Crack Displacement Calculator
Turns two crack-gauge or tape-extensometer readings into a signed displacement and, if you enter the elapsed time, a movement rate in mm/day and mm/week. Used by geotechnical engineers, mine geotechs and monitoring technicians to track opening or closing of tension cracks against TARP trigger levels.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the initial reading r0 (mm) and the current reading r1 (mm) from the same gauge.
- Optionally enter the elapsed time Δt in days to get the displacement rate.
- Read the displacement (positive = opening, negative = closing) and the rate, then compare against your TARP triggers.
How it works
Displacement is the change in gauge reading: displacement = r1 − r0. The sign carries the meaning — positive means the crack has opened (widened), negative means it has closed (narrowed).
When an elapsed time Δt is supplied, the rate is displacement ÷ Δt (mm/day), and the weekly rate is that value × 7. A steady or accelerating opening rate is one of the clearest field warnings of developing slope instability.
Worked example
Tension crack opening 3 mm in two weeks. A crack gauge reads r0 = 10 mm, then r1 = 13 mm fourteen days later. Displacement = +3 mm (opening). Rate = 3 ÷ 14 = 0.214 mm/day, or 1.5 mm/week.
Common mistakes
- Swapping r0 and r1, which flips the sign and makes an opening look like a closing.
- Mixing gauges or reference points — r0 and r1 must be the same instrument at the same location.
- Reading a single displacement in isolation instead of trending the rate over several readings.
Frequently asked questions
Does a positive displacement mean the crack is opening?
Yes. Displacement = r1 − r0, so positive means widened (opened) and negative means narrowed (closed).
Do I have to enter the elapsed time?
No. Δt is optional — leave it blank for just the displacement, or enter it (in days) to also get the rate in mm/day and mm/week.
What units does it use?
Readings and displacement are in millimetres; the rate is in mm/day and mm/week. Keep both readings in the same unit.
Why is the movement rate important?
An increasing or accelerating opening rate is a leading indicator of slope failure, compared against pre-set TARP trigger levels to decide on inspection, slow-down or evacuation.
Can I use this for a tape extensometer?
Yes. Any two comparable readings from a crack gauge, pin array or tape extensometer work — enter the earlier reading as r0 and the later as r1.
Related tools
- Prism Movement Rate Calculator
- Prism Movement TARP Checker
- Radar Prism Displacement Comparison Tool
- Piezometer Trend Calculator
- Cumulative Movement Calculator
- Rainfall Trigger Checker
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Tip: Enter any known values to calculate the remaining results.
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