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Rock Bolt Pull Test Calculator

Evaluates a rock-bolt or ground-anchor pull-out test by converting the applied (or failure) load into the average bond stress on the grouted interface. Geotechnical engineers, mining ground-control crews and drilling contractors use it to verify anchors on site against a required design load.

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How to use this calculator

Enter Values

Minimum 0

Minimum 0

Enter your values and press Calculate to see results.

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

  1. Enter the applied or failure load P (kN), the bolt/bar diameter d (mm) and the bonded (grouted) length L (m).
  2. Optionally enter the required design load P_req (kN) to get a factor of safety and a pass/fail verdict.
  3. Read the average bond stress τ (MPa) and, if provided, whether the test met the required load.

How it works

The test load is assumed to spread uniformly over the cylindrical grout–bar (or grout–rock) interface. That surface area is π·d·L, so the mean bond (shear) stress is τ = P / (π·d·L).

Working in kN, mm and m, the load's ×1000 (kN→N) and the length's ×1000 (m→mm) cancel exactly, so τ in MPa equals P[kN] / (π·d[mm]·L[m]). This is a MEAN value — real bond stress peaks near the collar and decays with depth. If a required design load is given, the factor of safety is the test load divided by that requirement.

Worked example

200 kN on a 25 mm bar over 1.5 m, required 150 kN. Bonded area = π × 25 × 1500 = 117,810 mm². τ = 200,000 / 117,810 = 1.698 MPa. Against a 150 kN requirement, FoS = 200 / 150 = 1.33 → Pass.

Common mistakes

  • Mixing units — the diameter must be in mm and the bonded length in m for the τ(MPa) = P/(π·d·L) shortcut to hold.
  • Using the total bolt length instead of the BONDED (grouted) length — only the bonded zone transfers load.
  • Treating the average bond stress as the peak; real bond concentrates near the collar, so design to the mean with an adequate factor of safety.

Frequently asked questions

Is this the average or peak bond stress?

The AVERAGE (mean) bond stress over the bonded length. Load transfer is highest near the top of the bond zone and falls with depth, so peak stress is higher than this average.

Should I use the bar diameter or the borehole diameter?

Use the diameter of the interface you are assessing: the bar diameter for grout–bar bond, or the borehole diameter for grout–rock bond. The tool uses whatever d you enter as the perimeter π·d.

What is a typical acceptable bond stress?

It depends on rock and grout — cement-grouted bars in competent rock commonly show a few MPa of allowable bond. Always test against the project-specific required load.

What load should I enter?

The peak load reached in the test — the failure (pull-out) load if the anchor failed, or the maximum proof/acceptance load held if it did not.

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Cite this tool

Zerdly. (2026). Rock Bolt Pull Test Calculator. Retrieved from https://www.zerdly.com/tools/rock-bolt-pull-test-calculator/

Tip: Enter any known values to calculate the remaining results.

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