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11
reference
end
introduction
Installation
discussion
measurement Method
conclusion
In the name of God
Measurement of pore water pressure
professor:
Nader Shariatmadari,P.h.D
presented by:
Masoud Mohseninya and Omid Asadi
M.Sc student in Geo-Eng At IUST
What is Piezometer?
Describes the various types of piezometers and their uses?
Piezometer is an instrument to measure pore water pressure. it is one of the simplest forms of manometers which consists of a glass tube, whose one end is connected to a point where the pressure is to be measured and the other end is open to atmosphere. It can be used for measuring moderate pressures of liquids. The pressure at any point in the liquid is indicated by the height of the liquid in the tube above that point.
introduction of piezometer
Figure-1
Figure 1 Piezometer inside a pipe
deep piezometer
installation
Deep Piezometer Installation – considerations:
Deep piezometer installations create a range of challenges, with the deeper the installation the larger the impact. These challenges include the positioning of the response zone, placing of the piezometer, protection of the sensor and cable and backfilling of the borehole all of which will have significant impacts on a successful installation.
No one piezometer installation is the same and careful planning of each project should be carried to ensure a successful installation
There are major challenges to be considered when installing deep piezometers and the deeper the installation the more difficult these challenges become and therefore, careful consideration on how to overcome these challenges should be undertaken. The main considerations are:
• Borehole construction
• The piezometer response zone
• The sensor
traditional Method:
- Sand pocket to create response zone around piezometer tip, with a conventional bentonite seal to separate sand and grout.
Traditional Method
figure 1
Typical grout mixing equipment
Placing bentonite pellets
Measurement
Method
measurement Method`
Observation Wells:
The observation well consists of a perforated cross-section connected to a pipe and placed in a sand borehole.
The observation well should not be confused with the monitoring well.
Observation
Wells
Figure 1
Figure 2
Open Standpipe Piezometers:
An open standing piezometer requires sealing in a porous filter. These piezometers can be used in embankments and boreholes or can be inserted directly into the site.
The function of this device is to first fix the surface of the water inside it and eventually be determined by a probe. It is therefore important not to let rainwater into the system and also to ensure that it is open.
These piezometers are also called Casagrande piezometers.
open stand pipe
piezometer
(casagrande)
Application
APPLICATIONS:
• Control of ground water level
• Hydrological and water supply investigations
• Construction and stability control of rail and road embankments, earth dams and foundations
• Investigation of stability in natural and cut slopes
• Permeability tests for drainage and de-watering activities.
figure 1
figure 3
Figure 4
figure 2
Figure 5
figure 6
figure 1
figure 2
figure 3
figure 4
figure 5
figure 6
figure 7
figure 8
figure 7
Twin-Tube Hydraulic Piezometers:
- The two-tube hydraulic piezometers consist of two plastic tubes and a Bordon barometer at the end of each tube. The data readings are performed by a scanner.
- Make sure the pipes are empty of air.
Twin-Tube
hydrolic
piezometer
APPLICATIONS:
- Uplift and pore pressure gradients in foundations, embankments and abutments.
- Monitoring and control of dewatering and drainage.
- Hydrological investigation and water supply operations.
- Construction control, stability investigation and monitoring of earth dams, embankments, foundations, shallow underground works and surface excavations.
Application
figure 1
figure 2
Pneumatic Piezometers :
A pneumatic piezometer consists of a sensitive check valve and a diaphragm separating the pore water from the measuring system. In a two-tube version below, pressurized gas applied to the inlet tube causes the check valve to open and vent from the outlet tube when the applied ·gas pressure equals the pore water pressure. The pressure is read at the pressure Gage on the inlet tube.
The diaphragm displacement accuracy is about 0.01 mm.
pneumatic
piezometer
figure 1
figure 2
figure 3
APPLICATIONS:
- Assessing performance and investigating stability of earth fill dam embankments
- Slope stability investigations
- Monitoring water levels in wells and standpipes
- Monitoring pressures behind retaining walls
- Monitoring pressures during fill or excavation and F.S
Application
Vibration-wire strain
Gage
piezometer
Vibrating-Wire Strain Gage Piezometers :
The vibrating-wire strain gage piezometer has a metallic disc diaphragm separating the pore water from the measuring system. As shown below, a prestressed wire is attached to the midpoint of the diaphragm such that diaphragm deflections cause changes in wire tension. Consequently, a change in pore pressure deflects the diaphragm and changes the tension in the wire. The tension is measured by plucking the wire, using an electric coil, and measuring the frequency of its vibration. The wire vibrates in the magnetic field of a permanenl magnet causing an alternating voltage to be induced inside the plucking coil. The frequency of the output voltage is identical to the frequency of the wire vibration and is transmitted to a frequency-counting device.
*with this pizeometer We can be aware of the existence of possible cracks in the dams.
figure 1
figure 2
figure 3
figure 4
APPLICATIONS:
- Dams and fill embankments
- Measurement of ground water
- Landslides monitoring
- Natural or cut slope sites
- Monitoring of up-lift pressure
- Measurement for low pore pressure
Application
Application
Bonded
resistance
strain Gage
piezometer
Bonded Resistance Strain Gage:
Piezometers The bonded resistance strain Gage piezometer has an electrical pressure transducer with a diaphragm in direct contact with the pore water.
Strains inducted in the diaphragm by the pore pressure are sensed by strain gages, bonded to the diaphragm. Hence, the transducer output signal can be used as a direct measure of pore pressure.
this pizeometer use for high frequency readings and short term applications,
experimental and important project in small models.
figure 1
figure 2
APPLICATIONS:
- Measurement of pore pressure in saturated soils
- Monitoring over-pressure induced by overload (road and rail embankment)
- Monitoring groundwater levels in open pipe piezometers and wells
- very accurate pressure transducers suitable for high frequency readings and short term applications
- dynamic pore pressure monitoring
- Pore pressure in deep excavations
Result
ADVANTAGES AND LIMITATIONS
Observation Well
Observation Well
ADVANTAGES:
- Simple
- inexpensive
LIMITATIONS:
- Not applicable if artesian water tables present.
Open Standpipe Piezometer
Open Standpipe Piezometer
ADVANTAGES:
- Simple
- inexpensive
- reliable
- Long performance record
- Self-deairing if inside diameter of standpipe is greater than 0.4 in.
LIMITATIONS:
- Porous filter can plug due to repeated water inflow and outflow.
- Freezing problems.
- Subject to damage by construction equipment and consolidation of soil around standpipe.
Twin-Tube Hydraulic Piezometer
Twin-Tube Hydraulic Piezometer
ADVANTAGES:
- Simple
- reliable
- Long experience record
- less prone to damage than open standpipe piezometer
LIMITATIONS:
- Freezing problems
- Tubing must not be significantly above piezometric elevation
- Periodic deairing required
Pneumatic Piezometer
Pneumatic Piezometer
ADVANTAGES:
- Stable
- Minimum interference to construction
- Level of tubes and readout independent of level of tip
- No freezing problems
LIMITATIONS:
- Dry gas should be used
Vibrating-Wire Strain Gage Piezometer
Vibrating-Wire Strain Gage Piezometer
ADVANTAGES:
- Easy to read
- Minimum interference to construction
- Level of wires and readout independent of level of tip
- Suitable for automatic recording
- Frequency signal permits data transmission over long distances(no necessary to be in situ)
- No freezing problems
LIMITATIONS:
- Not suitable for dynamic readings
- More expensive than pneumatic piezometers.
Bonded Resistance Strain Gage Piezometer
Bonded Resistance Strain Gage Piezometer
ADVANTAGES:
- Easy to read
- Suitable for dynamic measurements and automatic recording
- Minimum interference to construction
- Level of wires and readout independent of level of tip
- No freezing problems
LIMITATIONS:
- Some versions sensitive to temperature
- Long-term stability not yet verified
- False data can result from transmission over long distances
- More expensive than pneumatic piezometers
case study
VW pizeometer in dam
Fg-1
Fg-2
Fg-3
Reference:
books:
- Piezometers for pore-pressure measurements in fine-textured soils by A.I. Johnson
- safety of existing dams -evalaution and improvement by robert B.Jansen(ch10)
- Geothechnical instrumentation for monitoring field performance by john dunnicliff
paper:
- Geotechnical investigation and field performance of a zoned earth dam in Italy(2016)
website:
- www.sisgeo.com
- www.geokon.com
- www.rstinstruments.com
- www.geosense.co.uk
committee:
- IRCOLD(iranian national comitte on large dam)