Estimation of uniaxial compressive strength of weak rocks using needle penetrometer

This is a preliminary study for estimating the uniaxial compressive strength (UCS) of weak rocks from the needle penetration resistance (NPR) obtained using the needle penetrometer, which is often used as non-destructive and portable testing method. In relationship between NPR and UCS, the UCS gradually increased until the NPR was less than 10 N/mm, then drastically increased from more than 10 N/mm. The NPR was widely distributed at the curing period of 3-day to 14-day, but was narrowly distributed at the curing period of 21-day to 28-day. In addition, the NPR of the curing period between 21-day and 28-day was nearly constant, but the UCS during those periods showed a big difference. In relationship between the estimated unaxial compressive strength and the measured uniaxial compressive strength, the gradient of UCS obtained from this study was situated in the upper limit of 1:1.7 and the lower limit of 1:0.6. Most of the values are closely located at the accurate estimation gradient of 1:1. As a result, it is considered that the UCS estimated from the NPR is reasonable enough.