distribution image. Relevant feature parameters are extracted from the reconstructed image to provide necessary data for process control.

[0013] The Resistivity Pore Pressure Static Cone Penetration Test (RCPTU) integrates the functions of conventional static cone penetration testing, pore water pressure testing, and soil resistivity testing. It mainly includes a static cone penetration machine, a static cone penetration probe, a resistivity pore pressure static cone penetration probe, a data acquisition module, a transmission module, and a display module. By analyzing the changes in four high-precision test indicators—resistivity, cone tip resistance, sidewall friction, and pore water pressure—it can obtain the changes in the physical and mechanical properties of the soil at a specific location. During in-situ static cone penetration testing, the static cone penetration machine in the system has a crossbeam, and a rotating component is mounted on the drive unit fixed to the crossbeam. The static cone penetration probe rotates under the action of the rotating component and moves downwards along its axis. During this movement, the resistivity pore pressure static cone penetration probe converts parameters such as tip resistance, frictional resistance, pore water pressure, and resistivity into electrical signals, which are transmitted to the computer via the transmission module. After a series of simulation calculations, a simulated profile of the site is obtained.

[0014] Resistivity pore pressure static cone penetration test (RCPTU) has advantages such as high testing accuracy, large data volume, continuous test results, no need for sampling, speed and convenience, small disturbance, and low cost. However, due to its own structural limitations, obtaining accurate and continuous formation data is not easy with static cone penetration technology. Summary of the Invention

[0015] Purpose of the invention: In view of the above-mentioned prior art, the present invention proposes a two-dimensional high-precision electrical resistivity test evaluation method combining RCPTU-ERT. By using the continuous high-precision longitudinal resistivity value of RCPTU as the calibration value to correct the coarse two-dimensional resistivity measurement value of ERT test, a two-dimensional high-precision resistivity test profile is formed. Combined with other test data of RCPTU test, the influence factors of site soil noise are eliminated.

[0016] Technical solution: A two-dimensional high-precision electrical resistivity tomography (EPT) test and evaluation method combining RCPTU and ERT, comprising:

[0017] Step 1: For the soil section to be tested, in-situ ERT testing technology is used to conduct two-dimensional resistivity profile testing of the soil section to obtain the coarse resistivity value of the soil. The coarse resistivity value of the soil includes resistivity index analysis noise, which is related to soil type, water content and density.

[0018] Step 2: Using in-situ RCPTU testing technology, perform longitudinal continuous testing at equal intervals on the already conducted ERT test profile;

[0019] Step 3: Using the longitudinal continuous high-precision resistivity index measured by in-situ RCPTU as the calibration parameter, correct the coarse soil resistivity measured in Step 1 to obtain a high-precision resistivity test profile.

[0020] Step 4: Based on the cone tip resistance index and sidewall friction index obtained in Step 2, and on the basis of the high-precision resistivity test profile in Step 3, perform resistivity index analysis and noise elimination related to soil layer type, water content and density.

[0021] Furthermore, in step 2, the RCPTU test points are arranged along the ERT test section, and the interval between test points is greater than or equal to the ratio of the lateral distance L of the ERT test section to the number of ERT test layers N.

[0022] Furthermore, step 3 includes the following specific steps:

[0023] Step 3-1: Invert the resistivity distribution of the two-dimensional region of the test site using the boundary voltage measured by ERT;

[0024] Step 3-2: Reconstruct a mathematical model from the discretized image of the boundary voltage using the following matrix equation:

[0025] P = WF

[0026] In the formula, P is the normalized voltage projection data vector; W is the weighting coefficient matrix; and F is the image grayscale vector that reflects the resistivity distribution of the imaging area.

[0027] Step 3-3: Substitute the continuous longitudinal resistivity data obtained from the RCPTU test into the normalized voltage projection data vector P in the matrix equation to correct the two-dimensional region resistivity distribution obtained by inversion.

[0028] Steps 3-4: Redraw and reconstruct the image based on the corrected two-dimensional region resistivity distribution to obtain a high-precision resistivity test profile.

[0029] Furthermore, in step 3-3, the data of each RCPTU test longitudinal axis is used to calibrate and correct the range of L / 2N on both sides, and the least squares method is used to correct the lubrication at the junction of the two sides of the range.

[0030] Furthermore, step 4 includes the following steps:

[0031] Step 4-1: After the site test begins, the soil index I is obtained in real time based on the cone tip resistance and sidewall friction obtained during the RCPTU penetration process, according to the modified soil classification formula (1). c This is used to classify soil layers and determine the soil layer type of the site cross section;

[0032]

[0033] Q t =(q t -σ v0) / σ′ v0 ,F t =f s / (q t -σ v0 )

[0034] In the formula, I c Q is the soil index. t For the normalized cone tip resistance, F t To normalize the sidewall friction, q t For the cone tip resistance, σ v0 For the total stress, f s For the sidewall friction, σ′ v0 Effective stress;

[0035] Step 4-2: Based on the real-time output of soil layer classification, select the target soil layer, extract the corresponding soil sample, seal it after extraction, and send it indoors for moisture content calibration test and soil density calibration test.

[0036] The power function relationship between resistivity index and soil moisture content index was established based on the calibration experiment, as shown in equation (2).

[0037] ρ = aw b (2)

[0038] In the formula, ρ is resistivity, w is water content, and a and b are the coefficient parameters of the power function.

[0039] Based on the calibration experiment, a logarithmic function relationship between density and site-normalized cone tip drag was established, as shown in equation (3).

[0040] ψ = cd log Q t (3)

[0041] In the formula, ψ is the density, and c and d are the coefficient parameters of the logarithmic function, respectively;

[0042] Step 4-3: Substitute the resistivity value of ERT after step 3 into formula (2) to obtain the specific water content of the target soil layer; substitute the measured normalized cone tip resistance of the target site into formula (3) to obtain the density information of the target soil layer.

[0043] Step 4-4: For the entire cross-section of the ERT test, based on the water content w and density ψ of each soil layer obtained from the inversion in Step 4-3, adjust the resistivity ρ of the ERT after the correction in Step 3 according to Equation (4). c Numerical noise correction:

[0044]

[0045] In the formula, ρ T The true resistivity is the noise-corrected value; φ is the porosity. S w For saturation, Where, d s ρ is the specific gravity of soil particles. w ρ is the density of water; m is the cementing coefficient. n is the saturation index.

[0046] Beneficial Effects: This invention proposes a novel RCPTU-ERT combined two-dimensional high-precision electrical resistivity testing and evaluation technology. Using the longitudinally continuous high-precision resistivity index of RCPTU as a calibration value, it combines point-to-surface testing with ERT to provide accurate deep-seated data for subsequent ERT inversion simulations, improving the accuracy of two-dimensional resistivity parameters and solving the accuracy problem of single in-situ testing equipment. Simultaneously, it uniquely employs RCPTU test data as an indicator for eliminating noise in ERT soil layer testing, forming a two-dimensional high-precision RCPTU-ERT combined testing and evaluation technology based on electrical parameters.

[0047] The method of the present invention is simple, practical, easy to operate, fast and accurate, and is particularly suitable for use on engineering sites. It is of great significance for engineering design and construction and solves a technical problem that has long been unresolved in the prior art. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the structure of the present invention;

[0049] Figure 2 This is a flowchart of the present invention;

[0050] Figure 3 It is a soil layer classification diagram;

[0051] Figure 4 This is a graph showing the relationship between resistivity and water content.

[0052] Figure 5 This is a graph showing the relationship between normalized cone tip drag and density. Detailed Implementation

[0053] The invention will now be further explained with reference to the accompanying drawings.

[0054] like Figure 1 , Figure 2 As shown, a two-dimensional high-precision electrical resistivity tomography (EPT) test and evaluation method combining RCPTU-ERT includes:

[0055] Step 1: For the soil section to be tested, in-situ ERT testing technology is used to conduct two-dimensional resistivity profile testing of the soil section to obtain a rough resistivity value. This rough resistivity value includes resistivity analysis noise, which is related to soil type, water content, and density.

[0056] Step 2: Using in-situ RCPTU testing technology, perform longitudinal continuous testing at equal intervals on the ERT test profile.

[0057] Step 3: Using the longitudinal continuous high-precision resistivity index measured by in-situ RCPTU as the calibration parameter, correct the coarse soil resistivity measured in Step 1 to obtain a high-precision resistivity test profile.

[0058] Step 4: Based on the cone tip resistance index and sidewall friction index obtained in Step 2, and on the basis of the high-precision resistivity test profile in Step 3, perform resistivity index analysis and noise elimination related to soil layer type, water content and density.

[0059] Step 5: The resistivity profile after noise elimination can be used for further evaluation of site engineering characteristics.

[0060] Specifically, in step 1, the in-situ ERT test process is as follows:

[0061] Step A1: Lay out ERT test wire 1 in the survey area, and design the electrode spacing S2, the number of test layers N3 and the number of test electrodes 4 according to the exploration requirements.

[0062] Step A2: Connect the prepared test electrode 4 to the ERT test lead 1 and connect it to the resistivity tomography imager 5 to initialize the device.

[0063] Step A3: Turn on the resistivity tomography imager 5, check the coupling between all the deployed test electrodes 4 and the earth, and set the electrical exploration parameters.

[0064] Step A4: The excitation power supply generates an electrical signal. The electrical signal received by the test electrode 4 is transmitted to the resistivity tomography imager 5 through the ERT test lead 1, and the measurement data is transmitted to the computer 10. The measurement data is processed by the built-in processing software to obtain a rough measurement value of soil resistivity that includes noise such as soil type, water content and density. Based on the rough measurement value, a pseudo-profile of apparent resistivity is drawn.

[0065] In step 2, RCPTU test points are arranged along the ERT test section, and the interval between test points is greater than or equal to the ratio of the lateral distance L of the ERT test section to the number of ERT test layers N. The specific process of RCPTU testing is as follows:

[0066] Step B1: Use the resistivity pore pressure static cone penetration test probe 6 to perform steel ring testing. Check the operating status of the static cone penetration test device by observing the output results of the display module, and adjust and confirm the specific parameters.

[0067] Step B2: Use leveling equipment to level the site at the testing location.

[0068] Step B3: Install the static cone penetration test device 7 and perform leveling.

[0069] Step B4: Install the resistivity pore pressure static cone penetration probe 6 on the static cone penetration device 7 and conduct a static cone penetration test.

[0070] Step B5: Use the static cone penetration test equipment 7 to press the resistivity pore pressure static cone probe 6 into the ground surface. The resistivity pore pressure static cone test host 9 displays the parameter data such as tip resistance, frictional resistance, pore water pressure, and resistivity in the resistivity pore pressure static cone test borehole 8, and records the output results.

[0071] Step B6: After the penetration is completed, pull out the static cone probe and restore the ground surface.

[0072] Step B7: Transmit the data to computer 10, correct and verify the data, and draw a preliminary cross-sectional view of the production area after processing with professional software.

[0073] Step 3 is completed on computer 10 and includes the following specific steps:

[0074] Step 3-1: Invert the resistivity distribution of the two-dimensional region of the test site using the boundary voltage measured by ERT.

[0075] Step 3-2: Reconstruct the mathematical model from the discretized boundary voltage image using the following matrix equation:

[0076] P = WF

[0077] In the formula, P = [p1p2…p L ] T For the normalized voltage projection data vector; W = [w ij ] L×M It is the weight coefficient matrix; F = [f1f2….f M ] T It is an image grayscale vector that reflects the resistivity distribution of the imaging area; M and L are the number of subdivision units and the number of measurement data, respectively.

[0078] Step 3-3: Substitute the continuous longitudinal resistivity data obtained from the RCPTU test into the normalized voltage projection data vector P in the matrix equation to correct the two-dimensional resistivity distribution obtained from the inversion. The longitudinal axis data of each RCPTU test is used to calibrate and correct the range of L / 2N on both sides. The least squares method is used for lubrication correction at the boundary between the two sides of the range.

[0079] Steps 3-4: Redraw and reconstruct the image based on the corrected two-dimensional region resistivity distribution to obtain a high-precision resistivity test profile.

[0080] Step 4 includes the following steps:

[0081] Step 4-1: After the site test begins, the computer 10 uses the cone tip resistance and sidewall friction obtained during the RCPTU penetration process in real time to derive the soil index I based on the modified soil classification formula (1). c This is used to classify soil layers and determine the soil layer type of the site cross section;

[0082]

[0083] Q t =(q t -σ v0 ) / σ′ v0 ,F t =f s / (q t -σ v0 )

[0084] In the formula, I c Q is the soil index. t For the normalized cone tip resistance, F t To normalize the sidewall friction, q t For the cone tip resistance, σ v0 For the total stress, f s For the sidewall friction, σ′ v0 This is the effective stress. For example... Figure 3 The image shows a soil layer classification diagram.

[0085] Step 4-2: Based on the real-time output of soil layer classification, select the target soil layer, extract the corresponding soil sample, seal it, and send it indoors for moisture content calibration test and soil density calibration test.

[0086] The power function relationship between resistivity index and soil moisture content index was established based on the calibration experiment, as shown in equation (2).

[0087] ρ = aw b (2)

[0088] In the formula, ρ is resistivity, w is water content, and a and b are the coefficients of the power function obtained by the least squares method. The curve is shown in Figure 1. Figure 4 As shown.

[0089] Based on the calibration experiment, a logarithmic function relationship between density and site-normalized cone tip drag was established, as shown in equation (3).

[0090] ψ = cd log Q t (3)

[0091] In the formula, ψ is the density, and c and d are the coefficients of the logarithmic function obtained by the least squares method, as shown in the curve. Figure 5 As shown.

[0092] Step 4-3: Substitute the resistivity value of ERT after step 3 into formula (2) to obtain the specific water content of the target soil layer; substitute the measured normalized cone tip resistance of the target site into formula (3) to obtain the density information of the target soil layer.

[0093] Step 4-4: For the entire cross-section of the ERT test, based on the water content w and density ψ of each soil layer obtained from the inversion in Step 4-3, adjust the resistivity ρ of the ERT after the correction in Step 3 according to Equation (4). c Numerical noise correction:

[0094]

[0095] In the formula, ρ T The true resistivity is the noise-corrected value; φ is the porosity. S w For saturation, Where, d s The specific gravity of the soil particles is obtained by consulting the standard for specific gravity of soil particles; ρ w ρ is the density of water. w =1; m and n are respectively related to the soil index I c Relevant soil parameters, where m is the cementation coefficient. n is the saturation index.

[0096] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A two-dimensional high-precision electrical method test evaluation method of RCPTU-ERT combination, characterized in that, The application relates to a method for testing soil layer resistivity, and belongs to the field of geotechnical engineering. The method comprises the following steps: Step 1: for a soil layer section to be tested, in-situ ERT testing technology is used to test the two-dimensional resistivity profile of the soil layer section, and coarse soil layer resistivity values are obtained; wherein the coarse soil layer resistivity values contain resistivity index analysis noise, and the resistivity index analysis noise is related to soil layer types, water content and density; Step 2: in-situ RCPTU testing technology is used to continuously test in the longitudinal direction at equal intervals on the ERT testing profile; Step 3: the high-precision resistivity index obtained by the in-situ RCPTU is used as a calibration parameter to correct the coarse soil layer resistivity values obtained in step 1, and a high-precision resistivity testing profile is obtained; Step 4: according to the cone tip resistance index and the side wall friction resistance index obtained in step 2, the resistivity index analysis noise related to the soil layer types, water content and density is eliminated on the basis of the high-precision resistivity testing profile in step 3. The step 3 comprises the following specific steps: Step 3-1: the boundary voltage measured by the ERT is used to inversely calculate the two-dimensional regional resistivity distribution of the measured site; Step 3-2: a matrix equation is used to reconstruct the image of the discretized boundary voltage, and the matrix equation is as follows: P = WF In the formula, P is a normalized voltage projection data vector, W is a weight coefficient matrix, and F is an image gray vector capable of reflecting the resistivity distribution of the imaging region; Step 3-3: the continuous longitudinal resistivity data obtained by the RCPTU is substituted into the normalized voltage projection data vector P in the matrix equation, and the two-dimensional regional resistivity distribution inversely calculated is corrected; 2. The method of claim 1, wherein the RCPTU-ERT combined two-dimensional high-precision electrical method test evaluation method is characterized by, Step 3-4: according to the two-dimensional regional resistivity distribution after correction, an image is redrawn and reconstructed, and a high-precision resistivity testing profile is obtained.

3. The method of claim 1, wherein the RCPTU-ERT combined two-dimensional high-precision electrical method test evaluation method is characterized by, In the step 2, the RCPTU test points are arranged along the ERT testing section, and the interval between the test points is greater than or equal to the ratio of the ERT testing section transverse distance L to the ERT testing layer number N.

4. The method of two-dimensional high-precision electrical testing evaluation of RCPTU-ERT combined according to any one of claims 1-3, characterized in that, In the step 3-3, each RCPTU test longitudinal axis data is used to calibrate and correct the range of L / 2N on both sides of the range, and the least square method is used to lubricate and correct the junctions on both sides of the range. Step 4-1: After the start of the field test, the cone tip resistance and the side friction resistance obtained during the penetration process are obtained in real time by the RCPTU, and the soil classification index I is obtained according to the modified soil classification formula (1) c , so as to classify the soil and determine the soil type of the site section; Q t = (q t - σ v0 ) / σ' v0 , F t = f s (q t - σ v0 ) where I c is the soil index, Q t is the normalized cone tip resistance, F t is the normalized side friction, q t is the cone tip resistance, σ v0 is the total stress, f s is the side friction, σ' v0 is the effective stress; The step 4 comprises the following steps: Step 4-2: according to the real-time output soil layer classification, a target soil layer is selected, corresponding soil samples are cut, and the soil samples are sealed and sent into a room for indoor water content calibration test and soil layer density calibration test; p = aw b (2) According to the calibration test, a power function relationship between the resistivity index and the water content index in the soil is established, and the power function relationship is as shown in formula (2); In the formula, rho is the resistivity, w is the water content, and a and b are respectively the coefficient parameters of the power function; ψ = c - d log Q t (3) According to the calibration test, a logarithmic function relationship between the density and the normalized cone tip resistance of the site is established, and the logarithmic function relationship is as shown in formula (3) In the formula, psi is the density, and c and d are respectively the coefficient parameters of the logarithmic function; Step 4-3: the resistivity value of the ERT after correction in step 3 is substituted into formula (2), and the specific water content of the target soil layer is inversely calculated; the measured normalized cone tip resistance of the target site is substituted into formula (3), and the density information of the target soil layer is inversely calculated. Step 4-4: For the full cross-section of the ERT test, according to the water content w and the density ψ of each soil layer obtained by the inversion of Step 4-3, the resistivity p of the ERT after the correction of Step 3 is modified according to formula (4) c Numerical noise correction: In the formula, p T is the true resistivity after noise correction; φ is the porosity, S w is the saturation, wherein d s is the specific gravity of soil particles, p w is the density of water; m is the cementation coefficient, n is the saturation exponent,

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