Method and device for detecting post-grouting effect of cast-in-place pile based on elastic wave theory

Abstract

The application discloses a drilling bored pile post-grouting effect detection method and device based on elastic wave theory, which comprises the following steps: segmenting a reinforcement cage based on the length of a pile foundation, lowering the reinforcement cage into a pile body hole, and pouring concrete to the top of the pile; detecting the strength of the pile body, and obtaining a pile top flat surface; obtaining the center position of the pile top flat surface, the knocking ring range and the number of knocking points based on the pile top flat surface; knocking the top of the pile several times on the knocking ring after the pile foundation reaches the curing age, obtaining first and second elastic waves, and obtaining pre-grouting detection indexes based on the first and second elastic waves; performing post-grouting construction on the pile foundation, and curing to obtain a post-grouting pile foundation; based on the post-grouting pile foundation, collecting data again to obtain post-grouting detection indexes; comparing and analyzing the detection indexes of the two elastic waves before and after grouting, and evaluating the grouting effect. The device is simple, low in cost, and fills the gap of simple and rapid evaluation of the post-grouting effect of the pile foundation.

Description

Technical Field

[0001] This invention relates to the field of pile foundation construction technology, and in particular to a method and device for detecting the grouting effect of bored cast-in-place piles based on elastic wave theory. Background Technology

[0002] Pile foundations are widely used in high-rise buildings, bridges, and other engineering projects due to their high bearing capacity and wide applicability. However, excessive foundation deformation and poor local geological conditions can lead to a series of problems that endanger the safety of the superstructure, such as insufficient bearing capacity and uneven settlement.

[0003] Currently, post-grouting technology has been incorporated into the People's Republic of China industry standards "Technical Specification for Building Pile Foundations" (JGJ94) and "Design Specification for Highway Bridge and Culvert Foundations" (JTGD63). Post-grouting refers to the process where, after the bored pile is formed, grout is pumped in using a grouting pump through a grouting conduit pre-installed in the pile body and connected to grouting devices on the pile side and pile end. This grout penetrates, compacts, and fractures the mud skin around the pile, the sediment at the pile end, and the bearing layer at the pile end, thereby enhancing the strength of the soil around the pile and at the pile end, thus improving the bearing capacity of the pile foundation and reducing settlement.

[0004] Post-grouting processes have strict requirements regarding grouting pressure, grout volume, grout strength, and the number of unobstructed grouting pipes. However, currently, the evaluation of post-grouting effects can only be conducted through static load tests. There are two main types of common static load tests: one is the surcharge test, which requires a large number of counterweights, and the transportation, hoisting, and unloading of these counterweights is cumbersome, costly, time-consuming, and resource-intensive—a difficult task for bridge piles with a bearing capacity of several thousand tons. The second is the load cell test, where hydraulic jacks are pre-embedded at a specific location on the pile. The load cell utilizes the bearing capacity and self-weight of the upper and lower sections of the pile to achieve loading and conduct a bearing capacity test. However, load cells are often expensive, require pre-embedding, and cannot be reused; generally, only a few piles are selected for bearing capacity testing in a project. Post-grouting of bored piles is often carried out in batches, and currently, there is a lack of quick and easy methods for evaluating the improvement in bearing capacity (grouting effect) of post-grouting pile foundations.

[0005] Elastic wave technology is a type of non-destructive testing method based on the propagation of mechanical waves within a solid medium. Currently, some research has been conducted in China on using the impact elastic wave method for strength testing and durability assessment of concrete structures, but there are no reports on its application in testing post-grouting of pile foundations. This invention patent, based on elastic wave technology, develops a complete set of technologies and methods for testing the effect of post-grouting of pile foundations. Compared with static load testing, it not only has a simpler and lower-cost device, but also a simpler and easier-to-master testing method.

[0006] The invention patent "A Building Pile Foundation Inspection Technology" (CN108547332B) uses the side-hole transmission wave method to inspect the quality of pile foundations, improving the accuracy and reliability of pile defect detection. However, for post-grouting pile foundations, due to the uncertainty of the distribution of cement slurry on the pile side, it is difficult to effectively drill holes on the pile side without damaging the grouting effect. Moreover, drilling holes on the pile side for pile foundations over 100 meters long requires a large investment of time, manpower, machinery, and materials.

[0007] Therefore, it is very important to develop a low-cost, fast and simple evaluation technology for assessing the grouting effect after drilling and grouting. Summary of the Invention

[0008] The purpose of this invention is to solve the problems of the prior art by proposing a method and device for detecting the grouting effect of bored cast-in-place piles based on elastic wave theory.

[0009] This invention provides a method for detecting the grouting effect of bored piles based on elastic wave theory, comprising:

[0010] S1. According to the design length of the pile foundation, the steel cage is made in sections, wherein the pile foundation includes the pile bottom, the pile body and the pile top;

[0011] S2. Lower the steel cage into the pile hole and pour concrete to the top of the pile;

[0012] S3. Detect the concrete strength of the pile body, and based on the concrete strength of the pile body, treat the pile top to obtain a flat surface.

[0013] S4. Based on the flat plane of the pile top, obtain the center position of the pile top plane, the range of the tapping circle and the number of tapping points;

[0014] S5. After the pile foundation reaches the curing age, the top of the pile is tapped several times on the tapping ring. The first elastic wave of the tapping point is obtained based on the bottom of the pile, and the second elastic wave of the tapping point is obtained based on the center position of the top plane of the pile. The corresponding test indicators before grouting are obtained based on the first elastic wave and the second elastic wave.

[0015] S6. Perform post-grouting construction on the pile foundation and carry out curing to obtain the grouted pile foundation;

[0016] S7. Based on the grouting pile foundation, repeat S5-S6 to obtain the detection indicators after grouting corresponding to the two elastic waves.

[0017] S8. Compare and analyze the detection indicators of the two elastic waves before and after grouting to evaluate the grouting effect.

[0018] Preferably, the concrete strength of the pile body reaches 75% of the design strength, and the pile top plane is obtained by breaking the pile head and leveling the pile top surface.

[0019] Preferably, the process of determining the range of the tapping circle and the number of tapping points includes:

[0020] The range of the tapping circle is a circle with the center of the pile top plane as the center point and a radius of max(0.15D, 30cm), where D represents the diameter of the pile top plane in cm;

[0021] The number of striking points is three or more, and they are evenly distributed in the striking circle at the top of the pile.

[0022] Preferably, the grouting effect is evaluated based on the detection index of elastic waves before and after grouting;

[0023] The performance indicators for grouting pre-injection testing include: longitudinal wave velocity, energy attenuation ratio, dominant frequency, and centroid frequency.

[0024] Preferably, the formula for calculating the energy attenuation ratio is as follows:

[0025]

[0026]

[0027] In the formula, E i (i = 1, 2, ..., n) represents the signal energy received by the i-th detector, x i Let represent the time-domain signal received by the i-th detector, and s represent the energy attenuation ratio;

[0028] The longitudinal wave velocity was obtained based on on-site measurements.

[0029] The main frequency is obtained based on spectrum analysis.

[0030] The centroid frequency is obtained by averaging the frequencies based on the amplitude weighting within the first resonant frequency range of the spectrum.

[0031] This invention discloses a device for detecting the grouting effect of bored piles based on elastic wave theory, characterized in that it includes: a first detector, a second detector, a vibrating hammer, a signal acquisition instrument, a data analysis module, and several wires;

[0032] The first detector is installed at the bottom of the reinforcing cage to collect and receive elastic waves transmitted from the pile body to the pile bottom.

[0033] The second detector is installed at the center of the pile top plane to receive the elastic waves generated by the vibratory hammer at the top of the pile foundation;

[0034] The vibratory hammer is used to generate elastic waves at the top of the pile;

[0035] The signal acquisition instrument is connected to the first detector and the second detector via wires, respectively, and is used to receive the signals from the corresponding detectors;

[0036] The data analysis module is connected to the signal acquisition instrument and is used to analyze the longitudinal wave velocity, energy attenuation ratio, dominant frequency, and centroid frequency of the signals from the first detector and the second detector.

[0037] Preferably, both the first detector and the second detector include: a velocity detector and an acceleration detector;

[0038] The process of installing the second detector at the center of the pile top plane includes:

[0039] A handheld drilling rig is used to drill a hole at the center of the top plane of the pile, with a drilling depth greater than 40 mm; the diameter of the hole is less than or equal to the diameter of the tail cone of the second detector.

[0040] Fill the borehole with coupling agent;

[0041] Insert the tail cone of the second detector into the borehole to a depth of 20 mm or more, and ensure it is firmly attached to the borehole.

[0042] Preferably, the vibratory hammer includes a manual impact hammer and an automatic impact hammer.

[0043] The present invention has the following technical effects:

[0044] 1. Compared with the traditional method of using static load tests to detect grouting effect, the detection device used in this invention is simple and does not require the cooperation of other mechanical equipment, thus reducing construction difficulty and construction cost.

[0045] 2. The device of the present invention is simple and quick to install. It does not require a specific time to be arranged during the pile foundation construction process and can be tested during the construction interval.

[0046] 3. The present invention adopts a detection method based on elastic wave theory to obtain the detection index of the effect before grouting and the detection index of the effect after grouting. The method is simple, reliable and has high detection accuracy. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 This is a flowchart of the method for detecting the post-grouting effect of bored cast-in-place piles based on elastic wave theory in an embodiment of the present invention.

[0049] Figure 2 This is a schematic diagram of steps one through three in an embodiment of the present invention;

[0050] Figure 3 This is a schematic diagram of the pile top plane positioning point in an embodiment of the present invention;

[0051] Figure 4 The data are the data before and after grouting in the embodiments of the present invention, wherein (a) is the measured signal and spectrum of the first data acquisition before grouting, (b) is the measured signal and spectrum of the second data acquisition before grouting, (c) is the measured signal and spectrum of the first data acquisition after grouting, and (b) is the measured signal and spectrum of the second data acquisition after grouting.

[0052] Figure 5 This is a device for detecting the post-grouting effect of bored cast-in-place piles based on elastic wave theory, as described in this embodiment of the invention. Detailed Implementation

[0053] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0054] Example 1

[0055] like Figure 1 As shown in the figure, this embodiment discloses a method for detecting the grouting effect of bored cast-in-place piles based on elastic wave theory, including the following steps:

[0056] First step, such as Figure 2 As shown, according to the pile foundation length, the reinforcing cages are fabricated in sections at the reinforcing cage processing plant and numbered sequentially from bottom to top: reinforcing cage 1#, 2#, 3#, ...; 1# is installed at the bottom of reinforcing cage 1#. # Detector, 1 # The detector uses an accelerometer or speedometer, and the conductor length is equal to the pile length plus 500cm, and is tied vertically upward along the steel cage.

[0057] The second step is to lower the steel cage, simultaneously lowering 1... # The geophone's conductors are then encased in concrete. The conductors are fixed at least 2 meters outside the top of the pile; this location will not interfere with subsequent work such as pulling out the steel casing, breaking the pile head, and removing laitance. The conductor joints are protected with plastic sleeves, which can be either the conductor's own sleeve or a combination of PVC pipe and tape.

[0058] The third step is to use a rebound hammer to test the strength of the concrete at the top of the pile. Once the concrete strength of the pile body reaches 75% of the design strength, the pile head will be broken and the pile top surface will be leveled.

[0059] Step four, as Figure 3 As shown, determine the location of the vibratory hammer striking point at the top of the pile. The striking point should be at most (0.15D (D: pile diameter), 30cm) away from the center of the pile. There should be no less than 3 striking points, which should be evenly distributed on the top of the pile and permanently marked with red paint.

[0060] Step 5, install 2 # Detector, 2 # The detector's tail cone is a 75mm long steel or copper tail cone. First, a handheld drill is used to drill a hole to a depth greater than 40mm, with a diameter not exceeding half the diameter at the tail cone. Next, a coupling agent, yellow mud, or grease is filled into the drill hole. Finally, the steel tail cone is tightly inserted into the hole filled with coupling agent to a depth of not less than 20mm, ensuring a tight fit between the steel tail cone and the top of the pile.

[0061] Step 6, remove 1 # The plastic protective sleeve of the detector lead connector will 1 # Detector, 2 # The detector's terminals are connected to the data acquisition instrument, which is then started. The instrument records the detector's time-domain signal and calculates the result according to the formula... The signal energy is obtained, where E is the signal energy. i (i = 1, 2, ..., n) represents the signal energy received by the i-th detector; x i Let be the time-domain signal received by the i-th detector.

[0062] Step 7: Use a vibratory hammer to strike the determined striking point, record the striking force (the impact force should not be less than 5 kN), and simultaneously use a data acquisition device to collect the signal.

[0063] Step 8: Collect three sets of data repeatedly.

[0064] 2 recovered # Detector and acquisition unit, installed 1 # The detector wire connector has a plastic protective sleeve, and 1 # The geophone wires are fixed at the designated position on the top of the pile, and the pre-grouting inspection is completed.

[0065] The ninth step is to carry out post-grouting construction on the pile foundation according to the procedures specified in the grouting design drawings.

[0066] Step 10: Curing for 28 days. During this process, the data from the data acquisition instrument will be further processed to obtain the longitudinal wave velocity (Vp), energy attenuation ratio (s), dominant frequency (fm), and centroid frequency (fc) before grouting.

[0067] Step 11: Repeat steps 6 through 9 for the second data collection.

[0068] The twelfth step is to further process the data from the acquisition instrument to obtain the longitudinal wave velocity (Vp), energy attenuation ratio (s), dominant frequency (fm), and centroid frequency (fc) after grouting. The longitudinal wave velocity (Vp), energy attenuation ratio (s), dominant frequency (fm), and centroid frequency (fc) before and after grouting are compared and analyzed. The grouting effect of the pile foundation is mainly evaluated based on the energy attenuation ratio, and other parameters are used as auxiliary parameters.

[0069] Example 2

[0070] This embodiment discloses a method for detecting the grouting effect after drilling and grouting of bored piles based on elastic wave theory, such as... Figure 4 As shown in Table 1, a C30 concrete bored pile with a length of 60m and a diameter of 2.2m was tested at different locations on the pile top and bottom before and after post-grouting. The longitudinal wave velocity (Vp), energy attenuation ratio (s), dominant frequency (fm), and centroid frequency (fc) were analyzed.

[0071] Table 1

[0072]

[0073] After grouting, the energy attenuation increased significantly, by an order of magnitude compared to before grouting, and the longitudinal wave velocity also increased, indicating a significant grouting effect.

[0074] Meanwhile, after grouting, the signal main frequency and centroid frequency increase by about 8%, which can also help detect the grouting effect.

[0075] The static load test of the test pile showed that the bearing capacity before and after grouting was 3900 tons and 6000 tons, respectively. It can be seen that the energy ratio of the elastic waves at the pile bottom and pile top can objectively reflect the effect of post-grouting, and the longitudinal wave velocity, dominant frequency and centroid frequency can also reflect the effect of post-grouting.

[0076] Example 3

[0077] like Figure 5 As shown, this embodiment discloses a device for detecting the grouting effect of bored piles based on elastic wave theory. The detection device includes a vibratory hammer, 1 # Detector, 2 # Geometry detector, supporting software, and signal acquisition instrument. The vibratory hammer, consisting of an impact hammer with a signal transmission cable installed in the hammer handle, a force detector, an impact pad, and a set of elastic impact pads, is used to generate elastic waves at the top of the pile. The impact hammer weighs no less than 5 kg; 1 # A detector (at the pile bottom), employing an accelerometer or velocity meter, is for single use and is used to receive elastic waves transmitted through the pile; 2# A detector (pile top) is used to receive the elastic waves generated by the vibratory hammer at the top of the pile; a data acquisition unit and wireless data transmission are used to receive 1 # Detector and 2 # Detector signals; supporting software to analyze the longitudinal wave velocity, energy attenuation ratio, main frequency and center frequency of the received signals from detector #1 and detector #2. The energy attenuation ratio is used to reflect the effect of post-grouting. The larger the energy attenuation ratio, the better the grouting effect. The longitudinal wave velocity and frequency difference can also reflect the effect of post-grouting to a certain extent.

[0078] Elastic waves propagate outwards as energy, and experience attenuation upon reaching the detector. When the pile foundation cross-sectional dimensions are constant, the energy ratio obtained from the same source by different detectors per unit time is fixed. When only a single source exists, its energy can be calculated using the following formula:

[0079]

[0080] Among them: E i (i = 1, 2, ..., n) represents the signal energy received by the i-th detector; x i Let be the time-domain signal received by the i-th detector.

[0081] The energy attenuation ratio s is the elastic wave at the bottom of the pile. # The signal energy received by the detector accounts for 2% of the pile top. # The proportion of signal energy received by the detector.

[0082]

[0083] The main frequency fm is a characteristic parameter of the time-domain signal of the i-th detector obtained through spectrum analysis (Fourier analysis).

[0084] The centroid frequency fc is the amplitude-weighted average frequency within the first resonant frequency range of the spectrum, reflecting the center frequency position of the spectral energy distribution. The smaller the centroid frequency, the more the overall spectrum distribution shifts to the left, and the more concentrated the energy is in the low-frequency band.

[0085] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting the grouting effect of bored cast-in-place piles based on elastic wave theory, characterized in that, include: S1. According to the design length of the pile foundation, the steel cage is made in sections, wherein the pile foundation includes the pile bottom, the pile body and the pile top; S2. Lower the steel cage into the pile hole and pour concrete to the top of the pile; S3. Detect the concrete strength of the pile body, and based on the concrete strength of the pile body, treat the pile top to obtain a flat surface. S4. Based on the flat plane of the pile top, obtain the center position of the pile top plane, the range of the tapping circle and the number of tapping points; S5. After the pile foundation reaches the curing age, the top of the pile is tapped several times on the tapping ring. The first elastic wave of the tapping point is obtained based on the bottom of the pile, and the second elastic wave of the tapping point is obtained based on the center position of the top plane of the pile. The corresponding test indicators before grouting are obtained based on the first elastic wave and the second elastic wave. S6. Perform post-grouting construction on the pile foundation and carry out curing to obtain the grouted pile foundation; S7. Based on the grouting pile foundation, repeat S5-S6 to obtain the detection indicators after grouting corresponding to the two elastic waves. S8. Compare and analyze the detection indicators of the two elastic waves before and after grouting to evaluate the grouting effect; The process of determining the range of the tapping circle and the number of tapping points includes: the range of the tapping circle is a circle with the center of the pile top plane as the center point and a radius of max(0.15D, 30cm), where D represents the diameter of the pile top plane in cm; the number of tapping points is three or more, and they are evenly distributed in the tapping circle at the pile top. The grouting effect is evaluated based on the detection indicators of elastic waves before and after grouting; wherein, the detection indicators of the effect before grouting include: longitudinal wave velocity, energy attenuation ratio, dominant frequency, and centroid frequency; The formula for calculating the energy attenuation ratio is as follows: （1） （2） In the formula, (i = 1, 2, ..., n) represents the signal energy received by the i-th detector. Let represent the time-domain signal received by the i-th detector, and s represent the energy attenuation ratio; The longitudinal wave velocity was obtained based on on-site measurements. The main frequency is obtained based on spectrum analysis; The centroid frequency is obtained by averaging the frequencies based on the amplitude weighting within the first resonant frequency range of the spectrum.

2. The method for detecting the grouting effect of bored cast-in-place piles based on elastic wave theory according to claim 1, characterized in that, The concrete strength of the pile body reaches 75% of the design strength, and the pile top plane is obtained by breaking the pile head and leveling the pile top surface.

3. A device for detecting the grouting effect of bored piles based on elastic wave theory, characterized in that, The apparatus for implementing the method as described in any one of claims 1-2 includes: a first detector, a second detector (2), a vibratory hammer, a signal acquisition instrument, a data analysis module, and several wires; The first detector is installed at the bottom of the reinforcing cage to collect and receive elastic waves transmitted from the pile body to the pile bottom. The second detector is installed at the center of the pile top plane to receive the elastic waves generated by the vibratory hammer at the top of the pile foundation; The vibratory hammer is used to generate elastic waves at the top of the pile; The signal acquisition instrument is connected to the first detector and the second detector via wires, respectively, and is used to receive the signals from the corresponding detectors; The data analysis module is connected to the signal acquisition instrument and is used to analyze the longitudinal wave velocity, energy attenuation ratio, main frequency, and centroid frequency of the signals from the first detector and the second detector. Both the first detector and the second detector include: a velocity detector and an acceleration detector; The process of installing the second detector at the center of the pile top plane includes: A handheld drilling rig was used to drill a hole at the center of the top plane of the pile, with a drilling depth greater than 40 mm. The borehole diameter is less than or equal to the tail cone diameter of the second detector; Fill the borehole with coupling agent; Insert the tail cone of the second detector into the borehole to a depth of 20 mm or more, and ensure it is firmly attached to the borehole. The vibratory hammer includes manual impact hammers and automatic impact hammers.

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