Evaluation of Strength and Deformability of Soft Sedimentary Rocks in Dry and Saturated Conditions Through Needle Penetration and Point Load Tests: A Comparative Study


The preparation of standardized soft rock specimens to perform unconfined compressive strength (UCS) tests is typically difficult, expensive and time-consuming. Needle penetration test (NPT) was originally developed in Japan as an alternative for the indirect estimation of UCS of soft rocks. The needle penetrometer is a simple, portable and non-destructive testing device that measures applied load and penetration depth for the rock to calculate the needle penetration index (NPI). A complimentary, portable and widely used destructive test is the point load test (PLT), which measures regular and irregular specimens by the application of a concentrated load using two coaxial conical platens that yield the point load strength index (IS(50)). We investigated and compared the NPT and PLT in terms of measuring changes induced by water saturation and obtaining UCS and the static Young’s modulus (Est) for dry and saturated soft sedimentary rocks. The results point to significant correlation functions from which to infer UCS and Est in terms of NPI and IS(50) in dry and saturated soft rocks. Furthermore, both NPT and PLT are suitable tests for evaluating changes in strength and deformability induced by water saturation. We also found a good correlation between the NPI and Is(50).

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E st :

Static Young’s modulus

I s(50) :

Point load strength index

k :

Ratio between UCS and Is(50)

F :

Applied load in the NPT

D :

Penetration depth in the NPT

ρ d :

Dry density

ρ sat :

Saturated density

p o :

Open porosity

p :

Total porosity

W a :

Water absorption

χ :

Mean value

σ :

Standard deviation

R :

Coefficient of correlation

R 2 :

Coefficient of determination






Needle penetration index


Needle penetration test


Point load test


Unconfined compressive strength


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This research was supported by the Vice-rector of Research and Knowledge Transfer of the University of Alicante through predoctoral grant FPUUA53-2018 and projects UAUSTI18-21 and UAEEBB2018-09. The authors thank the Earth Sciences Department of the University of Alicante for allowing them to perform the thin-section analyses in their laboratories. Ailish M. J. Maher is gratefully acknowledged for the language editing of a version of this manuscript.

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Rabat, Á., Cano, M., Tomás, R. et al. Evaluation of Strength and Deformability of Soft Sedimentary Rocks in Dry and Saturated Conditions Through Needle Penetration and Point Load Tests: A Comparative Study. Rock Mech Rock Eng 53, 2707–2726 (2020). https://doi.org/10.1007/s00603-020-02067-6

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  • Needle penetration test
  • Dry and saturated conditions
  • Soft sedimentary rocks
  • Strength and deformability
  • Point load test