Some problems should be paid attention in the process of PIT testing
Basic principles of low strain detection
The reflected wave method, also known as the stress wave method, is to hit the pile top with a shock device such as a hand hammer or a force bar to generate a longitudinal stress wave signal to propagate along the pile body, and the sensor (velocity or acceleration) picks up the pile body defects and different interfaces. The reflected signal is used to determine the quality of the pile through a series of analysis and processing. Since this method is affected by various factors such as the external environment and personnel quality, the collected signals are often dynamic signals containing multiple frequency components, so corresponding measures and ways should be taken for each step of pile foundation detection to obtain the pile body.The real signal of the response. The low-strain reflected wave method pile inspection can be divided into two stages: on-site data collection stage and indoor data analysis and processing stage.
Several factors affecting the detection of low strain reflected wave method
1. Pile top treatment
For low strain measurement, the basis for judging the relative change in pile impedance is the impedance at the pile head. When dealing with pile heads, the conditions of the pile top and the quality of the pile head processing directly affect the quality of the test signal, so be sure to perform pile head processing. After treatment, the material and strength of the pile head should be the same as the pile body, and the cross-sectional size should not be significantly different from the pile body. The top surface of the pile should be flat, dense and substantially perpendicular to the pile axis. Cast-in-situ piles shall be chiseled with grouting or loose and damaged parts on the top of the pile to expose the hard concrete surface; the surface of the pile shall be flat and clean without water accumulation; the exposed main ribs of the pile which hinder normal testing shall be cut off. For the prestressed pipe pile, when the flange is tightly coupled with the concrete of the pile body, no treatment is required, otherwise, the pile head should be sawn flat with an electric saw.
When the pile head is connected to the cap or cushion, it is equivalent to a large section impedance change at the pile head, which will affect the test signal. Therefore, the pile head should be disconnected from the concrete cap during the test; when the side of the pile head is connected to the cushion, it should be disconnected unless it does not affect the test signal.
It should be noted that the pile body cannot be split to leave hidden cracks, the broken part of the pile head should be completely removed, and the pile head surface should be a complete horizontal plane. In particular, the knocking point and the sensor installation point should be ground flat, so as to avoid false signals during the detection process and affect the judgment result. When the signal repeatability of multiple hammering is poor, it is mostly related to the knocking or uneven installation part.
2. Selection and installation of sensors
The speed or acceleration sensor is selected when the low strain reflected wave method is used to test the foundation pile. Among them, the speedometer has poor amplitude-frequency and phase-frequency characteristics in the low frequency band. In the process of signal acquisition, the installation resonance frequency is excited by the shock, and parasitic oscillation is generated. It is easy to collect the waveform curve with oscillation, and the response to shallow defects is not obvious. Compared with the speedometer, the accelerometer has great advantages in terms of frequency response and output characteristics, and it also has the advantage of high sensitivity. Therefore, the waveform curve collected by the high sensitivity accelerometer is tested without oscillation or defects. The response is obvious. Therefore, it is recommended to use high-sensitivity accelerometers when testing foundation piles with low strain reflected wave method. The installation of the sensor has a greater impact on the on-site signal collection. In theory, the lighter the sensor, the closer to the pile surface, the greater the contact stiffness between the pile surface, the better the transmission characteristics, and the closer the test signal is to the particle vibration of the pile surface. This is required for all dynamic tests. For the test of solid piles, the sensor should be installed at a radius of 2/3～3/4 from the pile core; for the test of hollow piles, the hammering point and the sensor installation position should be on the same horizontal plane and form a connection with the center of the pile. The angle of 90°, the sensor installation position should be 1/2 of the pile wall thickness.
3. Selection of shock method
The strength of the shock signal also has a greater impact on the field signal collection. For the test of solid piles, the shock point should be selected at the center of the pile. Due to the construction of ultra-high buildings in recent years, the length of foundation piles has become longer and longer and the diameter of piles has become larger and larger, so more and more tests of long and large piles have been conducted. For the test of long and large piles, a force rod should generally be used to vibrate. Its weight is large, energy is large, pulse width, frequency is low, and attenuation is small. It is suitable for the detection of pile bottom and deep defects. The signal reflection of pile bottom and deep defects is strong. But this can easily lead to misjudgments and missed judgments of shallow defects and minor defects. When an abnormality in the shallow part is found based on the signal, it is recommended to use a small hand hammer to vibrate. Because of its small weight, low energy, narrow pulse, and high frequency, the degree and location of the shallow defect can be determined more accurately.
4. The influence of the soil around the pile
When the low-strain reflected wave method is used to test the foundation pile, the influence of the soil layer around the pile on the collected waveform curve should be fully considered. Inspectors often only notice the signal reflection caused by the change in the wave impedance of the pile, and ignore that when the stress wave propagates in the pile, it is not only affected by the pile material, stiffness and defects, but also by the elastic modulus of the soil around the pile. When the soil around the pile changes from a soft soil layer to a hard soil layer, the collected waveform curve will produce a reflection wave similar to the diameter expansion at the corresponding position. When the soil around the pile changes from a hard soil layer to a soft soil layer, the collected waveform curve will produce a reflected wave similar to the reduced diameter at the corresponding position. If the influence of the soil around the pile on the collected waveform curve is not considered, and the geology of the pile side is not understood, it is easy to misjudge the foundation pile.
Therefore, it is necessary to do the following two points:
1. Analyze the integrity of foundation piles based on geological data and construction records.
The pile type and construction process have a great influence on the integrity of the foundation pile and the type of defects. For example, it is impossible to reduce the diameter of prefabricated piles and manual digging piles; many defects or quality accidents occur at flowing water or stratum changes; stratum changes will also affect the waveform (reflective waves will be generated) and so on. Therefore, viewing geological data and understanding construction records are very helpful to determine the location of defects.
2. Comprehensive analysis of all tested piles of the same project. The geology and construction conditions of the same project are roughly the same. By looking for the commonality between the tested piles, and then analyzing the situation of each pile, the analysis effect can often be effectively improved.
5. Signal processing
Filtering is one of the important means of waveform analysis and processing. It is to process the collected original signals. It is to filter out the useless or secondary components in the test signal, so that the waveform is easier to analyze and judge. In actual, low-pass filtering is often used, and the selection of the upper limit of the low-pass filtering frequency is particularly important. If the selection is too low, it is easy to cover shallow defects, and if the selection is too high, it will not play the role of filtering. For the vibration of the installed resonance field of the speedometer, appropriate digital filtering can complement it, thereby extending the frequency range of the sensor. Therefore, the digital filtering method for this signal is based on frequency domain analysis to filter and install the principle of resonance field oscillation. It is possible to verify the proper filtering by comparing the amplitude spectrum curves before and after filtering, but generally speaking, it is advisable to select the frequency of the first valley on the left side of the resonant peak as the low-pass cutoff frequency. Generally, the frequency of the speedometer should not be low. Digital filtering can also be used for accelerometers. The principle of use is the same as that of speedometers. However, the filtering should be performed after integration and should not be lower than 1500hz. Although digital smoothing is similar to the filtering function, it cannot be completely equivalent. The integrated signal of the accelerometer is ideal, generally based on the principle of just eliminating high-frequency burrs.
Project example
1. The influence of manual digging pile wall protection on the measured waveform
A factory dormitory adopts manual digging piles, the pile diameter is 1,000mm, the design pile length is 10.0m, and the height of each section of the protective wall is about 1.0m. The measured waveform of the reflected wave started to oscillate periodically at about 1.3m, but the reflection at the bottom of the pile can be clearly seen. Since the depth of the judged defect is in the shallow part, we recommend that the pile defect be removed by the construction site, and then re-tested, and the integrity of the parts below the defect should be further investigated. The construction site Party A and the construction party provided active cooperation, but no segregation, honeycomb and other defects were found to the judged defect location, but it was found that the nearby location was just at the border of the retaining wall, and the vertical section of the pile body was trapezoidal. . At this time, the pile was flattened and polished and re-tested, and there was no defect reflection wave in the shallow part, so it can be determined that the periodic reflection wave detected for the first time was caused by the protective wall.
2. The influence of vertical cracks on the measured waveform
In a bridge reconstruction project, the foundation still uses the original foundation bored piles with a pile diameter of 1200mm. In order to understand the integrity of these old piles, the reflected wave method was used to conduct an integrity survey. During the survey, it was found that some of the pile dynamic measurement curves were messy. The rule is initially judged to be a shallow defect, and the specific type of the defect has not been determined. While conducting dynamic testing, our center conducts core-pulling sampling of these old piles to understand their concrete strength. During the core-pulling sampling process, the three pile core samples were found to be vertically cracked. After the pile heads were thoroughly washed, vertical cracks were visible across the entire pile section. Later, it was found that the piles with messy dynamic curves have this phenomenon. According to our speculation, the vertical cracks may be caused by directional blasting when the old bridge was demolished. Therefore, we suggest to cut these piles down 2m and re-test. The re-test results show that the shallow defects have disappeared, the reflection at the bottom of the pile is clear, and the dynamic test effect is very satisfactory.
3. The influence of shallow transverse cracks on the test waveform
A plant project used pre-stressed pipe piles with a diameter of 400mm. During the reflected wave test, it was found that the shallow part of the pile 1.3m above the ground by the roadside was broken. In order to further ascertain the integrity of the pile, with the cooperation of the construction party, the broken part was sawed and re-tested. From the curve analysis, there were still crack defects at 1.2m. However, it is worth noting that after excavation and cleaning of the part, no cracks were found. After all parts above 1.5m from the pile head were cleaned, the problem finally emerged. In fact, the crack was only 0.3 meters away from the pile head.
4. The influence of soil resistance on the measured waveform
A factory uses prestressed pipe piles with a diameter of 300mm and a design pile length of 15m. The top-down stratum conditions are: ① plain fill, ② silty clay, ③ silt soil. The 21 piles were tested dynamically at the site. No matter we used a 1kg nylon-head small hand hammer or a 15kg force rod, we could not get the ideal pile bottom reflection. A garden in Shatou also uses prestressed pipe piles with a pile diameter of 500mm and a designed pile length of 42m. The stratum is from top to bottom: ① plain fill, ②silty clay, ③silty soil, ④silt sand, ⑤silty soil , ⑥ gravel sand, ⑦ gravel sand mixed pebble, ⑧ strongly weathered argillaceous siltstone. In November 2007, the reflected wave method was used for dynamic measurement of five of the piles on the site. From the dynamic measurement curve, the reflection at the bottom of the pile was clear and obvious. Through the comparison of these two construction sites, it can be seen that although the low-strain impact energy is small and the soil resistance around the excited pile is small, the soil resistance around the pile, especially the moving soil resistance, has a very significant influence on the stress wave propagation, causing the stress wave to attenuate rapidly.
5. The influence of human factors
A company’s office building expansion project uses bored piles with a total number of 20 piles, with a design pile length of 34m, pile diameter 800mm, and pile body strength C25. The average wave speed of the reflected waves of the 20 piles is about 3700m/s. Two of the piles showed no reflection at the bottom of the pile at the pile length provided by the construction party. However, there is an in-phase reflected wave at 25.7m (calculated at the average wave speed). According to the "Technical Specification for Building Pile Foundation Inspection" (JGJ106-2003), these two piles belong to Class III piles, and other methods should be taken to further randomize them. Usability, specific methods include core pulling, high strain pile inspection or static load test. If these methods are adopted, the construction period will be delayed, and the construction site is on the side of the road, which has already been excavated, and the soil is easy to collapse during the rainy season. For this reason, we asked the construction party about the situation. After re-examination, the actual length of the two piles was 27m (approximately 26m after the broken pile reached the bottom of the cap). According to the actual pile length, the two piles are analyzed again. The reflected wave at 25.7m is reflected at the bottom of the pile, and the wave speed is close to 3700m/s, which belongs to the category I pile. After the report is submitted, the construction will proceed as scheduled.
