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Where, l is the effective length of tensioned vibrating wire, h is the thickness of the pressure diaphragm, r is the material density of the tensioned vibrating wire, m is the Poisson ratio of the pressure diaphragm, R is the effective radius of the pressure diaphragm, f0 is the initial natural frequency of the tensioned vibrating wire and fi is the natural frequency of the tensioned vibrating wire effected with the effect of the pore water pressure s acted on the pressure diaphragm.
DOI: 10.2172/12818275
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[2]
Where, K is the sensor coefficient and , its value is a constant which is related to the size and the material properties of the pressure diaphragm and the tensioned vibrating wire. The value of K and f0 are given by actual calibration before the sensor is sold. Signal Sampling System The sensor of each monitoring point is connected with a port of the signal acquiring device through a wire. The signal acquiring device is connected with a computer through a serial port in order to realize controlling the signal acquiring device and data transmission, see Fig.2. Serial communication protocols were programmed in JAVA language to realize the connection between the signal acquiring device and the computer serial port [[] Simin Zhang, Weina Liang: Java programming practice tutorial (Tsinghua University Press, Beijing 2006), in Chinese. ]. The code for creation of serial port connection and a cycle of data acquiring is given below. Fig. 2 Schematic diagram of signal sampling system public void runx() {
SB = new SerialBean(i); //To create interaction object with the serial port, i is the COM number of the computer. Msg = ""; //To initialize the serial port object. SB.WritePort(lianji); //To send command for connection to the serial port. SB.clear(); //To clear the cache of the serial port object. } SB.WritePort(caiji); //To send command for acquiring data to the serial port. SB.WritePort(lianji); //To send command for connection to the serial port once more. SB.WritePort(shangchuan); //To send command for uploading data to the serial port. Msg = SB.ReadPort(n); //To read the data whose length is n. String Msg1 = Msg.replace('N', '0'); //To replace N by 0 in the acquired data. Where, lianji, caiji, and shangchuan are defined according to the command strings of the signal acquiring device. Msg1 is the signal data from sensor which could be extracted, calculated, and saved. Data Calculation The relationship between water press and water level is given below.
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[3]
Where, s is the water press, rw is the density of underground water, g is the acceleration of gravity and L is the height difference between water level and sensor. The signal data fi (see Eq.2) is the frequency of sensor, which should be further calculated to gain the change of water level. The calculation parameters are shown in Fig. 3. Fig. 3 Schematic diagram of calculation parameters
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[4]
Where, f1 is the frequency of sensor and L1 is the height difference between water level and sensor at the initial time.
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[5]
Where, ft is the frequency of sensor and Lt is the height difference between water level and sensor at any time t.
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[6]
Where, DLt is the water level change relative to the initial time.
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[7]
Where, H is the height difference between water level and wellhead at the initial time and Ht is the height difference between water level and wellhead at any time t. Time Adjustment The acquiring device acquires signal in turn in accordance with the port number after receiving the acquisition command. The signal data is uploaded to a computer to be saved and recalled according to the uploading command after a cycle of acquiring. The time corresponding to the data received by computer is the uploading time rather than the acquisition time. There is a time difference between uploading time and acquisition time which is generally short. The time difference can be ignored when the system is applied in engineering with water level changing relatively slowly. On the contrary, the time difference can't be ignored for the engineering with water level changing rapidly such as pumping test. So time adjustment should be taken to avoid the decreasing of data accuracy caused by time difference, see Eq.8 and Eq.9.
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[8]
Where, N is the quantity of open port, D is the time length needed for each port to be acquired, n is the number of port and Dtn is the time adjustment of port n.
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[9]
Where, is the time after adjustment of port n and tn is the time before adjustment of port n. Engineering Application Expansion supporting airport transportation center project of Tianjin Binhai International Airport is located in the north side of the terminal, the inbound and outbound square and the empty underground of the east side. It is mainly composed of some projects such as Airport Station and part of the range project of Subway Line 2, Airport Station and part of the range reserved project of Beijing-Tianjin Intercity Railway, underground parking garage, transfer channel, connection channel with the T1 terminal which is being used, connection distributing centre with the T2 terminal which is being constructed and so on. The airport transportation center project has some characteristics such as large excavation area, deep excavation, complex soil conditions, important surrounding buildings and great social impact. The project has a total construction area of 135,600m2 and an excavation depth of 23m. And it is surrounded by the airport terminal. The underground has silt micro-confined aquifer 82, 92, 112, 114, 122 and lack of impermeable layer with continuous distribution. A water seal curtain need to be set conservatively to 63.6m-71.9m if the hydrogeology parameters are not adequate. The deepest water seal curtain in Tianjin can lead to huge project cost, great construction difficulty, difficulty to guarantee seal effect and increase of duration. There would be a risk of affect airport security operation caused by ground subsidence increase if ventured to shorten the water seal curtain. In summary, special pumping test is needed to provide scientific basis for engineering design and provide guarantee for the security of engineering progress. The silt layer water of the micro-confined aquifer 92, 112, 114, 122 would be Long-term pumped down in the process of the foundation pit. The single well and group well pumping test was operated to get the water head distribution of the micro-confined aquifer, hydrogeology parameters, and hydraulic connection with adjacent soil layer, provide basis for engineering design and analyze the impact on surrounding environment in the construction process [[] Zongyuan Lin: Geotechnical testing and monitoring manual (China Architecture & Building Press, Beijing 2005), in Chinese. ]. The transportation center was divided into 7 regions in the process of the engineering and the pumping test was located in region 2, region 3 and region 4. The pumping test in region 2 is shown as an example. There were a total of ten wells in the single well and group well pumping test in region two. In the single well pumping test, the well S12-1 was set as pumping well and the wells G9, G12, S11-1, S11-2, S11-3, S11-4, S12-2, S12-3, S12-4 were set as observation wells. In the group well pumping test, the wells S11-1, S11-2, S11-3, S11-4 was set as pumping well and the wells G9, G12, S12-1, S12-2, S12-3, S12-4 were set as observation wells. The plane positions of the ten wells are shown in Fig. 4. Fig. 4 Plane position schematic diagram of the wells in region 2 During the pumping test, especially the period of time when water level changed violently, in addition to the automatic monitoring system for continuous monitoring of water level, artificial method was used to check on the water level stochastically. Part of the check results are shown in Table 1. Reliability of the results monitored from the automatic monitoring system is proved by the acceptable difference. Table 1 Comparison of data from the two different methods Time
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s]
Well name (S)-single (G)-group
Data from monitoring system
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