Uniaxial and triaxial compression
- In uniaxial compression, a cylindrical specimen is loaded axially without lateral confinement to study strength and stress-strain response.
- In triaxial compression, a cell adds lateral pressure; repeating the test at different confinements shows how strength changes.
Preparation and instrumentation
- Use specimens with dimensions, faces, and parallelism that meet the method; record lithology, orientation, defects, and moisture.
- Measure load and axial strain; strain gauges or other local sensors can provide radial strain and Poisson’s ratio.
- For triaxial tests, check the sleeve seal, cell alignment, and stability of confining pressure.
Failure and envelope
- Identify peak strength and strain; where control permits, observe post-peak response or residual strength.
- A series of confining pressures can define failure points and estimate a strength envelope within the tested range.
- The Brazilian test loads a disc diametrically to estimate indirect tensile strength; geometry and contact must meet the procedure.
Safety and application
- Rock may fail brittly and eject fragments. Install guards and keep people outside the projection zone.
- Interpret results alongside discontinuities and rock-mass scale; intact-rock strength is not automatically rock-mass strength.
Intact-rock strength and strain
- Uniaxial compressive strength: σc = Fmax/A0. Use the initial cross-sectional area and record length-to-diameter ratio and failure mode.
- Axial strain: εa = ΔL/L0. Secant modulus E50 = (σ50−σ0)/(ε50−ε0) is calculated between points defined by the procedure; tangent modulus uses the local slope.
- Poisson’s ratio: ν = −εlateral/εaxial, with strains measured over the same stress interval.
- For triaxial testing, plot major principal stress σ1 at failure against confinement σ3. Do not extrapolate an envelope beyond the tested range or convert intact-rock strength into rock-mass strength without modelling discontinuities.
- For a Brazilian disk test, indirect tensile strength is estimated as σt = 2Fmax/(π·D·t), with diameter D and thickness t; check applicable geometry and contact criteria.
Educational reference guide. Check the current standard, calculation assumptions, and project specification before applying a procedure or acceptance criterion.