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Lectures on Rock Mechanics. • SARVESH CHANDRA. SARVESH CHANDRA. Professor. D t. t f Ci il E i i. Department of Civil Engineering. Indian Institute of. Rock Mechanics and Rock Engineering x Overview Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses. Introduction to Rock Mechanics bestthing.infon Second Edition, John Wiley and Sons Some knowledge of rock mechanics is vital for civil engineers.
It is less well known how great an impulse rock mechanics got from progress and research in civil engineering. Among civil engineers, dam designers were the first to become interested in the progress of rock mechanics. They soon realized that rock foundations were part of the design of any dam. They developed their own techniques for measuring the strength and elasticity of rock masses: in situ, at the rock surface, in trenches and in galleries.
They recognized the paramount importance of the joints,fissuresand faults in the rock masses and contributed to the development of methods for tridimensional representation of families of fissures, testing their shear strength and curing them. The latest tendency of dam designers is to direct their efforts towards a more precise description of the type of rupture occurring at different depths inside the rock masses, and an estimate of the static and dynamic effect of water seeping through the joints andfissures.
New chapters of rock mechanics were opened by the joint efforts of dam designers and rock specialists. More recently, vibrations of the earth's crust and earthquakes were analysed by them. For many years, tunnel designers worked from empirical rules. At the beginning of the century interest in the strength, elastic and plastic properties of rocks arose. The notion of stress and strain patterns developing about empty galleries or caverns, or around pressure tunnels, is more recent: the sudden undertaking, for many purposes, of large underground works, forced specialists to consider new methods of stress and strain analysis.
The finite element method allows the analysis of jointed rock masses, crossed by fissures or faults. More recently still, the effects of rock relaxation about cavities and the drop of the modulus of deformation in relaxed rock were analysed. These entirely new concepts about the behaviour of rock masses are initiating new lines of research in rock mechanics. And progress in engineering is linked with active research in rock mechanics. Part one of the book chapters 1 and 2 stresses the importance of the geologist's work, without which the science of rocks would not exist.
Courses No.(404G &425 G) Dr / Waleed A. Ogila 1
Part two chapters 3 to 8 co-ordinates the knowledge of the physical and mechanical properties of rock acquired from the ample information submitted to the First and Second Congresses and the Sixth Symposium on Rock Mechanics. Chapters 3 to 4 deal with the rock material—laboratory samples— with no major fractures, and chapters 5 to 8 study the properties and behaviour of rock masses in situ, and analyse strains and stresses in such masses. Abstract knowledge of rock properties is of limited use to engineers.
It is vital to bridge the gap between the accumulated scientific data on rocks and the requirements of design and field engineers. The second half of the book deals with practical applications.
Part three analyses the diverse aspects of rock slope stability chapter 9 and the strains about cavities excavated in the rock and the modern techniques of underground works chapter Chapter 11 discusses the very controversial problem of dam rock abutment design. Part four chapters 12 to 14 describes typical case histories, which illustrates the more important points developed in parts two and three.
Pulfy, March Part One Introduction to rock mechanics 1 The historical development of rock mechanics 1. Mining engineers and tunnel experts watching rock bursts and rock squeezing in tunnels and galleries, suggested that some 'residual forces' were still at work in rock at great depth.
The German tunnel expert Rziha was probably the first to be concerned with the horizontal component of the forces acting in many tunnels. A few years later Heim Professor at Zurich University and at Zurich Federal Institute of Technology suggested that the horizontal force component must be of the same order of magnitude as the vertical component and he forcefully stressed this opinion in several papers It took many decades for geologists and engineers to realize the importance of the ideas of Heim and Rziha.
In , the Ritom tunnel, which had just been built south of the Alps by the Swiss Federal Railways, was severely damaged. Inspection showed many longitudinal fissures running along the tunnel.
The rock strata had a general dip towards the valley and it was feared that water seepage could cause a rock slide. The tunnel was repaired. They decided to start pressure tests in this second tunnel.
Introduction to Rock Mechanics
A dead end of the gallery was sealed off with a concrete plug provided with a manhole and steel cover and wasfilledwith water under pressure. The tunnel diameters were measured by a spider with six branches and the length variations of the six radii versus time were recorded on a rotating disc. The varying water pressure was also recorded and strain-pressure diagrams traced. The bulk modulus of elasticity was estimated as a ratio of stress versus deformation. This was probably the first recording of the elastic deformations of rock masses.
A few years later J. Schmidt a, b published a thesis in which he  2 Historical development of rock mechanics cleverly combined Heim's ideas about residual stresses in rock, with the newly formulated ideas of rock elasticity to produce the first attempt at a theory of rock mechanics.
It was at this time that steel linings for tunnels and shafts were first introduced, and several authors Jaeger, , in different countries, produced papers estimating the stresses in the lining as a function of the relative elasticity of the steel and the rock. A few years later the Chilean geologist Fenner published a thesis which in many respects is similar to that by Schmidt.
These two pioneering works were ignored by most engineers until many years later. During the preceeding ten years, research in the field of rock mechanics had been slowly gaining momentum, and Talobre's treatise was most timely. Important research had been going on on both sides of the Atlantic mainly in connection with the mining industry. American mining schools and the U. Bureau of Mines were very active and so too were their European counterparts.
They were concerned with theoretical problems of stress around rectangular-shaped cavities but were also faced with many practical problems. Techniques were being developed for measuring strains and rock deformations, rock elasticity and convergence of the walls of galleries and cavities. Most American experts regard as the year in which systematic research into rock mechanics began in the USA.
American mining schools and universities were increasingly active, mainly on the national level.
Introduction to Rock Mechanics
Nation-wide symposia were organized; the U. Bureau of Reclamation at Denver was leading world research on the properties of rock material and rock masses; and an American Society of Engineering Geologists was created, one of its aims being the development of rock mechanics, which for many years was also the concern of the American Geophysical Society, the American Society for Testing and Materials and their research committees.
In several American universities the teaching methods in engineering geology were modernized and adapted to the requirements of the petroleum and mining industries. In Europe during the years to , the most active centre of research outside the mining schools was probably the University of Vienna, where European and American efforts 3 Stini created an Austrian Society for Geophysics and Engineering Geology.
This expanded rapidly and the 'Austrian School' became well known for its efforts in precisely describing and defining the faults and fissures in rock, far more exactly than is usual in engineering geology. Engineers from many European countries congregated in increasing numbers at the annual congress organized in Salzburg. After deciding against the possibility of linking its efforts with the International Conference on Soil Mechanics, the Salzburg group expanded on its own, forming the core of an independent International Society for Rock Mechanics.
As early as , dam designers started a parallel effort, when a suggestion was submitted by the author to the International Commission on Large Dams ICOLD to create a sub-committee on rock mechanics. Up to this time geologists had been extremely careful in deciding where to build dams. Dams were of relatively moderate size and there were few problems on the stability of rock abutments.
Isolated cases of dam rupture were explained by uplift forces or by failing shear strength of the rock. These two points initiated the development of techniques of dam foundation. The demand for more and more electric power, led to larger dams and to the introduction of bold arch dams. The problems of the strength of rock abutments were becoming more pressing and more difficult.
It became imperative to include the elasticity and plasticity of the rock abutments in the mathematical analysis of the arch dams and to consider with greater care the strain and stress distribution in the rock masses. The techniques of testing rock in situ, of analysing test results and of exploring rock abutments with galleries attained a high degree of precision. It was felt that dam designers and hydro-power engineers were fully responsible for the structures they were designing and that they could not leave the task of developing methods for rock testing, tunnel construction and dam foundation design to others.
In , a small committee of experts, headed by G. Westerberg Stockholm , submitted a report recommending the formation, within the organization of ICOLD, of a 'Committee on underground work' whose aim would be to solve the most urgent problems of rock foundation for large dams.
On 3 December , the dam of Malpasset burst, killing about people. Tensor: a quantity with magnitude and direction, and with reference to a plane it is acting across e.
Both mathematical and engineering mistakes are easily made if this crucial difference is not recognized and understood. These forces create the stress tensor. The normal and shear stress components are the normal and shear forces per unit area.
It should be remembered that a solid can sustain a shear force, whereas a liquid or gas cannot. A liquid or gas contains a pressure, which acts equally in all directions and hence is a scalar quantity. In fact, the strict definition of a second-order tensor is a quantity that obeys certain transformation laws as the planes in question are rotated.
F1, F2, …, Fn. Consider now the forces that are required to act in order to maintain equilibrium on a small area of a surface created by cutting through the rock.
Although there are practical limitations in reducing the size of the area to zero, it is important to realize that the stress components are defined in this way as mathematical quantities, with the result that stress is a point property. Note that the force and displacement have been scaled respectively to stress by dividing by the original cross-sectional area of the specimen and to strain by dividing by the original length.
It is important to realize that the compressive strength is not an intrinsic property. Intrinsic material properties do not depend on the specimen geometry or the loading conditions used in the test: the uniaxial compressive strength does. These features are crucial in the design and analysis of underground excavations.
If the ratio of sample length to diameter is kept constant, both compressive strength and brittleness are reduced for larger samples. Rock specimens contain microcracks: the larger the specimen, the greater the number of microcracks and hence the greater the likelihood of a critical flaw and effects associated with crack initiation and propagation. In other words, rock has strength in tension, compression and shear.
Brazilian tensile test, triaxial compression test, etc. Most rocks are therefore strengthened by the addition of a confining stress. As the confining pressure is increased, the rapid decline in load carrying capacity after the peak load is reached becomes less striking until, after a mean pressure known as the brittle-to-ductile transition pressure, the rock behaves in a near plastic manner.
In most cases, however, it is the effect of pore water pressure that exerts the greatest influence on rock strength. If drainage is impeded during loading, the pores or fissures will compress the contained water, raising its pressure. Creep — strain continues when the applied stress is held constant.
Relaxation — a decrease in strain occurs when the applied stress is held constant.
Fatigue — an increase in strain occurs due to cyclic changes in stress. The limited test data does show though, that increasing temperatures reduces the elastic modulus and compressive strength, whilst increasing the ductility in the post-peak region.
Building on the history of material testing, it was natural to express the strength of a material in terms of the stress present in the test specimen at failure i. A tension cutoff has been introduced to the Mohr-Coulomb criterion to predict the proper orientation of the failure plane in tension.
They can be considered linear only over limited ranges of confining pressures. Despite these difficulties, the Mohr-Coulomb failure criterion remains one of the most commonly applied failure criterion, and is especially significant and valid for discontinuities and discontinuous rock masses.
Since this is one of the few techniques available for estimating in situ rock mass strength from geological data, the criterion has become widely used in rock mechanics analysis. Discontinuities have been introduced into the rock by geological events, at different times and as a result of different stress states.
Very often, the process by which a discontinuity has been formed may have implications for its geometrical and mechanical properties. Very often major discontinuities delineate blocks within the rock mass, and within these blocks there is a further suite of discontinuities. Thus, we might expect that a relation of the form: should exist.
There is, however, no standardized method of measuring and characterizing rock structure geometry, because the emphasis and accuracy with which the separate parameters are specified will depend on the engineering objectives. Frequency i. An important feature for engineering is the overall quality of the rock mass cut by these superimposed fracture systems.
For this reason, the concept of the RQD was developed. It is often useful to present this data in a graphical form to aid visualization and engineering analysis. It must be remembered though, that it may be difficult to distinguish which set a particular discontinuity belongs to or that in some cases a single discontinuity may be the controlling factor as opposed to a set of discontinuities. In practice, the persistence is almost always measured by the one dimensional extent of the trace lengths as exposed on rock faces.A number of the papers deal with erosivity of rain only one paper is about erosivity of wind , whilst fewer are about the erodibility of the soil.
It is from this point of view that this book has been written. The finite element method allows the analysis of jointed rock masses, crossed by fissures or faults.
We will assume that deformation is made up of two components: The limited test data does show though, that increasing temperatures reduces the elastic modulus and compressive strength, whilst increasing the ductility in the post-peak region. In Europe during the years to , the most active centre of research outside the mining schools was probably the University of Vienna, where European and American efforts 3 Stini created an Austrian Society for Geophysics and Engineering Geology.
These entirely new concepts about the behaviour of rock masses are initiating new lines of research in rock mechanics. Pages With increasing fractures, the material tends to become isotropic in strength, like a granular soil.
As the depositional load increased, lower layers suffered a volume reduction largely attributable to a reduction of the pore volume of the rock.
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