Strength measurement of rock requires testing that must be carried out on test specimens with particular sizes in order to fulfill testing standards or suggested methods. Often, the coring process breaks up the weaker core pieces, and they are too small to be used in either index tests or conventional strength tests such as point load index (Is) and Brazilian tensile strength (BTS). One of the index tests to indirectly determine the rock strength is the block punch index (BPI) test, which requires flat disc specimens without special treatment. This study aimed to evaluate the applicability of the BPI test for predicting the uniaxial compressive strength (UCS), BTS and IS of the sandstones by empirical equations. Also, we have compared the performance of the BPI and IS for predicting the UCS and BTS. It was experimentally shown that BPI is a reliable method for predicting the UCS, BTS and Is of the sandstones under study. Moreover, the results indicate that BPI could be utilized with same importance as Is for predicting the UCS, while predicting the BTS by Is appears to be more reliable than BPI.

Introduction Determination of the mechanical properties of rock materials has been remained as a challenge for engineering geologists. In-situ tests are rarely used to determine the mechanical properties of rocks due to difficulties in sample preparation, performing and interpretation of the results, high costs as well as the required long time for doing the experiments. The common approach to determine the mechanical properties of rock materials is through conducting laboratory experiments and estimation the in-situ properties based on these laboratory results. This approximation, which is called scale effect, has been remained as a challenge for engineering geologists and practical rock engineers for decades...

Nowadays, the need to enhance the exploitation of hydrocarbon resources due to importance and increase in the rate of consumption of this material is more crucial than ever. On the other hand, the rock formations with low effective porosity, which are considered as hydrocarbon reservoirs prevented to have a suitable productivity of these reservoirs. Hydraulic fracturing is known as a process of initiation and propagation of fractures caused by fluid injection into the part of the boreholes in rock formations, which has an important role in increasing of the exploitation of hydrocarbon reservoirs with low efficiency. Initiation and propagation fractures have a high dependency to the parameters and properties of reservoir rocks. Among these parameters can mention, such as rock material, rock tensile strength, pore pressure and etc. Importance tensile strength of rock for predicting the breakdown pressure of hydraulic fracturing test is remarkable and this parameter is a function of lithology of reservoir rock. Hence, in this study with sampling at typical section of Asmari formation, evaluation of breakdown pressure parameter as a function of rock tensile strength and lithology parameters has been paid. The ratio of hydraulic fracturing tensile strength to Brazilian test tensile strength and Point load test allows prediction of the tensile strength and is an important parameter for simulation of hydraulic fracturing test results. Comparison of these ratios shows that the tensile strength parameter is dependent on the test method. Rock tensile strength that obtained from hydraulic fracturing test is 20.08 MPa and proportion between tensile strength of hydraulic fracturing test to Brazilian test is equal to 2.35. Slight differences in the distribution of data and regression coefficient 0.9 are due to the small karst cavities, relatively low porosity and intragranular voids because of fossils in the rock structure. All evidences, including the results of physical and mechanical properties reveal the influence of lithology on the results of tensile strength obtained from hydraulic fracturing test.

Brazilian tensile strength (BTS) is an important parameter in mining activities, particularly in conditions that rocks are under tensile stresses. This test measures the indirect tensile strength of rocks, which is crucial for understanding the mechanical behavior and quality of rocks in the mining context, including slope stability analysis, blast design, rock support systems, excavation and equipment selection, fracture propagation, and hydraulic fracturing and drilling. So far, no classification of tensile strength of rock for mining applications has been presented. In the present study, a new rock classification based on BTS for the various rocks was proposed. To achieve this purpose, by a reviewing previous studies, uniaxial compressive strength (UCS) and BTS of various rock classes, including igneous, sedimentary, and metamorphic were collected. For each rock class, the correlation equations between UCS and BTS were developed using simple regression analysis. Using data analyses, the rocks was categorized into to seven BTS classes. The findings revealed that igneous, sedimentary, and metamorphic rocks have a wide range of BTS values, and subsequent fall into the different BTS classes. The validity of BTS classification was verified using data of BTS and UCS of various rock classes published in the literature, and results showed that BTS can be as a suitable indicator for preliminary assessment of rock quality. This can lead to a better understand from the strength behavior of the rock under tensile stresses in site a mining activity, and therefore, a more accurate design of a mining project.

Rock anisotropy plays an important role in engineering behavior of rocks. Slates are anisotropic rocks which have long been used for gable roof, floor tiles, borrow materials, and other purposes. The slates studied in this research are calcareous and have a porphyro-lepidoblastic texture. To determine the role of the anisotropy on the tensile strength and fracture pattern, two variables including Y (the core axis angle to foliation) and b(the angle between the axis of loading and foliation) in the Brazilian tests were used. The angles were selected at 15o intervals.Thus, for both Y and b, seven angles of 0o, 15o, 30o, 45o, 60o, 75o, and 90o were selected (i.e., there are 43 possible modes). In order to name and examine the failure pattern, 11 models were proposed. The average value of the failure strength for the three stations varies from 3.21 MPa to 20.94 MPa. Based on the obtained results, there is a direct relation between the average tensile strength and density. A comparison between Brazilian test data under dry and saturation conditions shows that the saturated Brazilian tensile strength is 30.8% less than the dry Brazilian tensile strength. Moreover, the changes in fracture length with thechanges inψ and b indicate an inverse relation. Eventually, the average of tensile strength(st) and strength anisotropy index (Ia) demonstrates thatthe influence of orientation angle (Y) is much larger than that of foliation-loading angle (b).

This study deal with the effect of anisotropy on strength behavior and failure patterns of Borujerd area slates under Brazilian test conditions. For this purpose, 6 rock blocks were taken from different locations in north and east of Borujerd. After sample preparation and assessment of mineralogical and physical properties, the Brazilian tensile strength test was performed on the disc-shaped samples in 7 anisotropy angles (between the loading direction and anisotropy plane) including 0, 15, 30, 45, 60, 75 and 90° . The results indicate that the maximum BTS is observed at β = 90° for all samples. While, the lowest value of BTS corresponds to anisotropy angles (β ) equal to 15° and 30° . The observation of failure patterns of samples after Brazilian test shows that there are three types of dominant failure patterns including parallel to the anisotropy (PA), across the anisotropy (AA) and curved fracture (CF). Transitional angle, which indicates the change in dominant failure pattern from parallel to the anisotropy to across the anisotropy, were also determined. Finally, the relationship between fractures length and Brazilian tensile strength was determined for selected slates.

The most important criteria needed for the investigation and characterization of a rock mass on site in a geotechnical project are its uniaxial compressive strength (UCS) and tensile strength (TS). The UCS and TS of rocks are determined directly by complex laboratory or field tests that require specialized prepared samples and equipment. Therefore, the UCS and TS of rocks are estimated through several index parameters via regression analysis. The point load index (PLI) due to its simplicity and quickness is a common parameter for estimating the UCS and TS of rocks. In this study, data mining tools are used to estimate the UCS and TS [determined through the Brazilian tensile strength (BTS) test] of rock using PLI. The statistical parameters, including mean absolute error (MAE), root mean squared error (RMSE), and correlation coefficient (r), are used to evaluate the performance of each data mining tool. The validity and accuracy of platforms' data mining tools were verified according to the statistical parameters. The results indicated that all three platforms' data mining tools exhibited remarkable ability to predict UCS and BTS using PLI. Finally, using platforms' data mining tools obviates the need to perform the UCS and BTS tests as time-consuming and laborious efforts.