INFLUENCE OF LOCAL PERTURBATION ON REGIONAL STRESS AND ITS IMPACT ON THE DESIGN OF MAJOR UNDERGROUND STRUCTURE IN HYDROELECTRIC PROJECT

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1 INFLUENCE OF LOCAL PERTURBATION ON REGIONAL STRESS AND ITS IMPACT ON THE DESIGN OF MAJOR UNDERGROUND STRUCTURE IN HYDROELECTRIC PROJECT *D.S. Subrahmanyam National Institute of Rock Mechanics, Bangalore unit, Karnataka, India *Author for Correspondence ABSTRACT Based on the data available from different stress indicators like earthquake focal mechanism, borehole breakouts and other stress measurement methods, two orders of stress distributions are identified; first order or regional stresses and second order or local perturbation of the regional stresses (Zoback et al., 1989, Zoback, 1992). While measuring stress regimes at two sites separated by 10 km distance in one of the hydroelectric projects located in Himalayas of India, we have identified both first order and second order stresses. The direction of the maximum principal horizontal stresses ( H ) at one of the locations viz., desilting complex of the hydroelectric project is measured as N30 which is quite consistent with the World Stress Map direction in the North Eastern quadrant of India. This direction is regional one and can be called as first order stress as per the definition. However at the second location viz., power house area of the same hydroelectric project, the data indicates a second order perturbed stress with 70 rotation of the regional stress. This perturbation has been attributed to the presence of a nearby prominent fault. We tried to understand the phenomena by the application of numerical modelling. Based on 2-D distinctelement modelling, we reconstructed the local stress field inside and around the fault in actual geological context. The resulting stress distribution reveals that major directional stress changes occur around the surrounding and stress deviations can reach values as large as 35. We have established simple relationships controlling stress changes around a pre-existing fault zone as a function of the remote stress ratio σ 1 & σ 3, the friction coefficient on the discontinuity, and the strike of the discontinuity relative to the far-field stress. These studies confirmed that a major geological structure can considerably influence the stress setup around it. Any major underground structures that are designed within the influence of this perturbed stress may have to be designed accordingly due to the presence of geological structure as the case with the powerhouse of the hydroelectric project enlightened in this paper. Keywords: Focal Mechanism, Maximum Principal Horizontal Stress, Desilting Complex INTRODUCTION Though considerable advances have been made in the measurements of in-situ stress over the past 25 years the interpretations of these measurements with respect to geological structures have received significant attention. From different hydro and mining sites all over the world a large number of observations implicating the rotation of stress orientation as much as by 90 near geological structures are reported. A number of investigators have also postulated various theories to explain the phenomena. Haimson (1977) observed rotation of stress by 25 in the central Sierra Nevada Mountains and by 45 at the Nevada test site compared to stress direction along the San Andreas Fault based on hydro fracturing tests and focal mechanism. Zoback (1992) presumed that a strength contrast between a low frictional resistances of fault inside a strong crust is responsible for rotation of stress in the San Andreas Fault in Central California. Chandler and Martin (1990) found stress rotation from 40 to 90 in Underground Research laboratory, Canadian Shield by triaxial overcore tests. Sengupta et al., (1998) observed rotations in the order of 24 to 87 near faults at two project sites in Himalayas. This paper presents the results of an in-situ stress measurement programme at the proposed desilting chamber and powerhouse of a hydroelectric project in Himalayas The stress measurements at these two Copyright 2014 Centre for Info Bio Technology (CIBTech) 6

2 sites indicate that the trend of the compression H is N40 W on average at the proposed powerhouse near the fault, whereas they are N30 E in the surrounding areas including at the proposed desilting chamber. We have tried to understand the problem by parametric study using numerical modelling (2D-UDEC code) incorporating the rock and fault properties along with the horizontal stress ( H ) ratios and angle between the maximum horizontal principal stress orientation and the fault as input parameters. A comparison between the results of the tectonic study and those of theoretical modelling suggests that the 70 counter clockwise deviation is directly related to the role of mechanical decoupling along pre-existing zones of weakness i.e., fault. MATERIALS AND METHODS Background Introduction The proposed hydroelectric project envisages construction of 65 m high concrete gravity dam an underground power house to be located on the right bank through a 8.8 m dia Head Race Tunnel, 13.4 km long, with 130 m deep upstream surge shaft having a dia of 22 m, to generate 444 MW of power. Geological Setting The project area the rocks are composed mainly of calc arenaceous rocks with basic intrusive and migmatite bodies, while around Dam site, low to medium grade metamorphic rocks are exposed. Regional or first order stress province The regional or first order stress orientation is based on the focal mechanism and is N30 E. The measurements by hydrofracture methods at two locations at the desilting chamber/ dam site revealed a N30 E direction which can be summarised as follows Table 1: Orientation of primary or regional maximum horizontal stress ( R ) Latitude Longitude Direction of ( R) Reference N E N30 E NIRM Technical report 2009 N30 E World stress map 2008 Average direction of ( R) = N30 E Second order or Local Perturbed stress province A local fault zone of 10m width trending N30 E and dipping sub- is the closest major geological discontinuity at the test site. RESULTS AND DICUSSION Experimental Results Four locations were selected two at the proposed desilting chamber and two at the proposed powerhouse. The stress tensors evaluated at the four sites are summarised and is given in table 2 Table 2: Stress tensors as revealed by hydrofracture stress measurement Location Lat. Principal Stress Magnitudes (MPa) Principal horizontal Remarks /Long H h v Stress orientation H Desilting Chamber N30 o N30 E Primary or (2 sites) E79 o (Rock cover =305m) Regional stress Powerhouse N N40 W Secondary or (2 sites) E (Rock cover =365m) Perturbed stress Copyright 2014 Centre for Info Bio Technology (CIBTech) 7

3 Observation The observation made from study is given in table 3 Table 3: Maximum Horizontal Principal Stress Rotation ( ) at the study area The orientation of the regional maximum principal horizontal stress ( R ) The orientation of maximum horizontal principal Stress ( H ) at the vicinity of the fault where the powerhouse is proposed The total rotation of the maximum horizontal principal stress ( ) 70 N30 E N40 W A plan and rose diagram showing orientations of the intrusive in concern vis a vis regional and local perturbed maximum horizontal principal stresses are shown in Figure 1 and Figure 2 Figure 1: Plan Showing Orientation of the fault vis a vis Regional and Perturbed Local Maximum Horizontal Principal Stress Numerical Analyses A two dimensional program UDEC is used for analysis. To analyse the factors that are responsible for rotation of the stress direction, the following assumptions are made: i) The model incorporates a plane strain configuration ii) The discontinuity contact is Coulomb slip model (elastic perfectly plastic material ) iii) Regional stress acting on the boundaries are assumed to be the boundary stress directions Copyright 2014 Centre for Info Bio Technology (CIBTech) 8

4 Maximum Horizontal Regional Stress Direction ( R ) Minimum Horizontal Regional S Monitoring zone International Journal of Geology, y Earth & Environmental Sciences ISSN: (Online) x iv) The intact rock blocks are Mohr Coulomb model which behave as elastic/plastic material v) Monitoring of maximum horizontal principal stress direction and magnitude are assumed to be the perturbed stress direction and magnitude around the discontinuity Geometry and boundary condition Block Geometry, Boundary Condition and Location of A block with an intersecting intrusive has been considered in the present analysis which is subjected to a Monitoring Zone for the Computational Model for Fault Case bilateral tectonic stress field (Figure 2). In accordance with the field conditions, the dimensions, configuration and the initial boundary conditions are applied. Minimum Horizontal Regional Stress Direction ( r ) a Intrusive ß H Country Rock h Monitoring zone y x Figure 2: Block Geometry, Boundary Condition and Location of Monitoring Zone for the Computational Block Geometry, Model for Boundary intrusive Condition case and Location of Monitoring Zone for the Computational Model for Intrusive Case Modelling Strategy for Analysis The analysis was carried out by varying the following parameters: i) Angle ( ) between the boundary/regional stress direction and the strike of the discontinuity (the strike of the discontinuity relative to the far-field stress) ii) cohesion (C ) of the parent rock, intrusive and the intrusive/parent rock contact iii) bulk moduli (K) and shear moduli (G) of both the parent rock and the intrusive iv) Normal stiffness (Kn) and shear stiffness (Ks) of the fault contact v) friction angle ( ) of intact rock, intrusive and intrusive contact (the friction coefficient on the discontinuity) vi) the remote stress ratio (σ 1 and σ 3 )/ boundary stress ratio (K o ) The fault is treated as a low modulus material with definite boundary surrounded with high modulus isotropic material. Simulation results Out of six parameters above only three parameters viz., contact frictional angle ( c ), ( ) and (K o ) are found to have significant influence on stress perturbation. i) The maximum stress rotation of 32 is observed from numerical modelling is when the intrusive body is aligned 10 to the maximum horizontal principal stress (Figure 3) Copyright 2014 Centre for Info Bio Technology (CIBTech) 9

5 ß in degrees ii) The maximum stress rotation of 35 is observed when (K o ) is around 3.5 (Figure 4) iii) The stress rotation due to influence of friction angle of the discontinuity contact ( c ), attains maximum of (degrees) (degrees) Figure 3: Variation of with for intrusive case Variation of with for Intrusive Case K o ( H / v ) 3 4 Figure 4: Variation of with K o for intrusive case RESULTS AND DISCUSSION Variation of with K Both the models and the parametric study give an idea regarding o for Intrusive Case stress perturbation in and around the fault and the likely causes of such perturbations. The higher deflection of second order stress in the field compared to the simulated conditions can be attributed to 1) Scale effect 2) Anisotropic characters of both geological structure and the surrounding rock in actual field condition and may be 3) topography effect which has not been incorporated in the modelling. We have established simple relationships controlling stress changes around a pre-existing intrusive zone as a function of (1) the remote stress ratio σ 1 and σ 3, (2) the friction coefficient on the Copyright 2014 Centre for Info Bio Technology (CIBTech) 10

6 discontinuity, and (3) the strike of the discontinuity relative to the far-field stress. The study indicates how a second order stress orientation can be changed vis a vis orientation of the first order resulting in the change of orientation of the long axis of the powerhouse with respect to the desilting chamber. ACKNOWLEDGEMENTS We are thankful to the Director National Institute of Rock Mechanics (NIRM), KGF, India for permission to publish this paper. We are also thankful to the authorities of project for rendering help during field measurements. REFERENCES Chandler and Martin (1990). Stress heterogeneity and geological structures. International Journal of Rock Mechanics & Mining Sciences & Geomechanics Abstracts 30(7) Gowd TN, Srirama Rao SV and Gaur VK (1992). Tectonic Stress Field in Indian Subcontinent. Journal of Geophysical Research 97, N.B Haimson BC (1977). Crustal Stress in the Continental United States as derived from Hydrofracturing Tests, Geophysical Monograph 20, the Earth Crust, American Geophysical Union Washington, D.C., Sengupta S, Joseph D and Nagraj C (1997). Stress perturbation near two fault zones in Himalayas-field measurements and numerical simulation. Proceedings of the Third International Conference on Mechanics of Jointed and Faulted Rock- MJFR-3, Zhang YZ, Dusseault MB and Yasir NA (1994). Effects of rock anisotropy and heterogeneity on stress distribution at selected sites in North America. Engineering Geology Zoback ML (1992). First and second order patterns of stress in the lithosphere. The World Stress Map Project, Journal of Geophysical Research 97(11) Copyright 2014 Centre for Info Bio Technology (CIBTech) 11