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Relationship between Shear Strength and Component Content of Fault Cores

단층핵 구성물질의 함량과 전단강도 사이의 상관성 분석

  • Yun, Hyun-Seok (Department of Earth and Environmental Sciences, Chungbuk National University) ;
  • Moon, Seong-Woo (Department of Earth and Environmental Sciences, Chungbuk National University) ;
  • Seo, Yong-Seok (Department of Earth and Environmental Sciences, Chungbuk National University)
  • 윤현석 (충북대학교 지구환경과학과) ;
  • 문성우 (충북대학교 지구환경과학과) ;
  • 서용석 (충북대학교 지구환경과학과)
  • Received : 2019.01.16
  • Accepted : 2019.01.30
  • Published : 2019.02.28

Abstract

In this study, simple regression and multiple regression analyses were performed to analyze the relationship between breccia and clay content and shear strength in fault cores. The results of the simple regression analysis performed for each rock (andesitic rock, granite, and sedimentary rock) and three levels of normal stress (${\sigma}_n=54$, 108, 162 kPa), reveal that the shear strength is proportional to breccia content and inversely proportional to clay content. Furthermore, as normal stress increases, the shear strength is influenced by the change in component content, correlating more strongly with clay content than with breccia content. In the multiple regression analysis, which considers both breccia and clay content, the shear strength is found to be more sensitive to the change in breccia content than to that of clay. As a result, the most suitable regression model for each rock is proposed by comparing the coefficients of determination ($R^2$) estimated from the simple regression analysis with those from the multiple regression analysis. The proposed models show high coefficients of determination of $R^2=0.624-0.830$.

Keywords

simple regression analysis;multiple regression analysis;breccia and clay content;shear strength;fault core

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Fig. 1. Sample equipments used to minimize disturbance of fault core during specimens preparation for the direct shear test.

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Fig. 2. Representative outcrop photographs of the fault cores used in this study. (a) and (b) in andesitic rock are composed of mixed gouge and cataclasite of various colors, and the fault breccia and damage zone are partially observed. (c) and (d) of granite in Ulsan Fault are developed with low dip of reddish brown and pinkish gouge with a maximum thickness of 30 cm, and no cataclasite or breccia is observed. (e) in the Cretaceous sedimentary rock is composed of whitish gouge and greenish gray cataclasite and (f) located in Danyang is contains dark gray to black gouge due to the influence of coaly shale.

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Fig. 3. Box plots for shear strength of fault cores in andesitic rock, granite, and sedimentary rock under normal stress (a) 54 kPa, (b) 108 kPa, and (c) 162 kPa. The gray boxes are IQRs (inter-quartile ranges).

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Fig. 4. Photographs showing (a) a fault core specimen after the direct shear test and (b) the procedure of sieve test (using sieve No. 4 and No. 200) and soil washing test.

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Fig. 5. Box plots for particle size of fault cores in andesitic rock, granite, and sedimentary rock. The gray boxes are IQRs (inter-quartile ranges).

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Fig. 6. Results of simple regression analysis between the shear strength and content of breccia and clay in (a) andesitic rock, (b) granite, and (c) sedimentary rock under normal stress 54 kPa. The dotted lines are the UCI (upper confidence interval) and LCI (lower confidence interval) for 95% confidence Interval.

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Fig. 7. Results of simple regression analysis between the shear strength and content of breccia and clay in (a) andesitic rock,(b) granite, and (c) sedimentary rock under normal stress 108 kPa. The dotted lines are the UCI (upper confidence interval)and LCI (lower confidence interval) for 95% confidence Interval.

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Fig. 8. Results of simple regression analysis between the shear strength and content of breccia and clay in (a) andesitic rock,(b) granite, and (c) sedimentary rock under normal stress 162 kPa. The dotted lines are the UCI (upper confidence interval)and LCI (lower confidence interval) for 95% confidence Interval.

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Fig. 9. Comparative analysis of coefficients of determination (R2) estimated from simple regression analysis.

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Fig. 10. Comparative analysis of the regression coefficients on content of (a) breccia and (b) clay in each normal stress (σn=54, 108, and 162 kPa) estimated from simple regression analysis. Abbreviations of rock type are as follows: andesitic rock (AR), granite (GR), and sedimentary rock (SR).

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Fig. 11. Comparative analysis of coefficients of determination (R2) estimated from simple regression analysis and multiple regression analysis. Abbreviations of rock type are as follows: andesitic rock (AR), granite (GR), and sedimentary rock (SR).

Table 1. Shear strength values of fault cores in andesitic rock, granite, and sedimentary rock under normal stress (σn) 54 kPa, 108 kPa, and 162 kPa. Abbreviations of rock type are as follows: andesitic rock (AR), granite (GR), and sedimentary rock (SR)

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Table 2. Results of particle size analysis for fault cores in andesitic rock, granite, and sedimentary rock. Abbreviations ofrock type are as follows: andesitic rock (AR), granite (GR), and sedimentary rock (SR)

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Table 3. Results of the multiple regression analysis showing the regression coefficients of the independent variables (breccia and clay) for each rock type under normal stress 54 kPa

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Table 4. Results of the multiple regression analysis showing the regression coefficients of the independent variables (breccia and clay) for each rock type under normal stress 108 kPa

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Table 5. Results of the multiple regression analysis showing the regression coefficients of the independent variables (breccia and clay) for each rock type under normal stress 162 kPa

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Table 6. Regression equations for the shear strength of fault core estimated by simple regression analysis and multiple regression analysis

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Acknowledgement

Supported by : 한국연구재단, 충북대학교

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