New method for estimating unconfined compressive strength (UCS) using small rock samples

In restricted sampling conditions such as oil well drilling, appropriate rock samples do not exist for measuring unconfined compressive strength (UCS) using conventional standard methods during the excavation. Loading on small rock samples is one of the new feasible methods for assessment of UCS in restricted sampling conditions. In this paper 510 small rock samples of three types of sedimentary rocks (micritic limestone, crystalline limestone and sandstone) are tested in three different sizes (3-5 mm). All samples were loaded with flat ended cylindrical indenters (1 mm diameter) which can apply load on two parallel faces of cubic shaped small rock samples. The rate of displacement in performing load on samples was 1 mm per minute so it can be said that the condition of loading was quasi-static. The test was named as modified point load (MPL) and the obtained results from the MPL test introduced as modified point load force (MPLF). Obtained MPLF from different sizes of small rock samples was correlated with UCS. The high values of coefficient of determination (R2≥0.9) indicate the accuracy of empirical relations. Each group of small rock samples with different sizes had different empirical relations because of the dimension effect of small rock samples. Dimension effect of obtained MPLF is omitted by exchanging the values to tensile stress (σMPL). The general empirical relation is generated by establishing the UCS-σMPL correlation (R2=0.91). The accuracy of empirical relation was evaluated by assessment of UCS from obtained MPLF values. The estimated UCS values comparatively have 89% conformity with the measured UCS. Therefore, the defined empirical relations have adequate accuracy in estimating UCS from small rock samples.