This article briefly introduces several commonly used pile foundation detection technologies. For specific projects, hole-forming quality detection, static load test detection, low-strain dynamic detection and high-strain dynamic detection are used to inspect the project. The foundation piles were inspected and the quality of the pile foundations was evaluated to ensure the quality of the construction project.
0 Introduction
Pile foundation is a hidden project that supports structures on the ground. It is the foundation of buildings, and its quality directly affects the safety of these buildings. During the construction process of pile foundation, pile foundation inspection is an indispensable link. In recent years, pile foundations have been widely used in high-rise buildings and railway construction. As construction units increase their requirements for project quality, foundation pile detection technology will play an increasingly important role.
1 Pile foundation detection technology
1.1 Hole quality detection In the construction of piles, the quality of the holes directly affects the quality of the piles after concrete pouring: the quality of the pile holes If the hole diameter is too small, the bearing capacity of the entire pile will be reduced; the expansion of the upper part of the pile hole will cause the resistance of the upper side of the pile to increase, while the resistance of the lower side cannot be fully exerted; the deflection of the pile hole will weaken the effective performance of the bearing capacity of the foundation pile. ; The sediment at the bottom of the pile is too thick, which reduces the effective pile length. Therefore, hole-forming quality inspection is particularly important for controlling pile-forming quality. The content of hole quality inspection mainly includes pile hole location, hole depth, hole diameter, verticality, sediment thickness, etc.
1.2 Detection of pile bearing capacity
1.2.1 Static load test method The static load test method is used to detect the bearing capacity of foundation piles. The static load test method includes vertical and horizontal piles. For bearing capacity testing, vertical static load tests are often used in engineering. The significant advantage of the static load test method is that its stress conditions are relatively close to the actual stress conditions of pile foundations. The static load test is mainly suitable for testing the bearing capacity of engineering piles, and destructive testing cannot be done for engineering pile testing. The detection accuracy is high and the relative error is within 10%.
1.2.2 High Strain Change Testing Method The pile foundation high stress change test is to use a heavy hammer to make a transient impact on the top of the pile to cause plastic deformation of the soil around the pile. The actual force and speed are measured at the pile head. Time history curve, through stress wave theoretical analysis, obtains relevant parameters of the pile-soil system, reveals the working performance of the pile-soil system when it is close to the ultimate stage, analyzes the quality of the pile body, and determines the ultimate bearing capacity of the pile.
1.3 Integrity detection of piles
1.3.1 Low strain variation measurement method The low strain variation measurement method of foundation piles is to apply low excitation energy to the top of the pile, causing The micro-amplitude vibration of the pile body and surrounding soil is measured and recorded with instruments at the same time. The vibration speed and acceleration of the pile top are measured and recorded, and the recorded results are analyzed using wave theory or mechanical impedance theory to test the construction quality of the pile foundation and judge the pile body. Integrity, estimated pile bearing capacity and other purposes.
1.3.2 Acoustic wave transmission method The acoustic wave transmission method uses the acoustic parameters of ultrasonic waves propagating in concrete, such as the changes and waveforms of sound speed C, frequency F, amplitude A, to analyze the continuity and faults of the pile concrete. , the size and location of defects such as sand inclusions and honeycombs.
2 Application of pile foundation detection technology in engineering
A certain office building is a high-rise office building with fourteen floors above ground and one underground floor. It adopts a frame structure and has a total construction area of ??38818.6m2. , its foundation adopts reinforced concrete precast piles. After exploration, the site foundation was divided into four layers from top to bottom based on differences in its engineering characteristics, which are described as follows: silt layer, silty clay layer, gravel layer and strongly weathered mudstone layer. The design parameters of foundation piles are as follows: pile diameter is φ500mm; pile length is 10-12m; total number of engineering piles is 170; single pile bearing capacity characteristic value is 2000kN; concrete strength grade: C40; pile end bearing layer is gravel layer . In the practice of this project, based on the site environment and geological conditions, the following detection methods were mainly used: ① Hole quality inspection, the number of tests was 40; ② Test pile load test, the number of test piles was tested 3; ③ High strain dynamic test , the detection quantity is 10 pieces; ④ low strain dynamic detection, the detection quantity is 30 pieces.
2.1 Hole forming quality inspection The instruments and equipment used in the hole forming quality test of foundation piles in this project mainly include JJC-1A type aperture meter, JNC-1 type sediment meter, JJX-3A type well inclination meter, It is composed of depth recorder (charging pulse generator), electric winch, orifice wheel, etc. The hole depth, hole diameter, hole inclination and sediment thickness of the holes were tested respectively. Test results: The designed hole depth ranges from 10.45m to 11.94m, and the measured hole depth ranges from 10.60m to 12.20m. All test piles are greater than the designed hole depth. The measured local minimum aperture ranges from 451mm to 471mm, and the local maximum aperture ranges from 524mm to 633mm. There is no pile hole with a minimum aperture of lt; 550mm. The measured verticality ranges from 0.68 to 0.97, all less than 1. The measured thickness of sediment at the bottom of the hole ranges from 80 to 100mm, all less than 150mm. Based on the statistical analysis of the above data, the four indicators of pile hole quality inspection (hole depth, hole diameter, hole inclination, and sediment thickness) can all meet the specification requirements.
2.2 Static load test inspection In this project, according to the design requirements, single pile vertical static load tests were conducted on the three test piles during the test pile inspection process. The main equipment used in this test is: RS-JYB, a complete set of static load test equipment produced in Wuhan, which mainly includes a host, a repeater, a load control box, a 5000kN jack, a displacement sensor, etc. There are also steel beams, pressure plates, etc. The detection method is as follows: In this vertical static load test, the combined loading method of anchor pile reaction device and counterweight is used, that is, a jack is placed on the top of the test pile, and then the main beam and secondary beam are placed, and the secondary beam is connected to 4 anchor piles. At the same time, prefabricated piles are stacked on the secondary beams as counterweights. The loading method of piles adopts the rapid maintenance load method, that is, loading step by step. After loading, the number is read every 15 minutes, and the loading time of each step is 2 hours. It is expected that the loading will be 8 levels, and the load increment at each level is 500kN. If failure load occurs in the middle, stop loading. The test results show that the average ultimate bearing capacity of the three piles is 4000kN, and the maximum range is 0, which is not greater than 30 of the average value. Therefore, the characteristic value (standard value) of the single pile bearing capacity is 4000=2.0=2000kN, which meets the design requirements. .
2.3 Low-strain dynamic testing According to the "Technical Specifications for Inspection of Construction Pile Foundations", the low-strain method is suitable for testing the integrity of concrete piles, judging the extent and location of pile defects, and requires The body integrity test results are given to give the pile body integrity category of each pile. In this engineering practice, low-strain dynamic tests were conducted on 30 piles in the engineering piles. The detection instrument consists of FDP204PDA dynamic measurement analysis system, acceleration sensor and force rod. The detection method is: place an acceleration sensor on the top of the pile, receive the acceleration signal generated during the hammering process, amplify and A/D conversion through the FDP204PDA pile foundation dynamic measurement system, and turn it into a digital signal and transmit it to the microcomputer, and the signal is processed by the computer. Finally, the measured waveform is displayed on the screen, with one collection point for each pile cloth, and 5 to 6 hammer signals are collected at each point. The test signals stored on the disk are processed in the time domain. According to the stress wave reflection, the measured velocity signal is equivalently passed through the time domain and assisted by the frequency domain. The reflection signals at different parts are analyzed. Based on this, the integrity of the pile body of each pile is analyzed. sex. Test results: Among them: 28 type I piles met the design requirements; 2 type II piles met the design requirements.
2.4 High-strain dynamic testing In this project, *** low-strain dynamic testing was conducted on 10 piles in the engineering piles. The testing instrument adopts FEI-C3 dynamic measurement analysis system, which is composed of 486/40 microcomputer, 12-bit A/D converter, acceleration sensor, force sensor and weight. The detection method is: two accelerometers and two strain gauge force sensors are installed symmetrically on the pile side surface respectively. The hammer falls freely and hits the top of the pile. The acceleration and force signals generated by the instantaneous impact force pass through the FEI-C3 pile foundation. The dynamic measurement system amplifies and A/D converts it into a digital signal and transmits it to the microcomputer. The signal is processed by the computer software and stored in the disk. At the same time, the measured waveform is displayed. Then, the test signal stored on the disk is played back (force, speed) , using FEIPWAPC software to perform curve fitting analysis, and obtain the vertical ultimate bearing capacity of a single pile.
Test results: The basic values ??of the single pile vertical ultimate bearing capacity of the 10 tested piles are all between 2178kN and 2342kN. The average single pile vertical ultimate bearing capacity is 2260kN. Therefore, the single pile is comprehensively judged based on the results of this high strain test. The ultimate bearing capacity of the pile is 2260kN.
3 Summary
The foundation piles of an office building project were inspected using technologies such as hole-forming quality inspection, static load test inspection, low-strain dynamic inspection and high-strain dynamic inspection. Understand the integrity of the pile body and the concrete quality of the pile body under test, and initially judge the strength of the soil support at the pile end, and then evaluate the quality of the pile foundation to ensure the quality of the construction project.
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