Method for evaluating pile bottom depth based on first arrival wave travel time fitting

Abstract

The invention discloses a method for evaluating the depth of a pile bottom based on first arrival wave travel time fitting, which comprises the steps of preparation work, field survey, data analysis and processing and detection report compiling, and is characterized in that the position of the pile bottom is detected through a side hole transmission wave method for data analysis and processing; according to the principle, transmission waves descending along a pile body can be diffracted at the pile bottom, so that a symmetric curved surface with the depth where the pile bottom is located as the center is formed, and the curved surface is approximately in a hyperbola shape and can be determined through fitting by adopting a hyperbola standard equation. According to the pile bottom depth fitting evaluation method based on the first arrival wave travel time, fitting can be carried out only by reading not less than 6 time-depth points at the initial position of the curved surface, the method does not have strict requirements on the positions of data points like a traditional method, effective data can be easily obtained under the condition of a complex site, and the method is suitable for popularization and application. The pile bottom depth is determined through the center point position obtained through hyperbolic equation fitting, and errors caused by linear fitting deviation are reduced.

Description

Technical Field

[0001] The invention relates to the technical field related to pile bottom depth detection, and in particular to a method for evaluating pile bottom depth based on first arrival wave travel time fitting. Background Art

[0002] In the process of renovating existing projects under the background of urban renewal, it is often necessary to determine the pile length to evaluate the bearing capacity. For the pile foundation where the pile top connects the pedestal and the superstructure, the side-hole transmission wave method is an effective method to detect the pile length. However, the existing method of evaluating the pile bottom depth based on the first arrival wave travel time fitting still has certain defects when used.

[0003] During use, the existing method for evaluating the pile bottom depth requires precise determination of the upper and lower straight lines. A slight deviation will lead to inaccurate determination of the final pile bottom depth. When determining the fitting of the lower straight line, it is required to use a time-depth point that is greater than 5 times the pile hole distance from the estimated pile bottom depth. Therefore, in some cases where site conditions are complex or the distance between the measured hole bottom and the pile bottom is small, it may be difficult to obtain enough suitable data points, affecting the feasibility of the detection. Once the fitted straight line has a large deviation, the deviation between the pile bottom position determined after correction and the actual position will also be large, and the accuracy requirements of the correction value itself are also high, which is difficult to control in actual operation.

[0004] In view of the above problems, it is urgent to make innovative designs based on the original method of evaluating the pile bottom depth based on the first arrival wave travel time fitting. Therefore, we proposed a method of evaluating the pile bottom depth based on the first arrival wave travel time fitting, which can solve the above problems well. Summary of the invention

[0005] The purpose of the present invention is to provide a method for evaluating the pile bottom depth based on the first arrival wave travel time fitting, so as to solve the problem that the existing method for evaluating the pile bottom depth on the market proposed in the above background technology needs to accurately determine the upper and lower straight lines, and a slight deviation will lead to inaccurate determination of the final pile bottom depth. When determining the fitting of the lower straight line, it is required to adopt a time-depth point that is greater than 5 times the pile hole distance from the estimated pile bottom depth. Therefore, in some cases where the site conditions are complex or the pile spacing is small, it may be difficult to obtain enough suitable data points, affecting the feasibility of detection, and once the fitted straight line has a large deviation, the deviation between the pile bottom position determined after correction and the actual position will also be large, and the accuracy requirement of the correction value itself is also high, which is difficult to control in actual operation.

[0006] To achieve the above-mentioned purpose, the present invention provides the following technical solutions: a method for evaluating the depth of pile bottom based on first arrival wave travel time fitting, including preparatory work, on-site survey, data analysis and processing, and test report preparation, characterized in that the pile bottom position is detected by a side-hole transmission wave method for data analysis and processing, and the principle is that the transmission wave traveling down the pile body will diffract at the pile bottom, thereby forming a symmetrical surface centered on the depth of the pile bottom, and the surface is approximately a hyperbola, which can be fitted and determined using the standard equation of the hyperbola.

[0007] Preferably, the preparatory work includes data collection, on-site survey, instrument and equipment preparation and drilling arrangement. The data collection requires the knowledge of pile foundation type, size, construction technology and site geological conditions.

[0008] Preferably, the instrument and equipment preparation requires the preparation of side-hole transmission wave method detection instruments and equipment, such as excitation equipment, receiving sensors and data acquisition instruments, to ensure that the instrument performance is good and has been calibrated and debugged.

[0009] Preferably, the drilling arrangement requires drilling a detection hole parallel to the pile body beside the pile to be tested, the drilling depth exceeds the expected pile bottom depth, and the diameter is determined according to the sensor size and the site to avoid damaging the pile body through drilling.

[0010] Preferably, the on-site survey includes installing sensors, vibration operation, data acquisition and lifting sensors. The sensor installation includes connecting the receiving sensor to the data acquisition instrument, slowly lowering the sensor to the bottom of the detection hole to avoid colliding with the hole wall, and installing trigger sensors on the upper structure such as the pile top.

[0011] Preferably, the vibration operation uses a vibration device to vertically strike the pile top or superstructure to generate stress waves, and the magnitude of the vibration force and the action time are adjusted according to the pile type, size and site.

[0012] Preferably, the data acquisition instrument records the first wave time, waveform and other data of the stress wave arrival, reasonably sets acquisition parameters, and performs multiple acquisitions to eliminate accidental errors.

[0013] Preferably, the data analysis and processing includes time-depth relationship drawing, pile bottom position determination, wave velocity calculation, correction and verification. The time-depth relationship drawing calculates the average wave velocity of the pile body and the average wave velocity of the pile bottom bearing layer according to the slope of the time-depth curve. The measurement results can be corrected by methods such as wave velocity testing of the soil layer between the pile and the hole.

[0014] Preferably, the preparation of the test report includes content organization and report preparation. The preparation of the test report requires the organization of test information, data, charts and analysis results, and the preparation of the test report according to specifications and standards to provide a basis for the renovation of existing projects.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: the method for evaluating the pile bottom depth based on the first arrival wave travel time fitting is based on the characteristics of the hyperbolic surface formed by the pile bottom diffraction, which is more in line with the physical process of stress wave propagation at the pile bottom, and can more accurately reflect the pile bottom position than the traditional linear fitting, and only needs to read no less than 6 time-depth points at the starting position of the surface to perform the fitting, and does not have strict requirements on the data point position like the traditional method, and can also easily obtain effective data under complex site conditions, and the pile bottom depth is determined by the center point position obtained by fitting the hyperbolic equation, thereby reducing the error caused by the linear fitting deviation. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a flow chart of the overall method of the present invention; Figure 2 Prepare a workflow diagram for the present invention; Figure 3 This is a flow chart of the on-site survey of the present invention; Figure 4 The data analysis and processing flow chart of the present invention; Figure 5 Prepare a flow chart for the test report of the present invention; Figure 6 This is a test diagram of the side hole transmission waveform under the saturated foundation of the present invention. DETAILED DESCRIPTION

[0017] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0018] Embodiment: In this embodiment, as Figure 1-Figure 5The technical solution shown includes preparatory work, on-site investigation, data analysis and processing, and preparation of a test report. It is characterized in that the pile bottom position is detected by the side-hole transmission wave method for data analysis and processing. The principle is that the transmission wave descending along the pile body will diffract at the pile bottom, thereby forming a symmetrical surface centered on the depth of the pile bottom. The surface is approximately a hyperbola and can be fitted and determined using the standard equation of the hyperbola. The preparatory work includes data collection, on-site investigation, instrument and equipment preparation, and drilling arrangement. Data collection requires the mastery of the pile foundation type, size, construction technology, and site geological conditions. Instrument and equipment preparation requires the preparation of side-hole transmission wave method detection instruments and equipment, such as excitation equipment, receiving sensors, and data acquisition instruments to ensure that the instrument performance is good and calibrated. The drilling arrangement requires drilling a detection hole parallel to the pile body next to the pile to be tested. The drilling depth exceeds the expected pile bottom depth. The diameter is determined according to the sensor size and the site to avoid drilling damage to the pile body. The on-site investigation includes installing sensors, excitation operation, Data collection and lifting sensors. Sensor installation includes connecting the receiving sensor and the data acquisition instrument, slowly lowering the sensor to the bottom of the detection hole to avoid colliding with the hole wall, and installing trigger sensors on the upper structures such as the pile top. The excitation operation uses the excitation equipment to vertically knock on the pile top to generate stress waves. The excitation force and action time are adjusted according to the pile type, size and site. The data acquisition instrument records the first wave time, waveform and other data of the stress wave arrival, reasonably sets the acquisition parameters, and collects multiple times to eliminate accidental errors. Data analysis and processing include time-depth relationship drawing, pile bottom position determination, wave velocity calculation, and correction and verification. Time-depth relationship drawing calculates the average wave velocity of the pile body and the average wave velocity of the pile bottom bearing layer according to the slope of the time-depth curve. The measurement results can be corrected by methods such as pile-hole soil layer wave velocity testing. The preparation of the inspection report includes content organization and report preparation. The preparation of the inspection report requires the organization of inspection information, data, charts and analysis results, and the preparation of the inspection report according to specifications and standards to provide a basis for the renovation of existing projects.

[0019] Collect relevant information of existing projects, including original design drawings, construction records, and geological survey reports, to understand the type, size, construction technology, and geological conditions of the pile foundation; then survey the test site to determine the pile position, pedestal position, and surrounding environment, check the connection between the pile top and the pedestal and the superstructure, and ensure that the site has the conditions for testing; prepare the instruments and equipment required for the side-hole transmission wave method test, such as excitation equipment (hammer or force rod), receiving sensors, data acquisition instruments, and other instruments and equipment, and ensure that the instruments and equipment are in good performance and have been calibrated and debugged; drill a test hole parallel to the pile body next to the pile to be tested. The drilling depth should exceed the expected pile bottom depth. The drilling diameter should be determined based on the sensor size and the actual site conditions. Generally, it should not be too small to ensure that the sensor can be lowered and lifted smoothly. The drilling position should be as close to the pile body as possible, but the drilling process should avoid damage to the pile body.

[0020] Connect the receiving sensor to the data acquisition instrument through a cable, and then slowly lower the sensor to the bottom of the detection hole. During the lowering process, care should be taken to avoid collision between the sensor and the hole wall, and ensure that the sensor is in a vertical state and can work normally. At the same time, install a trigger sensor on the pile top or on the upper structure such as the pedestal and pile cap connected to the pile top to record the excitation moment. Use the excitation equipment to knock vertically on the pile top (or the designated excitation point) to generate stress waves. The magnitude of the excitation force and the action time should be appropriately adjusted according to the type, size and site conditions of the pile to ensure that the generated stress wave can be effectively propagated in the pile body and the surrounding soil layer and be detected by the detection hole. The sensor in the detection hole receives the stress wave signal clearly. After the sensor receives the stress wave signal, the data acquisition instrument will record the first wave time, waveform and other related data of the stress wave. When collecting data, it is necessary to ensure that the collection parameters are set reasonably, such as sampling frequency, gain, etc., to ensure that the collected data is accurate and reliable. At the same time, multiple collections should be performed on each detection point to eliminate accidental errors. After completing an excitation and data collection, the sensor is lifted to a certain height in the detection hole, and the excitation and data collection are performed again. The sensors are lifted in turn at a certain interval (such as every 0.5 meter or 1 meter) until the sensor traverses each detection point of the set height in the detection hole from bottom to top.

[0021] According to the collected data, the stress wave travel time (i.e., the first wave arrival time minus the excitation time) and the corresponding detection point depth of each detection point are sorted out, and a time-depth relationship diagram is drawn. Among them, the existing side-hole transmission wave method uses the time-depth point linear fitting of the first wave transmitted by the side-hole to determine the upper and lower straight lines, and the pile bottom depth is determined after the intersection of the two lines and correction. However, this method has high requirements on the accuracy of the fitted straight line, and when using this method to determine the fitting of the lower straight line, in order to ensure the fitting accuracy, the time-depth point that is greater than 5 times the pile hole distance from the estimated pile bottom depth must be used for fitting. When the fitting deviation of the fitting straight line is large, the intersection point is determined and corrected in this way and the position is consistent with the actual pile bottom depth. The bottom position deviation is large, and the accuracy of the correction value is required to be high; the side-hole transmission wave method of the present application method for detecting the pile bottom position is based on the fact that the transmission wave descending along the pile body will diffract at the pile bottom, thereby forming a symmetrical surface centered on the depth of the pile bottom. The surface is approximately a hyperbola and can be fitted and determined using the standard equation of the hyperbola. By reading no less than 6 time-depth points at the starting position of the surface and fitting using the hyperbola equation a(t-t0)^2-b(z-L0)^2=1 (a and b are coefficients to be determined, t0 is a constant to be determined, and L0 is the pile bottom depth to be determined), the center point of the fitted hyperbola equation is the depth of the pile bottom.

[0022] In summary, it is found that the traditional method needs to accurately determine the upper and lower straight lines. A slight deviation will lead to inaccurate determination of the final pile bottom depth, and when determining the fitting of the lower straight line, it is required to use a time-depth point that is more than 5 times the pile hole distance from the estimated pile bottom depth. In some cases where the site conditions are complex or the measured hole depth is small, it may be difficult to obtain enough suitable data points, affecting the feasibility of the detection. In addition, once the fitted straight line has a large deviation, the deviation between the pile bottom position determined after correction and the actual position will also be large, and the accuracy requirement of the correction value itself is also high, which is difficult to control in actual operation. The characteristics of the hyperbolic surface formed by the diffraction of the pile bottom are more in line with the physical process of stress wave propagation at the pile bottom. Compared with the traditional linear fitting, it can more accurately reflect the pile bottom position, and only needs to read no less than 6 time-depth points at the starting position of the surface to perform fitting. It does not have strict requirements on the data point position like the traditional method, and it is easier to obtain valid data under complex site conditions. The pile bottom depth is determined by the center point position obtained by fitting the hyperbolic equation as a whole, which reduces the error caused by linear fitting deviation, can more accurately determine the pile bottom depth, and provide a more reliable basis for pile length evaluation in the reconstruction of existing projects.

[0023] According to the slopes of different stages in the time-depth curve, the average wave velocity of the pile body and the average wave velocity of the bearing layer at the bottom of the pile are calculated respectively. The average wave velocity of the pile body can be used to verify the uniformity and integrity of the pile body material, and the wave velocity of the bearing layer at the bottom of the pile can assist in determining the properties of the soil layer at the bottom of the pile. Considering that the soil layer around the pile may be uneven in actual engineering, some methods can be used to correct the measurement results. For example, by conducting a wave velocity test in the soil layer between the pile and the hole, the measured first wave arrival time can be corrected to improve the accuracy of the pile length measurement. At the same time, it can also be combined with the results of other detection methods (such as the low-strain method, the core drilling method, etc.) for comparative verification to ensure the accuracy and reliability of the detection results.

[0024] Organize the various information, data, charts and analysis results collected during the inspection process, including project overview, inspection purpose, inspection method, inspection instruments and equipment, inspection results, analysis and conclusions, and prepare a detailed inspection report in accordance with the requirements of relevant specifications and standards. The report should be concise and accurate in language, with detailed data, clear charts and conclusions, and reasonable analysis and evaluation of the inspection results should be conducted to provide a reliable basis for the renovation of existing projects.

[0025] Based on the characteristic of the hyperbolic surface formed by the diffraction at the pile bottom, it is more in line with the physical process of stress wave propagation at the pile bottom. Compared with the traditional linear fitting, it can more accurately reflect the pile bottom position. And only by reading no less than 6 time-depth points at the starting position of the surface can the fitting be carried out. It does not have strict requirements on the position of data points like the traditional method, and it is also easier to obtain effective data under complex site conditions. The pile bottom depth is determined by the center point position obtained by fitting the hyperbolic equation, reducing the error caused by the deviation of linear fitting, and can more accurately determine the pile bottom depth, providing a more reliable basis for the pile length assessment in the renovation of existing projects.

[0026] The test of the crosshole transmission waveform diagram under the saturated foundation is as Figure 6 shown. The designed pile length is 30m, the depth of the measuring hole is 42m, the distance between the crosshole and the pile is 2m, and the detection interval of the geophone is 0.5m. The position shown by the red ellipse in the figure presents the characteristics of the right half branch hyperbola that is symmetric up and down. Assume that the equation of this hyperbola is a(t - t0)^2 - b(z - L)^2 = 1 (a and b are coefficients to be determined, t0 is a constant to be determined, and L is the pile bottom depth to be determined). Read 7 time-depth points from the start of the wave jump of the surface in the figure, and perform standard hyperbola equation fitting according to these points. The expression of the fitted equation is 504100(t - 0.00818)^2 - (z - 30)^2 = 1. From this, it can be determined that the center point of the hyperbola is located at (30m, 0.00818s), that is, the pile bottom depth is 30m, which is consistent with the designed depth.

[0027] Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for assessing pile bottom depth based on first arrival wave travel time fitting, including preparation work, on-site survey, data analysis and processing, and test report preparation, characterized in that: The pile bottom position is detected by the side-hole transmission wave method for data analysis and processing. The principle is that the transmission wave traveling down the pile body will diffract at the pile bottom, thus forming a symmetrical surface centered on the depth of the pile bottom. The surface is approximately a hyperbola and can be fitted and determined using the standard equation of the hyperbola.

2. A method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 1, characterized in that: The preparation work includes data collection, on-site survey, instrument and equipment preparation and drilling arrangement. The data collection requires understanding of the pile foundation type, size, construction technology and site geological conditions.

3. A method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 2, characterized in that: The instrument and equipment preparation requires the preparation of side-hole transmission wave method detection instruments and equipment, such as excitation equipment, receiving sensors and data acquisition instruments, to ensure that the instrument performance is good and has been calibrated and debugged.

4. A method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 2, characterized in that: The drilling arrangement requires drilling a detection hole parallel to the pile body next to the pile to be tested, the drilling depth exceeds the expected pile bottom depth, and the diameter is determined according to the sensor size and the site to avoid damaging the pile body through drilling.

5. The method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 1, characterized in that: The field survey includes installing sensors, vibration operation, data collection and lifting sensors. The sensor installation includes connecting the receiving sensor and the data acquisition instrument, slowly lowering the sensor to the bottom of the detection hole to avoid colliding with the hole wall, and installing trigger sensors on the upper structure such as the pile top.

6. A method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 5, characterized in that: The excitation operation uses an excitation device to vertically strike the pile top or superstructure to generate stress waves, and the magnitude of the excitation force and the action time are adjusted according to the pile type, size and site.

7. The method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 1, characterized in that: The data acquisition instrument records the first wave time, waveform and other data of the stress wave arrival, reasonably sets acquisition parameters, and performs multiple acquisitions to eliminate accidental errors.

8. The method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 1 is characterized in that: The data analysis and processing includes time-depth relationship drawing, pile bottom position determination, wave velocity calculation, correction and verification. The time-depth relationship drawing calculates the average wave velocity of the pile body and the average wave velocity of the pile bottom bearing layer according to the slope of the time-depth curve. The measurement results can be corrected by methods such as wave velocity testing of the soil layer between the pile and the hole.

9. The method for evaluating pile bottom depth based on first arrival wave travel time fitting according to claim 1, characterized in that: The preparation of the test report includes content organization and report preparation. The preparation of the test report requires the organization of test information, data, charts and analysis results, and the preparation of the test report according to specifications and standards to provide a basis for the renovation of existing projects.

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