Rock Properties & Testing

Rock strength and stiffness control every excavation. Learn uniaxial compressive strength (UCS), tensile strength (Brazilian test), Young's modulus and Poisson's ratio, the point load index test and its size correction, density and porosity, and the complete stress–strain curve of rock.

Rock ranges from weak shale (UCS < 25 MPa) to very strong quartzite and basalt (> 250 MPa). Laboratory tests on drill core give the parameters used in classification systems, pillar design, slope stability and blasting.

An analogy

Testing rock is like testing chocolate: squeeze it (compression), pull it apart (tension), and see how much it bends before snapping (stiffness).

Strength tests

UCS: a core with length/diameter ≈ 2.5 is loaded axially to failure. Brazilian test: a disc loaded across its diameter splits in tension (σ_t ≈ UCS/10 – UCS/20). Point load test: a quick field test between two cones — ideal for many samples.

Deformation

Stress–strain curves show crack closure, elastic region (E, ν), crack initiation (≈ 40 % UCS), crack damage (≈ 80 %), peak and post-peak softening. Brittle rocks lose strength suddenly; this stored energy drives rockbursts.

Triaxial strength

Confinement increases strength dramatically. Hoek–Brown criterion: σ₁ = σ₃ + σ_c (m_i σ₃/σ_c + 1)^0.5 for intact rock; extended with GSI for rock masses.

Scale and anisotropy

Larger samples are weaker (more flaws); foliated rocks (schist, shale) are weaker parallel to foliation. Rock-mass strength can be only 10–30 % of intact UCS because of joints.

Formulas

• UCS: σ_c = P / A
• Brazilian tensile: σ_t = 2P / (π D t)
• Point load index: I_s = P / D_e², I_s(50) = (D_e/50)^0.45 × I_s
• UCS estimate: σ_c ≈ 20–25 × I_s(50)
• Modulus: E = Δσ / Δε

Worked example: Point load test

Given: Failure load 10 kN; Equivalent diameter 54 mm

Find: I_s(50) and UCS estimate.

• I_s = 10 000/54² = 3.43 MPa
• F = (54/50)^0.45 = 1.035 → I_s(50) = 3.55 MPa
• UCS ≈ 24 × 3.55 = 85 MPa

Answer: ≈ 85 MPa (strong rock)

Worked example: Modulus from test data

Given: Stress rises 50 MPa while strain rises 0.001

Find: E.

• E = 50/0.001 = 50 000 MPa

Answer: 50 GPa

Common mistakes

• Wrong: Using intact UCS as rock-mass strength. — Right: Joints reduce strength — use classification-based rock-mass parameters.
• Wrong: Converting point load with a single universal factor. — Right: Calibrate the factor with UCS tests for each rock type.

Applications

• Geotechnical logging: Field strength estimates along drill core.
• Design inputs: UCS and E feed classification and numerical models.
• Rockburst assessment: Brittleness and stored energy.

Key takeaways

• UCS = P/A; tensile ≈ UCS/10–20.
• I_s(50) × 20–25 ≈ UCS.
• E from the linear stress–strain slope.
• Rock mass ≠ intact rock.

Going further

• Hoek–Brown criterion: Nonlinear strength envelope for rock.
• Post-peak behaviour: Class I vs Class II brittle rocks.
• Acoustic emission: Detecting crack initiation in lab tests.