Document Type : Research Paper
Wave propagation is used as an advanced tool of determining elastic properties of rocks. In spite of being cheap and non destructive tests of rock mechanics, due to its modernity method is not yet replacing the traditional dear and destructive tests. In order to pave ways of replacing this test and making best use of it all effective factors of such method are to be assessed. One fundamental factor affecting the test result is the shape of the specimen tested. The existing standards do not seriously take the sample shape into consideration. It was pointed out elsewhere that in spite of popular taught elastic dynamic properties do vary seriously due to variation of sample shape (height to diameter ratio). As this fact was revealed for limestone in our previous investigation this assessment was extended to other types of rocks for the possibility of true judgment. Therefore, ultrasonic wave propagation tests were conducted on four various rock types with height to diameter ratio of 0.5 to 3. Analyzing of the test results showed that shear wave velocity and elastic dynamic constants are noticeably dependent on sample height/diameter ratio. The variation of height/diameter ratio from 0.5 to 3 caused the variation of shear wave velocity and Poisson’s ratio up to 70%, Elasticity modulus up to 160% and shear modulus up to 200% as follows in details:
Variation in shear wave velocity: Limestone, 24%; Siltstone, 70%; Green Tuff, 17% and Granodiorite, 20%.
Variation in dynamic elastic modulus: Limestone, 26%; Siltstone, 65%; Green Tuff, 73% and Granodiorite, 33%.
Variation in Poisson’s ratio: Limestone, 45%; Siltstone, 160%; Green Tuff, 20% and Granodiorite, 35%.
Variation in shear modulus: Limestone, 53%; Siltstone, 200; Green Tuff, 38% and Granodiorite, 45%.
It was also observed that the longitudinal wave velocity was not following the height/diameter ratio and showed no specific trend of variation.
As seen from the above details, the variation of modulai is much grater for siltstone in compares to the 3 other rocks. It is most likely that this obvious difference is due to inhomogenity of the siltstone texture which causes different behavior in this particular rock.