Displacement is a measure of how much an object or surface has changed in position. In the field of geotechnical engineering, displacement monitoring is important to ensure that structures such as buildings and bridges remain stable and safe.
Displacement monitoring can help geotechnical engineers to identify potential problems early before they cause serious damage. It can also be used to track the progress of construction projects or to monitor how a structure is settling over time.
There are many challenges and limitations associated with managing displacement, but it remains a key indicator of an asset’s or structure’s health. Here we’ll explore how displacement is measured and monitored, and why it’s so important for engineers working in this field to be armed with accurate, reliable, and precise data.
What is displacement?
Displacement is defined as the movement of a point or object relative to its surroundings. In geotechnical engineering, displacement is commonly monitored in order to assess the performance and stability of structures built on or within the ground, such as buildings, dams, bridges, and embankments.
The cause of displacement could be one of several factors, including settlement, soil consolidation, creep, thermal expansion/contraction, and traffic loading. Weather, especially rainfall, can also affect the displacement of surfaces or structures.
How is displacement measured?
Displacement monitoring involves taking measurements at regular intervals from fixed points, and then calculating the amount of movement that has occurred between readings.
As displacement is a relative measure, it is important to first establish what the zero point or datum is, and how this will be monitored. The most common reference datums used in geotechnical engineering are the original ground level (OGL) or pre-construction ground level (PCGL), and the top of structure/asset (TOS/TOA). These datums are usually established by surveying.
There are many tools for measuring displacement, including:
- Inclinometers
- Extensometers
- Interferometric synthetic aperture radar (InSAR)
- Tiltmeters
- Laser scanning
- GPS
- Photogrammetry
Each method has its advantages and disadvantages, which will be discussed in more detail below.
Inclinometers
Commonly used to measure displacement in geotechnical engineering. They are installed vertically within boreholes drilled into the ground and measure the inclination of the borehole wall concerning vertical. Inclinometers can be used to monitor both horizontal and vertical displacement and can be installed in both soft and hard soils. However, they’re not suitable for use in areas with a high water table or where there is a risk of flooding.
Extensometers
Also commonly used to measure displacement in geotechnical engineering. They are installed horizontally within boreholes drilled into the ground and measure the change in length of the borehole wall with respect to time. Extensometers can be used to monitor both horizontal and vertical displacement, but are only suitable for use in soft soils.
InSAR
A remote sensing technique that can be used to measure very small displacements, even down to millimetres. It uses satellite-based radar images to create detailed maps of ground surface deformation. InSAR can be cost-prohibitive for many applications in measuring displacement, so doesn’t always make sense to deploy.
Tiltmeters
Used to measure small changes in the inclination of a structure with respect to the horizontal plane. They can be used to monitor both horizontal and vertical displacement, but are only suitable for use in structures constructed of concrete, masonry, or steel.
Laser scanning
A relatively new technology that is being used increasingly in geotechnical engineering. It involves the use of a laser to scan the surface of a structure and create a three-dimensional point cloud model. This point cloud can then be used to measure the displacement of the structure with respect to a reference point.
GPS (Global Positioning System)
A satellite-based navigation system that can be used to measure the position of a point on the ground with great accuracy. It can be used to monitor both horizontal and vertical displacement but is only suitable for use in open areas where there is a clear line of sight to the satellites.
GPS, combined with IoT and cloud computing. has allowed the emergence of high accuracy, low-cost solutions for displacement monitoring like Kurloo. In the case of Kurloo, the signals of multiple satellites (i.e. Global Navigation Satellite System (GNSS) are used to provide an even higher degree of precision.
Photogrammetry
This is the process of taking measurements from photographs. It can be used to measure the displacement of a point on the ground by taking two or more photographs from different positions and using them to calculate the change in position of the point.
The choice of method for measuring displacement will depend on factors like the type of soil, the type of structure, the accuracy required, and the budget. In general, inclinometers and extensometers are the most commonly used methods for measuring displacement in geotechnical engineering, but new technologies such as laser scanning and GPS are becoming increasingly popular.
Solutions like Kurloo mean measuring displacement in difficult-to-access areas is made easier, safer and more affordable than ever before.
Accurate, reliable, and precise data is crucial when measuring displacement
There are many things that can affect how accurate displacement measurements are. This includes how the measurement is taken (manually or using a sensor), what type of object or surface is being measured, and where the measurement is taking place (e.g. inside a laboratory or out in the field).
Whichever method is used, it’s important to gather reliable, accurate, and precise data. This will help to ensure that any problems are identified early and that the correct course of action can be taken to rectify the situation.
One of the key challenges of managing displacement is that it’s often difficult to know how much movement is too much. This can vary depending on the type of structure or asset, and its intended purpose. For example, a small amount of displacement in a building might not be cause for concern, but the same amount in a dam or bridge could pose a serious problem.
This is where the precision of the data becomes so crucial. Because it empowers geotechnical engineers to accurately assess the situation and its risk factors, and establish an appropriate maintenance and repair program.
Risky business
One of the biggest challenges with displacement measurements is that they are often needed in difficult or dangerous environments, such as steep embankments or slopes. Often this makes capturing accurate and reliable displacement data a risky and potentially expensive exercise to obtain.
Steep slopes can be difficult to access with specialised equipment, particularly during high-risk weather events. This is when measured displacement data is even more critical – to proactively monitor for any risk of damage or collapse.
Despite the difficulty in measuring, it’s important to keep track of how much displacement has happened. Displacement is a key indicator of how healthy an object or surface is. It tells us how much it has moved over time. It’s this powerful information about the behaviour in an asset lifecycle that can be used to cost effectively assess its stability and identify potential problems that require more detailed investigations.
For example, if the displacement of a building is increasing over time, this could be an indication that the foundation is settling or that the structure is becoming less stable. By monitoring displacement, engineers can identify potential problems early and take steps to prevent them from becoming bigger issues.
When managing assets or structures, displacement is a crucial metric that needs to be measured and monitored in order to assess the structural health. Displacement data provides essential information for understanding how an asset or structure is performing, how it will perform in the future, and how best to manage its risk.
There are many ways to measure displacement, each with its own advantages and disadvantages. The most important thing is to choose the method or combination of methods that best suits your needs.
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