Strength Prediction of Underground Hard Rock Pillars: Effects of Geometric Ratio, complex and straightforward Discontinuities with different Dip Angles

Stability of hard rock pillars underground is significantly influenced by discontinuities and geometrical factors, but not much is understood about their coupled effects and interaction mechanisms. Stability design approaches do not have adequate tools for strength prediction, especially in presence of several discontinuities.Laboratory tests were carried out on cement-sand blocks having different width-to-height (W/H) ratios (0.5, 1.0, 2.0) with single and double discontinuities at various dip angles (0°, 30°, 45°, 60°, 90°). Test procedure included Uniaxial Compressive Strength (UCS) testing, ultrasonic wave velocity testing, numerical model verification, and statistical analysis for developing equations.Results indicate that wide pillars (W/H=2.0) demonstrated superior stability, retaining 55-82% of intact strength (26.67-48.22 MPa range), while slender pillars (W/H=0.5) showed higher sensitivity with 46-88% strength retention (13.78-26.67 MPa range). The 45° discontinuity orientation was consistently most critical, producing strength reductions of 29%, 44%, and 44% for wide, cubic, and slender pillars respectively. Double discontinuity configurations, particularly 30°-45° combinations, induced severe strength losses up to 54% in slender pillars (minimum strength:13.78 MPa), significantly exceeding single discontinuity effects. P-wave velocity reductions strongly correlated with strength degradation, reaching 14.4% and 15.1% for critical double discontinuity combinations in wide and slender pillars respectively. Two empirical equations were derived for strength prediction and showed excellent agreement with experimental data (R²= 0.949 and 0.924, respectively). The study offers validated pillar strength prediction tools using destructive and non-destructive approaches. The formulated equations and established relationships between discontinuity effects and geometry improve pillar design methodology, providing practical guidelines for underground mine operations.