Chain Inclinometer or Probe? How to Choose and How to Read the Profile

An inclinometric probe and a chain inclinometer (in-place) measure the same thing, the tilt of a pipe in soil or in a wall, but they do it differently. A probe gives a dense profile when the operator comes on site. A chain gives a sparser profile, but it does so automatically, every 15 minutes by default, or more often, with thresholds and alarms. Choose a probe for control checks and secondary boreholes. Choose a chain where you cannot afford to be blind between readings.

In brief

• Both instruments measure angle; horizontal displacement is calculated with Δ = L·sin θ and summed from the anchor, the lower fixed end of the pipe, upward.
• Probe: one instrument for many boreholes, dense increment, typically 0.5 m, but subject to operator error, someone must be at the borehole, and there is no alarm between readings.
• In-place chain: segments fixed in the pipe, in manufacturers’ systems 0.25 to 3 m, automatic reading, logger every 15 minutes by default, or more often, thresholds and SMS, but measurement points only where the segments are installed.
• You read the profile in two ways: the cumulative curve says “how much”, the incremental curve says “where”.
• Decision pattern: how quickly you need to know, how long the sensitive phase lasts, how many boreholes you have. On an urban excavation: chain on the decisive boreholes, probe for control and the rest.

How an inclinometer works: definitions and conversion of tilt to millimetres

An inclinometer is an instrument that measures the angle of deviation from vertical of the axis of a pipe installed in soil, a diaphragm wall, a pile wall, or an embankment, from which the horizontal displacement of that pipe at different depths is calculated by geometry. The sensor itself does not measure millimetres. It measures angle, in milliradians (mrad), degrees, or as sin θ. Millimetres are the result of a calculation, and that calculation has assumptions you need to know.

The inclinometer pipe has four guide grooves. They force sensor orientation and allow measurement of two perpendicular tilt components.

Axis A is the direction in the inclinometer pipe set during installation to match the expected main movement direction, for example perpendicular to the wall, toward the excavation, while axis B is perpendicular to it and is used for control and to capture “lateral” movement.

The anchor is the lower end of the inclinometer pipe, embedded below the zone where movement is expected, treated in calculations as the fixed point from which displacements are summed upward.

From angle to millimetres: the formula

The horizontal displacement of one pipe section of length L, inclined by an angle θ relative to vertical, is:

Δ = L · sin θ

Angles in geotechnical monitoring are small, so sin θ ≈ θ in radians:

Δ ≈ L · θ[rad]

Illustrative example: a 1 m section inclined by 1 mrad (0.001 rad) moves about 1 mm at the top; a 2 m section at the same angle moves about 2 mm. That is why the segment length and the probe increment must be known and constant.

What matters to you is how much a point at a given depth has moved relative to the anchor. You calculate it like this:

1. Assign zero displacement to the anchor.
2. For each section from bottom to top, calculate Δ_i = L_i · sin θ_i.
3. The displacement at depth z is the sum of all Δ_i below that depth: D(z) = Σ Δ_i.

Practical note: usually you are not interested in absolute tilt, only in the change relative to the zero reading taken before work started. The displacement profile is the difference between the current profile and the baseline profile. Without a reliable zero reading, every later number hangs in the air, which is the same point made in the deep excavation geotechnical monitoring guide.

What this means for you: before comparing a probe with a chain, define the two common points for both, where the anchor is and when you take the zero reading. Every instrument will fail there if you get it wrong.

Inclinometric probe: advantages, drawbacks and operator errors

An inclinometric probe is a portable tilt sensor with guide wheels that the operator lowers on a cable into the pipe and stops at a fixed increment equal to the wheel base (500 mm according to Geokon 6000/6100 and Sisgeo datasheets), reading the angle at each depth in two opposite orientations.

The operator runs the probe in A0 orientation, rotates it by 180°, and repeats the pass as A180. Averaging the two removes sensor zero error, and the sum of A0 + A180 at each depth, the checksum, is the first quality control tool: if it jumps, something went wrong.

What a probe does well

It gives a dense profile: a measurement point at each probe increment along the full length of the pipe, so a thin slip zone will not pass between points. One probe serves many boreholes, so the instrument cost is spread across all pipes. Nothing remains in the pipe permanently, so if the pipe is sheared, you lose the pipe, not the sensor. The manufacturers state the systematic accuracy of MEMS probes at ±2 mm over 25 to 30 m of pipe, according to the Geokon 6100 and Sisgeo datasheets, a good figure when the procedure is correct.

Where a probe falls short

A probe measures when the operator comes on site. Between readings there are no data, no thresholds, and no alarm. If you read once a week, nobody knows what is happening in the pipe for a week, and the moment of highest risk, for example removal of another layer or prestressing of anchors, can fall between measurements.

The second weakness is operator error. A proper reading requires stabilising the probe at each depth, using the same reference depth, the same attachment point at the head, a clean pipe, and the same probe and cable as in the zero reading. It takes only a different probe, a shortened cable, or a new person on the team for a “jump” to appear in the profile with no relation to the ground. Systematic probe errors, zero offset, depth error, and rotation error, are described in the literature, including Dunnicliff, and in manufacturers’ instructions. If you do not correct them, they distort the trend.

The third issue is the person at the borehole. Boreholes on an urban excavation are often at the edge, in the road reserve, among equipment. Every reading means entering the work zone. The fourth issue is time: a measurement takes several tens of minutes per borehole, and then a spreadsheet still has to be processed. It can take a day or more from reading to plot.

Conclusion: a probe is a good control instrument if you have a stable team, stable equipment, and a stable procedure. It is not an early warning system.

Chain inclinometer (in-place): continuity, automatic profile and what about cost

A chain inclinometer (in-place, IPI) is a chain of tilt sensors permanently installed in an inclinometer pipe, connected by rigid sections of known length and joints, read automatically by a logger and giving a displacement profile without operator involvement.

Each segment is a sensor, today most often MEMS, mounted on wheels in the same grooves used by the probe, measuring in one axis (A) or in two (A and B). Segment length is selected to suit the ground profile: shorter where you expect a shear zone, longer where only the overall shape of deflection matters. Manufacturers offer segments from 0.25 to 3 m: Geokon 6180, standardly 0.5, 1, 2 and 3 m with a minimum spacing of 0.5 m, RST 0.5 to 3 m, Sisgeo LT-Inclibus a 2 m rod with two sensors every 1 m, and Measurand SAAV (ShapeArray type chain) 0.25 or 0.5 m, according to manufacturers’ datasheets; RST, according to the distributor’s datasheet.

What a chain does well

The main advantage is continuity in time. Automatic reading, with the logger every 15 minutes by default, or more often, means the profile always exists, not just “since Thursday”. On each segment and on the total you can set warning and alarm thresholds, and the notification goes by SMS or e-mail before anyone comes to site.

The chain also removes operator error by definition. The same sensor, in the same position, in the same groove, measures each time. Repeatability belongs to the sensor, not to the person. Geokon states for the 6180 a resolution of 0.00025° and a precision of ±0.0075°, which is about 0.13 mm per metre of segment, according to the datasheet. Each reading has a time stamp and comes from an identifiable device, which matters when the data must serve as evidence. And nobody enters the work zone: the pipe head is secured, and the logger stands aside and sends the data to the platform.

Where a chain has limitations

Spatial resolution is only as fine as the segment length. If the slip zone is half a metre and the segments are two metres, you will see it as a blurred tilt over one section, not a sharp kink. That is why a chain is designed after the ground has been understood: you know where the weak layer is, and you densify there.

Pipe damage is more expensive: if the wall cuts the pipe, the chain stays in the pipe. That is why chains are installed so they can be extracted, and on boreholes at high risk of shearing, a probe is considered first, then a chain after stabilisation.

What about cost

This is where your first concern sits: overpaying. A chain is an upfront expense for each borehole that will be monitored continuously. A probe is one instrument, but each reading has a cost: travel, two people’s time, and processing. We do not give amounts because they depend on the number of boreholes, pipe lengths, reading frequency, and location. The rule is simple: the more often you need a profile and the longer the sensitive phase lasts, the faster the chain pays back. If you read once a month on a quiet site, the probe wins. For an excavation in a built-up area, where you want to know every day, the arithmetic goes the other way. In the language of the SHM pillar: a weekly probe reading gives a 672x longer time from event to knowledge than automatic reading every 15 minutes by default, or more often. That is not a judgement on the probe, only arithmetic.

Comparison table: probe vs chain in 12 criteria

Footnotes: [1] Geokon 6000/6100 probe datasheets (500 mm base; ±2 mm/30 m for 6100 MEMS, ±6 mm/30 m for 6000) and Sisgeo Digital MEMS Inclinometer System (±2 mm/25 m). [2] Geokon 6180 datasheet (0.5/1/2/3 m; resolution 0.00025°, precision ±0.0075°), RST MEMS Digital IPI (0.5 to 3 m, according to distributor), Sisgeo LT-Inclibus, Measurand SAAV (0.25 or 0.5 m). Links in “Sources”.

The table does not decide which is better. It decides what each is suited for. Show rows 2, 8 and 9 to the manager: they decide whether you learn about wall movement today or next week.

How to read the displacement profile

The displacement profile is a plot with depth on the vertical axis and displacement in millimetres on the horizontal axis, usually with several curves for successive dates. Look at four things.

Anchor and summation from the bottom

Assume that the bottom of the pipe is fixed and sum the Δ_i upward. That leads to the first design rule: the pipe must extend clearly below the zone that may move. If the anchor “floats”, the entire profile shifts as a rigid body and, instead of ground movement, you see anchor movement. Equal displacement along the full length, including the bottom, means either the pipe is too short or there is a systematic error.

When the bottom of the pipe cannot be treated as fixed, the top is anchored geodetically, by measuring the head position, and the summation is done downward, less convenient, but sometimes there is no other option. It is still better to add a few metres of pipe than to anchor the head later.

Axis A and axis B

Axis A should be parallel to the expected displacement: on an excavation wall, perpendicular to the wall, on a slope, in the direction of fall. Then axis B shows “noise” without a clear trend. If axis B develops a trend comparable to axis A, either the ground is moving differently than the design assumed, or the pipe is rotated relative to the assumed direction. The second case is more common and can be excluded easily by measuring groove orientation at the head.

The resultant displacement is √(A² + B²), and the direction is arctg(B/A). In practice, you mainly look at axis A because that is what you compare with design values.

Cumulative curve vs incremental curve

The cumulative profile, or total displacement profile, is a plot in which each depth shows the sum of the displacements of all sections below it, and therefore the real shape of the deflected pipe relative to the anchor.

The incremental profile, or section displacement profile, is a plot in which each depth shows only the displacement of one section, Δ_i, without summation, so a local jump identifies the exact depth at which the ground is shearing.

The cumulative curve says “how much”, the incremental curve says “where”. A smooth arc on the cumulative curve and a flat incremental curve mean a wall that bends like a beam, which is expected and can be compared with calculations. A sharp kink on the cumulative curve and a spike on the incremental curve at one depth mean a slip surface. These are two different mechanisms and two different decisions.

Maximum deflection depth

Maximum deflection depth is the depth at which the cumulative profile reaches its greatest displacement value, and which you compare with the calculation model, for an excavation wall with struts or anchors the maximum usually occurs below the lowest support, not at the head. If the maximum moves downward as the excavation advances, that is expected. If it jumps to a place where nothing has changed, first check the data, then the ground.

On the profile, mark the excavation base level for the given phase, the support levels, and the top of the weak layer from the geotechnical record. Without that context, the profile is only a nice curve.

For you this means one thing: read the profile in pairs, cumulative and incremental, and always against the construction phase. A single curve without context leads to either a false alarm or false calm.

Typical pitfalls

Pipe rotation during installation. An inclinometer pipe with couplings can twist spirally along its length. As an illustration, on a 30 m pipe, a few degrees per coupling give a significant rotation at the bottom. Axis A at the bottom is then not the same axis A as at the top, and part of the movement escapes onto B. Protection: measure spiral rotation with a spiral probe after installation and before the zero reading, and with a chain, route the segments carefully and note groove orientation.

No zero reading, or zero reading taken too late. A baseline reading taken after excavation has started is not a zero reading. Everything that happened earlier disappears from the profile. Two or three zero readings before the works, and comparing them, is the minimum.

Temperature. MEMS sensors have temperature coefficients. In the ground at a depth of several metres, temperature is almost constant, but the upper segments and the logger are subject to daily and seasonal fluctuations; on the plot it looks like a tilt that “breathes” with the day-night rhythm. Before you call it movement, plot tilt and temperature on one chart. If the curves match, it is thermal behaviour, not geotechnics.

Damaged or not fully seated segment. One segment with a defect, a wheel outside the groove, a loose joint, or a moisture-affected sensor, gives a constant offset or noise at one depth which, when summed upward, shifts the entire cumulative curve above that point. On the incremental profile this appears immediately as a solitary spike that does not grow with time. That is why a chain is checked segment by segment, not only through the total.

Different probe, different cable, different person. The most common cause of “jumps” between probe sessions. The reading record, probe number, cable, operator, and pipe condition should be part of the protocol.

Threshold set on the wrong quantity. A threshold on maximum cumulative displacement is one thing. Assessing rate of increase, mm per day, is another and often more important. A stable value is not dangerous; a rising rate is. We explain how to set thresholds in the article on warning and alarm thresholds.

How to choose: probe, chain, or both

Let us name the concern directly. You are worried about two things at once: paying for chains that will draw a straight line for half a year, or saving on a probe and finding that the wall moved on Wednesday while your reading is on Friday. Both concerns are valid, and both can be controlled by decision, not hope.

Answer three questions:

1. How quickly do you need to know? If the monitoring plan requires a response in hours, for example stopping the excavation or adding a strut, the measurement must be more frequent than the possible state change. Eurocode 7, PN-EN 1997-1:2008, p. 2.7, observational method, requires the monitoring plan to reveal exceedances early enough for emergency actions to be taken, and the response time of the instruments and analysis procedures to be short enough relative to the possible evolution of the system. A weekly probe usually does not meet that condition. More in the observational method article.
2. How long does the sensitive phase last? Several weeks of excavation and anchoring is a different arithmetic from two years of slope observation after stabilisation.
3. How many boreholes do you have, and which ones decide? On an excavation, usually two to four boreholes are placed in the most loaded wall sections or in the area of the weak layer; the rest are for control.

The answer gives the pattern:

The regulation of 25 April 2012 on determining geotechnical conditions for the foundation of construction objects (§ 4) classifies excavations, retaining structures and ground anchors as at least geotechnical category II, and for categories II and III requires a geotechnical design which, according to § 10 point 10, defines the scope of necessary monitoring of the structure, neighbouring structures and surrounding ground. An excavation for several underground levels in dense urban development often meets the conditions of category III; the designer decides. That is where, not in the instrument order, the decision “probe or chain” is made: the design specifies the frequency and response time, and you choose the technique that will meet them.

Most often the mixed setup wins: chains on the decisive boreholes, a probe on the secondary ones and periodically in the pipes with chains as independent control. If both techniques show the same profile, you have the redundancy expected at elevated risk, and an argument for the board: you did not buy an expensive gadget, you bought response time where it is needed.

What this looks like in Inclify

Chain inclinometers are one of the sensor types selected and installed by the Inclify team, with more than 40 hardware types in the offer, and the platform calculates the profile itself. It takes the tilts from the segments, axis A and axis B, with axis Z used for quality control, knows the configured segment lengths, and calculates the displacement profile from the anchor upward, without a spreadsheet. After the first correct frame from the logger, it creates a dashboard with a vertical profile, A, B, or resultant, and a trend over time for each depth. The zero reading is the reference in the platform: you set it from the device or manually, and a change in pipe length or the addition of a segment creates a new, dated configuration version, so the history is always interpreted with the configuration valid for the measurement time.

You can set WARNING and ALARM thresholds, with hysteresis, on the segment channel, on head displacement, and on maximum resultant displacement. The depth at which that maximum occurs is read from the profile. OK / WARNING / ALARM / NO_DATA states are visible in the object list, SMS and e-mail go to authorised people, and the ticket keeps history: who acknowledged, who silenced, and until when. Tilt can be plotted on one chart with temperature so daily breathing can be separated from a lasting trend.

We say two things plainly. The platform does not import probe readings, so the probe profile must be compared with the platform profile outside the platform. There is also no separate alarm on the rate of increase: thresholds are thresholds on value, and the rate is assessed from the trend per depth and in the 7/30-day risk report. If you already have pipes with chains and a logger sending data over HTTP/JSON, connecting to the platform takes a few days. More on the inclinometric monitoring in Inclify page.

Sources and further reading

• PN-EN 1997-1:2008 Eurocode 7: Geotechnical design - Part 1: General rules (p. 2.7 observational method, chapter 4 monitoring) - sklep.pkn.pl. In the second generation of the standard (EN 1997-1:2024), the numbering of clauses is different.
• Regulation of the Minister of Transport, Construction and Maritime Economy of 25 April 2012 on determining geotechnical conditions for the foundation of construction objects (Journal of Laws 2012 item 463; § 4 geotechnical categories, § 10 point 10 scope of monitoring) - isap.sejm.gov.pl
• Borecka A., Stopkowicz A., Sekuła K., “Metoda obserwacyjna i monitoring geotechniczny w świetle przepisów prawa…”, Przegląd Geologiczny 2017, vol. 65, no. 10/2.
• Geokon - model 6180 Vertical In-Place Inclinometer and 6180 datasheet (segment lengths, resolution, precision).
• Geokon - 6000/6100 series inclinometer probe datasheets (system accuracy, 500 mm base).
• Sisgeo - in-place inclinometers (LT-Inclibus); Measurand - ShapeArray SAAV, ordering guide (segment lengths 250/500 mm).
• Dunnicliff J., Geotechnical Instrumentation for Monitoring Field Performance, Wiley, New York 1988 (1993 ed.) - monograph on geotechnical instruments, including inclinometers and probe errors.
• Inclify - Structural monitoring (SHM): complete guide
• Inclify - Deep excavation geotechnical monitoring: what to measure, where, how often

What next

If you are designing monitoring for an excavation or slope and wondering on which boreholes to place chains and where a probe is enough, do it before the zero reading, because a chain installed after works start has no baseline. The lowest entry point is one chain on a decisive borehole as a pilot, or connecting the pipes and loggers you already have, in a few days. We will show you the profile from the anchor, axis A/B, and maximum deflection on a live dashboard, on a site similar to yours. See the inclinometric monitoring in Inclify page or book a call and we will respond within 24 hours.