Description

Construction of hydroelectric power projects have become the need of the hour today in India. Consequently, large, and small-scale Hydro Power Projects are growing in various parts of India, particularly in hilly areas. The planning and successful completion of these projects requires not only the geological mapping but the large- and small-scale structural analysis of the areas are of prime importance. The paleo and neo-stress tensors in relation to major and minor structures of projects also are important in successful running of the projects.

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 many observations implicating the rotation of stress orientation as much as by 90° near geological structures are reported. Several 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 resistance 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.

Hence it is clarified that, the in-situ stress parameters are controlled by major geological structures like folds, faults and intrusives. Besides this, the magnitude and distribution of in situ stresses are also governed largely by the rock mass structure and its cumulative geological history. The role played by scale effect and the boundary conditions due to the composite nature of rocks from micro scale to the regional scale in rock masses is important to civil, petroleum and mining engineers. The in-situ stresses in rock mass can be so intricate that local stresses may be quite different from regional stress. Consequently, the stress results may vary for different techniques and with deviation at different times of experiments, which should be treated as intrinsically non uniform but not as anomalies. At many construction sites, rotation of stress orientation was as high as 900 to the geological structures. Due to proximity of these perturbed stresses to the underground structures, it is expected that these stresses will have a greater impact on the overall stability of the underground structure than regional stresses. Thus, stress perturbations due to local geological structures at the site of underground projects must be clearly understood before stress values are taken as input parameters for design of any structure.

Now the stress orientation is one of the most important factors to be determined at the proximity to any new underground project. In most of these underground sites detailed structural maps are available clearly marking the major discontinuities like fold fault and intrusive bodies. The stress direction can be evaluated from these discontinuities by applying classical approach by which the relation between stress direction and the discontinuities are well established. But this technique of estimating the orientation of the current state of stress should be approached with caution, since the stresses that created the geological structures may have been modified over time due to additional tectonic events, erosion, glaciation etc. Thus, establishing stress direction based on geological discontinuities may give only paleo stress as opposed to neo stress/ in-situ stress, which is our concern. Though some investigations were carried out to understand the relationship between paleo stress and neo stress vis a vis geological structure, it is notable that there has been comparatively little effort devoted to understanding the relationship.

The important role played by in-situ stresses in the mechanics of development of geological formations like fractures, faults, folds, intrusions, etc. has been acknowledged by structural geologists and geophysicists for a long time. In general estimating in situ stresses requires a detailed characterisation of the rock and considerable judgement for past geological events. Igneous materials like dykes generally originate from a magma chamber. Until the pressure in the magma chamber surpasses the overburden stress, momentous upward movement of the magma in the country rock is unlikely to occur. But at depth, it is believed that the mechanism for the emplacement of dykes is by forceful injection which happens because of hydraulic fracturing of the crust. To evaluate the stress regime, the in-situ stress measurements were conducted by hydraulic fracturing method at one of the Underground Pump house project sites near Hyderabad, India. Dyke intrusion has been observed where an underground pump house is proposed to be constructed. The stress measurements must be conducted to delineate the Maximum Principal Stress direction to orient the long axis of the pump house parallel to it. However, it is important to note whether there is any influence of dyke on the in-situ stress parameters, or the stress direction is in compromising alignment with the direction of the dyke. In this area dyke has been formed in compromising direction with the Maximum Principal Stress at N1500. Hence it is believed that the mechanism for the emplacement of dyke by means of forceful injection was occurred because of hydraulic fracturing of the crust. Hence there was strong influence of Stress for the formation of Dyke (Price and Cosgrove, 1990).

The direction of maximum principal horizontal stress effects the orientation, geometry, and shape of the underground opening/structures. Some of the design solutions requiring in-situ stress measurements in the hydroelectric projects depend on the ability of the rock formation to support the excavation. With judiciously orienting the excavation geometry vis a vis maximum principal stress direction the stability of the excavation can be made optimum. It can be concluded that the over a geological time the orientation of palaeostress derived from fault intrusive kinematics cannot remain the same. This may be due modification of stresses which created the geological structures. The modification may be due to additional tectonic events, erosion, glaciation etc. Hence neostress and palaeostress may or may not be correlated at all. Therefore, it is necessary to seek out the most recent geological structures among all the structures present which is difficult to ascertain. It is, therefore, a comprehensive study to determine the relation between palaeostress, neostress and geological structures must be carried out. It is but natural that any stress changes will leave its imprint in the rock. Following steps may be followed for further study.

• Study could be carried out to those places where the actual neostress are determined in the field by modern methods, which include hydraulic fracturing method-
• By knowing the exact orientation of neostress the geological structure formed may be

demarcated by fault kinematics.

• This must be done in numerous places till a specific correlation is obtained.
• The same correlation may be applied to a new place to obtain the stress orientation
• The actual measurement may be carried out to that new place to check the confidence limit.

Reference

• Haimson, B.C. (1984)., Pre-excavation in-situ stress measurements in the design of large underground openings, Proc. ISRM Symposium on ground openings, in Proc ISRM Symposium on design and performance of underground Excavations, Cambridge, British Geotechnical Society London, pp.183-90
• Hudson, J.A. and Cooling, C.M (1988) In-situ rock stresses and their measurement in the UK- Part I. The current state of Knowledge. Int. J. Rock.Mech. Min.Sci. & Geomech. Abstr., 25, 363-70
• Pollard, D.D. and Segall, P (1987) Theoretical displacements and stresses near fractures in rock: with applications to faults, joints, veins, dikes and solution surfaces, in Fracture Mechanics of Rock, Academic Academic Press, London,pp.277-349
• Sengupta, S (1998) Influence of geological structures on in-situ stress, Ph.D Thesis IIT, Delhi
• Sengupta,S., Chavan A.S, Joseph D, Raju, N.M (1995) Influence of topography on in-situ stress under a river valley- a case study. Proceeding 35th U.S. Symposium on Rock Mechanics, Lake Tahoe, Organised by Mackay School of Mines, University of Nevada, Reno, USA, pp 511-516
• Sengupta S., Sinha RK., Subrahmanyam DS. and Shyam G.(2010). Assessment of Impact of Nearby Excavations on the Deformability and Stress Conditions Around a Proposed Powerhouse by Field and Numerical Modeling. Accepted for Presentation and Publication in Proceedings: 45th U.S. Rock Mechanics/Geomechanics Symposium, American Rock Mechanics Association, San Francisco, to be held on Jun 26-29. Paper ID ARMA 11-428.
• Sengupta S., Chakrabarti S., Gupta RN., Subrahmanyam DS., Joseph D., Sinha RK., Kar A., Sanyal K., Prasad B., and Wanarey RM. Measurement of In-situ Stress by Hydrofracture Method and Investigations on Redistribution of In-situ Stress Due to Local Tectonics and Methods of Working at Tandsi and Thesgora Mines, WCL to Devise a Suitable Support Plan SSR. A coal S&T project, funded by Ministry of Coal, Govt. of India, Project No. MT 117.
• Zobak M.L et al (1989) Global patterns of tectonic stress, Nature, 341, 291-8.
• Anderson, E. M. 1951a. The Dynamics of the formation of Cone-Sheets, Ring-Dykes and Cauldron-Subsidences, Proc. Roy. Soc., Edinburgh, 56, 2p.
• Cornet, F.H. and Vallette, B. 1984. In-situ stress Determination from hydraulic Injection Test data, J. Geol. Res. 89(B13). 11527-11537.
• Ramsay, G and Hubber, Martin, I 1998 The Techniques of Modern structural geology, vol 2 Folds and Fractures, Academic Press 564-566.
• Savage, W. Z. 1978. The Development of Residual stress in cooling Rock Bodies, Geophysical Research letters, Vol. 5, No. 8.633-636.
• Price, N. J and Cosgrove, J.W. 1990. Analysis of geological structures, Cambridge University Press, Cambridge.

Date Conducted

December 2021

Contributors

DS Subrahmanyam

Owned by organization

National Institute of Rock Mechanics