Abstract: With the development of China's capital construction, pile foundation projects are increasing day by day, various types of concrete cast-in-place piles are widely used, and many new quality problems have appeared. Therefore, the testing workload of piles is very large. The traditional method is the static load test of the pile. Due to its high cost and long time, the number of testings usually only reaches about 1% of the total number of piles. Therefore, high-strain dynamic load test has been widely promoted and applied in recent years because of its relatively advanced technology and simple operation.
Introduction
In the pile foundation detection technology, a heavy hammer is used to impact the pile top, the velocity and force time history curves of the pile top are measured, and the vertical compressive bearing capacity of a single pile and the integrity of the pile body are determined through the analysis of wave theory. It is the high strain pile dynamic load test method. The main function of the method is to determine whether the vertical compressive bearing capacity of a single pile meets the design requirements. Pile foundation dynamic test has the advantages of low cost, rapidity, lightness, and suitability for general survey, which greatly promotes the research and application.
1. Principle
The high-strain PDA test is to impact the pile top with a heavy hammer to cause elastic-plastic deformation of the soil around the pile, and collect the force and velocity time-history curves of the sections near the pile top. Through the theoretical analysis of stress wave, the bearing capacity of the pile and the integrity of the pile body are calculated.
The specific method of high-strain dynamic load test method is as follows:
(1) Actual assessment of pile-soil system with high-energy impact load. Generally speaking, the peak value of the instantaneous dynamic strain of the pile body under impact should not be less than the static strain value from the static load test to the ultimate bearing capacity.
(2) During the actual testing, the time-history curve of the axial strain of the representative pile body section near the pile top and the acceleration of the pile body movement were collected. Through the necessary layout and calculation, the axial average internal force Fm(t) and the axial average velocity Vm(t) of the section are obtained.
(3) The collected data contains segmental layered information of pile impedance and soil resistance.
(4) According to the actual working mechanism of the pile-soil system, a mathematical model is established, and the collected data is analyzed by using the one-dimensional wave equation, and the results about the pile integrity and the bearing capacity of the pile-soil system can be obtained.
(5) On the basis of long-term and a large number of static and dynamic comparisons, the ultimate bearing capacity of a single pile can be inferred from the above collected data and analysis results.
2. on Site Test Technology
2.1 The influence of pile top handling on the data curve.
The quality of the pile head directly affects the wave propagation effect. For the treatment of the pile head, the broken layer and weak concrete on the top of the pile should be chiseled first. For cast-in-place piles, concrete precast piles with severely damaged pile heads and steel piles with deformed pile heads, the pile heads should be repaired or reinforced before the test. The top surface of the pile head should be level and smooth, and the central axis of the pile head should coincide with the central axis of the pile body. The cross-sectional area of the pile head should be the same as the cross-sectional area of the original pile body. Within the range of 1 times the pile diameter from the top of the pile, it should be surrounded by 3mm-5mm steel plates or set up stirrups within the range of 1.5 times the pile diameter from the top of the pile, and the spacing should not be greater than 100mm. The top of the pile should be provided with 2-3 layers of steel mesh, with a spacing of 60mm-100mm. The strength of the pile head concrete should be 1-2 grades higher than that of the pile body concrete, and should not be lower than C30. The pile head should be 2-3 times the pile diameter higher than the soil around the pile, and the pile should be leveled and compacted within 1.2m around the pile. The strength grade of the pile head concrete should be 1-2 grades higher than that of the pile body concrete, and should not be lower than C30.
2.2 Impact of hammer energy on high-strain testing.
During the high-strain dynamic test, the hammering energy is large, and the processing requirements for the pile head are also higher, which must be taken seriously. For concrete bored piles, it is necessary to carefully cut off the slurry or the non-compact part, and then connect a section of equal-section pile head, whose length is generally not less than 2 times the pile diameter, and the concrete strength grade is increased by 1-2 grades (not less than C30) .
When the pile head is externally connected, the main reinforcement of the pile body should be extended to the top of the pile. The top of the pile should also be provided with two or three layers of reinforced mesh, with a mesh spacing of about 5cm. The pile head is cracked to ensure the success of the test and to protect the sensor from damage. The geometric axis of the pile head should be coincident with the pile body, the surface of the pile head should be horizontal, and the outer wall of the pile head should be smooth and dense, so as to facilitate the installation of sensors and collect good stress-strain signals.
2.3 Influence of sensor installation on high-strain testing.
The signal directly measured by the sensor is the signal of strain and acceleration on the detection surface, and the force and velocity signals can only be obtained after calculation according to the set values of other parameters. If the detection section is improperly selected, such as the sensor is too close to the top of the pile or near the variable section, the measured strain is not representative. The poor quality of the local concrete where the sensor is installed is unfavorable for the fixation of the sensor, serious inelastic deformation may occur under the action of the hammering force, and the impedance of the section is also inaccurately estimated, which will affect the calculation result of the bearing capacity. Especially for the installation of the cast-in-place pile sensor, the following points should be met. ①The sensor should not be installed near the variable section, because the stress concentration of the section will cause the measured force to be inaccurate. The installation point should not be too close to the top of the pile, generally 1.5D-2.0D is better, so as to reduce the influence of the eccentricity of stress concentration. ② The center of the strain sensor and the acceleration sensor should be on the same horizontal line; the horizontal distance between the strain sensor and the speed sensor on the same side should not be greater than 80mm, and the central axis of the sensor should be parallel to the central axis of the pile, so that the measured signal can be measured. Averaging is performed to remove the effect of hammer eccentricity. ③ The material of the mounting surface of the sensor should be uniform, dense, stable, and parallel to the axis of the pile, otherwise it should be smoothed by a grinder. If there are cracks or plastic deformation near the installation point, the velocity at the peak of the waveform is likely to be greater than the force, and the tail of the force wave does not return to zero. ④The drilling holes of the mounting bolts are perpendicular to the surface of the pile side. After installation, the sensor should be close to the surface of the pile body, and it should not be loose or bad contact due to hammer vibration, so as to ensure that the sensor and the pile body are deformed together.
3. Data Analysis
3.1 Analysis of the test curve
The data curve should reflect the integrity of the pile and the dynamic and static characteristics of the pile. Judging whether the data curve is qualified is mainly based on the following aspects:
1) The pros and cons of the data curve: including whether the hammer is eccentric, whether the sensor works normally; whether the start point of the F(t) curve and the ZV(t) curve coincide, etc.
2) Whether the hammering energy is sufficient: the penetration of the pile is not enough, the degree of separation of the curve is small, and the reflection at the bottom of the pile is not obvious. At this time, the drop weight distance should be increased and the test should be repeated.
3) Whether the data curve can be reasonably explained: When the data curve cannot be explained, the test should be re-tested. If the results of multiple tests cannot be interpreted, tests on similar piles should be considered.
3.2 Determination of velocity
The meaning of velocity refers to the propagation speed of the elastic wave at the installation point of the sensor in the pile body. Its size directly affects the impedance of the pile, which also affects the ultimate bearing capacity of the pile. In the current test, there is no reliable method to directly measure the wave velocity of the pile body. Some testers use the wave velocity of the small strain test in their calculations, which is not appropriate. Because the mechanical wave speed is related to its frequency, the frequency of the two is very different, and the wave speed is necessarily different. Furthermore, since the wave velocity measured by the small strain is calculated according to the pile length provided by the construction unit, which itself is not very reliable, it is difficult to obtain a relatively accurate wave velocity. In actual work, if the length of the pile is known, in the testing process, the up and down waves can be used to find the reflection at the bottom of the pile, and the average wave speed of the pile body can be obtained, which is relatively accurate. If the pile bottom reflection is obvious, the wave speed can also be calculated by the pile bottom reflection.
3.3 Jc Value
Whether the Jc value is selected accurately or not directly affects the ultimate bearing capacity of the pile. There is a recommended value range of Jc under different foundation soil conditions internationally, in general, we can apply it. In fact, Jc is a comprehensive parameter related to many factors such as pile type, pile diameter, foundation soil type, and mechanical properties of the pile passing through the soil layer. The pile type of a construction site is rammed pile, and the bearing layer is hard plastic silt. The results are obtained under different Jc values. According to the international recommendation table, the damping coefficient of the pile tip soil should be within the range of 0.15 to 0.40. between. According to the pile foundation testing procedures, the Jc values in the same site should be roughly the same, and the maximum difference should not exceed 0.1. By comparison, if the recommended value is used in this construction site, the results are small, and the value is one level higher than the recommended value (0.1), and the dynamic and static comparison results tend to be consistent. Later engineering pile testing proved that such a value is appropriate.
4. Conclusion
Pile foundation test is a rigorous and meticulous work. Any negligence in test may cause the failure of the test, and any parameter error during data processing may bring super-precision errors to the results, or even get erroneous results. Therefore, it requires testers not only to have superb professional skills, serious work attitude, but also to have rich practical experience, to be able to find and solve problems in a timely and accurate manner.
