Geomechanics and Engineering

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Determining N value from SPT blows for 30 cm penetration in weathered strata

  • Sun, Chang-Guk (Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources) ;
  • Cho, Hyung-Ik (Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources) ;
  • Kim, Han-Saem (Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources) ;
  • Lee, Moon-Gyo (Earthquake Research Center, Korea Institute of Geoscience and Mineral Resources)
  • Received : 2021.12.02
  • Accepted : 2022.01.24
  • Published : 2022.03.25
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Abstract

The standard penetration test (SPT) obtaining the N value of the number of blows has been widely used in various subsurface conditions, including in weathered soil and rock on fresh bedrock, in geotechnical studies pertaining to the design of foundations and earth structures. This study examined the applicability of SPTs terminated conventionally after 50 blows for a penetration of less than 30 cm, particularly in weathered strata, at four sites in Korea. The N values obtained during practical SPTs are typically extrapolated linearly at 30 cm penetration, despite the possibility of a nonlinear relationship between blow counts and penetration. Such nonlinearity in weathered strata has been verified by performing special SPTs ensuring 30 cm penetration. To quantify the nonlinearity in dense strata, we conducted statistical regression analyses comparing the differences (DN) between the N values measured by the special SPTs and those extrapolated using the practical approach with the differences (DP) between the 30 cm penetration and the penetration during 50 blows. Bi-linear relationship models between DN and DP were subsequently proposed for determining the N values at 30 cm penetration in weathered strata. The N values reflecting nonlinearity could be determined from the linearly extrapolated N values by adding a modeled DN value.

Keywords

Acknowledgement

This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM).

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