GS-P01
2024-11-30 GeoSitter 0
The vibrating wire piezometer is widely regarded as the preferred choice for measuring pore water pressure and liquid levels due to its exceptional accuracy and high reliability. This advanced sensor has become a staple in various industries where precision and durability are critical. If you are familiar with piezometers, you already understand their importance. Now, let’s dive deeper into the specific applications of vibrating wire piezometers and explore how they are used across different fields.
Vibrating Wire Piezometer
Vibrating Wire Piezometers (VWP) as one type of piezometer are the geotechnical sensors that are used to measure pore water pressure (piezometric level) in the soil, earth/rock fills, foundations, and concrete structures. After all, vibrating wire piezometers are commonly used in a wide range of civil engineering and environmental monitoring applications, including:
· Monitoring in Dam Projects: Vibrating wire piezometers can monitor seepage pressure both inside and around dams, helping to assess the dam's safety and stability. Real-time monitoring of seepage pressure allows for the early detection of potential safety hazards, enabling the implementation of reinforcement measures to ensure the safe operation of the dam.
· Monitoring in Tunnels Projects: They can monitor groundwater pressure during both the construction and operational phases of tunnels, preventing leakage and water ingress accidents. Data shows that the use of vibrating wire sensors in tunnel projects has effectively reduced the incidence of water inrush accidents.
· Monitoring in Embankments Projects: Vibrating wire piezometers can monitor seepage pressure within embankments, ensuring their stability and load-bearing capacity. For high embankments, these sensors can accurately track internal pressure changes, providing a scientific basis for the design and construction of the embankments.
· Monitoring in Slopes Projects: By monitoring changes in pore water pressure within slopes, vibrating wire piezometers help assess slope stability and prevent landslides and other geological hazards. For example, in hillside road construction, vibrating wire sensors have played a crucial role in early warning systems, preventing potential landslide accidents and ensuring the safety of lives and property.
· Monitoring in Underground Pipeline Projects: In the construction of urban infrastructure, the layout, material, and construction methods of underground pipelines are crucial. Using vibrating wire piezometers to monitor the surrounding soil around pipelines can help prevent issues such as pipe fractures or water leakage.
GeoSitter Vibrating Wire Piezometer basically consists of:
A sensitive stainless steel diaphragm. It offers high sensitivity, accuracy, and stability for long-term use.
A magnetic, high tensile strength stretched wire, one end of which is anchored and the other end is fixed to a diaphragm.
A porous stone filter with 50-micron pores is made of sintered stainless steel with 50-micron pores, which facilitates the release of air from the piezometer's cavity.
Thermistor for temperature readings
Stainless steel body with resistance to rusting or corrosion against several kinds of dissolved impurities found in water under field conditions. For saline water application, a special sensor with additional protection is provided.
Vibrating Wire Piezometer
The vibrating wire piezometer measures pressure using a method based on the tension of a wire. It consists of an elastic diaphragm with a wire stretched across it, anchored at both ends within the instrument’s body. When groundwater pressure is applied to the diaphragm, it causes a change in the wire's stress, making it vibrate. The frequency of these vibrations is directly proportional to the applied pressure. This frequency is detected by an electromagnetic coil, and the signal is sent to a reading device to determine the water pressure.
Upon receipt, the piezometer readings should be checked and recorded (refer to Section 4 for details on reading methods). Each instrument is provided with calibration coefficients, including a temperature correction factor (see Table 2-1 for an example of a calibration chart).
The following procedure can be used to verify the calibration coefficients provided in the calibration chart (Table 2-1):
Saturate the porous stone filter and fill the cavity between the porous stone filter and the diaphragm with water.
Lower the piezometer to the bottom of the measurement hole using a cable to determine the actual depth.
Allow the piezometer to achieve thermal equilibrium for 15–20 minutes, then record the reading of the liquid level using a reader
Raise the piezometer by a known height, record the reading, and calculate the coefficient to determine the change in pressure and readings. Compare the result with the values in the calibration chart. Repeat the test if necessary.
Each GS-P01 piezometer is supplied with a calibration chart (Table 2-1), which includes important instrument parameters.
Table 3-1 Calibration table of vibrating wire piezometer GS-P01
Each piezometer requires an accurate zero pressure reading (initial reading), which will be used for later data processing (unless monitoring relative pressure). The value recorded before applying pressure during installation serves as the zero pressure reading.
To ensure an accurate zero pressure reading, perform the following checks:
Achieve thermal equilibrium: Allow the piezometer to stabilize for 15–20 minutes to reach the environmental temperature of the monitoring point.
Saturate the porous stone filter and the piezometer cavity with water: partial saturation can lead to inaccuracies due to surface tension effects.
Ensure water level equilibrium: When monitoring water levels in vertical wells or standpipes, the water level should be allowed to stabilize. For example, with a long cable and small borehole diameter, a GS-P01 piezometer placed 15m below the water surface in a piezometer tube with a 25mm outer diameter (22mm inner diameter) can cause the water level to rise by approximately 1m. Allow sufficient time for stabilization.
Record temperature and barometric pressure when obtaining the zero pressure reading to allow for corrections if needed.
For water level measurements, obtain the zero pressure reading in air.
After saturating the porous stone filter and recording a zero pressure reading, the piezometer can be lowered into the piezometer standpipe to the desired depth using a cable. Depth markers should be made on the cable to ensure the piezometer's tip is precisely positioned. For installations in piezometer wells, the same procedure as for piezometer standpipe can be followed. If necessary, the piezometer can be protected with a steel or PVC pipe with perforations at the bottom. Ensure that the cable is securely fixed at the top of the piezometer tube to prevent the piezometer from slipping into the well, which could cause reading errors.
Fig. 3-2 Standard installation for water level monitoring
Piezometers can be installed either individually or in multiple units within a borehole (see Fig. 2-3). For monitoring micropore pressure in a specific area, special attention must be given to sealing the borehole. Materials that settle rapidly over time, such as loose backfill, should not be used during installation.
The borehole should be drilled 15–30 cm deeper than the designated position for the piezometer and thoroughly cleaned. Fill the bottom of the hole with clean fine sand up to 15 cm below the piezometer tip. Then, insert the piezometer, ideally enclosed in a sandbag to keep it clean. Saturate the sand with water, place the piezometer into position (marking the cable), and surround the piezometer with clean sand, filling up to 15 cm above the tip (see Fig. 2-3).
Once the “Response Zone” is established, seal the borehole using one of two methods:
Backfill alternating layers of bentonite and sand (approximately 25 cm thick), then backfill the rest with regular soil.
Use an impermeable mixture of bentonite and cement grout.
For multiple piezometers installed in a single borehole, bentonite and sand should be backfilled to just below the upper piezometer, alternating layers at intervals corresponding to the distance between piezometers.
Since vibrating wire piezometers are essentially non-flow instruments, the size of the water collection zone does not need to be large. The piezometer can be in contact with most materials, as their particles cannot pass through the filter.
Fig. 3-3 Standard Borehole Installation
For installations in earth dams, clay core walls, or embankments, create a trench in the backfill layer as shown in Fig. 2-4 to form a water collection zone. Install the piezometer with sand filling the trench or wrap the piezometer in a sandbag.
When installing in earth dams or clay core walls, ensure that the piezometer cable is laid in a groove and backfilled with fine-grained material. Compact the material around the cable manually and create water-stopping plugs at specified intervals (recommended not exceeding 1 m) using bentonite to prevent seepage along the cable trench.
Fig. 3-4 Earth Fills Installation
Before installation, air must be removed from the porous stone filter cavity of the piezometer. Otherwise, serious lag, measurement errors, or even unstable readings may occur after installation.
The method is to remove the porous stone filter from the front end of the piezometer, then completely submerge the piezometer in a container filled with clean water. Underwater, the porous stone filter is slowly reattached to the piezometer, and it should remain submerged in water until installation.
Alternatively, sandbags can be used to wrap the piezometer, and this process can be done simultaneously with the one described above. The wrapped piezometer should also remain submerged in water until installation.
Before soldering, strip the outer insulation from the cable end for about 8 cm to expose the core wire. Sand or abrade the remaining cable insulation for about 3 cm. A φ12mm heat shrink tubing (approximately 14 cm long) is placed over the outer part of the cable. Use wire stripping pliers to remove 0.5 to 0.8 cm of insulation from the core wire, and then slip φ2mm heat shrink tubing over the core wire. The corresponding colored core wires are joined and twisted together, then soldered with a soldering iron. During soldering, care should be taken to avoid cold solder joints, and burrs should be removed. All five core wires need to be soldered, ensuring that:
3.7.1 The core wire joints are staggered.
3.7.2 All core wire lengths are consistent to ensure uniform force distribution when the cable is pulled.
After soldering, the exposed length of the core wire should be about 7 cm. Once soldering is completed, the φ2mm heat shrink tubing is pushed to the joint area and heated with a hot air gun to shrink it. Finally, the φ12mm heat shrink tubing is pushed to the cable joint and shrunk in place with a hot air gun. The φ12mm heat shrink tubing should cover approximately 3 cm of the sensor cable’s outer insulation on each end. When using the hot air gun to shrink the tubing, the temperature must be controlled to ensure that the internal hot-melt adhesive of the shrink tubing melts into a transparent, flowing state, completely filling the joint. If the temperature is too high, it may melt the core wire insulation, causing a short circuit, or the shrink tubing may become carbonized and brittle.
Note: After soldering the core wires, a reader should be used to check the readings, and a multimeter should be used to measure the resistance between the core wires. This ensures there is no short circuit or open circuit in the core wires due to the soldering process.
Fig. 3-5 Cable Welding Diagram
The piezometer is equipped with a three-terminal plasma surge arrester inside, which prevents peak voltages from entering through the wires.
If the instrument uses a portable reader for manual readings, the lightning protection method is to ensure that the cable is properly grounded at all times.
The GS-P01 piezometer can be read using the GS-801 readout device. Please select the "B" setting during the reading process.
For example, consider a piezometer with the following parameters:
Current reading R1: 6200
Initial reading (zero pressure reading) R0: 8153
Current temperature T1: 15℃
Initial temperature T0: 25℃
Temperature coefficient K: -0.033 KPa/℃
Calibration coefficient G: -0.08937 KPa/Digit
Coefficients:
A=0.0000004443194773A
B=−0.094878415363
C=744.2276C
(Refer to the calibration table for coefficients.)
P=G×(R1−R0)+K×(T1−T0)
=(-0.08937) ×（6200-8153）＋(-0.033)×(15-25)
=174.86961 KPa
P＝AR12+BR1+C＋K(T1-T0)
＝(0.0000004443194773)×62002＋(-0.094878415363)×6200＋744.2276
＋(-0.033) ×(15-25)
＝173.3910654568 Kpa
Note: Due to changes in barometric pressure or environmental conditions, the C value may vary. When this happens, C needs to be recalculated using the zero pressure reading obtained on-site, by setting P=0:
C=-AR02-BR0
Once the new C value is determined, it can be used in the polynomial formula for subsequent calculations.
5. How to Troubleshoot and Maintenance
After installation, regular monitoring and maintenance of the piezometer and its connections are necessary to ensure continued accurate functioning. Schedule regular checks to verify system integrity, recalibrate the instruments as needed, and check for signs of wear or damage.
Common issues might include signal loss due to cable damage or errors in frequency reading due to electronic malfunctions. So if issues arise during reading, follow these steps for troubleshooting:
Check the coil resistance: Under normal circumstances, the coil resistance should be 180 ± 10 ohms, plus the resistance of the cable.
a) If the resistance is too high or infinite, suspect an open circuit in the cable.
b) If the resistance is too low or close to zero, suspect a short circuit.
c) If the resistance is normal but none of the sensors provide readings, suspect an issue with the reader and consult us.
d) If all resistances are normal but only one sensor fails to provide readings, suspect an issue with that specific sensor and consult us.
If an open or short circuit is detected in the cable, reconnect it following the recommended cable connection procedures.
Note: The instrument should be stored in a dry and well-ventilated room.
For more information about GeoSitter monitoring instruments, get in touch with us today on +8613868064820 or info@geositter.com.
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