Deep excavation changes the balance of forces within the ground.
As excavation proceeds, an earth-retaining wall may deflect toward the excavation and the surrounding ground may deform. Inclinometer monitoring provides engineers with one of the most useful ways of measuring this lateral movement with depth.
In Singapore, lateral ground movement forms part of the instrumentation monitoring framework associated with ERSS works, and BCA documentation recognises inclinometer monitoring in relevant ERSS applications.
But what exactly does an inclinometer measure?
The Basic Principle
An inclinometer system normally consists of:
Inclinometer casing installed vertically in the ground or structure.
Reference grooves within the casing that control the orientation of the probe.
Inclinometer probe containing tilt sensors.
Readout or data acquisition system that records measurements at successive depths.
The casing itself moves with the surrounding ground or structural element.
By measuring small changes in inclination along the casing and integrating those changes over depth, the system calculates a lateral displacement profile.
The key point is:
An inclinometer does not simply tell us that a wall moved 10 mm.
It can tell us how displacement varies from the surface to the bottom of the casing.
Why the Baseline Reading Matters
Before excavation influences the monitoring location, an initial survey is taken.
This becomes the baseline.
Future measurements are compared with that baseline:
Current profile − Baseline profile = Change in lateral position
Without a reliable baseline, later displacement calculations become much less meaningful.
For this reason, baseline readings should be obtained after installation has stabilised and before the relevant construction activity begins, subject to the project monitoring procedure.
A-Axis and B-Axis
Inclinometer casing typically contains orthogonal measurement directions.
One axis is normally orientated approximately toward the expected principal direction of movement.
For a retaining wall, this may mean orientating the primary axis perpendicular to the wall.
However, both directions should be considered because real ground movement does not always follow a perfectly predetermined direction.
Correct casing orientation and installation records are therefore important parts of the monitoring system.
From Tilt to Displacement
At each depth interval, the probe measures inclination.
Conceptually:
small inclination × measurement interval
= incremental lateral displacement
The increments are then accumulated along the casing to generate a displacement profile.
This is why inclinometer results are commonly presented as:
Incremental displacement
and
Cumulative displacement
versus depth.
A cumulative displacement plot can reveal the shape of the deformation rather than just its maximum value.
What Different Profiles Can Tell Engineers
Movement concentrated near the surface
This may indicate relatively shallow deformation or local ground response.
Progressive deflection through the retained depth
This can be associated with broader deformation of the retaining system and surrounding ground.
Distinct change at a particular depth
A change in profile shape may correspond to geology, support level or a zone of concentrated deformation.
Apparently sudden displacement across the entire profile
This should not immediately be assumed to represent real ground movement.
It may indicate:
- reference instability;
- casing-head disturbance;
- measurement orientation issues;
- probe positioning problems;
- data-processing errors.
For that reason, monitoring interpretation should consider both engineering behaviour and measurement quality.
Why One Reading Is Rarely Enough
Suppose the maximum inclinometer movement changes:
8 mm → 9 mm → 10 mm → 16 mm.
The 16 mm reading immediately attracts attention.
But good monitoring asks several questions:
Was excavation taking place?
Did groundwater change?
Did nearby settlement markers also move?
Did strut loads change?
Was the probe checked?
Does the displacement occur at a credible depth?
Is the change repeated in the next survey?
The engineering significance therefore comes from trend + location + construction activity + corroborating instruments.
Manual vs Automated Inclinometer Monitoring
Traditional inclinometer monitoring uses a portable probe moved manually through the casing.
Its advantages include flexibility and the ability to survey a complete casing using one instrument.
Automated systems use fixed sensors, such as in-place inclinometer arrays, to provide measurements more frequently.
The choice depends on:
- required monitoring frequency;
- project risk;
- accessibility;
- duration;
- expected deformation;
- cost;
- need for remote alerts.
Neither method is automatically “better”. The correct solution depends on what the project needs to know and how quickly it needs to know it.
Common Sources of Poor Inclinometer Data
High-quality interpretation starts with high-quality measurement.
Common issues can include:
Incorrect casing orientation
Damaged or obstructed casing
Casing twist
Insufficient embedment or unstable reference
Probe positioning inconsistencies
Wrong depth interval
Incorrect baseline
Transcription or processing errors
This is why a sophisticated dashboard cannot rescue poor field data.
The chain remains:
installation → measurement → validation → interpretation → decision.
Inclinometers in an Integrated Monitoring Programme
For a deep excavation, inclinometer readings are usually more valuable when considered alongside:
- settlement monitoring;
- piezometers;
- water standpipes;
- strut load monitoring;
- survey prisms;
- building monitoring;
- construction sequence.
This creates a more complete picture of excavation behaviour.
For example:
Increasing lateral movement + groundwater drawdown + adjacent settlement
provides much more engineering information than any of those observations considered in isolation.
Conclusion
Inclinometer monitoring works because it converts a series of very small angular measurements into a vertical profile of lateral displacement.
For deep excavation, the value is not merely the maximum movement number.
The important questions are:
Where is movement occurring?
At what depth?
At what rate?
How is the profile changing?
What construction activity occurred at the same time?
Do other instruments support the same interpretation?
When these questions are answered systematically, inclinometer monitoring becomes much more than a compliance measurement—it becomes a practical tool for understanding excavation performance.
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