Development And Application Of A Series Of Instruments Relevant To Testing Of Foundation Piles

Various field testing methods have been accepted as standards to evaluate the quality and the bearing capacity of bored piles,however,the prevalent testing equipment and testing instruments for bored piles still need improvements.As for common needs in field testing,currently available sensors of stress or strain,measuring devices of reinforcing cage length and pile dynamic testing instruments have their respective disadvantages:a large quantity of measuring points of stress or strain could result in large bundles of cables,thus make the installation and the data acquisition more difficult and less efficient,or even damage the integrity of the pile;the vertical component of the magnetic anomaly around the end of an reinforcing cage could be too weak to give an accurate result of reinforcing cage length;low integrity test and parallel seismic test need dedicated instruments,respectively.To deal with the aforementioned issues,a kind of serial axial strain sensor,a device for reinforcing cage length inspection based on 3D magnetic field measurement and a pile dynamic testing instrument with Wi-Fi have been proposed and developed,and tests have been conducted in the field.The main work accomplished includes the following aspects:(1)According to the principle of resistive strain gauge,a kind of sensor that supports serial communication is developed for the measurement of stress and strain of bored piles.The sensor has digital outputs,thus has good noise tolerance.All sensors of the same type along a pile can share a common cable and there is no actual limits to pile length.The installation is simple,convenient and efficient.The development includes the design of the sensor structure and the water-proof protection structure,the selection of materials and components,the design and debug of hardware and software.Two batches of sensors were calibrated after being assembled,and the calibration results have shown their functionality and reliability.(2)To test the performance of serial axial strain sensors,a pre-piling instrumentation test and a post-piling instrumentation test were conducted respectively with static load tests in the field.The results of both field tests have shown the functionality and feasibility of the application of the serial axial strain sensors in different test conditions,and the performance is comparable with that of the vibrating wire sensors.(3)The post-piling instrumentation test method with serial axial strain sensors does not involve any procedure in the construction period of piles,and this method has an extra advantage that it can make use of the pre-installed pipes after they have served their intended purposes.Test results suggest that the grouting procedure with cement mortar is critical for the normal performance of sensors.(4)Based on the principle of magnetic method for reinforcing cage length inspection,various factors which can influence the total magnetization of the reinforcing cage as well as the shortcoming of the traditional method of reinforcing cage length inspection are discussed and analyzed.Then a reinforcing cage length inspection method based on 3D magnetic field measurement is proposed to deal with the condition that the signature on the curve of the vertical component of the magnetic anomaly around the end of a reinforcing cage could be too weak for reliable length inspection.And a device for reinforcing cage length inspection based on 3D magnetic field measurement is designed and developed.The work includes the design of the probe structure,the selection of materials and components,the design of the circuit with a 3D magnetometer of the probe and the circuit of the inspection controller,and the design and debug of the software.A field test was conducted and the test results have shown the functionality of the device.(5)A pile dynamic testing instrument with Wi-Fi is developed,which satisfies requirements of both the low strain integrity testing and the parallel seismic testing of piles.Experiments with bored piles were carried out for both testing methods and test results have shown the functionality of the instrument.