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Progressive failure of jointed rocks
ARTICLE IN PRESS
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 389
SINOROCK2004 Paper 1A 30
Progressive failure of jointed rocks L.G. Thama,*, T.H. Yangb, C.A. Tangb a
Department of Civil Enginering, Rock Engineering Center, The University of Hong Kong, Pokfulam, Road, Hong Kong, China b Center for Rock Instability and Seismicity Research, Northeastern University, Shenyang, China
Abstract As jointed rocks consist of joints embedded within blocks of ‘intact’ rock, the behaviour of these rocks will depend on the behaviour of the joints and the intact rock blocks. In jointed rocks, there are two levels of heterogeneity: (1) the differences in properties between the rock blocks and the joints and (2) the heterogeneity within the rock blocks due to the differences in the degrees of weathering and the distributed flaws. Furthermore, as water normally exists in jointed rocks, the pressure and flow of the water will also have a significant effect on these parameters. In this paper, a finite element model, based on Biot’s theory, is developed to predict the behaviour of jointed rocks by taking into account the above factors. The joints are modelled by elements with low modulus and strengths; whereas the heterogeneity of the rock properties of the ‘intact’ rock is taken into account by assuming that they follow Weibull’s distribution. Four specimens with joints inclined at different angles, namely 0 , 45 , 90 and 135 are employed to study the effect of the orientation of the joints on the slip surface. The joints are discontinuous and they are modelled by adopting a weaker material without considering the effect of the pore fluid flow. The results show that a rock specimen with joints normal to the shear direction is more brittle. Furthermore, the shear strength is largest when the pre-existing joints are normal to the shear direction. It is minimum when the joints are horizontal. The minimum shear resistance is only about 60% of the uniaxial compressive strength, indicating the strength of the rock mass can be very much reduced by the joints (Fig. 1). An example of a vertical rock slope is used to demonstrate the effects of the water table on the failure of slopes.
Keywords: Jointed rock; Heterogeneity,Biot’s theory; Weibull’s distribution; Water pressure; Flow
90°
· ·
240 MPa 135°
·
45°
184 MPa
·
240 MPa
·
·
174 MPa
·
180° 240 MPa
·
0
42 MPa
240 MPa
132 MPa
·
42 MPa
Shear stress, τMPa Fig. 1. Relations between peak shear stress and joint gradient.
*Corresponding author. Tel.: +852-2859-1965; fax: +852-2559-5337. E-mail address: [email protected] (L.G. Tham). For full length paper see CD-ROM attached. doi:10.1016/j.ijrmms.2003.12.048
·
240 MPa
0°
International Journal of Rock Mechanics & Mining Sciences 41 (2004) 389
SINOROCK2004 Paper 1A 30
Progressive failure of jointed rocks L.G. Thama,*, T.H. Yangb, C.A. Tangb a
Department of Civil Enginering, Rock Engineering Center, The University of Hong Kong, Pokfulam, Road, Hong Kong, China b Center for Rock Instability and Seismicity Research, Northeastern University, Shenyang, China
Abstract As jointed rocks consist of joints embedded within blocks of ‘intact’ rock, the behaviour of these rocks will depend on the behaviour of the joints and the intact rock blocks. In jointed rocks, there are two levels of heterogeneity: (1) the differences in properties between the rock blocks and the joints and (2) the heterogeneity within the rock blocks due to the differences in the degrees of weathering and the distributed flaws. Furthermore, as water normally exists in jointed rocks, the pressure and flow of the water will also have a significant effect on these parameters. In this paper, a finite element model, based on Biot’s theory, is developed to predict the behaviour of jointed rocks by taking into account the above factors. The joints are modelled by elements with low modulus and strengths; whereas the heterogeneity of the rock properties of the ‘intact’ rock is taken into account by assuming that they follow Weibull’s distribution. Four specimens with joints inclined at different angles, namely 0 , 45 , 90 and 135 are employed to study the effect of the orientation of the joints on the slip surface. The joints are discontinuous and they are modelled by adopting a weaker material without considering the effect of the pore fluid flow. The results show that a rock specimen with joints normal to the shear direction is more brittle. Furthermore, the shear strength is largest when the pre-existing joints are normal to the shear direction. It is minimum when the joints are horizontal. The minimum shear resistance is only about 60% of the uniaxial compressive strength, indicating the strength of the rock mass can be very much reduced by the joints (Fig. 1). An example of a vertical rock slope is used to demonstrate the effects of the water table on the failure of slopes.
Keywords: Jointed rock; Heterogeneity,Biot’s theory; Weibull’s distribution; Water pressure; Flow
90°
· ·
240 MPa 135°
·
45°
184 MPa
·
240 MPa
·
·
174 MPa
·
180° 240 MPa
·
0
42 MPa
240 MPa
132 MPa
·
42 MPa
Shear stress, τMPa Fig. 1. Relations between peak shear stress and joint gradient.
*Corresponding author. Tel.: +852-2859-1965; fax: +852-2559-5337. E-mail address: [email protected] (L.G. Tham). For full length paper see CD-ROM attached. doi:10.1016/j.ijrmms.2003.12.048
·
240 MPa
0°