Reliability assessment of Needle Penetration Index for estimating compressive strength of some sedimentary rocks from the Qom Formation, Central Iran

In this paper, the needle penetrometer test was utilized to explore the reliability of the Needle Penetration Index (NPI) for estimating the Unconfined Compressive Strength (UCS) of sedimentary rocks including gypsum, marl, siltstone and sandstone collected from the Qom Formation. Following the UCS and NP test, regression analyses were carried out to control the predictive performances of NPI. Statistic performance indices such as determination coefficient (R2) the Root Mean Square Error (RMSE) and Variance Account For (VAF) are calculated. Regression analyses suggest meaningful relationships between UCS and NPI, for gypsum, marl, siltstone and all rock types as whole. The results are reasonably meaningful, excluding for sandstone which NPI does match with values of UCS prediction. As NPI could only penetrate to a maximum of 10 mm depth, it is not representative of intrinsic properties (e.g. texture and mineralogy) that control their UCS. Cross-plot of UCS/NPI vs. UCS and regression analysis was carried out to overcome this uncertainty. Results obtained from this purposed approach suggest that it is more reliable than those achieved from UCS vs. NPI. The derived equations are in good corresponding with those suggested by other researchers. Additionally, control data test was applied to make sure their validation.