What Is an Inclinometer and How Does It Work?

An inclinometer is a system for measuring lateral ground movement made up of two parts: a guide casing installed vertically in a borehole, with 4 internal guide grooves, and a tilt probe with wheels that run in the grooves. The operator lowers the probe to the bottom of the casing, then pulls it up in 0.5 m intervals, and at each interval the probe reads the inclination of the casing. When the ground moves, the guide casing deforms with it, so the measured inclination changes from the baseline. This traversing-probe method follows ASTM D6230.

The calculation has 3 steps. First, convert the tilt angle θ of each interval into a lateral deviation using d = L × sin θ, where L is the 0.5 m reading interval. Second, accumulate the deviations of all intervals from the bottom of the casing up to the top. Third, subtract the baseline profile from the result. What remains is the lateral ground displacement at each depth. The 4 grooves in the guide casing form 2 perpendicular axes: the A axis is aligned with the expected direction of ground movement, and the B axis is perpendicular to the A axis.

The 2D diagram below shows the whole principle in one picture; read it from left to right. On the left is a borehole section with the guide casing deforming with the ground, in the middle is a close-up of one reading interval and a plan of the casing grooves, and on the right is the cumulative displacement plot obtained from the measurements.

2D model of how an inclinometer works, from the guide casing in the borehole and interval-by-interval tilt readings to the cumulative displacement profile versus depth Conceptual cross-section. On the left is a borehole with a grooved guide casing grouted in place with cement-bentonite. The bottom of the casing is embedded in a stable, non-moving layer. The upper soil layer moves to the right, so the guide casing bends with the ground. The vertical dashed line is the casing position at the baseline reading. Inside the casing is a probe whose wheels run in the grooves, connected by cable to a readout unit at the ground surface. At the top center is a close-up of one reading interval: the probe length L equals 0.5 m and is tilted at an angle theta from vertical; the lateral deviation of that interval equals L times sine theta. At the bottom center is a plan of the guide casing with 4 guide grooves; the A0 axis points in the expected direction of ground movement and the B axis is perpendicular to the A axis. On the right is a plot of cumulative displacement versus depth: the vertical dashed line is the baseline and the orange line is the latest reading. The cumulative value starts at zero at the bottom of the casing and increases up to the top. Values are hypothetical, for illustration only. The bottom strip summarizes the three calculation steps: read the tilt every 0.5 m, convert to distance, and accumulate from the bottom up. 2D model: an inclinometer reads tilt, then sums it into displacement Not to scale Readout unit + cable Moving soil Stiff soil Stable 1 Guide casing Guide Casing 2 Grout around cem.-bentonite 3 Probe Wheels in slots 4 Casing toe in stable layer Original line (baseline) 5 1 interval θ d = L × sin θ L = 0.5 m Read interval Vertical 6 Casing plan: 4 grooves A0 A180 B180 B0 A0 = expected movement dir. 7 Cumulative displacement Cumulative displacement → Depth ↓ Top Casing toe Toe = 0 (reference) Base Latest Sum upward Hypothetical · conceptual A Read tilt every 0.5 m Pull probe up per interval B Convert: L × sin θ Deviation per interval C Sum from bottom up Minus baseline = movement
2D model — ① grooved guide casing ② grout around the casing (cement-bentonite) so the casing moves with the ground ③ probe with wheels running in the grooves ④ casing toe embedded in a non-moving layer as the reference point ⑤ one reading interval, deviation d = L × sin θ with L = 0.5 m ⑥ plan of the 4-groove guide casing, with the A0 axis pointing in the expected direction of ground movement ⑦ cumulative displacement plot relative to the baseline; values in the plot are hypothetical, for illustration only (Image: SPN Soil Engineering)

What sets an inclinometer apart from surface measurements is that it tells you the "depth" of movement. A surface settlement point can only show that the ground surface has moved, but an inclinometer shows at which level the movement is concentrated, which is the location of the slip plane. This information is used both to diagnose the cause and to choose a remedy. The inclinometer is one of the 10 main geotechnical instruments; for an overview of the whole system, see What Is Geotechnical Instrumentation?

SPN team drilling and installing an inclinometer guide casing in an embankment at a hydropower project
Drilling and installing an inclinometer guide casing in an embankment — the casing toe must extend down to a layer considered non-moving to serve as the reference point (Image: SPN Soil Engineering)

How Do You Install an Inclinometer So the Readings Are Reliable?

Installing an inclinometer means drilling a borehole, installing the guide casing and grouting around it so the casing moves with the ground, with the casing toe in a non-moving layer. Data quality is decided on installation day, because a guide casing cannot be corrected once it is grouted in. The actual sequence has 5 steps.

1. Drill the borehole about 100–150 mm in diameter, at least 3 m deeper than the expected zone of movement, down to a layer considered non-moving, subject to each project's specification. The reason is that the cumulative calculation starts from the casing toe; if the toe moves with the ground, the result is a relative movement that underestimates the true movement.

2. Install the guide casing using ABS casing with an outside diameter of about 70 mm, joined section by section and lowered into the hole. Align the A-axis grooves with the expected main direction of movement, for example perpendicular to the excavation line or pointing down the slope.

SPN team lifting an inclinometer guide casing and lowering it section by section into a borehole next to a sheet pile line in an excavation
Lifting the guide casing, joining it section by section and lowering it into the borehole — the A-axis grooves must stay aligned with the expected main direction of movement in every section (Image: SPN Soil Engineering)

3. Grout around the casing with cement-bentonite grout so the casing moves together with the surrounding soil, then wait at least 24 hours for the grout to set before taking readings.

4. Take the baseline readings on both the A and B axes every 0.5 m, repeated several times until the values are stable, and complete them before construction activities begin. The baseline is the reference for every subsequent reading; if it is poorly taken, the whole data set cannot be interpreted.

5. Protect the casing top with a cap and a protective chamber against site equipment, and record the coordinates and reference level of each monitoring point.

SPN team joining inclinometer guide casing couplings and fixing the casing top at the borehole collar next to a sheet pile line
Joining guide casing couplings and fixing the position at the borehole collar — every joint must be tight and the guide grooves kept in line so the probe can run continuously over the full depth (Image: SPN Soil Engineering)
Diagram of the 4 steps to install an inclinometer in a borehole: open the hole and advance casing to the stiff layer, wash-bore to the required depth, grout with Bentonite:Cement:Water 1:5:16, and push the inclinometer casing to the bottom of the hole and withdraw the casing
Installation sequence for an inclinometer in a borehole (Installing Inclinometer in Borehole) — (1) open the borehole and advance casing to the stiff layer (2) wash-bore to the required depth (3) grout the borehole with Bentonite:Cement:Water at a ratio of 1:5:16 until full (4) push the inclinometer casing down to the bottom of the hole and withdraw the casing, then hold it down with weights until the cement hardens
Inclinometer installation drawing showing a 70 mm guide casing in a 4-inch borehole grouted with cement-bentonite, with probe details and a Section A-A of the guide grooves
Inclinometer installation drawing (Schematic of Inclinometer Installation) — 70 mm diameter guide casing in a 4-inch borehole, grouted with cement-bentonite, with probe details and Section A-A showing the A-axis guide grooves aligned with the main direction of movement

How Do You Read Inclinometer Results?

Inclinometer results are read from 2 types of plot used together: the cumulative plot and the incremental plot. Both are always relative to the baseline.

  • Cumulative displacement plot shows the total displacement at each depth relative to the baseline; it is used to see the overall shape of movement over the full depth.
  • Incremental displacement plot shows the movement of each 0.5 m interval separately. If there is a distinct slip plane, the plot shows a clear spike at a single level, and that level is the depth of the slip plane.

What engineers actually look at is the trend over time, not a single value. Movement that increases at a steady rate and then starts to accelerate is a signal that requires an immediate response, even if the absolute value has not yet reached the threshold.

Every reading must be taken in two directions, 0° and 180°, to check the sum (checksum) and confirm that the probe and that set of readings are reliable. Common practice is to use the same probe and cable, insert in the same direction and stop at the same depths every time, so that each set of readings can be compared directly.

How Is It Used for Deep Excavations and Retaining Walls?

In deep excavations, inclinometers measure the deflection of the retaining wall and the movement of the soil behind it at each excavation stage. There are 2 ways to install them: the first embeds the guide casing in the wall itself when it is cast, for example in a diaphragm wall; the second installs it in a borehole in the soil behind the wall line. Each reading shows how much the wall has deflected and at what level the deflection is greatest. Readings must be taken at every key stage — each excavation lift, strut installation and strut removal. See a real application in Case Study: Temporary Earth Retaining System for a Deep Excavation, and see the layout of the full instrumentation system in Deep Excavation Monitoring in Bangkok

The 3D view below shows both installation types in a braced excavation. Note that both casing toes are in the stiff layer, and the purple casing lines bow away from the dashed lines, which show the casing position at the baseline reading. The right panel summarizes the types of work that use inclinometers and 4 points that make the readings reliable.

3D model of inclinometer locations in a deep excavation with a retaining wall and struts Conceptual 3D cutaway of a deep excavation. On the left is the soil behind the wall: soft clay on top and a stiff layer below. In the middle is the retaining wall with its toe embedded in the stiff layer. On the right is the excavation with walers and steel struts at two levels. The wall deflects toward the excavation, most of all near the excavation base, drawn exaggerated. The purple inclinometer casings are in two locations: the first is embedded in the wall and bends with it; the second is installed in a borehole in the soil behind the wall, with its toe extending down into the stiff layer. The vertical dashed lines show the casing position at the baseline reading. At the ground surface a readout unit is connected by cable to the casing top. The right panel summarizes the work that uses inclinometers — deep excavations and retaining walls, slopes, embankments on soft ground, and dams — and four points that make readings reliable: casing toe in a non-moving layer, baseline read before work starts, same probe and same direction every time, and readings in two directions, 0 degrees and 180 degrees. 3D model: inclinometers in a deep excavation Not to scale Soft clay Stiff soil Pit base 1 Soil inclinometer Drilled behind wall 2 Wall inclinometer Casing cast in wall 3 Struts + walers Strut / Waler Readout unit 4 Toe in stiff layer Non-moving reference 5 Wall bows into pit Exaggerated Where used · How to read Where it is used Deep excavation · Retaining wall Slopes and hillside cuts Embankments on soft soil Dams and levees 4 keys to reliable data Toe in a stable layer Sum starts at the toe Baseline before work All reads vs. baseline Same probe, same dir. Cuts systematic error Read 0° and 180° Checksum test Project alert levels Set by design engineer Conceptual view, not to scale
3D model — ① inclinometer installed in a borehole in the soil behind the wall line, cabled to a readout unit at the ground surface ② inclinometer embedded in the retaining wall ③ struts and walers at two levels ④ casing toe embedded in the stiff layer as a non-moving reference point ⑤ wall deflecting toward the excavation, drawn exaggerated to show the shape (Image: SPN Soil Engineering)
SPN team installing an inclinometer guide casing next to a sheet pile line in an excavation, project in Bang Phli, Samut Prakan
Installing an inclinometer guide casing next to a sheet pile line, project in Bang Phli District, Samut Prakan (Image: SPN Soil Engineering)

A deep excavation monitoring plan sets thresholds at 3 levels: alert, alarm, and stop-work to review measures. The values for each level come from the design engineer's analysis for each project; there is no universal standard value that applies to every job. What every project has in common is that the thresholds and reporting chain must be agreed before excavation starts — not decided on site after the readings have already moved.

How Is It Used for Slopes, Embankments and Dams?

On slopes, inclinometers are used to find the depth of the slip plane, which is the starting input for every type of remedial measure — soil nails, stabilizing piles or slope regrading. These measures must extend through the slip plane into stable ground to be effective. Without knowing the depth of the slip plane, remedial design is just guesswork. Read more about slope failure mechanisms in Landslide and Slope Failure Prevention

Slopes are usually read more often in the rainy season, because a rising groundwater level reduces the slope factor of safety (FS), so movement tends to accelerate during prolonged heavy rain. For embankments on soft soil and dams, inclinometers are used for long-term monitoring over the service life, installed together with piezometers to track movement and pore water pressure in the embankment at the same time. Many projects connect probes to a datalogger for automatic readings and remote alerts. See real projects of both types in Case Study: Inclinometer on a Slope, Kraseaw Hydropower and Case Study: Inclinometer in a Deep Excavation, Bang Phli

Diagram of 6 inclinometer applications: slopes and landslides, embankments, D-wall and sheet pile retaining walls, tunnels, laterally loaded piles and permanent retaining walls
Applications of inclinometers in different types of work — slopes and landslides, embankments/dams, retaining walls (diaphragm wall / sheet pile), tunnels, laterally loaded piles and permanent retaining walls (Source: Slope Indicator Company, 1994)

What Is the Difference Between an Inclinometer and a Tiltmeter?

An inclinometer measures ground movement over the full depth through a guide casing in a borehole, while a tiltmeter measures the tilt of a structure at the point where the sensor is mounted. Both measure "tilt", but they provide different data and answer different questions. The table below summarizes the main differences.

← Scroll to see the full table →

TopicInclinometerTiltmeter
What it measuresLateral soil movement at each depthTilt of the structure at the sensor point
InstallationRequires a borehole and guide casingFixed to the structure surface; no drilling
Data obtainedFull-depth profile; locates the slip planeSingle-point tilt, high resolution of ±0.01°
Best forSoil behind retaining walls, slopes, embankments, damsBuildings next to excavations, the retaining wall itself

Urban deep excavations therefore often use both: inclinometers monitor movement of the soil and wall, while tiltmeters are fixed to adjacent buildings to monitor the tilt of the structures that must be protected.

What Standard Applies to Inclinometers and How Much Does It Cost?

Traversing-probe inclinometer monitoring follows ASTM D6230. There is no specific DPT standard for geotechnical instrumentation, so ASTM is used together with the project specification and the probe manufacturer's manual. SPN's service covers drilling and installing the guide casing, baseline readings, periodic readings, through to data processing and reporting. Reports are certified by a licensed civil engineer (Council of Engineers Thailand), with signature and license number, and can be submitted to the relevant authorities. See the full scope of services on the Geotechnical Instrumentation and Monitoring page

Fees depend on casing depth, number of installation points and reading frequency, so they are assessed per project. Contact us for a free quotation within 24 hours. Prices do not include travel and equipment mobilization, which are charged extra based on the distance to the site.

⚠️ This article provides general information

This content explains how inclinometers work and how they are used, for general understanding. Installation locations, casing depth, reading frequency and alert levels for each project must be set by the responsible engineer based on the specific conditions of that job.