Paper contribution

The effect of confining pressure on the needle penetration index for three different pyroclastic rocks

Sair Kahraman; Anıl Kaya

Proceedings
EUROCK 2026
Pages
587-592
Submitted
28 January 2026
Accepted
15 June 2026
Published
12 August 2026

Summary

Abstract

The needle penetration test is a non-destructive index test that is applicable in both the field and in the laboratory and requires no special preparation of the sample. The test was used to estimate the physical and mechanical properties of soft or hard rock. Several researchers have used the needle penetration index to estimate the uniaxial tensile strength of rock. Several researchers also studied the relationship between needle penetration index and other rock parameters, such as the Brazilian tensile strength, elasticity and seismic velocities, cohesion and the angle of internal friction. However, there is no study in the literature about the effect of confining pressure on the needle penetration index. In underground rock engineering applications, the effect of confining pressure on the properties of the rock is of considerable importance. This study investigates the influence of confining pressure on the needle penetration index for pyroclastic rocks. Block samples of pyroclastic rock from three different locations and core samples were taken. Smooth-cut core samples were put into the Hoek cell and needle penetration index tests were carried out at the different confining stress levels such as 1, 2, 3 and 4 MPa. At least three samples were tested at each stress level. The uniaxial compressive strength values were estimated from the needle penetration index. The results were evaluated using regression analysis. It was seen that both the needle penetration index and uniaxial compressive strength values were increased significantly with increasing confining pressure. The increments in both the needle penetration index and uniaxial compressive strength values were about 100 % for the stress level of 4 MPa compared to the unconfined values. It was concluded that the derived equations could be used for the estimation of both the needle penetration index and uniaxial compressive strength values for the different confining stress levels.

Contributors

Authors

  1. Sair Kahraman Hacette University, Turkiye ORCID 0000-0001-7903-143X
  2. Anıl Kaya Hacettepe University

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References

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