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<p>The Point Load Index (PLI) test has always been attractive in the indirect determination of rock strength. Unlike uniaxial compression tests, this test becomes even more important for the anisotropic rocks as it won ’t require extensive specimen preparation and costly test setup. Although the PLI test has become an important tool for assessing the strength of isotropic rocks, its applicability to anisotropic rocks remains inadequately evaluated. This is primarily due to challenges in achieving the accurate specimen geometry (Length/Diameter "t/d" ratio), resulting in inaccurate strength measurements and invalid failure modes. Addressing this issue, an experimental investigation has been carried out to investigate the effects of specimen geometry (t/d ratio) on the I<sub>S(50)</sub> value of the Srinagar/Chandpur phyllite across various foliation angles (β) using an axial PLI test. The results indicate that geometry has a significant impact on the strength and failure mode of phyllite. More specifically, specimens with t/d ratios of 1.0 and 0.8 exhibited increasing strength as the β angle increased up to 60°; thereafter, strength declined with further increases in β. For these two geometries, valid failure modes occurred only up to β = 30°; beyond that, failure became invalid with larger β angles. In contrast, specimens with a t/d ratio of 0.6 showed a continuous increase in strength from β = 0° to 90°, consistently exhibiting valid failure modes across the entire β. The mathematical analyses (empirical modelling, statistical validation, and Student ’s t-test &amp; F-test) further validated the experimental results. The experimental results and statistical validations demonstrated that a specimen having t/d ≈ 0.6 is a preferred geometry for PLI testing of anisotropic phyllite rock masses. Additionally, a combined empirical equation incorporating both β and t/d was developed. The negative interaction of the equation reveals that geometry effects become increasingly critical at higher foliation angles. This experimental investigation with analytical validation for understanding the effect of specimen geometry (t/d ratio) in axial PLI testing of anisotropic rocks is crucial for the design, analysis, and safety evaluation of engineering structures in complex geological formations where anisotropy plays a significant role.</p>

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Influence of Specimen Geometry on the Point Load Strength Anisotropy of Phyllite

  • 期刊论文

  • 发表日期：

    2025年10月21日

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  • Divyanshoo Singh,
  • Kumar Nilankar,
  • Bikash Kumar Ram,
  • Hemant Kumar Singh,
  • Akash Sharma,
  • Pranjal Srivastava

摘要

The Point Load Index (PLI) test has always been attractive in the indirect determination of rock strength. Unlike uniaxial compression tests, this test becomes even more important for the anisotropic rocks as it won ’t require extensive specimen preparation and costly test setup. Although the PLI test has become an important tool for assessing the strength of isotropic rocks, its applicability to anisotropic rocks remains inadequately evaluated. This is primarily due to challenges in achieving the accurate specimen geometry (Length/Diameter "t/d" ratio), resulting in inaccurate strength measurements and invalid failure modes. Addressing this issue, an experimental investigation has been carried out to investigate the effects of specimen geometry (t/d ratio) on the IS(50) value of the Srinagar/Chandpur phyllite across various foliation angles (β) using an axial PLI test. The results indicate that geometry has a significant impact on the strength and failure mode of phyllite. More specifically, specimens with t/d ratios of 1.0 and 0.8 exhibited increasing strength as the β angle increased up to 60°; thereafter, strength declined with further increases in β. For these two geometries, valid failure modes occurred only up to β = 30°; beyond that, failure became invalid with larger β angles. In contrast, specimens with a t/d ratio of 0.6 showed a continuous increase in strength from β = 0° to 90°, consistently exhibiting valid failure modes across the entire β. The mathematical analyses (empirical modelling, statistical validation, and Student ’s t-test & F-test) further validated the experimental results. The experimental results and statistical validations demonstrated that a specimen having t/d ≈ 0.6 is a preferred geometry for PLI testing of anisotropic phyllite rock masses. Additionally, a combined empirical equation incorporating both β and t/d was developed. The negative interaction of the equation reveals that geometry effects become increasingly critical at higher foliation angles. This experimental investigation with analytical validation for understanding the effect of specimen geometry (t/d ratio) in axial PLI testing of anisotropic rocks is crucial for the design, analysis, and safety evaluation of engineering structures in complex geological formations where anisotropy plays a significant role.