Estimation of Unconfined Compressive Strength (UCS) of Carbonate Rocks by Index Mechanical Tests and Specimen Size Properties: Central Alborz Zone of Iran

Central Alborz Zone (CAZ) is one of the most important zones in Iran, which comprises a wide variety of carbonate rocks such as limestone, dolomite, and their compounds. Therefore, determining engineering geological characteristics of carbonate rocks is essential for proper use in civil, mining, and geotechnical engineering. The purpose of this work was to investigate the relationships between the uniaxial compressive strength (UCS) and the point load index (PLI), Schmidt hammer rebound (SHR) value, and Brazilian tensile strength (BTS). In addition, the UCS value was predicted based on the young modulus (UCS-E) and sample size factors in terms of length, length to diameter (L/D), and volume. In this study, samples were collected from the type section of 12 formations of the CAZ, and all of them were classified based on the petrographic study as carbonate rocks (Limestone, dolostone, etc.). The results showed significant positive linear equations within the 95% prediction bands for correlating UCS to PLI, BTS, SHR, and E. A conversion factor of carbonate rocks in the linear regression without intercept for UCS-PLI, UCS-BTS, UCS-SHR, and UCS-E was observed as 14.32, 7.26, 1.71, and 3.52, respectively. This study concentrated on the carbonate rocks; therefore, the results compared to previous studies were conducted on carbonate rocks. Also, the relation between the modulus ratio (M-R), UCS, and maximum axial strain (epsilon(a max)) studied for all rock samples and results showed that the UCS has a partial effect on M-R and epsilon(a max) has a strong correlation with M-R. Concerning the sample size, results showed that with an increase in the length, L/D, and volume of the specimens, the UCS values are decreased. This reduction is because of an increase of internal defects (increase in length and diameter), end effects, and lateral deformation. The decrease of the UCS value is more sharply in the L/D ratios between 2 and 2.5, and for more than 2.5, the reduction rate is much lower.

Automated summary

The study investigated the engineering geological characteristics of carbonate rocks in the Central Alborz Zone in Iran, finding significant positive linear relationships between UCS and PLI, BTS, SHR, and E. Additionally, UCS had a partial effect on M-R and epsilon(a max) showed a strong correlation with M-R. Increasing specimen size led to decreased UCS values, with a sharper reduction in L/D ratios between 2 and 2.5.