How to use electromagnetic flowmeter to measure correctly in tunnel grouting project

1. Overview of the background of electromagnetic flowmeter use
           With the implementation of China's Belt and Road economic strategy, the connection between the inland and the coast, and between China and foreign countries will become closer and closer. Various types of material trade will greatly promote the development of the country's land transportation lines, and railways and highways will have a development climax. China's terrain trend is high in the west and low in the east. Most of the west is mountainous, and the plains in the east are only a small part of China's entire territory. Therefore, many tunnels and dams will need to be dug along the westward transportation route. In the grouting construction of the tunnel, dynamic and accurate measurement of cement grouting volume is crucial. The electromagnetic flowmeter, with its unique advantages, greatly improves the measurement accuracy of the slurry volume in the tunnel consolidation grouting construction, and plays a great role in ensuring the authenticity of the data and ensuring the quality of the project.
　
   　The Daxigou Reservoir Project in this article is a leading reservoir on the Urumqi River in Xinjiang. The reservoir is 68km away from Urumqi City. It is a water conservancy hub project mainly for flood control. The hub is mainly composed of buildings such as clay core wall dam, spillway, flood discharge and diversion tunnel, and water release irrigation tunnel. The project is of Grade I and of Large (1) type. The flood discharge and diversion tunnel is a Grade 1 structure, arranged in the right bank rock mass. The tunnel is responsible for diversion during the construction period and flood discharge, sand flushing and emptying during the normal operation period. The flood discharge and diversion tunnel includes a 103.305m long circular pressure tunnel section and a 316.695m long non-pressure tunnel section. The pressure tunnel has a diameter of D=4.6m. The non-pressure tunnel is a city gate tunnel with a cross-sectional size of 5.0m×6.0m. The tunnel is lined with reinforced concrete with a lining thickness of 0.5m. The
　　surrounding rock of the tunnel section is tuff, which is extremely thick and layered, and the rock mass presents a mosaic and fragmented structure. The comprehensive assessment is mainly Class III. The surrounding rock of the tunnel is first backfilled and grouting is carried out, and then consolidation grouting is carried out. The consolidation grouting holes are arranged in a plum blossom shape, with a spacing of 3.0m×3.0m and a depth of 3.5m into the bedrock.
 


2. Working principle of electromagnetic flowmeter
　　The flow measurement of slurry and water in the grouting construction process is very important, especially the liquid flow detection instrument used, which has its special requirements, such as being easy to clean frequently and bearing the friction of particles in high-speed slurry and harsh environment. Among many liquid flow instrument products, electromagnetic flowmeter has the advantages of measurement not being affected by fluid temperature, pressure, density, viscosity, smooth internal straight-through, direct electrical measurement, high measurement accuracy, fast response speed, etc.; and its detection part has no moving parts and will not leak. The lining can be made of polytetrafluoroethylene plastic and alumina ceramic, which has strong corrosion resistance. In recent years, it has become the preferred instrument for grouting engineering flow measurement. The working principle of
　　electromagnetic flowmeter (hereinafter referred to as EMF) is based on Faraday's law of electromagnetic induction. When the measured medium flows perpendicular to the direction of the magnetic field lines, an induced electromotive force E is generated in the direction perpendicular to both the medium flow and the magnetic field lines. The induced electromotive force E is proportional to the measured medium flow Q, and is related to the magnetic induction intensity B and the measuring inner diameter D, but has nothing to do with the changes in other physical parameters. The transmitter output E of the measurement system is a weak alternating signal, which contains various interference components, and the signal internal resistance varies up to tens of thousands of Ω. Therefore, the converter is required to be an AC mV converter with high input impedance and can suppress various interference components, converting the induced electromotive force into a unified signal of 4-20mA. The converter consists of a high input impedance differential amplifier, a main amplifier, an orthogonal interference suppressor, a phase-sensitive detector, a DC amplifier, a Hall multiplier, etc. The final output signal current is Io=K·Q, where Io is the output signal current and K is the instrument constant.
 
3. Use in grouting engineering
　　3.1 Tunnel consolidation grouting construction process
　　Drilling, hole washing → water pressure test → grouting → grouting → sealing
　　3.2 Construction method
　　3.2.1 Drilling, hole washing
　　Drilling with a hand pneumatic drill, after the completion of the single hole drilling, a large flow of water is introduced into the conduit, and the method of flushing from the bottom of the hole to the outside of the hole is used to flush until the return water becomes clear.
　　3.2.2 Water pressure test
　　The simple water pressure test is to send water into the hole at the pressure required by the design, measure the corresponding flow value, and calculate the water permeability of the rock mass based on this. The calculation result is related to the evaluation of the permeability characteristics of the rock mass and the collation of grouting results. This measurement point is very important and sensitive. Accuracy is the primary indicator. Water has a certain conductivity and meets the measurement requirements of EMF. The caliber of EMF needs to be considered, because the same EMF is used for water pressure test and grouting.
　　3.2.3 Grouting
　　The grouting materials are weighed by scales, and the ZJ-800 high-speed mixer is used to mix cement slurry. The slurry should be sieved before use. The time from preparation to use should be less than 4h.
　　3.2.4 Grouting
　　The consolidation grouting adopts the hole closed circulation grouting method. The BW-250 grouting machine is used to grout the entire hole at one time. The grouting pressure is 0.3~0.5MPa. During the grouting process, a special person is assigned to check the pressure gauge. The pressure value should be read from the median value of the pressure gauge pointer swing.
　　The grouting slurry should be changed step by step from dilute to concentrated. The water-cement ratio of consolidation grouting adopts four ratios: 2:1, 1:1, 0.8:1, and 0.6:1.
　　The grouting pump hydraulically delivers cement slurry with a certain water-cement ratio into the hole. A part of it enters the cracks and diffuses, and the remaining slurry returns to the outside of the hole through the return pipe and flows back to the slurry mixer; the consolidation grouting hole is under the specified pressure. When the injection rate is not more than 0.4L/min, continue to grout for 30 minutes, and the grouting can be completed.
　　Each drilling equipment requires two EMFs to record the flow rate of inflow and return grouting respectively. The grouting volume is equal to the inflow volume minus the return volume. The on-site pipeline and EMF installation layout are shown in the figure below:
　　Figure 1 Grouting project on-site pipeline and EMF installation layout diagram
　　Since the on-site grouting pump has a pumping volume of 100L/min, the EMF range is selected as 100L/min. Since the cement slurry contains cement solid particles, considering the wear on the EMF lining and electrodes, the flow rate is selected to be ≤5m/s. On the other hand, the cement slurry has the characteristics of easy adhesion, precipitation and scaling, so the flow rate in the EMF measuring tube should not be less than 0.15m/s to play a self-cleaning role on the electrodes and lining of the electromagnetic flowmeter. In order to ensure the detection accuracy of the electromagnetic flowmeter, the flow rate is controlled between 0.15 and 5m/s.
　　During grouting construction, the amount of grout suction is generally 0-100L/min, and the corresponding flow range of EMF on the inlet and return grout is 30-100L/min. It is more appropriate to select DN25 as the EMF caliber. At the same time, the time constant of EMF should also be set smaller, generally 1-3s, to improve the sensitivity of measurement.
　　During the grouting process, the grouting automatic recorder is used to complete data acquisition and processing, and the flow, water-cement ratio and total amount of cement injected are displayed on the screen. The time, flow and water-cement ratio are printed out at set time intervals. When the grouting is completed, the total amount of cement injected, the total amount of slurry and the amount of cement consumed per unit are automatically calculated and printed.
　　3.2.5 Sealing of grouting holes
　　After the full hole grouting is completed, the catheter grouting method is first used to replace the remaining slurry in the hole with a thick slurry with a water-cement ratio of 0.5, and then the grouting plug is plugged at the hole mouth. This slurry is continued to be used for pure pressure grouting and sealing. The pressure of the sealing grouting is generally not less than 1MPa.
　
IV. Application Notes
　　4.1 Influence and treatment of the amount of grout in the measuring pipe
　　After each grouting, the residual grout in the electromagnetic flowmeter measuring tube should be removed in time, otherwise the cement grout will easily produce different degrees of cementation in the measuring pipe, and even block the electromagnetic flowmeter measuring pipe and the connected grouting pipe. The adhesion layer in the electromagnetic flowmeter measuring pipe will cause additional relative error. The conductivity of the adhesion cement layer is extremely low. The greater the thickness of the attachment, the greater the error.
　　4.2 Influence and treatment of bubbles in the medium
　　Due to the process or the medium itself, the measured liquid often contains some bubbles. When the bubbles pass through the electrode surface, there is a friction process, which generates a spike pulse interference potential, and its value is much larger than the normal flow signal. Usually, the electromagnetic flow converter cannot effectively handle such interference, resulting in unstable measurement values. Therefore, the installation position of the electromagnetic flowmeter should be considered to prevent the generation of bubbles. It is generally installed at the discharge end of the pump. It is best to install it vertically, and the slurry flows from bottom to top. When installed horizontally, the electrode axis should be parallel to the horizon, not perpendicular to the horizon, because the electrode at the bottom is easily covered by sediments, and the top electrode is easily rubbed by occasional bubbles in the liquid to cover the electrode surface.
　　4.3 Impact and treatment of harsh construction site environment
　　The environment of the grouting construction site is relatively harsh most of the time, such as high temperature, humidity, high dust, etc. Condensed water and dust are easily accumulated in the EMF junction box over time, or penetrate into the EMF shell through the poorly sealed joint surface, which will cause the impedance of the EMF converter input circuit to decrease, attenuate the flow signal to be transmitted to the amplifier; or destroy the insulation of the excitation circuit and the signal circuit, causing serious failure of the EMF. In order to avoid such failures, insulating materials can be poured into the junction box. When repairing and debugging the EMF, water must be avoided, and the junction box must be kept dry and clean. During use, it must be avoided from being immersed in water or slurry.
　　
V. Conclusion of this article
　　The use of electromagnetic flowmeters greatly improves the accuracy of measuring the volume of slurry with a ruler, avoids many human factors, and plays a great role in ensuring the authenticity of the data and ensuring the quality of the project. However, the EMF should be inspected and calibrated before use to ensure that the required accuracy is achieved.
　　At present, my country is in a new upsurge of engineering construction. The amount of cement grouting is very large. Dynamic and accurate measurement of cement grouting is very important. Electromagnetic EMF has become the preferred instrument for flow measurement of grouting engineering with its unique advantages and can be widely promoted and used.