Point Load Strength Index: Test & Formula

The point load strength index is a quick rock strength test that measures the load needed to break a rock sample between two pointed platens, reported as Is(50) in megapascals (MPa). You enter the failure load and the distance between the platens, and the calculator returns the uncorrected index and the size-corrected Is(50) value used in engineering classification.

Formula

The uncorrected point load strength index is calculated as:

Is = P / D²

• Is = point load strength index (MPa)
• P = failure load (kN, converted to N in the calculator)
• D = distance between the two platen contact points at failure (mm)

Because the index depends on sample size, the standard practice is to correct it to a reference diameter of 50 mm. The size correction factor recommended by the International Society for Rock Mechanics (ISRM) is (D/50)^0.45, giving the size-corrected index Is(50). This lets you compare results from cores of different diameters.

Worked Example

Using the default values from the calculator: P = 15 kN, D = 50 mm.

1. Convert load to newtons: 15 kN × 1000 = 15000 N
2. Compute D²: 50 × 50 = 2500 mm²
3. Uncorrected index: Is = 15000 / 2500 = 6.00 MPa
4. Size correction: (50/50)^0.45 = 1.00, so Is(50) = 6.00 MPa

The calculator shows 6.00 MPa for both outputs, matching the hand calculation.

How the Test Works

A rock sample—core, block, or irregular lump—is placed between two conical or spherical steel platens. Load is applied steadily until the sample fractures. The failure load and the distance between the platen contact points at failure are recorded. The test is popular because it requires minimal sample preparation and can be done in the field with a portable apparatus.

Samples can be tested in three orientations:

• Diametral: load applied across the diameter of a core (most common)
• Axial: load applied along the core axis
• Irregular lump: any roughly equidimensional piece

For valid results, the sample should break along a single plane that passes through both loading points. If the fracture is irregular or passes through an obvious flaw, the test is discarded.

Why Size Correction Matters

Larger samples generally give lower point load strength values because they contain more microcracks and defects. Without correction, a 40 mm core would give a higher index than a 70 mm core of the same rock, even though the rock is identical. The size correction normalizes all results to a 50 mm reference, making it possible to compare tests from different projects and laboratories.

The correction exponent 0.45 is the ISRM-recommended value based on extensive testing. Some older standards use 0.5, which gives slightly different results for samples far from 50 mm. Always note which correction was applied when reporting results.

How to Read the Result

Is(50) values are used to estimate uniaxial compressive strength (UCS) and to classify rock hardness. A common empirical relationship is:

UCS ≈ 20–25 × Is(50)

This is only an estimate; the actual multiplier varies with rock type. For example:

• Weak shale: Is(50) ≈ 0.5–2 MPa
• Sandstone: Is(50) ≈ 2–6 MPa
• Granite: Is(50) ≈ 6–12 MPa
• Very strong quartzite: Is(50) > 12 MPa

These ranges are rough guides. The point load test is best used for classification and correlation, not as a substitute for direct UCS tests on critical projects.

Common Mistakes and Limitations

• Unit errors: Using kilonewtons instead of newtons, or millimeters instead of meters, leads to wildly wrong results. Always convert to N and mm.
• Ignoring size correction: Reporting uncorrected Is when comparing samples of different sizes is invalid.
• Testing wet vs. dry: Saturation reduces strength significantly. Report the moisture condition.
• Anisotropic rocks: Schists, slates, and gneisses give different results depending on orientation relative to foliation. Test in multiple directions.
• Broken or flawed samples: Discard tests where the fracture does not pass through both loading points.

The test is not suitable for very soft or very hard rocks outside the typical range, and it does not replace a full laboratory UCS suite for design purposes.

Frequently Asked Questions

What is the difference between Is and Is(50)?

Is is the raw index calculated from the failure load and platen distance. Is(50) is the size-corrected value normalized to a 50 mm diameter. Always use Is(50) when comparing different samples.

Can I use this test for all rock types?

It works for most hard and soft rocks, but very weak rocks (like some clays) may deform instead of fracture, and very hard rocks may cause platen damage. The test is best for rocks with UCS between about 1 and 100 MPa.

How many tests should I do?

ISRM recommends at least 10 valid tests per rock type, with results averaged. For anisotropic rocks, test both parallel and perpendicular to foliation.

What is the relationship to uniaxial compressive strength?

A common estimate is UCS ≈ 24 × Is(50), but the multiplier ranges from about 15 to 30 depending on rock type. It is a screening tool, not a precise prediction.

Can I use the point load index for foundation design?

Yes, as a preliminary site characterization tool, but final design should use direct UCS tests and other site-specific data. The point load test is too approximate for critical load-bearing calculations.

Why does my result seem too high or too low?

Check that you entered the load in kN and the distance in mm. Also verify the sample broke cleanly and that you used the correct size correction. Moisture content and rock anisotropy can also shift results significantly.

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in P | Failure load | kN | 15 | 0
in D | Distance between platens | mm | 50 | 0
out Is | Uncorrected index Is | MPa | (P * 1000) / (D * D) | 2
out Is50 | Size-corrected Is(50) | MPa | ((P * 1000) / (D * D)) * (D / 50)^0.45 | 2