Keeping it Level: All About Tiltmeters in Civil Engineering

In the world of civil engineering, precision reigns supreme. Even the slightest rotational movement in a structure can have significant consequences, affecting safety, stability, and long-term performance. This is where tiltmeters come in – smart little devices that measure minute changes in ground or structural inclination.

So, what exactly are tiltmeters (otherwise known as tilt meters or tilt sensors), and how can they benefit engineers and construction professionals?

The Lowdown on Tiltmeters

A tiltmeter, essentially a super-sensitive inclinometer, measures deviations from the vertical plane to a high degree of precision.

There are two main types of tiltmeters commonly used in construction:

• Mechanical Tiltmeters: These traditional devices utilise a spirit level or a plumb bob suspended on a precise pivot. While reliable, they can be less sensitive than their electronic counterparts.
• Electronic Tiltmeters: Modern tiltmeters leverage various technologies like vibrating wires, MEMS (Micro-Electro-Mechanical Systems), or electrolytic sensors. These offer superior sensitivity, data logging capabilities, and often, wireless communication.

This article will focus on modern tiltmeters, such as the wireless MEMS-based tilt sensor nodes developed by Senceive. These wireless devices measure movement at a resolution of 0.0001 degrees, which equates to sub-millimetre measurement in most engineering applications. The civil engineering sector has adopted this IoT monitoring technology extensively over the last two decades and there are probably in excess of 100,000 such devices in use at one time worldwide. Benefitting from a programme of continuous development and improvement, advanced tiltmeters such as the Senceive NanoMacro can send wireless updates to remote users every 30 minutes for between 10 and 15 years. It also has the capability to send updates at sub-minute frequency where users need to keep track of dynamic situations such as a movement of a flood-damaged bridge or a structure affected by tunnelling activity.

Triaxial Tiltmeters

Triaxial tiltmeters, also known as a 3-axis tiltmeter, have an integrated triaxial tilt monitor sensor to measure tilt from different angles, depending on the installation orientation.  The three-axis measurement is based on how the sensor is positioned with respect to gravity.  Two axes are usable for measurements at any given time.  The movement data that is reported is based on the two axes of rotation from the horizontal plane.

Triaxial tiltmeters can be mounted at any angle to virtually any material, but they must be securely fixed.  Triaxial tiltmeters such as the Nano Triaxial Tilt Sensor Node can be mounted with different plates and mounts for different applications, such as magnetic mounts for steel or cast iron, two-part mounts for trackbed, or stake mounts for geotechnical monitoring.  Tilt beams can also be used and can be daisy chained to provide movement profiles.

Applications in Construction

As part of a remote condition monitoring system, tiltmeters play a crucial role in various stages of construction, from monitoring ground stability to ensuring structural integrity. Just a few examples of construction applications are listed below:

Structural Monitoring: Tiltmeters are invaluable for monitoring the stability of buildings, bridges, and other structures during and after construction. They can detect rotational movement, differential settlement and other deformations, allowing engineers to intervene before structural integrity is compromised.

Tunnelling: Tiltmeters are used extensively in tunnel construction monitoring, providing critical data within the tunnel such as lining movement close to the tunnelling face and longer-term convergence, as well as deformation above the tunnel that could affect the stability of buildings and structures.

Excavation: Engineers use tiltmeters to monitor the stability of deep excavations to improve the safety and efficiency of the process.

Dam Safety: As dams fill with water, the immense pressure can cause minute ground and structural movements. Tiltmeters placed on the dam can alert engineers to potential issues.

Applications in Rail and Infrastructure

Landslide Monitoring: Geotechnical engineers use tiltmeters in areas prone to slope failure risk due to landslides, debris flows, rockfall or washout due to flooding to detect ground movement. It is common practice to mount the tilt sensors on metal stakes to detect shallow ground movement. 

Rail Track Monitoring: Because modern tiltmeters are so robust and precise they are widely used to monitor changes in railway track geometry. It is possible to measure or derive internationally recognised track geometry parameters including cant, twist and differential settlement from tilt data.

Bridges, Buildings and other Infrastructure:  Tiltmeter technology is recognised as a valuable part of the structural health monitoring (SHM) toolkit. As such, it is widely used to provide long-term asset condition data, particularly from buildings and structures where concern has arisen relating to condition or structural integrity.

Tiltmeter Selection and Deployment

Choosing the right tiltmeter for your project requires careful consideration of several factors including:

• Resolution: This is the smallest change in a quantity being measured that causes a perceptible change in the output. High resolution is critical to many structural monitoring applications, but is less so for detecting high magnitude movements such as landslides.
• Repeatability: This effectively tells you how noisy the data is and is important because it is an indication of precision under a set of repeatability conditions.
• Environmental Conditions: Consider the operating environment – temperature fluctuations, vibration, and exposure to elements – when selecting a tiltmeter. You should expect a tiltmeter to be certified to IP68/69K and to operate across a temperature range comfortably exceeding the expected highs and lows of your site.
• Wireless capability: Tiltmeters are of most use when deployed as part of wireless monitoring system. It is therefore important to consider factors such as the maximum distance from the cellular gateway, the number of sensors that can be deployed per gateway and the lag between a tilt reading being taken in the field and received by the user.  
• Battery life: It is important to be confidant that the battery life of the tiltmeter will exceed the planned duration of your monitoring programme – and that you have considered factors such as sampling frequency that affect battery performance.

Conclusion

Tiltmeters are powerful tools when applied to civil engineering applications. Their ability to measure tilt movement to a very high degree of precision and repeatability provides invaluable insights into ground and structural behaviour. By incorporating wireless tiltmeters into their projects, civil engineers can benefit from virtually real time updates and alerts from remote sites - supporting safer, more efficient construction and maintenance of a wide range of infrastructure.