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1997 | Buch

Introduction Kapitel lesen Erstes Kapitel lesen

Rock Stress and Its Measurement

verfasst von: Bernard Amadei, Ove Stephansson

Verlag: Springer Netherlands

Enthalten in: Professional Book Archive

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Über dieses Buch

Rock masses are initially stressed in their current in situ state of stress and to a lesser natural state. Whether one is interested in the extent on the monitoring of stress change. formation of geological structures (folds, faults, The subject of paleostresses is only briefly intrusions, etc. ), the stability of artificial struc­ discussed. tures (tunnels, caverns, mines, surface excava­ The last 30 years have seen a major advance our knowledge and understanding of rock tions, etc. ), or the stability of boreholes, a in the in situ or virgin stress field, stress. A large body of data is now available on knowledge of along with other rock mass properties, is the state of stress in the near surface of the needed in order to predict the response of rock Earth's crust (upper 3-4km of the crust). masses to the disturbance associated with those Various theories have been proposed regarding structures. Stress in rock is usually described the origin of in situ stresses and how gravity, within the context of continuum mechanics. It is tectonics, erosion, lateral straining, rock fabric, defined at a point and is represented by a glaciation and deglaciation, topography, curva­ second-order Cartesian tensor with six compo­ ture of the Earth and other active geological nents. Because of its definition, rock stress is an features and processes contribute to the current enigmatic and fictitious quantity creating chal­ in situ stress field.

Inhaltsverzeichnis

Frontmatter
1. Introduction
Abstract
Unlike artificial materials such as concrete and steel, natural materials such as rocks and soils are subject to natural (virgin) stresses called in situ stresses. Stress is an enigmatic quantity which, according to classical mechanics, is defined at a point in a continuum and is independent of the constitutive behavior of the medium. The concept of stress used in rock mechanics is consistent with that formulated by Cauchy and generalized by St Venant in France during the 19th century (Timoshenko, 1983). A summary of the continuum mechanics description of stress is presented below. More details can be found in Appendix A or in Mase (1970).
Bernard Amadei, Ove Stephansson
2. Estimating in situ Stresses
Abstract
Before measuring virgin stresses with some of the methods discussed in the following chapters, an attempt should be made to obtain an estimate of the in situ stress field. This can be done, for instance, from stress versus depth relationships or observations obtained from stress measurements made in the past in the region of interest or by extrapolation from regions with similar geological and tectonic settings. Information can also be derived from the topography, the geology, the rock fabric, the rock loading history, the first motion analysis of earthquakes, the occurrence of stress release phenomena (squeezing, pop-ups, buckling, etc.), breakouts in boreholes, tunnels and shafts, rock bursts, and the presence of stratification, heterogeneities or geological structures (faults, folds, shear zones, uncomformities, volcanic vents and dikes). Estimating in situ stresses can be useful in the early stage of engineering design, for the planning process and when selecting stress measuring methods and the location of those measurements.
Bernard Amadei, Ove Stephansson
3. Methods of in situ Stress Measurement
Abstract
Compared with other rock mass properties, rock stress is a difficult quantity to measure. As pointed out by Leeman (1959), ‘It is impossible to measure stress directly since, in fact, it is a fictitious quantity. It is only possible to deduce the stresses in a solid body from the results of measurements using some indirect method’. Since stress can be represented by a secondorder Cartesian tensor, determination of the complete in situ stress field in three dimensions requires at least six independent pieces of information.
Bernard Amadei, Ove Stephansson
4. Hydraulic Methods
Abstract
The main objective of hydraulic methods is to measure in situ stresses by isolating a section of a borehole and applying a hydraulic pressure on its wall. The applied pressure is increased until existing fractures open or new fractures are formed. The fluid pressure required to open, generate, propagate, sustain and reopen fractures in rock at the test horizon is measured and is related to the existing stress field. The direction of the measured stresses is usually obtained by observing and measuring the orientation of the hydraulically induced or opened fractures.
Bernard Amadei, Ove Stephansson
5. Relief Methods
Abstract
The main idea behind relief methods is to isolate (partially or wholly) a rock sample from the stress field in the surrounding rock mass and monitor its response (Merrill, 1964). This can be achieved by different methods such as overcoring or undercoring holes and cutting slots. The stresses are not related to applied pressures such as in hydraulic methods. Instead, the stresses are inferred from strains or displacements created by the relief process and measured on isolated rock samples, in boreholes or on the surrounding rock associated with the relief process. The successful interpretation of stress relief tests depends to a great extent on the ability (1) to establish a stress—strain (or displacement) relationship for the rock, (2) to be able to determine rock mass properties from tests on samples and (3) to have instrumentation sensitive enough to capture small strains or displacements. It is common practice to relate strains or displacements to the stress field components through equations derived from the theory of linear elasticity for isotropic media.
Bernard Amadei, Ove Stephansson
6. Jacking Methods
Abstract
Jacking methods are sometimes called ’stress compensating’ methods. The equilibrium of a rock mass is disturbed by cutting slots on the surfaces of rock excavations (quarries, galleries, pillars, etc.). This in turn creates deformations that are measured with reference pins or strain gages placed on either side of the slots. Finally, equilibrium is restored by inserting a device such as a jack in the slots. Then the jack is pressurized until all deformations have vanished. One of the most widely used jacking methods is the flat jack (or flatjack) method.
Bernard Amadei, Ove Stephansson
7. Strain Recovery Methods
Abstract
When a piece of rock is removed from the in situ state of stress, it tends to relax and thereby deform. The relaxation consists of an instantaneous elastic component and a timedependent (anelastic) recovery. Field measurements have shown that anelastic strain recovery occurs in drill cores after drilling and coring and is usually accompanied by the opening and propagation of preferential microcracks. Upon relief from an anisotropic in situ stress field, core samples tend to expand most in the direction of maximum stress relief and least in the direction of minimum stress relief. Thus by proper instrumentation of recovered oriented cores, the orientation of the principal in situ stresses can be inferred from the directions of measured maximum and minimum strains (Fig. 7.1a). Determination of the magnitude of the stresses is more difficult and requires a constitutive model for the rock. This relatively recent stress measurement method is called the ‘anelastic strain recovery’ (ASR) method.
Bernard Amadei, Ove Stephansson
8. Borehole Breakout Method
Abstract
Spalling of the walls of boreholes or wellbores due to stress concentration produces elongated intervals with non-circular crosssections whose long axes share common average direction. Such intervals are defined as breakouts or breakout zones when the shorter diameter of the borehole corresponds to, or is close to, the diameter of the drillbit. The orientation of the major and minor horizontal in situ stresses around a vertical borehole can be inferred from the orientation of breakouts as it is usually assumed that breakouts occur in two diametrically opposed zones along the direction of the minimum horizontal in situ stress (Fig. 8.1).
Bernard Amadei, Ove Stephansson
9. Case Studies and Comparison between Different Methods
Abstract
The previous five chapters have described various techniques of rock stress measurements. In this chapter we present several case studies where some of those techniques have been used at the same site either in a complementary manner or as a cross-check. Several examples of comparison between different methods are presented. Such comparisons are recommended in practice as they provide an additional measure of consistency and reliability with regard to the methods used and the stresses measured.
Bernard Amadei, Ove Stephansson
10. Monitoring of Stress Change
Abstract
Monitoring of rock stress change with time is important when assessing the short- and longterm performance of a rock mass to the construction and exploitation of underground engineering structures such as tunnels, caverns and mines. A large amount of literature on stress change monitoring deals with the stability of mines and more particularly mine pillars. Of particular interest to mining engineers is the optimization of mine layout and pillar design and the prediction of rock loads, rockbursts and mine stability upon blasting (Maleki, 1990). The magnitude and distribution of stress changes in a rock mass due to excavation can be quite complex, as some regions in a rock mass experience stress increases and others experience stress decreases (Kaiser and Maloney, 1992). As an example, Fig. 10.1 shows a pattern of stress changes measured by Lee, Abel and Nichols (1976) in jointed and foliated gneiss ahead of an advancing crosscut at the Colorado School of Mines Experimental Mine in Idaho Springs, Colorado. They found that stress changes could be detected as far as 7.5 diameters ahead of the crosscut face.
Bernard Amadei, Ove Stephansson
11. The State of Stress in the Earth’s Crust: From Local Measurements to the World Stress Map
Abstract
The last 30 years have seen a major advance in our knowledge of in situ stresses in rock. A large body of data on the state of stress in the near-surface part of the Earth’s crust (upper 4–5 km of the crust) is now available. Regional stress data from various continents have been collected in separate databases and later compiled into a worldwide database. The first part of this chapter deals with the organization and database of the World Stress Map Project, and the ‘big picture’ of the state of stress in the Earth’s crust. The second part of this chapter deals with the effect of scale on in situ stresses and in situ stress measurements, and the relationship between local stress measurements and the global stress field.
Bernard Amadei, Ove Stephansson
12. Using Stresses in Rock Engineering, Geology and Geophysics
Abstract
Rock masses are initially stressed in their natural state. Whether one is interested in natural geological structures (folds, faults, intrusions, etc.) or artificial structures (tunnels, caverns, mines, surface excavations, etc.), a knowledge of the in situ or virgin stress field (along with other rock mass properties) is needed when predicting the response of rock masses to the disturbance associated with those structures. The response can take multiple forms such as deformations of the walls of a surface or underground excavation, stresses and breakouts in the walls of a shaft or borehole, creep of a salt pillar, initiation of a microearthquake, shearing of a fault or glacial rebound of a glaciated terrain. Today, there exist a variety of analytical solutions to many of the geological, geophysical and geoengineering problems. Computer-based numerical methods for stress, strain and strength analysis are also available to handle problems with more complex geometries and/or constitutive behavior. Many of the analytical methods and numerical codes use stress (or traction) as a possible boundary condition. Hence, a proper determination, or at least a good estimation, of the state of stress in situ is needed in order to reach reliable solutions to the problem of interest.
Bernard Amadei, Ove Stephansson
Backmatter
Metadaten
Titel
Rock Stress and Its Measurement
verfasst von
Bernard Amadei
Ove Stephansson
Copyright-Jahr
1997
Verlag
Springer Netherlands
Electronic ISBN
978-94-011-5346-1
Print ISBN
978-94-010-6247-3
DOI
https://doi.org/10.1007/978-94-011-5346-1