Any structure (or system) must have a certain response (or response) when it is subjected to dynamic action (excitation). In addition to the excitation conditions, this reaction depends on the dynamic characteristics of the structure. In daily life, the power measurement method is often used inadvertently. For example, pat the skin of a watermelon with your hands (excitation) and listen to its sound (response) to judge the ripeness of the melon; After the train arrives at the station, the inspector uses a hammer to knock on the axle, axle box and supporting spring of the wheel, and listens to its sound to determine whether there are cracks; The hammering sound changes during piling, and an experienced piling worker can find that the pile has been broken, etc.
Listening to the sound is also a dynamic detection method.
The so-called low strain method refers to the detection method of using the low-energy transient excitation method to excite the pile top, measure the velocity time history curve or velocity admittance curve of the pile top, and judge the integrity of the pile through wave theory or frequency domain analysis.
The low-strain method is a method based on the propagation characteristics of stress waves in the pile. The method assumes that the pile is a one-dimensional homogeneous member with continuous elasticity, and the elastic wave propagates downward along the pile when the pile is vertically excited at the top of the pile, and when there is an obvious wave impedance difference interface in the pile (that is, when the pile has defects, such as fracture, diameter reduction, mud inclusion, segregation or encounter the soil layer at the bottom of the pile) or the cross-sectional area of the pile changes, the reflected wave will be generated, and the reflected information from different parts can be identified through reception, amplification, filtering and data processing. Through the analysis and calculation of the reflected signal, the integrity of the pile concrete is judged, and the degree and location of the pile defect are determined.
1. The important law of wave impedance change of pile body.
When Z1 Z2, it means that the pile section is uniform, no defects, and no defects are reflected.
When z1 > z2, it means that there is a decrease in wave impedance at the corresponding position of the pile (such as defects such as shrinking cross-section or poor concrete quality), and the reflected wave velocity signal is consistent with the incident wave velocity signal.
When z1 < z2, it means that there is an increase in wave impedance at the corresponding position of the pile (such as expansion diameter, etc.), and the reflected wave velocity signal is opposite to the incident wave velocity signal.
Summary of the law: the same indentation and reverse expansion.
2. The influence of wave impedance change at the bottom of the pile on the velocity response curve.
When the wave impedance at the bottom of the pile is weaker than that at the bottom of the pile, there are many reflections at the bottom of the pile. The reflected wave at the bottom of the pile is in the same direction as the incident wave;
When the wave impedance at the bottom of the pile is stronger than that at the bottom of the pile, there are many reflections at the bottom of the pile. The odd reflected wave is opposite to the incident wave, and the even reflected wave is in the same direction as the incident wave.
The greater the change between the wave impedance of the pile bottom and the wave impedance of the pile body, the more obvious the reflected wave at the bottom of the pile.
3. Important rules about multiple reflections.
For the position where the wave impedance of the pile is reduced, when the defect is serious, secondary or even multiple reflections will occur; Its primary, secondary, or even multiple reflected wave velocity signals are in phase consistent with the incident wave velocity signals.
For the position where the wave impedance of the pile is increased, when the wave impedance increases very seriously, secondary or even multiple reflections will occur. The odd reflected wave velocity signal is inverted with the incident wave velocity signal, and the even reflected wave velocity signal is consistent with the incident wave velocity signal.
When the difference between the wave impedance of the upper and lower sections of the pile interface is larger, the larger the reflection coefficient and the more obvious the measured reflected wave, which can be used as the basis for judging the degree of wave impedance change.
The following 16 waveforms are the theoretical curves obtained under the idealized condition and the propagation path of the stress wave.
Complete piles. 
Bottomed complete piles.
End bearing complete pile.
Reducer piles. 
Expansion piles. 
Shallow reducer piles.
Middle reducer pile.
Deep reducer piles.
Middle expansion pile.
Gradually expand the diameter and shrink the pile.
Shallow enlarged head pile.
Defective piles near the pile head.
Shallow shrinkage and deep expansion pile.
Shallow expansion and deep shrinkage pile.
Shallow shrinkage and deep shrinkage pile.
Shallow shrinkage and deep expansion pile.
The complete pile is only reflected from the bottom of the pile, and the reflected wave and the incident wave are in phase.
The cross-sectional area of the pile at the fracture becomes smaller, which is manifested as in-phase reflection.
Shallow cracks in the pile.
Shallow fracture of the pile.
The middle part of the pile is partially fractured (the fracture area is 1 3 of the cross-sectional area of the pile).
The middle part of the pile is partially fractured (the fracture area is 2 3 of the cross-sectional area of the pile).
The middle of the pile is broken.
The pile is deeply fractured.
3) The cross-sectional area of the pile changes: the cross-sectional gradient pile is not easy to judge, the cross-sectional gradient process is similar to the reflected wave with the increase of side resistance, and the reflected wave and the incident wave at the end of the gradient are in phase.
The cross-section of the pile is increased
The cross-section of the pile is increased
The cross-section of the pile gradually increases.
The cross-section of the pile body is reduced
The cross-section of the pile is partially reduced
The cross-section of the pile is gradually reduced.
4) Segregation or mud inclusion pile: the reflected wave and the incident wave at the beginning part are in phase, the reflected wave and the incident wave at the end of the mud inclusion and segregation are inversely phased, and the friction pile with mud inclusion and diameter reduction is not serious, the reflection at the bottom of the pile, and the reflection wave and the incident wave are in the same phase.
Pile segregation or mud inclusion.
5) Belled pile: For friction piles, the reflected wave and the incident wave at the beginning of the belling are inverted, and the reflected wave and the incident wave at the end are in the same phase.
Belled piles. 6) Rock-socketed pile: piles with good rock-socketing effect, the reflected wave at the bottom of the pile and the incident wave are inversely phased.
Well socketed piles.
There is sediment at the bottom of the pile.
Due to the complexity of the types of pile defects and the limited technical level of the measured curve interpreters, the interpretation of the measured data is a difficult task. The following is a summary of the interpretation methods of the measured curves of the reflected wave method through the reflected wave characteristics of various common defects of the pile.
1. To judge the existence or absence of pile defects, it is necessary to distinguish whether there is a defective reflected signal in the measured curve and distinguish the reflected signal at the bottom of the pile, which is helpful for the qualitative and quantitative interpretation of the defect. The reflection at the bottom of the pile is obvious, which generally indicates that the integrity of the pile is good, or the defect is slight and the scale is small. In addition, the average longitudinal wave velocity of the pile can be converted to evaluate whether the pile has defects and its severity.
In addition, the stratigraphic data should also be analyzed to eliminate the "false reflection" phenomenon caused by factors such as excessive changes in the wave impedance of the soil layer around the pile.
Intact pile (with obvious reflection at the bottom of the pile).
Defective piles (weakened reflection at the bottom of the pile).
2. Multiple reflection and multi-layer reflection problems.
When there are multiple reflected waves on the measured curve, it should be judged whether it is a multiple reflection of the same defect surface, or a multi-layer reflection of multiple defects between piles, the former, that is, the defect reflection wave is reflected back and forth between the top surface of the pile and the defect surface, and its main characteristics: the reflected wave increases exponentially in time, and the reflected wave energy decreases regularly. The latter tends to be messy and does not have the regularity described above.
The occurrence of multiple reflection phenomena generally indicates that the defect is in the shallow part, or the reflection coefficient is large (such as broken pile). It is strong evidence of severe segregation or fracture at the top of the pile. The multi-layer reflection not only indicates that there may be multiple defects, but also the nature and relative scale of the upper defects can be inferred from the relative difference in the energy of the reflected waves of the lower defects.
If serious defects are found after testing, attention should be paid to timely retesting, and geotechnical engineering investigation data and construction records should be consulted. Sometimes the test results are inconsistent with the actual situation due to poor pile head treatment, sensor installation is not firm, etc., or misjudgment is caused by the influence of the formation. Therefore, it is very important to collect geotechnical investigation data and construction records of the inspected piles.
1) Ultra-long piles in soft soil areas with a large length-to-diameter ratio.
2) The soil constraint around the pile is very large, and the stress wave attenuation is very fast.
3) The impedance of the pile body is well matched with the impedance of the bearing layer.
For example, if the rock-socketed section of the rock-socketed pile is long, the rock-socketed section is well connected with the bedrock or is basically integrated, and the wave impedance of the rock is not much different from the wave impedance of the pile, and it is difficult to see the obvious reflection of the pile bottom in many cases.
4) The impedance of the cross-sectional area of the pile body changes significantly or gradually changes along the pile length.
5) Influence of precast pile joint gaps
When there is no reflection of the pile bottom due to the above reasons, the determination of pile integrity can only be further detected by combining experience, referring to the comprehensive analysis of the same type of pile in the site and the region, or using other methods.
As we all know, the reflected wave method is to judge the quality of the pile by using the principle that the impedance change of the pile has an impact on the signal curve, but in addition to the factors that the impedance change of the pile will affect the signal curve, the soil around the pile will also inevitably affect the signal curve. The signal curve of the reflected wave method reflects not only the change of the impedance of the pile, but also the result of the generalized impedance.
Effect of soil resistance around pile on waveform curve:
1) It leads to the rapid attenuation of the stress wave, and the effective test depth is reduced during detection.
2) It affects the amplitude of the reflected wave of the defect, so that the error of the defect analysis increases.
3) The soil resistance wave is generated at and near the junction of the soft and hard soil layers, which interferes with the reflected wave of the pile, and the reflected wave of the soil resistance is easily confused with the reflected wave of the defect of the pile, thus causing misjudgment.
When analyzing the curve, the influence of the soil layer around the pile on the collected waveform curve should be fully considered. In the low-strain test, inspectors often pay attention to the defect judgment caused by the superposition of their own test waveforms, but ignore that when the stress wave propagates in the pile, it is not only affected by the pile material, stiffness and defects. The better the soil mechanical properties of the soil layer around the pile, the greater the loss of the stress wave in the soil layer around the pile. At the same time, affected by the magnitude of the soil modulus of the soil layer around the pile, a reflected wave similar to the diameter expansion will be generated in the hard soil layer, and a reflected wave similar to the diameter reduction will be generated in the soft soil layer due to the small transmission loss of the stress wave. If you do not consider the influence of the soil layer around the pile on the collected curve, and do not understand the soil quality on the side of the pile, it will sometimes cause misjudgment.
Measured curves of prefabricated piles in air
Measured curve of prefabricated piles buried in soil.
In the low-strain reflection wave test, a kind of high-frequency interference that is not related to the frequency characteristics of the measurement system often occurs, especially when the pile diameter is larger and the pulse is narrower, and its amplitude decays slowly with time. It has a strong masking effect on defect reflection, including pile bottom reflection.
When the top of the pile is struck, in addition to the downward wave, it also generates a wave that propagates along the top surface of the pile. If the diameter of the pile is large enough and the excitation pulse is narrow enough, the measurement is similar to a semi-infinite body when the wave propagating along the top surface of the pile travels to the periphery. At this time, the stress wave generated at the top of the pile after excitation can be divided into compression wave, shear wave and Rayleigh wave, in which the Rayleigh wave occupies most of the energy and attenuates slowly, followed by the shear wave, which has the smallest energy and the fastest attenuation. The high-frequency interference wave seen in the measurement is the coupling of two high-frequency waves formed by the back-and-forth reflection of shear wave and Rayleigh wave on the surface of the pile top, and the frequency difference between the two is not large, and only one high-frequency peak between the two is reflected in the frequency domain.
The strength of high-frequency interference is related to the location of the sensor. For a cylinder, the transient concentrated force acts at the center of the circle, and although the frequency of the first-order high-frequency interference is the same at points at different distances from the center of the circle, the velocity amplitude is different, and the minimum point of the high-frequency interference amplitude is about 2r 3 from the center of the circle. For pipe piles, the 90° point is not sensitive to odd-order mode shapes, while in conventional tests, as long as the excitation pulse is not very narrow, the second-order and above radial higher-order mode shapes will not be excited. Of course, if the excitation pulse is wide, the first-order mode shape will also be excited. Therefore, this point is the ideal sensor mounting point.
Schematic diagram of sensor installation point and excitation point layout.
Although the high-frequency interference in the pile top signal can be filtered out by analog or digital filtering, the error caused by the flat section assumption caused by the size effect of the large diameter pile due to the size effect of the narrow pulse excitation is inherent in the pile. Therefore, when testing the integrity of large-diameter piles, a heavy hammer with appropriate cushioning should be used to widen the width duration of the force pulse, that is, mechanical filtering. The use of digital filtering alone can easily lead to distortion of the stress wave signal.