Piezometers: Types, Functions, & How it Works?

Piezometers

Types, Functions, & How it Works?

TABLE OF CONTENT

1

INTRODUCTION

8

HOW TO INSTALL PIEZOMETER IN EARTH AND ROCK FILL DAM?

2

WHAT IS A PIEZOMETER?

9

HOW TO INSTALL PIEZOMETER IN CONCRETE DAM?

3

WHAT ARE THE TYPES OF PIEZOMETERS?

10

HOW TO INSTALL PIEZOMETER IN SOFT SOIL?

4

WHAT IS THE PORE WATER PRESSURE /PIEZOMETRIC LEVEL?

11

CAN A VW PIEZOMETER MEASURE NEGATIVE PORE WATER PRESSURE?

5

WHAT IS THE WORKING PRINCIPLE OF A VW PIEZOMETER?

12

APPLICATIONS OF PIEZOMETERS

6

PIEZOMETER TYPES ENCARDIO-RITE’S MODELS

13

WHAT ARE THE LIMITATIONS OF PIEZOMETERS?

7

HOW TO INSTALL PIEZOMETER IN BOREHOLE/FOUNDATION?

1

INTRODUCTION

Piezometers, also known as pore pressure meter, are used to measure pore water pressure (Piezometric level) in the soil, earth/rock fills, foundations and concrete structures.

It provides significant quantitative data on the magnitude and distribution of pore pressure and its variations with time.

It also helps in evaluating the pattern of seepage, zones of potential piping and the effectiveness of seepage control measures undertaken.

2

WHAT IS A PIEZOMETER?

Piezometers or pore pressure meters are the pressure transducers that are installed beneath the ground to measure the sub-surface piezometric level within groundwater level, soil, or rock.

Proper evaluation of pore pressure helps in monitoring the behaviour after construction and indicates potentially dangerous conditions that may adversely affect the stability of the structure, its foundation and appurtenant.

It also provides basic data for design improvement that will promote safer and more economical design and construction.

3

WHAT ARE THE TYPES OF PIEZOMETERS?

Piezometers are available in different shapes and sizes that are used to measure the pore pressure inside boreholes and beneath the earth’s surface.

https://www.encardio.com/blog/a-guide-on-structural-health-monitoring-shm/

3

WHAT ARE THE TYPES OF PIEZOMETERS?

Casagrande or Open Standpipe Piezometers One of the simplest instruments to measure the piezometric level in the ground is an Open Standpipe Piezometer or Casagrande.

It is a plastic pipe, with a small diameter (up to 20mm) and a porous section at the bottom

3

WHAT ARE THE TYPES OF PIEZOMETERS?

This pipe is installed inside a borehole while placing the porous section at the bottom where the piezometric level is to be measured.

The annulus between the porous filter and the borehole has sand in it, sealed from both the sides using bentonite and, the rest of the borehole is filled with a cement mortar.

The groundwater pressure pushes the water into the standpipe until the level of water inside the standpipe (h) is equal to the piezometric level (pore water pressure) in the ground at the elevation of the porous filter.

3

WHAT ARE THE TYPES OF PIEZOMETERS?

Electric Piezometers Electric piezometers comprise of a deflecting diaphragm and a porous filter divided by a small reservoir of water.

Vibrating wire or a strain gauge is used to calculate the deflections of the diaphragm. The calculated results are then converted to an equivalent pressure using a proper calibration The next procedure is exactly similar to the open standpipe piezometers. The electric piezometer is inserted into a borehole while the annulus between the porous filter and the borehole is filled with cement/bentonite grout.

3

WHAT ARE THE TYPES OF PIEZOMETERS?

Hydraulic Piezometers Hydraulic piezometers consist of a porous filter containing a reservoir of water, separated from a pressure gauge via resilient and water-filled tubes.

These tubes circulate water through the system to remove air and ensure that the reservoir remains filled with water.

The piezometer measures overall hydraulic pressure from which pore pressure can be calculated.

3

WHAT ARE THE TYPES OF PIEZOMETERS?

Pneumatic Piezometers Pneumatic piezometer is operated by gas pressure. It can be placed in a borehole, fills, or in a large diameter standpipe. The readings are then calculated with a pneumatic indicator.

Vibrating Wire Piezometers The Vibrating Wire Piezometers measure the pore water pressure in fully or partially saturated soil and rock.

3

WHAT ARE THE TYPES OF PIEZOMETERS?

A vibrating wire piezometer consists of:

• • • • • •

A sensitive stainless steel diaphragm A magnetic, high tensile strength stretched wire, one end of which is anchored and the other end is fixed to a diaphragm.

Ceramic low air filter (brass high air entry filters are also used depending on site requirements) Thermistor for temperature readings Glass to metal seal with four terminals for cable connection 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

4

WHAT IS THE PORE WATER PRESSURE/PIEZOMETRIC LEVEL?

The pore water pressure/Piezometric Level is the pressure experienced by water contained in pores of earth materials or concrete structures.

The use of piezometer has following purposes:

•

• • •

To study the effect of water in pores of soil or rock is to reduce load bearing capacity of soil or rock. Effect is more pronounced with higher pore water pressure leading eventually in some cases to total failure of load bearing capacity of the soil.

To determine level and flow pattern of ground water To determine the flow pattern of water in earth/rock fill, concrete dams, and their foundations.

To delineate the phreatic line.

5

WHAT IS THE WORKING PRINCIPLE OF A VW PIEZOMETER?

The vibrating wire piezometer basically consists of a magnetic, high tensile strength stretched wire, one end of which is anchored and the other end fixed to a diaphragm that deflects in some proportion to applied pressure.

Any change in pressure deflects the diaphragm proportionally and this, in turn, affects the tension in the stretched wire. Thus any change in pore pressure directly affects the tension in the wire.

The wire is plucked by a coil magnet. Proportionate to the tension in the wire, it resonates at a frequency ‘f’, which can be determined as follows: f = {[σg/ρ] ^1/2}/ 2l Hz Where σ = tension of the wire g = gravitational constant ρ = density of wire

5

WHAT IS THE WORKING PRINCIPLE OF A VW PIEZOMETER?

The resonant frequency, with which wire vibrates, induces an alternating current in the coil magnet.

The pore pressure is proportional to the square of the frequency and the Encardio-rite model EDI-51V readout logger (data logger) is able to display this directly in engineering units.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Model EPP-10SP/ Open Standpipe Piezometer or Casagrande The Model EPP-10 SP standpipe is used for measuring groundwater level and its variation with time.

The Encardio-rite model EPP-10SP standpipe piezometer consists of a pipe that is sealed along its entire length and installed in a borehole such that it must be open to water flow at the bottom and open to the atmosphere at the top.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

EPP-10/6 Water Level Sounder The Model EPP-10/6 water level sounder is designed to measure the elevation of groundwater in boreholes, standpipes and wells.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

It is robust, light weight and convenient to use. The water level sounder probe is lowered from the surface with the help of the connecting flat cable for taking observations.

The length of the cable is commensurate with the depth up to which the observation is required to be made.

The unit is battery operated complete with an on-off switch, buzzer, LED for power on and signal, flat cable connected to a probe, winding reel and carrying handle.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Model EPP-30V/ Electron Beam Welded Heavy-Duty VW Piezometer In the Model EPP-30V electron beam welded heavy-duty vibrating wire piezometer, the vibrating wire and coil magnet assembly are enclosed in a stainless steel body which is electron beam welded to the diaphragm.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

This result in a vacuum of around 1/1000 Torr inside the sensor making it immune to the effect of any ingress of water and other corrosive materials that may be present in the water.

As piezometer is of stainless steel construction, it is not affected by normal chemical corrosion at locations in which it is used.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Encardio-Rite Model EPP-40V Vibrating Slim Wire Piezometer The Encardio-rite Model EPP-40V piezometer is well known for its long term stability. This is achieved by:

• • •

Ageing pressure and thermal cycling Unique method of wire clamping By generating a vacuum of around 1/1000 Torr inside the sensor by electron beam welding. This results in effect of oxidation, moisture, environmental conditions and any ingress of water being completely eliminated.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Encardio-rite Model EPP-60V Vibrating Wire Low-Pressure Piezometer The Encardio-rite Model EPP-60V vibrating wire low range piezometer is specially designed to measure low water pressure and water levels in boreholes.

It provides significant quantitative data on groundwater level, groundwater pressure & its variations with time.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Proper evaluation of water level helps in monitoring the behaviour after construction & indicates potentially dangerous conditions that may adversely affect the stability of the structure and its foundation.

It also provides basic data for design improvement that will promote safer and more economical design and construction.

Two variants are available:

•

Gauge pressure sensor with vented cable, used for applications in which maximum hydrostatic head (height of water level above piezometer location) is less than 7 m over the sensor diaphragm.

•

Absolute pressure sensor with 20 m range, which is electron beam welded and comes with non-vented cable.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Model EPU-20V Uplift Pressure Meter Model EPU-20V sensor incorporates the latest vibrating wire technology to provide a remote digital readout of fluid and/or water pressure in standpipes, boreholes and embankments.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

It is similar to the model EPP-30V piezometer except that instead of the special filter, a 25 mm BSP adaptor is provided for the pipe connection.

The water oozing through internal pores or seams in rock formations of dam foundations, mass concrete /foundation soil of structures, reclaimed land soil etc. percolates upward through the pipe to the sensor.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Model EPU-20G Bourdon Gage System The EPU-20G uplift pressure measuring system incorporating Bourdon pressure gage is economical and easy to install.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

However, for taking readings, one has to go into the gallery. Remote measurement of uplift pressure is possible with Model EPU-20V vibrating wire uplift pressure meter.

The system enables reliable and fast measurement of uplift pressure. It also enables data storing in case the output is connected to the data acquisition system or automatic data logger.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Model EPP-50V/ Vibrating Wire Push-In Piezometer The Encardio-rite Model EPP-50V push-in vibrating wire piezometer is specially designed to measure pore water pressure in soft soil, clays and landfills in earth fill dam foundations & structures.

6

PIEZOMETER TYPES - ENCARDIO-RITE’S MODELS

Long term stability is ensured in the model EPP-50V Encardio-rite pore pressure meter by:

• • •

Pressure and thermal cycling The unique method of wire clamping By generating a vacuum of around 1/1000 Torr inside the sensor by electron beam welding. This results in the effect of oxidation, moisture, environmental conditions and any ingress of water being eliminated.

7

WHY HOW IS TOGEOTECHNICAL INSTALL PIEZOMETER MONITORING IN BOREHOLE/FOUNDATION? IMPORTANT? Installation of piezometer in a borehole is done with a fully grouted method.

Installing multiple piezometers in a borehole using the traditional method, which involves placement of sand pockets surrounding the piezometers, bentonite seals and grout columns to separate piezometers at different levels, is slow and complex.

7

WHY HOW IS TOGEOTECHNICAL INSTALL PIEZOMETER MONITORING IN BOREHOLE/FOUNDATION? IMPORTANT? It also poses a risk of unintended communication between piezometers. Fully grouted method is quick and easy and results in excellent zone isolation.

Vibrating wire piezometers are the electrical piezometers of choice to implement this method as these require very little volume of water to register a change in pore water pressure, thus resulting in fast response time.

8

HOW WHY IS TOGEOTECHNICAL INSTALL PIEZOMETER MONITORING IN EARTH IMPORTANT? AND ROCK FILL DAM? In an earth and rock fill dam, a number of piezometers and maybe other sensors are installed at selected elevations at different cross-sections, as shown in the figure below:

9

HOW WHY IS TOGEOTECHNICAL INSTALL PIEZOMETER MONITORING IN CONCRETE IMPORTANT? DAM? As access galleries are available in concrete dams, the cable from the sensors is first routed to the gallery.

These cables may be terminated in junction boxes inside the gallery. The data from the various sensors can then be taken or logged from the junction boxes with the help of a readout unit or data logger.

Alternatively, if required, the signals from the junction boxes may be carried through multicore cables to any observation room outside the dam structure.

10

HOW WHY TO IS GEOTECHNICAL INSTALL PIEZOMETER MONITORING IN SOFT IMPORTANT? SOIL? For installation in soft soils the Encardio rite model EPP-50V push-in piezometer is recommended.

It does not require any elaborate grout sealing below and above the piezometer as required for other piezometer models.

The EPP-50V has a hard conical tip that allows the piezometer to be pushed in to the required depth from the ground surface or the base of a drilled borehole.

11

WHY CAN A ISVW GEOTECHNICAL PIEZOMETER MONITORING MEASURE NEGATIVE IMPORTANT? PORE WATER PRESSURE? A vibrating wire piezometer is the only device that can measure negative pore pressures because it indicates absolute pressures.

Encardio-Rite's vibrating wire piezometers are fitted with high air entry porous filters that remove the air inside the piezometer so that the readings are accurate.

12 • • • • • •

WHY APPLICATIONS IS GEOTECHNICAL OF PIEZOMETERS MONITORING IMPORTANT? Construction control, stability investigation and monitoring of earth dams, embankments, foundations, shallow underground works and surface excavations.

Uplift and pore pressure gradients in foundations, embankments, abutments and fills.

Hydrological investigation, groundwater pressure study and water supply operations.

Pore pressure studies in relation to waste and environmental applications.

Monitoring of pore pressure for soil improvement & stability and for slope stability Reclamation of land in high population density areas in coastal regions for building highways, multi-storied structures, port trusts or recreational facilities. This requires extensive instrumentation for measurement of horizontal displacement, vertical movement and pore pressure.

12 • •

WHY APPLICATIONS IS GEOTECHNICAL OF PIEZOMETERS MONITORING IMPORTANT? In coastal areas, it is necessary to determine and implement a strategy to overcome salinity problems and to increase community understanding and involvement in finding and implementing a solution.

In many coastal areas, and occasionally inland, there is a danger that over pumping water from boreholes will cause saline water to replace the fresh water in the aquifer.

Aquifers can be complex and include several saline horizons at different depths.

Such horizons can be detected by multilevel piezometers and water sampling systems together with conductivity/flow logging in boreholes.

13 •

WHY WHATISARE GEOTECHNICAL THE LIMITATIONS MONITORING OF PIEZOMETERS? IMPORTANT? Use of sensors in saline water is one of the most difficult applications from the point of view of corrosion.

Great care should be taken for corrosion protection in case a piezometer is used for such applications.

This is a tricky application and precaution must be taken to protect both the body and the diaphragm of the sensor.

•

It cannot measure the pressure of the gas or vacuum. A piezometer is ideal to measure only the liquid pressure.

THANK YOU