Estimating elasticity modulus and uniaxial compressive strength of sandstone using indentation test

Elastic modulus (E) and uniaxial compressive strength (UCS) are very important engineering parameters that is required for many engineering projects. In some projects such as well drilling for oil production, deep underground tunnels in crushed zone, due to the depth of the well and fractures of rocks, there are some problems for obtained standard core samples. In this situations various methods have been developed for determining UCS and E using drilling cutting. Indentation test is one of these methods, in this method, an indenter penetrates into the stabilized samples in the surrounding material and then Critical Transition Force (CTF) and Indentation modulus (IM) determine. In the present study in order to developing the application of indentation testing for sandstone, and investigate the effect of particles size, shape and thickness, 14 sandstone blocks were collected from different formations in Iran and their physical characteristics, UCS and, E were measured using standard methods. After crushing the blocks and preparing 600 particles with dimensions of 3 × 3 × 3 mm3, 5 × 5 × 3 mm3, 5 × 5 × 5 mm3, 5 × 5 × 8 mm3, and 8 × 8 × 8 mm3 and irregular shape particles with a thickness of 5 mm, indentation tests were performed on each particle and CTF and IM were calculated for each particle separately. According to the tests carried out some empirical relationships were proposed between UCS - CTF (with R2 ≥ 0.86) and E - IM (with R2 ≥ 0.73) for different shapes and dimensions. Due to the differences in the dimensions of the produced particles, investigating the effect of surface and thickness of particle on the CTF and the IM revealed that the particle's surface had a considerable effect on the CTF and IM; however, the effect of thickness was negligible. Using the dimensionless surface parameter (S and Si), some empirical relationships between UCS - CTF/S (with R2 ≥ 0.80) and E - IM/Si (with R2 ≥ 0.77) were proposed for predicting UCS and E of sandstone particles with different shapes and dimensions. The accuracy of these relations was validated by performing standard uniaxial and indentation tests on 4 sandstone blocks and comparing measured UCS and E with those predicted using these relations. This comparison showed there is a good correlation between measured and predicted values. The proposed equations in this study were compared with results of previous studies. The relationship proposed in the present study to predict sandstone UCS has a similar accuracy with those proposed in by other researchers.