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PLT to UCS Correlation Calculator

Simple calculator to convert axial point load strength index (Is(50)) results to uniaxial compressive strength (UCS) using a correlation factor.

🧮 Calculator

🔍 What is the PLT to UCS Correlation Calculator?

This handy calculator takes your axial Point Load Test (PLT) results—often reported as Is(50)—and converts them into an estimated Uniaxial Compressive Strength (UCS). It uses a straightforward correlation factor to bridge the gap between quick field tests and traditional lab-based UCS measurements.

⏱️ Why and When to Use It

PLT is a brilliant on-the-spot test for rock strength, particularly when you need a rapid check without hauling cores back to the lab for full UCS testing. The correlation gives you a ballpark UCS figure—ideal for early design decisions, ongoing site evaluations, or when you simply don't have enough time or budget for lab work.

Our free tool does the maths for you, meaning you can pop in your Is(50) results and walk away with a concise UCS estimate in seconds, no complicated spreadsheets required.

📊 Technical Background

The correlation between Point Load Strength Index and UCS is typically expressed as:

UCS = K × Is(50)

Where:

UCS = Uniaxial Compressive Strength in MPa

Is(50) = Point Load Strength Index (size-corrected to 50mm) in MPa

K = Correlation factor (typically between 20-25 for most rock types)

The correlation factor (K) can vary depending on rock type, with sedimentary rocks typically having lower values than igneous or metamorphic rocks. For general purposes, a value of 24 is often used when specific correlations for the rock type are not available.

Key Benefits

  • Field Expedience: Get strength estimates directly from field testing
  • Cost Efficiency: Reduce reliance on expensive laboratory UCS testing
  • Quick Results: Make timely decisions with immediate strength data
  • Sample Conservation: Preserve core samples for other testing needs
  • Versatility: Apply to various rock types with appropriate correlation factors

🏗️ Applications

The PLT to UCS Correlation Calculator is particularly valuable for:

Mining & Tunneling

  • Excavation stability assessment
  • Support system design
  • Rock mass classification
  • Blast design optimization

Geotechnical Engineering

  • Foundation design
  • Slope stability analysis
  • Site investigation
  • Material characterization

📝 Best Practices

For the most reliable results when using PLT to UCS correlations:

  • Establish site-specific correlations by conducting both PLT and UCS tests on samples from your project area
  • Use the axial test method when possible, as it typically provides more consistent results than the diametral method
  • Perform multiple tests and use the average value to account for rock heterogeneity
  • Apply the size correction to standardize results to the equivalent 50mm diameter (Is(50))
  • Consider the rock type when selecting an appropriate correlation factor

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