An investigation on the relationship between rock strength and brittleness

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Brittleness is one of the most important engineering properties of the rocks. However, there is no available economical and practical direct method for measuring the brittleness feature of rock. Therefore, brittleness is commonly calculated as a function of rock strength. The purpose of this study is to investigate both the relationship between rock strengths -including the uniaxial compressive strength (UCS), the tensile strength (BTS) by an indirect method of Brazilian test- and rock brittleness measured in the laboratory using the punch penetration test. In present, the ratio of UCS and BTS could be used for estimating rock toughness and brittleness. In this study, 3 different brittleness indices were generated. Brittleness numbers; B1 is a ratio the ratio of UCS to BTS, B2 is the ratio of UCS minus BTS to UCS plus BTS and B3 is a brittleness value obtained from the punch penetration test. In order to investigate an empirical relationship between rock strength and brittleness, the rock samples were collected from 32 tunnel projects that were excavated throughout the world and database was established. A total of more than 1000 samples were prepared and tested for identifying rock strengths and brittleness of different rock types. In the database, rock strength tests were conducted on the collected samples according to the ASTM standards, however, brittleness test, which is also called as punch penetration test, was performed according to the standards used in industry.

The database was used for establishing relationships between the brittleness values B1, B2, and B3 and obtaining as new equation to estimate rock brittleness (B4) empirically. One of the commercial software package for standard statistical analysis was used to perform the multiple variable regressions among the parameters. In fact the relationship achieved between the variables is linear functions. In other words, the program finds the best-fit regression between the parameters in a linear combination. So, a new variable B4 can be empirically defined as a function of original parameter, as follows:

B4 = f(UCS, BTS) = A.UCS + B.BTS

In the above equation, B4 is an obtained rock brittleness index. UCS and BTS are independent variables (strengths); A and B are the calculated coefficients. As a result, it is concluded that reasonable relationship exists between the rock brittleness values and strengths. The rock brittleness index (B4) could be estimated as a function of commonly measured rock strengths, UCS and BTS.
Original languageEnglish
Number of pages1
Publication statusPublished - 2006
Event59th of Geological Congress of Turkey - Ankara, Ankara, Turkey
Duration: Mar 20 2006Mar 24 2020


Conference59th of Geological Congress of Turkey
Abbreviated titleTJK
Internet address


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