Cement lime soil roadbed compactness high-precision nondestructive testing method based on split type ground penetrating radar

Abstract

The invention belongs to the technical field of road and bridge construction, and particularly discloses a cement lime soil roadbed compactness high-precision nondestructive testing method based on a split type ground penetrating radar. The method comprises the following steps: calibrating dielectric constants epsilon x corresponding to a standard test piece under different compaction degree kx conditions by adopting a small-size test piece differential medium combination dielectric constant transmission wave calibration method in a laboratory, and obtaining a relation fitting formula f (kx, epsilon x) of the limestone soil roadbed compaction degree kx and the dielectric constants epsilon x through data fitting; carrying out calculation processing on field limestone soil roadbed test data based on Hilbert-Huang transform signals, and calculating to obtain a dielectric constant epsilon i under a field working condition; and substituting epsilon i into a fitting formula f (kx, epsilon x), and reversely deducing the field compaction degree ki. Due to the fact that the ground penetrating radar conducts whole-line continuous detection on site, the full-range compactness index of the roadbed in the test area is calculated according to the method, and lossless, rapid and high-precision detection of the compactness index of the cement limestone soil roadbed is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of road and bridge construction, relates to the detection of roadbed compaction, and particularly to a high-precision nondestructive detection method for the compaction of cement-lime soil roadbed based on a split-type ground penetrating radar. Background Art

[0002] With the rapid development of social construction, the scale of transportation infrastructure in various cities has shown a trend of continuous growth. The quality of roadbed is related to highway driving safety and economic benefits, and has received much attention from the country and society. In the current highway roadbed construction, cement lime soil has been widely promoted and applied due to its good stability, compaction and water stability, and the strength of the material can increase with age. As an important layer of highway engineering, the roadbed bears the load from the road surface and the erosion of natural environment temperature and water. The compaction quality of the roadbed is the key to the construction and acceptance of highway engineering. The compaction quality of the roadbed is crucial to the strength, durability and stability of the roadbed. Insufficient compaction quality will reduce the structural strength and bearing capacity. The compaction degree of the roadbed is the core indicator of the quality control of the roadbed compaction. The compaction degree is closely related to the compaction effect of the road. If the compaction degree does not meet the standard, it will cause roadbed and pavement diseases. Compactness detection is the core task of roadbed quality inspection and acceptance. At present, the roadbed compaction detection is still mainly based on conventional destructive coring detection methods such as the ring knife method and the sand filling method. The above-mentioned traditional manual detection methods are simple and fast, but the efficiency is low and there are the following defects: (1) Random detection has poor representativeness; (2) Limited point sampling cannot fully reflect the overall compaction quality of the roadbed; (3) Drilling coring is a destructive detection method, which is destructive and easy to leave hidden dangers for subsequent projects. Therefore, it is of great engineering significance to find a fast and non-destructive compaction detection method to control the compaction quality during roadbed construction, and to reflect the overall compaction quality of the base to a certain extent.

[0003] Ground penetrating radar is a nondestructive testing method that uses radar antennas to emit radio electromagnetic waves to detect media. Electromagnetic waves will be transmitted and reflected in media with different dielectric properties, and the energy of the corresponding radio waves will change. As the electromagnetic waves attenuate, their waveforms change. The dielectric constant of the electromagnetic waves in the material can be converted through calculation. There are differences in the dielectric constant of materials in roadbeds with different compaction degrees. By using the correlation between the dielectric constant and the compaction degree, the distribution of the compaction degree of the roadbed can be quickly detected and evaluated, thereby realizing rapid, nondestructive and continuous detection of the compaction degree during the roadbed construction process.

[0004] At present, ground penetrating radar has been used in non-destructive testing of roadbed compaction and has achieved certain results. There are many theoretical studies, but there are two main problems in practical application: (1) The transmitting and receiving antennas of most radars are integrated. The corresponding laboratory calibration requires the preparation of standard large specimens with specified compaction. However, there is no special compaction control equipment and method for laboratory calibration of standard large specimens. It is very difficult to accurately control the compaction of specimens. It is impossible to accurately control the compaction accuracy under large-scale laboratory conditions, so it is difficult to accurately obtain the dielectric constant of standard specimens with different compaction degrees, and it is difficult to provide accurate dielectric constant reference comparison for field compaction test results. (2) There are many interference factors in field testing. Since the radar test signal of cement lime soil roadbed material is non-stationary and nonlinear, it is difficult for the radar test raw data to directly identify the propagation time of electromagnetic waves in the medium, and ultimately it is difficult to accurately calibrate the field roadbed compaction dielectric constant. In summary, at present, there are certain difficulties in the calibration and testing of dielectric constants in both the laboratory and field for non-destructive testing of roadbed ground penetrating radar, and ultimately the test accuracy of ground penetrating radar is difficult to guarantee. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a high-precision nondestructive detection method for the compaction of cement-lime-soil roadbed based on a split ground penetrating radar. Based on the equipment feature of the split ground penetrating radar that the generating antenna and the receiving antenna are separated, a high-precision nondestructive detection method for the compaction of cement-lime-soil roadbed based on a split ground penetrating radar is proposed. A cement-lime-soil compaction detection process based on a split ground penetrating radar is studied and constructed, a transmission wave calibration method for the dielectric constant of a differentiated medium combination of small-size specimens suitable for laboratories is proposed, and a key technology for high-precision analysis of the dielectric constant signal of a field roadbed based on the Hilbert-Huang transform is designed, which effectively solves the problem that large-volume specimens in the laboratory are difficult to meet the compaction standards. The complex signal analysis obtained from the field roadbed compaction test is converted into a finite number of linear steady-state signals through the Hilbert-Huang transform, and the instantaneous characteristic parameters in the signal are effectively extracted, finally forming a high-precision nondestructive detection method for the compaction of cement-lime-soil roadbed based on a split ground penetrating radar.

[0006] The present invention is achieved through the following technical solutions:

[0007] A high-precision nondestructive detection method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar comprises the following steps:

[0008] (1) Establish the compaction degree k of cement lime soil x and dielectric constant ε x The fitting formula f(k x , ε x )

[0009] (1.1) Preparation of different compaction degrees k in the laboratoryx The small-volume cement-lime-soil specimen is required to be compatible with the outer dimensions of the transmitting antenna and the receiving antenna of the split-type ground penetrating radar, and the cross section should be able to completely cover the cross section of the transmitting and receiving antennas; the compaction range of the cement-lime-soil specimen design should be wide, and the compaction of the roadbed of different grades of roads is usually more than 94%. The dielectric constant of cement-lime-soil specimens with different compaction degrees is calibrated in the laboratory. Considering the underpressure or overpressure in some areas of the actual on-site compaction, the design compaction range is recommended to cover 86%-100%;

[0010] (1.2) Split-type ground penetrating radar uses a small-scale specimen with a differentiated dielectric combination dielectric constant transmission wave calibration method to obtain the propagation time t of the magnetic wave in the specimen. x , according to formula (1), the dielectric constant ε of specimens with different compaction degrees is calculated x :

[0011]

[0012] Where x is the designed compaction degree of the specimen, %; h x - Measured height of the specimen, m; t x - the time it takes for the perspective wave to enter the lime soil specimen and then pass through the lime soil, ns; c- the speed of light, 3*10 8 m / s;

[0013] Based on the structural characteristics of the split antenna of the ground penetrating radar signal, the laboratory uses a small cylindrical specimen. The compaction degree of the specimen can be controlled by the compaction test benchmark and the molding workload is small. The specimen diameter is 15cm, the design height is 15cm, and the dielectric constant test adopts the transmission wave principle. However, considering the later identification, it can accurately determine the electromagnetic wave entering and leaving the specimen, and accurately calculate the propagation time t of the electromagnetic wave in the specimen. x The key to identification lies in the differences in materials at different layers, that is, the dielectric constant of the material before the electromagnetic wave enters the cement lime soil specimen is significantly different from that of the specimen. Figure 1 As shown in the figure, the dielectric constant transmission wave calibration method of the small-scale specimen with differentiated dielectric combination is shown in the figure. A cylindrical iron sheet is set at both ends of the specimen. The dielectric constant ε of the iron is a The dielectric constant ε of lime soil x The difference is large. This operation can clearly identify the propagation time t of the electromagnetic wave in the specimen during data processing. x .

[0014] (1.3) Compactness k of lime soil specimen x and dielectric constant ε x Relationship Fitting

[0015] Based on the above test process, the specimens with different compaction degrees and dielectric constants were obtained. The results are shown in Table 1.

[0016] Table 1 Test results of specimen compaction and dielectric constant

[0017] Compaction <![CDATA[k1]]> <![CDATA[k2]]> <![CDATA[k3]]> …… <![CDATA[k x ]]> Dielectric constant <![CDATA[ε1]]> <![CDATA[ε2]]> <![CDATA[ε3]]> <![CDATA[ε x ]]>

[0018] The data results were fitted using origin software to obtain the compaction degree k x and dielectric constant ε x The fitting formula f(k x , ε x ).

[0019] (2) On-site cement lime soil roadbed data collection and processing

[0020] (2.1) The on-site radar uses the reflected wave and the GPR data management, integration and data display software EKKO_Project to obtain the unprocessed radar data;

[0021] The on-site dielectric constant test of ground penetrating radar adopts the principle of reflected wave transmission. The propagation process of electromagnetic waves in cement lime soil roadbed is as follows: Figure 2 As shown. The ground penetrating radar electromagnetic waves send a series of pulse electromagnetic waves to the cement lime soil roadbed through the transmitting antenna. When encountering the interface between the roadbed and the underlying layer, there is a difference in the dielectric constant of the material between the upper and lower layers, and the electromagnetic waves are reflected from the interface. The ground penetrating radar receiving antenna receives the reflected electromagnetic waves.

[0022] (2.2) Using the Hilbert transform function of the radar software to perform Hilbert transform on the preprocessed data, convert the time domain into the frequency domain, and obtain the preprocessed data;

[0023] (2.3) Through Dewow+SEC2 Gain filtering, useful signals are separated from noise, the signal's anti-interference ability and signal-to-noise ratio are improved, and the drift of the radar wave signal's initial arrival time point is eliminated. The mean filtering method is used to remove the background in the radar profile image. The upper of the two dark horizontal lines represents the electromagnetic wave entering the roadbed surface, and the lower one represents the electromagnetic wave reaching the roadbed bottom layer. After the processed data is obtained, the electromagnetic wave propagation time t is determined. i ,like Figure 3 As shown, the thickness of the designed roadbed is h i , the distance between the GPR transmitting and receiving antennas is d, assuming that the electromagnetic waves propagate at the same speed in the roadbed;

[0024] According to formulas (2) and (3), the dielectric constant ε corresponding to the on-site roadbed is calculated i ;

[0025]

[0026] In the formula, h i- designed roadbed thickness, d-the distance between the ground penetrating radar transmitting and receiving antennas;

[0027] (3) The ε calculated by the on-site lime soil roadbed test i Substitute into the fitting formula f(k x ,ε x ), and inversely deduce the on-site compaction degree k i ;

[0028] (4) The corresponding points on site are cored and dried using the ring knife method to obtain the moisture content and dry density, and the actual compaction degree k of the corresponding points on site is obtained. j , k i With k j The error should be less than 1.5%, otherwise the cause analysis should be carried out and correction methods should be proposed to ensure the accuracy of the radar test results.

[0029] A further improvement of the present invention is:

[0030] The moisture content of the cement-lime-soil specimen, the theoretical optimum moisture content and the actual moisture content on site must be consistent, with the error controlled within ±2%.

[0031] Furthermore, the process of analyzing the ground penetrating radar signal of cement lime soil roadbed by Hilbert transform is as follows:

[0032] (1) Using the Empirical Mode Decomposition (EMD) method, complex signals are decomposed into a finite number of Intrinsic Mode Functions (IMFs) from high frequency to low frequency; Empirical Mode Decomposition (EMD) is to adaptively decompose complex signals into Intrinsic Mode Function (IMF) components for Hilbert transform. The purpose of EMD empirical mode decomposition is to decompose any signal (especially non-stationary, nonlinear time series signals) into linear steady-state signals. Its core is to decompose any free signal into several Intrinsic Mode Functions (IMFs, also known as Intrinsic Mode Functions) and a residual component. Each IMF represents the oscillation change of the original signal in different frequency bands, reflecting the local characteristics of the signal, while the final residual component reflects the slow change in the signal.

[0033] (2) Perform Hilbert transform on the IMF components obtained by EMD decomposition to construct the analytical signals of each IMF component and its instantaneous amplitude a i (t), instantaneous phase θ i (t) and the instantaneous frequency ω i (t).

[0034] Further, such as Figure 4 As shown, the process of step (1) empirical mode decomposition is:

[0035] Step 1: Assume that the original signal data is x(t), find all local maxima and local minima in x(t), and use cubic spline interpolation to connect the local maxima into the upper envelope e max (t), connect the local minima to form the lower envelope e min (t);

[0036] Step 2: Calculate the average value of the upper and lower envelopes at each time to obtain the mean envelope m1(t):

[0037]

[0038] Step 3: Subtract the mean envelope from the original signal data x(t) to get the first score h1(t):

[0039] h1(t)=x(t)-m1(t) (5)

[0040] Here, if h1(t) satisfies the condition of the intrinsic mode function, then h1(t) is the first-order intrinsic mode function of the original signal. If h1(t) does not satisfy it, proceed to step 4;

[0041] Step 4: Treat h1(t) as the original signal and perform a second screening, namely:

[0042] h 11 (t) = h1(t) - m 11 (t) (6)

[0043] Where: m 11 (t) is the mean of the upper and lower envelopes of h1(t);

[0044] Repeat the screening until h1k(t)=h1(k-1)(t)-m1(k-1)(t) satisfies the definition of the intrinsic mode function, and the first-order intrinsic mode function C1(t)=h1k(t) is obtained.

[0045] Step 5: Subtract C1(t) from the original signal x(t) to get the residual r1(t), that is:

[0046] r1(t)=x(t)-C1(t) (7)

[0047] Step 6: Treat r1(t) as the new original signal and re-execute steps 1 to 5 to obtain the new residual r n (t), repeat this process n times, and when the nth residual quantity r nWhen (t) becomes a monotonic function or a constant and cannot be decomposed into intrinsic mode functions, the entire EMD process ends;

[0048] The original signal x(t) can be expressed as n intrinsic mode function components and an average trend component r n (t), that is:

[0049]

[0050] Where: C k (t) is the intrinsic mode function component, r n (t) is the residual component, which generally represents the average trend of the signal and is a constant sequence or a monotonic sequence.

[0051] Furthermore, the IMF component and the residual term r n (t) can be reconstructed into the original signal x(t), which can be expressed as

[0052]

[0053] IMF is a single-component signal with only one frequency at any moment, namely the instantaneous frequency. The two necessary conditions for the intrinsic mode function IMF include: in the entire data set, the sum of the number of local maxima and local minima must be equal to the number of zero crossings or differ by at most one; at any time point, the upper envelope defined by the local maximum and the lower envelope defined by the local minimum have a mean of zero, that is, the waveform of the entire sequence must be locally symmetrical.

[0054] Furthermore, when the Hilbert transform is performed on the IMF component in step (2), the residual term r n (t) is a constant or monotonic signal, which contains little information, so r is discarded. n (t) The original signal x(t) can be obtained by calculating the real part of its analytical signal, that is,

[0055]

[0056] When the amplitude in equation (7) is expressed as a function of time and frequency, the time-frequency distribution of the amplitude is called the Hilbert spectrum, denoted as: H(ω,t);

[0057] The Hilbert transform is defined as:

[0058]

[0059] Among them, pv represents the Cauchy principal value, that is, the integral value of the defect integral;

[0060] From the above definition, we can see that the Hilbert transform of a function u is the convolution of the function and The convolution is:

[0061]

[0062] By calculating the given sin function, we get:

[0063]

[0064] The Hilbert transform extracts the instantaneous amplitude, instantaneous phase and instantaneous frequency of the signal, which are very helpful in improving the resolution and interpretation accuracy of radar signals. By effectively extracting the instantaneous characteristic parameters in the signal, the Hilbert transform can not only improve the accuracy of data interpretation, but also enhance the detection capability of underground structures.

[0065] The instantaneous increase calculation formula is:

[0066]

[0067] The instantaneous frequency is calculated as:

[0068]

[0069] The calculation formula for the instantaneous phase is:

[0070]

[0071] Using convolution, we can get:

[0072]

[0073] The real and imaginary parts of the complex domain signal are defined as f(t) and The corresponding analytical signal is:

[0074]

[0075] Compared with the prior art, the present invention has the following beneficial effects:

[0076] (1) The present invention innovatively proposes a nondestructive detection method and process flow for the compaction of cement-lime-soil roadbed based on a split-type ground-penetrating radar. Based on the dielectric constant test of the test section roadbed and the dielectric constant comparison method of the laboratory standard specimen, a continuous nondestructive detection method and process flow for the compaction of cement-lime-soil roadbed based on a split-type ground-penetrating radar are designed.

[0077] (2) Based on the separate structural characteristics of the transmitting antenna and receiving antenna of the ground penetrating radar signal, and using the transmission theory of transmitted waves, we made small-scale specimens of cement lime soil with different compaction degrees by using laboratory compaction tests and reliable equipment. Compared with the compaction degree calibration of large specimens, we proposed a transmission wave calibration method for the dielectric constant of differentiated medium combinations. This method can clearly determine the propagation time of electromagnetic waves in the specimens of designed thickness, and thus accurately obtain the different compaction degrees k. x The dielectric constant ε corresponding to the standard specimen under the conditions x , the compaction degree k of lime soil roadbed is obtained by data fitting x and dielectric constant ε x The relationship fitting formula f(k x ,ε x ).

[0078] (3) The present invention relies on a split-type ground-penetrating radar to emit high-frequency electromagnetic waves to the cement-lime soil roadbed to be inspected. Based on the principle of electromagnetic wave reflection waves, the time difference between the electromagnetic waves emitted and received by the split antenna of the ground-penetrating radar, and the Hilbert-Huang transform signal processing algorithm, the propagation time ti of the electromagnetic wave in the cement-lime soil roadbed with a compaction degree ki on site is calculated, the propagation speed vi of the electromagnetic wave in the lime soil roadbed is identified, and the dielectric constant εi of the electromagnetic wave under on-site working conditions is calculated. The εi calculated according to the on-site lime soil roadbed test i Substitute into the fitting formula f(k x ,ε x ), and inversely deduce the on-site compaction degree k i Since the ground penetrating radar performs continuous detection along the entire line on site, the compaction index of the entire range of the roadbed in the test area is calculated according to the above method, realizing non-destructive, rapid and high-precision detection of the compaction index of the cement-lime soil roadbed.

[0079] (4) After the roadbed is paved, the inspection of the present invention can achieve high-precision detection of the entire roadbed and calibrate the roadbed compaction degree compared to traditional damage point monitoring, fully reflecting the compaction quality and effect of the paved roadbed. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Figure 1 The schematic diagram of the calibration principle of the transmission wave of the differentiated medium;

[0081] Figure 2 It is a schematic diagram of the principle of electromagnetic wave reflection;

[0082] Figure 3 This is the model diagram for calculating the dielectric constant of the on-site roadbed;

[0083] Figure 4 This is the EMD empirical mode decomposition flow chart;

[0084] Figure 5 This is a map of the project site;

[0085] Figure 6 To adjust the test parameters;

[0086] Figure 7 is the original radar data;

[0087] Figure 8 is the transformation function diagram;

[0088] Fig. 9 For gain filter adjustment;

[0089] Fig.10 For processed radar data; Fig.11 Figure 2 is a diagram of raw materials and equipment used in the present invention;

[0090] Fig.12 To prepare the molded test piece;

[0091] Fig.13 This is a test chart of 90% compaction;

[0092] Fig.14 This is a test chart of 94% compaction;

[0093] Fig.15 This is a test chart of 98% compaction;

[0094] Fig.16 is the compaction degree k x and dielectric constant ε x The fitting curve of

[0095] Fig.17 is the relationship between distance and compaction degree;

[0096] Fig.18 is the relationship between distance and compaction degree. DETAILED DESCRIPTION

[0097] The present invention is described in detail below in conjunction with specific embodiments.

[0098] 1. Field data acquisition and preprocessing

[0099] This example is based on the "Lianshui County Jianshe Road (Zhongou Road ~ Qinglian Avenue) Construction Project" and is determined to be constructed using the standards for urban secondary trunk roads, with a roadbed width of 40m and a design speed of 40km / h. Figure 5 As shown, the roadbed is mainly filled with silt. In order to ensure the strength and stability of the roadbed and meet the compaction standard of heavy compaction, the construction experience of the highways under construction or already built in the project area is drawn upon, and the construction and other factors are taken into consideration. The design of the roadbed filling material of the entire line is to use inorganic binders such as lime and cement for technical treatment.

[0100] After the roadbed material is sprinkled with water and mixed evenly, a quick sample is taken and the moisture content of cement lime soil is tested on site in the project department laboratory to check whether it is within the allowable range of the optimum moisture content ±2%. If it is not within this range, the moisture content on site is adjusted. If it is within this range, this moisture content is used as the actual moisture content of the indoor compaction test piece.

[0101] After the on-site roadbed is compacted and formed, remove the equipment and obstacles on the test route to keep the route unobstructed. According to the ground penetrating radar operation process, complete the system's distance calibration, parameter setting and debugging, record the mileage pile number of the starting point, start the measurement work, complete the full-line inspection of the cement lime soil roadbed, and conduct lane-by-lane inspection as required. After the inspection is completed, save the inspection data for background software data processing.

[0102] The radar equipment uses Canadian EKKO geological radar. Because the thickness of the test roadbed is relatively shallow, the higher the antenna frequency, the shallower the detection depth, but the higher the resolution. Therefore, the on-site radar uses a high-frequency antenna 1000MHz reflected wave. The on-site test parameters are adjusted as follows: Figure 6 shown.

[0103] Using GPR data management, integration and data display software EKKO_Project, unprocessed radar data such as Figure 7 shown.

[0104] The Hilbert transform function of the radar software is used to perform Hilbert transform on the raw data, such as Figure 8 As shown, the instantaneous amplitude, instantaneous frequency and instantaneous phase are obtained. Through Dewow+SEC2 Gain gain filtering, the useful signal is separated from the noise, the anti-interference ability and signal-to-noise ratio of the signal are improved, and the drift of the initial arrival time point of the radar wave signal is eliminated. The mean filtering method is used, as shown in Fig. 9 As shown in the figure, the background in the radar profile image is removed. The upper one of the two dark horizontal lines represents the electromagnetic wave entering the roadbed surface, and the lower one represents the electromagnetic wave reaching the roadbed bottom layer. The electromagnetic wave propagation time t is determined. i , the processed data is as follows Fig.10 shown.

[0105] The total length of the field test is 600 meters. A measurement point is taken every meter. The dielectric constant ε corresponding to each meter of the roadbed is calculated according to formula (3): i .

[0106] 2. Calibration of dielectric constant of specimens with different compaction degrees in the laboratory

[0107] (1) Fig.11As shown in the figure, through laboratory tests, the natural moisture content of the soil is 12.28%, and the designed cement lime soil ratio is 2%:4%:100%. Through compaction tests, the target moisture content of the designed mix material is 14%, 15%, 16%, 17%, 18%, 19% and 20%. Analysis of the compaction test results shows that the optimal moisture content of the mix cement lime soil is 15.8%, and the maximum dry density is 1.515g / cm 3 .

[0108] (2) The compaction degrees of the designed specimens are 88%, 90%, 92%, 94%, 96% and 98% respectively. Table 2 shows the specific material mass mix ratios.

[0109] Table 2 Material quality ratio

[0110]

[0111] (3) Test piece Fig.12 shown.

[0112] (4) After the specimens are formed, the height of each specimen is tested and recorded. The dielectric constant transmission wave calibration method of the differential medium combination of small-size specimens is used to test the dielectric constants of the specimens with compaction degrees of 88%, 90%, 92%, 94%, 96%, and 98%. The compaction degrees of 90%, 94%, and 98% are selected. Some test figures are shown in the figure below. Figure 13-15 shown.

[0113] The test analysis results are shown in Table 3.

[0114] Table 3 Summary of dielectric constant test results of specimens with different compaction degrees

[0115] Compactness(%) 88 90 92 94 96 98 Dielectric constant 50.89 47.93 48.2 45.56 38.11 25.79

[0116] (4) Compactness k of cement-lime-soil specimens x and dielectric constant ε x Relationship Fitting

[0117] Based on the above test process, the specimens with different compaction degrees and dielectric constants were obtained. The data results were fitted using the origin software. The fitting curve is shown in the figure. Fig.16 As shown, the compaction degree k is obtained x and dielectric constant ε x The fitting formula f(k x , ε x ).

[0118] The fitting formula of compaction degree and relative dielectric constant is shown in (19):

[0119] k i =78.69197+1.30726ε i -0.02187εi 2 (19)

[0120] 3. Compactness of lime soil roadbed k i Back calculation and verification

[0121] The dielectric constant ε corresponding to each meter of roadbed on site i , substituted into the relationship between compaction degree and dielectric constant (19) fitted in the laboratory, and the dielectric constant ε calculated on site i By comparison, the on-site compaction degree k is obtained. i The highest is 98.6%, the lowest is 90.9%, the average compaction is 95.06%, and the compaction that meets the engineering requirements is not less than 94%. Fig.17 shown.

[0122] The total length of the roadbed test is 600 meters. For easy viewing, the 600 meters are divided into 6 sections, each 100 meters. The compaction degree of each meter of roadbed is as follows: Fig.18 shown.

[0123] At the starting position of the on-site road surface, 0 meters, 300 meters in the middle, and 600 meters at the end, the core was taken and dried using the ring knife method to obtain the moisture content and dry density. The dry density was divided by the maximum dry density to obtain the compaction degree kj. The compaction degree at 0 meters was 96.5%, 300 meters was 94.3%, and 600 meters was 94.9%. The average compaction degree kj was 95.23%, and the error between the compaction degree ki measured by the radar method and the compaction degree kj measured by the ring knife method was 0.17%, both meeting the engineering requirement of not less than 94% compaction degree.

[0124] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent transformation or modification made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-precision nondestructive testing method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar, characterized in that: The following steps are involved: (1) Establish the compaction degree k of cement lime soil x and dielectric constant ε x The fitting formula f(k x , ε x ) (1.1) Preparation of different compaction degrees k in the laboratory x The small-volume cement-lime-soil specimen is required to be compatible with the outer dimensions of the transmitting antenna and the receiving antenna of the split-type ground penetrating radar, and the cross section should be able to completely cover the cross section of the transmitting and receiving antennas; the compaction degree of the cement-lime-soil specimen is required to cover the range of 86%-100%; (1.2) Split-type ground penetrating radar uses a small-scale specimen with a differentiated dielectric combination dielectric constant transmission wave calibration method to obtain the propagation time t of the magnetic wave in the specimen. x , according to formula (1), the dielectric constant ε of specimens with different compaction degrees is calculated x : Where x is the designed compaction degree of the specimen, %; h x - Measured height of the specimen, m; t x - the time it takes for the perspective wave to enter the lime soil specimen and then pass through the lime soil, ns; c- the speed of light, 3*10 8 m / s; (1.3) The data results are fitted using the origin software to obtain the compaction degree k x and dielectric constant ε x The fitting formula f(k x , ε x ); (2) On-site cement lime soil roadbed data collection and processing (2.1) The on-site radar uses the reflected wave and the GPR data management, integration and data display software EKKO_Project to obtain the unprocessed radar data; (2.2) Using the Hilbert transform function of the radar software to perform Hilbert transform on the preprocessed data, convert the time domain into the frequency domain, and obtain the preprocessed data; (2.3) Through Dewow+SEC2 Gain filtering, useful signals are separated from noise, the signal's anti-interference ability and signal-to-noise ratio are improved, and the drift of the radar wave signal's initial arrival time point is eliminated. The mean filtering method is used to remove the background in the radar profile image. The upper of the two dark horizontal lines represents the electromagnetic wave entering the roadbed surface, and the lower one represents the electromagnetic wave reaching the roadbed bottom layer. After the processed data is obtained, the electromagnetic wave propagation time t is determined. i , according to formula (3), the dielectric constant ε corresponding to the on-site roadbed is calculated i ; In the formula, h i - designed roadbed thickness, d-the distance between the ground penetrating radar transmitting and receiving antennas; (3) The ε calculated by the on-site lime soil roadbed test i Substitute into the fitting formula f(k x ,ε x ), and inversely deduce the on-site compaction degree k i ; (4) The corresponding points on site are cored and dried using the ring knife method to obtain the moisture content and dry density, and the actual compaction degree k of the corresponding points on site is obtained. j , k i With k j The error should be less than 1.5%, otherwise the cause analysis should be carried out and correction methods should be proposed to ensure the accuracy of the radar test results.

2. According to claim 1, a high-precision nondestructive testing method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar is characterized by: The moisture content of the cement-lime-soil specimen, the theoretical optimum moisture content and the actual moisture content on site must be consistent, with the error controlled within ±2%.

3. The high-precision nondestructive testing method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar according to claim 1 is characterized by: The process of analyzing the ground penetrating radar signal of cement lime soil roadbed by Hilbert transform is as follows: (1) Using the empirical mode decomposition (EMD) method, complex signals are decomposed into a finite number of intrinsic mode functions (IMFs) from high frequency to low frequency; (2) Perform Hilbert transform on the IMF components obtained by EMD decomposition to construct the analytical signals of each IMF component and its instantaneous amplitude a i (t), instantaneous phase θ i (t) and the instantaneous frequency ω i (t).

4. According to claim 3, a high-precision nondestructive testing method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar is characterized by: The process of step (1) empirical mode decomposition is: Step 1: Assume that the original signal data is x(t), find all local maxima and local minima in x(t), and use cubic spline interpolation to connect the local maxima into the upper envelope e max (t), connect the local minima to form the lower envelope e min (t); Step 2: Calculate the average value of the upper and lower envelopes at each time to obtain the mean envelope m1(t): Step 3: Subtract the mean envelope from the original signal data x(t) to get the first score h1(t): h1(t)=x(t)-m1(t) (5) Here, if h1(t) satisfies the condition of the intrinsic mode function, then h1(t) is the first-order intrinsic mode function of the original signal. If h1(t) does not satisfy it, proceed to step 4; Step 4: Treat h1(t) as the original signal and perform a second screening, namely: h 11 (t)=h1(t)-m 11 (t) (6) Where: m 11 (t) is the mean of the upper and lower envelopes of h1(t); Repeat the screening until h1k(t)=h1(k-1)(t)-m1(k-1)(t) satisfies the definition of the intrinsic mode function, and the first-order intrinsic mode function C1(t)=h1k(t) is obtained. Step 5: Subtract C1(t) from the original signal x(t) to get the residual r1(t), that is: r1(t)=x(t)-C1(t) (7) Step 6: Treat r1(t) as the new original signal and re-execute steps 1 to 5 to obtain the new residual r n (t), repeat this process n times, and when the nth residual quantity r n When (t) becomes a monotonic function or a constant and cannot be decomposed into intrinsic mode functions, the entire EMD process ends; The original signal x(t) can be expressed as n intrinsic mode function components and an average trend component r n (t), that is: Where: C k (t) is the intrinsic mode function component, r n (t) is the residual component, which generally represents the average trend of the signal and is a constant sequence or a monotonic sequence.

5. According to claim 4, a high-precision nondestructive testing method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar is characterized by: The IMF components and the residual term r n (t) can be reconstructed into the original signal x(t), which can be expressed as 6. According to claim 3, a high-precision nondestructive testing method for compaction of cement-lime soil roadbed based on split-type ground penetrating radar is characterized by: When the Hilbert transform is performed on the IMF component in step (2), the residual term r n (t) is a constant or monotonic signal, which contains little information, so r is discarded. n (t) The original signal x(t) can be obtained by calculating the real part of its analytical signal, that is, When the amplitude in equation (7) is expressed as a function of time and frequency, the time-frequency distribution of the amplitude is called the Hilbert spectrum, denoted as: H(ω,t); The Hilbert transform is defined as: Among them, pv represents the Cauchy principal value, that is, the integral value of the defect integral; From the above definition, we can see that the Hilbert transform of a function u is the convolution of the function and The convolution is: By calculating the given sin function, we get: The instantaneous increase calculation formula is: The instantaneous frequency is calculated as: The calculation formula for instantaneous phase is: Using convolution, we can get: The real and imaginary parts of the complex domain signal are defined as f(t) and The corresponding analytical signal is:

Images