Investigation of the effect of impact direction on Schmidt rebound values by multivariate regression and neuro-fuzzy mod
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Investigation of the effect of impact direction on Schmidt rebound values by multivariate regression and neuro‑fuzzy model Hasan Karakul1 Received: 12 August 2020 / Accepted: 28 September 2020 © Springer Nature Switzerland AG 2020
Abstract Schmidt hammer test is a nondestructive test used in rock characterization and estimation of mechanical properties of rocks. It is an important advantage that the experiment can be carried out for different impact directions. However, a normalization should be performed on the test results because of variation of effect of gravitational forces on rebound value. On the other hand, some weak, highly porous and weathered rocks have different energy absorbance levels that may not confirm the assumption of normalization formula of the suggested method. So, in this study it is aimed to examine the prediction performance of suggested normalization formulation for weak and highly porous rocks. For this purpose, Schmidt hammer tests for five different impact directions were performed on eight different rock types. The test results indicated that, in general, the rebound values obtained for an impact direction other than horizontal can be converted to the rebound value of horizontal impact direction with a high accuracy by using suggested normalization formula and/or chart. However, existence of some exception was also determined for weak and highly porous rock types. Multivariate regression analyses and adaptive-neuro-fuzzy inference system (ANFIS) were used to explain the variation of ratio between the normalized and tested rebound values for rock types used in this study. The results obtained from statistical and soft computing methods showed that the ratio between the normalized and tested rebound values is predictable and higher for weak and highly porous rock types. Keywords Schmidt hammer test · Impact direction · Energy absorbance level · Multivariate regression analysis · Soft computing methods
1 Introduction Schmidt hammer test is a nondestructive test method which is extensively used for the estimation of geomechanical parameters (such as uniaxial compressive strength and modulus of elasticity) in rock engineering applications and rock material characterization. The test can be applied both in field and laboratory due to portable nature of the hammer. The nondestructive nature of the experiment ensures that deformations remain within elastic limits and there is no sample loss. On the other hand
the level of impact energy limits the usage range interval of hammer. Although it is considered as an important advantage that the experiment can be used both in the laboratory and in the field, this advantage also includes some limitations. The existence of different inclinations of geological layers and structural features in the field makes it inevitable to perform the Schmidt hammer tests in different directions. However, the rebound values obtained from different impact directions are varied due to gravitational forces. So, it is essential to eliminate the different effects of ex
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