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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 bilgedumarre.gqn Second Edition, John Wiley and Sons Some knowledge of rock mechanics is vital for civil engineers.
Courses No. Ogila 1 What is Rock Mechanics? Rock mechanics is a science that uses the principles of mechanics to describe the behavior of rock for engineering purposes.
Engineering Rock Mechanics
The Engineering Definition of Rocks Rock is the hard and durable material that cannot be excavated without blasting. Or The earth materials that do not slake when soaked into water. Ogila 2 Rock Groups and Classes Based on their geologic aspects, rocks are grouped by origin as igneous, sedimentary, and metamorphic, these rocks are classified according to their petrographic characteristics, which include their mineral content, texture, and fabric.
Geologists concern about the origin, classification, history, and the economic aspects of rocks. Rocks are identified mostly by its texture; mineral composition; field relationships; color; crystal size and crystal form, and other petrographic characteristics. Ogila 3 Engineering Considerations of Igneous Rocks 1 Fine-grained of alkali igneous rocks cannot be used as aggregates in Portland cement due to volume expansion caused by the Alkali- silica reaction.
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.
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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.
Rock Mechanics and Engineering
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.
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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. This obviously introduces a degree of sampling bias that must be accounted for in the interpretation of results. From the practical point of view, only one technique has received some degree of universality — the Joint Roughness Coefficient JRC.
This method involves comparing discontinuity surface profiles to standard roughness curves assigned numerical values. The geometrical roughness is naturally related to various mechanical and hydraulic properties of discontinuities. This parameter has mechanical and hydraulic importance, and a distribution of apertures for any given discontinuity and for different discontinuities within the same rock mass is expected.
In compression, the rock surfaces are gradually pushed together, with an obvious limit when the two surfaces are closed.In all of these subjects, the geological history has played its part, altering the rock and the applied forces. Rock specimens contain microcracks: Ogila 3 Engineering Considerations of Igneous Rocks 1 Fine-grained of alkali igneous rocks cannot be used as aggregates in Portland cement due to volume expansion caused by the Alkali- silica reaction.
Brazilian tensile test, triaxial compression test, etc.
Those out to better themselves should b u y this book. Full Text 72 Part 5 is called "Miscellany" twenty-eight papers.
Whilst the b o o k is printed in the English language, the presentation of the papers is uneven. Creep — strain continues when the applied stress is held constant.