Design Of Foundation Pit Leakage Detector Based On Flow-field Fitting Method

With the acceleration of the socio-economic development and the social process ofurbanization, in order to improve the urban living environment and traffic environment, moreand more people want to seek more space over the city and the underground. More and morelarge-scale high-rise buildings are being built, and the subway increased greatly. With theincreasing in large-scale projects, the need for more and more Deep Foundation Pit. Due tothe influence of construction quality or hydrological conditions, leading to the foundation pita variety of leakage problems, a direct impact on the stability and security of the foundationpit and the surrounding buildings. There are not commercialized specifically equipment forthe detection of the foundation pit leakage in the market. The main purpose of this paper is todesign an equipment for the detection of leakage before the excavation of foundation pit.The flow-fitting method was first put forward by the Central South University He jishanAcademician, which is a method for the detection of flow direction. Before the excavation offoundation pit, emission electrodes which transmit bipolar square wave with1Hz should laidin two sides of diaphragm retaining walls。The resistivity of piping’s location is lower thanthat of normal diaphragm retaining wall because of seepage of underground water, thereforethe leakage’s location becomes to main gallery of current. The distribution of current fieldcould be acquired by measuring every survey’s potential value. The current field of piping’slocation is abnormal as the corresponding potential value of which is increasing, then theseepage can be located through current field’s distribution pattern.Based on flow-fitting method，the article shows the design of instrument specially appliedto detection of foundation pit leakage。transmitter、receiver and host computer controllingsoftware are included in the foundation pit leakage detector。The foundation pit leakagedetector designed by the article has horizontal detection mode and vertical detection mode.The horizontal detection mode could ascertain leakage’s horizontal location above the ground,while the vertical detection mode could ascertain the vertical location of which. These two modes can provide effective leakage condition of foundation pit.System communication adopts wireless communication mode based on Zigbee.Transmitter adopts CPLD which is regarded as the drive signal generator, segregated byoptical coupler6N137and amplified by triode, drive the transmitting bridge that made up ofMOSFET directly to transmit bipolar square wave with1Hz from3V to12V freely. Design ofreceiver uses FPGA and MCU framework. Signal is amplified by the fore amplifier composedby AD620, whose amplification factor is ten times.Signal collection board adopts18-bit Serial Digital-to-Analog Converter AD7982whosecontrolling signal is provided by FPGA. The data collected by AD7982makes a fast fouriertransform in FPGA, the result is stored in FPGA provisionally and finally fetched by MCU,effective data is selected by wireless communication based on Zigbee to upload to hostcomputer. The host computer receives the uploading date, calculates the amplitude of1Hz、3Hz、5Hz、7Hz、9Hz harmonics and stores them in instrument number order. In horizontaldetection mode, surfer is used to draw potential contour map of harmonics under eachfrequency. The location of leakage could be ascertain through the distribution of potential。Invertical detection mode, each frequency harmonic potential are drawn curve associated withthe depth, through the waveforms of the potential value of each frequency can be analyzed pitleakage.In order to verify the reliability of the pit leakage detector designed in this paper, thecombination of the hardware structure of the instrument, the instrument performancecomparison test to verify the reliability of the pit leakage detector. In order to verify thereliability of the foundation pit leakage detector designed in this paper, comparison test ofeach receiver are made to verify the consistency and reliability of the foundation pit leakagedetector. Accord to the instrument’s design ideas, a laboratory model experiments are made,and getting a large number of experimental data, and proposing the method of analysis of thedata. The construction site of foundation pit experiments are made, proposing analyticalmethod and validation of experimental data, and improving the accuracy of the analysis of thefoundation pit leakage.