A piezometer stands out as a vital geotechnical sensor employed to gauge pore water pressure or piezometric level within the ground. These devices are meticulously crafted to assess pore water pressure in a variety of settings, including soil, earth/rock fills, foundations, and concrete structures.

Regarded as a cornerstone in geotechnical monitoring, piezometers not only play a crucial role in practical applications but also make for intriguing study material. In this article, we will read about the piezometer definition, what do piezometers measure, explore its diverse types, underlying working principles, practical applications, and essential functions in comprehensive detail. So, let's get started.

What Is Piezometer?

Piezometer, also known as pore pressure meter, is a term that is a fusion of two Greek-rooted components: the prefix "piezo-," signifying "pressure," and the root "meter," denoting "to measure." True to its etymology, a piezometer is used to measure the pressure. To be more specific, a piezometer is designed to gauge static water pressure, typically within two primary contexts. Firstly, it measures the pressure of groundwater held within the adjacent rock and soil, known as pore water pressure. Secondly, it assesses the pressure exerted by a body of water against the confines of its container.

Understanding pore water pressures is pivotal because they constitute a fundamental element of effective stress, an important concept in soil mechanics. The principle of effective stress indicates that any measurable impacts resulting from a stress change, such as compression distortion and alterations in shearing resistance, are exclusively attributed to changes in effective stress, factoring in time effects. While reasonable estimates can be derived for total stresses (despite the existence of sensors for measuring them), the measurement of pore water pressures is indispensable. This is where the significance of piezometers truly shines—they play a crucial role in accurately gauging pore water pressures, making them indispensable in geotechnical applications. 

A thorough examination of pore pressure is crucial for monitoring post-construction behavior and identifying potential hazards that could jeopardize the stability of structures, foundations, and associated elements. Additionally, it furnishes essential data for enhancing design, contributing to safer and more cost-effective construction practices. Now that we've covered the basics of what a piezometer is let's explore its practical applications in more detail.

Why is a piezometer used?

The application of extensive instrumentation is crucial for the following geotechnical and environmental purposes, which is why the piezometers are used in these scenarios.

1. Construction control, stability investigations, and groundwater monitoring play a major role in ensuring the integrity of structures such as earth dams, embankments, foundations, shallow underground works, and surface excavations. Pore pressure gradients and uplift studies are essential considerations for foundations, embankments, abutments, and fills, contributing to the overall stability of these structures.
2. Hydrological investigations, groundwater pressure studies, and monitoring operations are integral for effective water supply operations. Pore pressure studies find relevance in waste and environmental applications, contributing to informed decision-making. Monitoring pore pressure is imperative for soil improvement, stability assessments, and slope stability analyses.
3. In coastal regions with high population density, extensive instrumentation becomes essential for reclaiming land and constructing highways, multi-storied structures, port trusts, and recreational facilities. This demands precise measurements of horizontal displacement, vertical movement, and pore pressure to ensure the safety and stability of the constructed infrastructure.
4. Addressing salinity challenges in coastal areas is another critical application. Implementing strategies to prevent saline intrusion due to over-pumping from boreholes is vital, as once this intrusion occurs, rectifying the problem becomes exceedingly difficult. Multilevel piezometers, water sampling systems, and conductivity/flow logging in boreholes are instrumental in detecting saline horizons at different depths in complex aquifers. This comprehensive approach helps safeguard freshwater resources and reduces the environmental footprint of human activities in these sensitive coastal environments.

Also Read: Water Conservation And Its Importance For Sustainable Businesses

What are the types of piezometers?

Diving into the diverse world of piezometers, these instruments come in various shapes and sizes. Each type of piezometer caters to specific needs, offering a range of options for accurately measuring and monitoring pore water pressure in diverse geological settings. Let's explore the different types of piezometers.

Open Standpipe Piezometers

The Open Standpipe Piezometer, also known as Casagrande, is a straightforward instrument designed to measure the ground's piezometric level. Comprising a plastic pipe with a small diameter and a porous section at the bottom, it is installed inside a borehole. The porous section is strategically placed at the bottom, surrounded by sand, and sealed with bentonite. The rest of the borehole is then filled with cement mortar. This design allows groundwater pressure to propel water into the standpipe until the inside water level equals the ground piezometric level.

Electric Piezometers

Electric Piezometers feature a deflecting diaphragm and a porous filter, separated by a small water reservoir. Measurement is conducted using a vibrating wire or strain gauge, and the results are converted to pressure through calibration. Installation mirrors that of open standpipe piezometers, with insertion into a borehole and the annulus filled with cement/bentonite grout. The operation involves groundwater pushing into the reservoir, causing deflections in the diaphragm until the pressure inside matches the pore water pressure in the ground.

Hydraulic Piezometers

Hydraulic Piezometers consist of a porous filter with a water reservoir, separated from a pressure gauge by water-filled tubes. These tubes circulate water, eliminating air and ensuring the reservoir remains filled. The instrument measures overall hydraulic pressure, from which pore pressure can be calculated.

Pneumatic Piezometers

Pneumatic Piezometers operate through gas pressure and find application in boreholes, fills, or large-diameter standpipes. Piezometer reading is calculated using a pneumatic indicator.

Vibrating Wire Piezometers

Tailored for measuring pore water pressure in saturated soil and rock, Vibrating Wire Piezometers include a sensitive diaphragm, a magnetic wire, a ceramic filter, a thermistor, and a stainless steel body. Special sensors are provided for applications involving saline water. These piezometers offer versatility for accurately measuring and monitoring pore water pressure across diverse geological settings.

How Does a Piezometer Work?

The fundamental components of a vibrating wire piezometer include a magnetic wire with high tensile strength, anchored at one end and fixed to a diaphragm at the other end. The diaphragm responds proportionally to applied pressure, causing a corresponding change in tension within the stretched wire. Consequently, any pore pressure alteration directly influences the wire's tension, forming the basis of the piezometer's functionality.

In the intricate mechanics of the system, a coil magnet plucks the wire, inducing resonance at a frequency 'f.' This frequency is determined by the formula: 

f = {[σg/ρ] ^1/2}/ 2l Hz 

In this formula, σ represents the wire tension, g is the gravitational constant, l is the length of the wire, and ρ is the density of the wire. The resonant frequency induces an alternating current in the coil magnet, elucidating the piezometer equation.

The Pros & Cons of Piezometers

While piezometers offer numerous advantages in monitoring fluid pressures in various applications, users must be mindful of their limitations and ensure proper selection and installation for optimal results. Here are some of the pros and cons of piezometer to keep in mind.

Pros

• Instant Pressure Measurement: Piezometers provide immediate readings of groundwater pressure at specific points, making them highly efficient for real-time monitoring.
• Data Logging Capabilities: These devices allow for the logging of data over time, offering a comprehensive understanding of fluid pressure variations.
• Versatility in Applications: Piezometers find extensive use in the geotechnical industry, aiding in determining groundwater levels. This is particularly valuable for monitoring ground conditions around construction sites, such as tunnels.
• Construction Site Applications: In construction settings, piezometers are instrumental in gauging groundwater pressure before excavation, ensuring the safety and stability of structures like walls and pillars.
• Effectiveness in Drainage System Evaluation: Piezometers play a crucial role in assessing the effectiveness of drainage systems by measuring water pressure and its impact on lateral walls.
• Pressure Rating: Piezometers typically have a pressure rating, guiding users on their application limits and ensuring reliable readings within the specified range.
• Variants for Different Needs: There are variants like drive-in piezometers, which can be directly pushed into the ground, offering flexibility for various applications.
• Portability: Piezometers' small and portable nature makes them convenient and adaptable for use in a wide range of construction scenarios.

Cons

• Pressure Limitations: Piezometers have pressure limitations, and readings beyond the specified range may not be feasible.
• Selection Challenges: Careful consideration is required in selecting the correct type of piezometer for a specific application, ensuring compatibility and accurate measurements.
• Dependency on Correct Installation: The effectiveness of piezometers depends on correct installation, and improper placement may lead to inaccurate readings.

Also Read: All You Need To Know About Water Flow Sensors!

Piezometer Installation for Various Applications

The piezometer installation method varies depending on the geography and its applications. Let's understand a few of them.

Borehole

The fully grouted method optimally achieves Installation in a borehole. On the other hand, the traditional approach, which involves intricate steps such as placing sand pockets, bentonite seals, and grout columns to separate piezometers at different levels, the fully grouted method proves to be both swift and uncomplicated. The installation is best executed using vibrating wire piezometers as the preferred piezometer sensor, as these piezometers demand minimal water volume.

Concrete Dams

In concrete dams equipped with access galleries, the initial routing of sensor cables involves directing them to the gallery. Within these galleries, the cables find termination points in junction boxes. Subsequently, the data collected from the diverse sensors can be conveniently accessed or logged directly from these junction boxes, facilitated by a readout unit or data logger. Alternatively, when necessary, the signals originating from the junction boxes have the flexibility to be transmitted through multicore cables, extending to an observation room located outside the dam structure.

Soft soil Installation

The push-in piezometer is engineered explicitly for insertion into soft soils using drill rods, either manually or with the hydraulic assistance of a rig. Throughout the insertion process, it's imperative to monitor the piezometer closely to prevent pressures from surpassing the calibrated range

Piezometer Buying Guide

When navigating the market for piezometers, it's essential to have a comprehensive understanding of the pricing range. In India, piezometer prices can vary significantly, ranging from 20,000 to 1,75,000 INR. To make an informed purchase, consider your specific requirements and budget constraints. Lower-priced options suit basic applications, while higher-end models with advanced features may be necessary for complex projects.

Additionally, assess the reputation of the manufacturer, warranty provisions, and customer reviews for insights into product reliability and performance. Also, periodic piezometer maintenance is mandatory for getting the correct results. By aligning your needs with available options and understanding various factors, you can make a well-informed decision when investing in a piezometer for your geotechnical or construction endeavors.

You can also reach out to us at business@kritsnam.com or +91-970340123 for queries.