Enginnering Geology- N Chennakesavulu
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Engineering Geology...
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2nd
Edition
C N Chenna Kesavulu, 1993, 2009
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All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission. Any person who does any unauthorized act in relation to this publication may be liable to criminal prosecution and civil claims for damages. First published, 1993 ·Reprinted, 1997-,2007(eight times) Second edition, 12009
MACMILLAN PUBLISHERS INDIA LTD. Delhi Bangalore Chennai Kolkata Mumbai Ahmedabad Bhopal Chandigarh Coimbatore Cuttack Guwahati Hubli Hyderabad Jaipur Lucknow Madurai Nagpur Patna Pune Thiruvananthapuram Visakhapatnam Companies and representatives throughout the world ISBN I 0: 0230-63870-8 ISBN 13: 978.0230-63870-9 Published by Rajiv Beni for Macmillan Publishers India Ltd. 2110, Ansari Road, Daryaganj, New Delhi 110 002 Typeset by Sri Krishna Graphics B-22, South Ganesh Nagar, Delhi 110 092 Printed at Baba Barkha Nath Printers 37, MIE, Bahadurgarh, Haryana t24 507
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CONTENTS
Preface to the Second Edition Preface to the First Edition 1.
3.
4.
5.
IX
GEOLOGY
1
I.I
2 5 7
1.2 1.3 2
Vil
Main and Allied Branches of Geology Importance of Geology in Civil Engineering Scope of Geology
GENERAl.GFOlOGY
11
2.1 2.2 2.3
12 14 27
Geological Agents Weathering of Rocks River as a Geological Agent
MINERALOGY
39 a
3 1
Defiojtjoo of
Mineral
3.2 33 3.4 3.5 3.6 3.7
Definition of a Crystal Mode of Foanation of Minerals Common Rock-forming Minerals and Their Abundance Different Methods of Study of Minerals Significance of Different Physical Properties in Mineral Identification Diagnostic or Distinguishing Physical Properties
39
41 43
44 45 48 (j()
COMMON ROCK-FORMJNG MINERALS
61
4.1 4.2 4.3 4.4
61 I 61 65
Types of Rock-forming Minerals Special Features of Silicate Minerals Brief Sketch of Common Rock-forming Minerals Important Details of Common Rock-forming Minerals
PETROl.oGY 5I 5.2 53 5.4 5.5 5.6
Shell Stn1cn1re
68 94
of rhe Earth
Definition of a Rock Classification of Rocks Sequence of Formation of Different Groups of Rocks Rock Cycle Civil Engineering Importance of Petrology
94
95 96
100 IOI
102
Gopynghted material
xu Contents 6.
7.
8.
IGNEOUS ROCKS Introduction 6.1 Forms of Igneous Rocks 6.2 Miscellaneous 6.3 Common Igneous Rocks and Relation of Their Constituent Minerals 6.4 Classification of Igneous Rocks 65
Stn1ct11res and
6.6 6.7
Suitability of Igneous Rocks for Building and Foundation Megascopic Description of Relatively Common Igneous Rock Types
Texh1res
SEOIMENTARYROCKS
109
1 II Ill
115 123
125 134
Introd11ction
134
7.1 7.2 7.3 7.4
134 135 145 150
Sedimentary Rocks on the Earth's Crust Classification of Sedimentary Rocks Common Structures and Textures of Sedimentary Rocks Descriptive Study of Common Sedimentary Rocks
METAMORPHIC ROCKS Introduction
8.1 8.2 8.3 8.4 9.
103 103 104
Metamorphism Common Structures and Textures of Metamorphic Rocks Classification of Metamorphic Rocks Descriptive Study of Common Metamorphic Rocks
SIRUCTURALGEOLOGY
161
161 162 171
175 176 190 190
Causes for Development of Structures
9.5 9.6
Unconformity Common Symbols to Indicate Some Geological Structures
I 0. IMPORTANCEOF GEOLOGICAL STRUCTURES
II.
222 226
227
Introduction
227
10. I 10.2 10.3
228 232 236
Effects of Folding and Their Civil Engineering Importance Effects of Faulting and Their Civil Engineering Importance Effects of Joints and Their Civil Engineering Importance
GROUNDWATER
Introduction I I.I Sources of Ground Water Supply I 11 Advantages of Using Ground Water 11.3 Distribution of Rainfall 11.4 Porosity and Permeability 11.5 Classification of Rocks Based on Porosity and Permeability 11.6 Water Table and Types of Ground Water I 1.7 Geological Controls on Ground Water Movement I I .8 Fluctuation of the Water Table Level in Unconfined Aquifers I 1.9 Ground Water Potential in Different Parts of India
240 240 241 242 244
245 248 250 253 254 256
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Contents
12.
13.
14.
I I.IO Ground Water Exploration 11.11 Effects of Excessive Tapping of Ground Water 11.12 Waterlogging
257 260
STRATIGRAPHY
26)
Introduction 12.I Aims of Stratigraphy 12.2 Principles of Stratigraphy 12.3 Geological Time Scale 12.4 Geological Divisions of India 12.5 Major Stratigraphical Units of India 12.6 Importance of the Study of Stratigraphy from the Civil Engineering Point of View
261 262 262 264 267
284
EARTHQUAKES
285
Introduction 13. I Earthquake Terminology 13.2 Classifications and Causes of Earthquakes 13.3 Seismic Belts and Shield Areas 13.4 Earthquakes and Faulting 13.5 Earthquake Waves 13.6 Intensity of Earthquakes 13.7 Magnitude of the Earthquakes 13.8 Locating the Epicentre of an Earthquake 13.9 Determining the Depth of the Focus of an Earthquake 13.10 Effects of Earthquakes 13.11 Civil Engineering Considerations in Seismic Areas 13.12 Plate Tectonics and Earthquake Distribution
285 286 286 288 288 289 290 291 293 294 294 295 298
LANDSLIDES
300
Introduction 14.1 Importance of Landslides 14.2 Classification of Earth Movements 14.3 Causes of Landslides 14.4 Effects of Landslides 14.5 Preventive Measures for Landslides
300 300 301 303 306 306
15. GEOPHYSICAL INVESTIGATIONS
16.
XIII
260
268
308
Introduction 15. I Branches of Geophysics 15.2 Necessity of Geophysical Investigations 15.3 Principles of Exploration Geophysics I 5.4 Classification of Geophysical Methods 15.5 Well-logging
308 308 309 311 312 329
GEOPHYSICAi. INVESTIGATIONS OF CIYII. ENGINEERING IMPDRTANCE
331
Introduction
331 332 345
16.1 16.2
Electrical Resistivity Method Seismic Refraction Method
c,opvnghted material
xiv
Contents
17.
ENGlNEERING PROPERTIES OF ROCKS
356
Introduction 17.1 Different 17 .2 Tests for 17.3 Tests for 17.4 Tests for
356 357 358 362
Engineering Rocks Used Rocks Used Rocks Used
Property Tests for Rocks as Foundation Sites of Constructions as Building Stones as Aggregates
18. DAMS
371 376
Introduction 18.1 Importance of Geology in Dam Construction 18.2 A Dam and Its Parts 18.3 TYPes of Dams and Bearing of Geology of Site in Their Selection 18.4 Purposes of Dams 18.5 Geological Considerations in the Selection of a Dam Site 18.6 Stages of Investigation in the Selection of a Dam Site I 8,7 Case Histories
376
377 380 381 383
384 396 398
19, RESERVOIRS
4JJ
lntroductjoo
411
19. I
412
19 .2
19.3 19.4
19.5 19.6
19. 7
Considerations for Successful Reservoirs Capacity of the Reservoir Effect of Evaporation Water-tightness and Influencing Factors Reservoir Silting Seismic Activity in Reservoir Areas Landslide Occurre;ices
20. TUNNELS
412
412 413 420
425 425 427
Introduction 20. I Purposes of Tunnelling 20.2 Effects of Tunnelling on the Ground 20.3 Lining of Tunnels 20.4 Economical Aspects of Tunnelling 20.5 Geological Considerations for Successful Tunnelling 20.6 Overbreak 20. 7 A Few Examples of Tunnels of Interest and Importance Bibliography Index
427 428 429
429 430
430 440
441 443
•
Copvnghted maienal
1 GEOLOGY
Introduction; I. I Main and Allied Branches of Geology; I. I. I Physical Geology; 1.1.2 Mineralogy; 1.1.3 Petrology; 1.1.4 Structural Geology; 1.1.5 Historical Geology; l.1.6 Palaeontology; 1.1.7 Economic Geology; 1.1.8 Engineering Geology; 1.1.9 Mining Geology; I. I. I 0 Geophysics; I. I.JI Geohydrology; 1.1.12 Geochemistry; 1.2 Importance of Geology in Civil Engineering; 1.3 Scope of Geology; 1.3.1 Academic Importance of Geology; 1.3.2 Importance of Applied Geology in Different Fields.
Aims: The aims of this chapter are: 1. To introduce the subject. 2. To list out the main and allied branches of geology, explain their subject matter and to briefly indicate their relevance from the civil engineering point of view. 3. To explore the scope of geology in terms of its academic significance. It is purely meant to inculcate interest and curiosity in the subject matter. 4. To give the importance of geology in the fields of: (a) exploration, estimation and exploitation of economic mineral deposits. and mining; (b) ground water studies, town-planning; (c) civil engineering, and so on.
INTRODUCTION Geology is the science of the earth (geo = earth, logos = study or science). It deals with different aspects of the earth as a whole such as (i) origin, age, interior structure and history of the earth; (ii) evolution and modification of various surface features like rivers, mountains and lakes along with their causes; and (iii) materials making up the earth. Geology is a relatively recent subject. In addition to its core branches, advances in geology in allied fields have led to specialized sciences like geophysics, geochemistry, geohydro1ogy, glaciology, seismology, oceanography, rock mechanics, photogeology, and remote sensing. t Similarly, based on the applied importance of geology in other fields, related subjects such as engineering geology, mining geology and so on have come into existence.
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Textbook of Engineering Geology
1.1 MAIN AND ALLIED BRANCHES OF GEOLOGY The vast subject of geology has been subdivided into the following branches for the sake of systematic study: Main branches:
Allied branches:
Physical geology Mineralogy Petrology Structural geology Historical geology (stratigraphy) Palaeontology
Engineering geology Mining geology Geophysics Geohydrology Geochemistry
Economic geology 1.1.1 Physical Geology This is also variously described as dynamic geology. geomorphology, etc. As the name suggests it deals with: (i) different physical features of the earth, such as mountains, plateaus, valleys, rivers, lakes, glaciers, and volcanoes in terms of their origin and development, (ii) the different changes occurring on the earth's surface, like marine transgression, marine regression, formation or disappearance of rivers, springs and lakes, (iii) geological work of wind, glaciers, rivers, oceans, ground water, and their role in constantly moulding the earth's surface features, and (iv) natural phenomena like landslides, earthquakes, and weathering. The main cause for surface changes is weathering. This is a natural phenomenon resulting directly or indirectly due to changes in the atmosphere. It disintegrates and decomposes rocks. This aspect is of special importance from the civil engineering point of view, because colour, appearance, strength and durability of rocks are adversely affected by weathering. Thus even granite which is considered ideal for most of the civil engineering works becomes weak and friable on thorough weathering. rendering it useless. Civil engineers deal with structures like darns which are artificial barriers to the natural flow of rivers. Proper understanding of the geological work of a river and its features will lead to their better utilization for engineering applications. 1.1.2 Mineralogy This deals with the study of minerals. Minerals are basic units with which different rocks and ores of the earth are made up of. Details of mode of formation, composition, occurrence, types, association, properties, uses, etc., of minerals form the subject matter of mineralogy. Knowledge in this branch of geology is necessary for a civil engineer because the properties of rocks -(which he is going to make use of in different ways) are to a large extent contributed by the properties and composition of their constituent minerals. For example, sometimes quartzite and marble resemble one another in shine, colour and appearance. But quartzite by virtue of its mineral composition is very hard, tough, strong and durable, while marble disintegrates and decomposes in a shorter period because of its mineral composition and properties.
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Geology
3
1.1.3 Petrology (Petro = rock, logos = study) Petrology deals with the study of rocks. The earth's crust, also called lithosphere, is made up of different types of rocks. Petrology deals with mode of formation, structure, texture, composition, occurrence, types, etc., of rocks. The composition and textural characters of rocks primarily contribute to their inherent strength and durability. Rocks based on their suitability can be used as foundation for dams, for tunnelling and as materials of construction. Hence this is the most important branch of geology from the civil engineering point of view. 1.1.4 Structural Geology The rocks which form the earth's crust undergo various deformations, dislocations and disturbances under the influence of tectonic forces. The result is the occurrence of different geological structures like folds, faults, joints and unconformities in rocks. The details of mode of forrnation, causes, types, classification, importance, etc., of these geological structures form the subject matter of structural geology. From the civil engineering point of view, it is as important as petrology because these geological structures modify the inherent physical characters of rocks rendering them more suitable or unsuitable for civil engineering purposes. For example, at a dam site sedimentary rocks with upstream dip provide a desirable geological set-up, while the same rocks with downstream dip make the geological set-up most undesirable. 1.1.5 Historical Geology (Stratigraphy) The earth's surface was always uneven and provided suitable conditions for the deposition of sediments at some place or the other. Therefore, there are sedimentary rocks on the earth representing the entire period of the earth's history. Proper investigations of these rocks reveal the chronological sequence of formation of rocks, evolution-migration-extinction details of different plant and animal life during the different periods of the earth's history. In addition, the climatic and geographical changes including tectonic events in the geological past can also be known from these investigations. This kind of study of the earth's history through the sedimentary rocks is called historical geology. It is also called stratigraphy (strata= a set of sedimentary rocks; graphy =description) because this subject deals with deiails and description of sedimentary rock sequences. This is a major branch of geology, having a lot of academic and applied importance. From the civil engineering point of view also it is relevant that except for the fact that older rocks are in general more stable (like shield areas of the Archaean era) and more competent due to more compaction and cementation. Further, study of stratigraphy can be utilized in predicting the underlying strata which may be very important in some specific civil engineering constructions. 1.1.6 Palaeontology If, under favourable conditions, animal or plant life gets embedded in sediments, it will be preserved partly or completely. Such relics and remnants of ancient life preserved in rocks by natural processes are known as "fossils". Details of the mode of formation of fossils, their types, occurrence, etc., form the subject matter of palaeontology. Like stratigraphy, this is also an important branch of geology. though it is not of much importance from the civil engineering point of view. But as fossils are rare
Lopynghted m2tennl
4
Textbookof Engineering Geology
and throw much light on the past history of the earth, a civil engineer should know some details regarding them so that he recognizes them as fossils. Whenever he comes across such finds during his work, he should report the matter to the person concerned, for necessary action. 1.1.7 Economic Geology Minerals can be grouped as general rock-forming minerals and economic minerals. Some of the economic minerals like talc, graphite, mica, asbestos, gypsum, magnesite, barytes, diamond and gems are useful as such or as raw materials in different industries. Some others like hematite, chromite, galena and pyrolusite are used as ores for the extraction of various metals, the uses of which are well known. The prosperity of a nation depends to a large extent on the rich reserves of economic mineral deposits it has. (For example, Gulf countries are rich because of their oil deposits; South Africa is rich because of its gold and diamond deposits.) The details of their mode of formation, occurrence, classification, association, varieties, concentration, properties, uses, etc., form the subject matter of "economic geology". This branch of geology, though it is very important by virtue of its economic importance, is not relevant for civil engineers for obvious reasons. It will be enough for them to know a few details as in the case of fossils, so that they will not be ignorant of them as and when they come across these in course of their civil engineering works like tunnelling and road cutting. Each of the foregoing branches deals with specific subject matter and comprises the main branches of geology proper. Further, based on application of geological knowledge in other fields there are many other allied branches collectively called earth sciences. Some of them described here are: Engineering geology. Mining geology. Geophysics. Geohydrology. Geochemistry. 1.1.8 Engineering Geology This deals with the application of geological knowledge in the field of civil engineering, for execution of safe, stable and economic constructions like dams, bridges and tunnels. As this is the branch with which we are most concerned, it has been explained in detail separately (refer to Sec. 1.2). 1.1.9 Mining Geology This deals with the application of geological knowledge in the field of mining. A mining engineer is interested in the mode and extent of occurrence of ores, their association, tenor, properties, etc. It is also necessary to know other physical parameters like depth, direction (strike), inclination (dip), thickness and reserve of ore bodies for efficient utilization. Such details of mineral exploration, estimation and exploitation are dealt with in mining geology. The importance of geology in mining may be cited with the following example. Sometimes, the lodes or seams of economic minerals suddenly get terminated. This might happen either due to the natural limit of the ore body or due to faulting. Geological studies will solve· this problem and, if it
~opyngh
ma· n81
Geology
S
is due to faulting, the continuity can be traced by ascertaining the direction and extent of displacement caused by faulting. The geological knowledge helps in planning the method of mining or quarring a deposit in an advantageous way.
1.1.10 Geophysics The study of physical properties like density and magnetism of the earth or its parts, to know its interior, forms the subject matter of geophysics. Broadly it is subdivided into general (or pure) geophysics and exploration (or applied) geophysics. Pure geophysics deals with general aspects of the earth as a whole and exploration geophysics deals with the study of upper layers of the earth's crust in order to (i) solve some civil engineering problems, (ii) locate oil and goes deposits, (iii) locate ground water, (iv) explore and estimate the ore deposits, etc., of underground. There are different types of geophysical investigations based on the physical property utilized, viz .. gravity methods, seismic methods, magnetic methods. Since these are quickly and easily done on the surface, large areas can be investigated economically and efficiently. Engineering geophysics is a branch of exploration geophysics which aims at solving civil engineering problems by interpreting subsurface geology of the areas concerned. Geophysical investigations are very useful in solving foundation problems, alignment of structures, leakage problems along canals, locating building materials like stones (where they are not available on the surface), etc. Electrical resistivity methods and seismic refraction methods are commonly used in solving civil engineering problems.
1.1.11 Geohydrology This may also be called as hydrogeology. It deals with occurrence, movement and nature (i.e., quality and quantity) of ground water in an area. It has applied importance because ground water has many advantages over surface water. This branch is closely related to geology because the very existence, movement of ground water, etc., are directly related to porosity, permeability, structure, texture and composition of the surface and underground rocks. Dykes may control the occurrence and movement of ground water. In general, geological, geophysical (electrical resistivity method) and hydrological studies are together taken up for ground water investigations.
1.1.12 Geochemistry This branch is relatively more recent and deals with the occurrence, distribution, abundance, mobility, etc., of different elements in the earth's crust. It is not important from the civil engineering point of view.
1.2 IMPORTANCE OF GEOLOGY IN CIVIL ENGINEERING The civil engineers aim at safety, stability, economy and life of the structures that they construct. Civil engineering constructions like dams and bridges will have their foundations on geological formations of the earth's surface. Therefore, their stability and safety depend on the competence of the in situ rocks of the sites concerned. Also, to be economical, such competent foundation rocks should be at a shallow depth. Further, for huge constructions like dams, building materials are required in very large
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120
Textbook of Engineering Geology
by the surrounding enormous cold sea water), then there may not be any time for crystallization to take place. Under such conditions, lava solidifies as completely amorphous or glassy matter without any minerals. On the other hand, if cooling time is intermediate, then the resulting rock will be composed partly of glassy matter and partly of minerals. Thus, depending on the nature of cooling, the resulting igneous rocks are: (i) completely crystalline (holocrystalline, holo = complete) i.e., completely made up of minerals without any glassy matter, or (ii) completely glassy (holohyaline, hyaline = glassy or amorphous) i.e., composed of only glass without any minerals, or (iii) partly crystalline and partly glassy (merocrystalline or hemicrystalline, hemi =half) i.e., some part composed of minerals and the rest being glass. Thus, the preceding three different types of crystallization give rise to three textures of igneous rocks namely: holocrystalline, holohyaline and hemicrystalline. Textures Based on Granularity Depending on the physical conditions that had prevailed during the crystallization of magma, mineral grains occur in different sizes. The presence of volatiles, low viscosity, slow cooling and great pressure help to grow large minerals. (Molecular concentration also influences the growth of a mineral, i.e., a mineral with larger grains is formed if its molecular concentration is more in the melt.) The absolute sizes of minerals vary widely. Some (microlites) are too small and can be seen only under powerful microscopes, while others are very large and heavy. A beryl crystal from Albany, Maine (USA) has been recorded to measure 18 feet in length and to weigh 18 tons. Similarly, beryl crystals weighing up to 20 tons have been found in pegmatite mines of Rajasthan in our country. Therefore, the ratio of extreme absolute sizes of minerals may be more than .1: l million. However, the size ratio of commonly occurring minerals is only I: 1000 (approximately). The following textures have been recognized based on the granularity of minerals. If minerals in the rock are big enough to be seen by the naked eye, the texture is described as phaneric texture. On the other hand, if minerals are too fine to be seen separately by the naked eye, the texture is described as aphanitic texture. The phaneric texture, for convenience of description, has been further classified into coarse, medium and fine. When the mineral grain size is more than 5 mm, the texture of the rock is called phaneric-coarse; when the grain size happens to be in between 1 mm and 5 mm, the texture is called phaneric-medium and when the grain size is less than 1 mm, the texture is called phaneric-fine. Similarly, the aphanitic texture is also classified as microcrystalline, cryptocrystalline and glassy. When the grains are recognizable under the microscope, the texture is called aphanitic-microcrystalline. If the rock is amorphous and minerals are not noticed at all under the microscope, the texture is described as aphanitic-glassy. If the minerals had undergone only very incipient growth, and are not distinguishable under the microscope but affect the polarized light giving a hazy outlook under crossed Nicols (unlike a black field in case of glassy rock). the texture is called aphanitic-cryptocrystalline. Textures Based on Shapes of Crystals These textures are of two different kinds: The first in terms of development of crystal faces or boundary outlines and the other with reference to the nature of the growth of the mineral. Minerals develop perfect crystal outlines when they crystallize in a thin liquid medium free from interference by neighbouring crystals. Pegmatite minerals like beryl tourmaline belong to this kind. Minerals also develop such outlines when they crystallize in early stages of magma solidification because then they would be surrounded only by a liquid melt on all sides, enabling the mineral to
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Igneous Rocks
133
in colour and texture. These genetically related rocks are called chamockite series. This series is also called Nilgiri gneiss or mountain gneiss.
Minerals Present All chamockites are characterized by an abundance of feldspars and pyroxenes, particularly hypersthene. Acidic chamockites are like hypersthene granites, rnineralogically, i.e., they mainly contain feldspars, quartz and hypersthene. The intermediate and basic types are equivalent to quartz norites and norites respectively. (Norite is like gabbro. The difference is that the pyroxene present in gabbro is augite, but in norite the pyroxene is hypersthene. Both have the labradorite type of plagioclase feldspar in addition.) The ultrabasic (i.e., silica-poor) type of chamockite is equivalent to pyroxenite.
Mode of Origin Though chamockites are considered as igneous rocks, they show peculiar characters diagnostic of both igneous and metamorphic rocks. Hence there is a controversy about their origin. Throwing off tongues and veins into surrounding rocks by the chamockite body and occurrence of features indicative of partial assimilation and hybridism strongly support the intrusive and igneous origin of chamockites. But (i) the occurrence (though occasionally) of typical metamorphic minerals Like cordierite and garnet, (ii) the occurrence of gneissose structure, (iii) the myrmekitic growth of quartz and feldspar and (iv) sporadic round quartz grains enclosed by feldspar strongly support the metamorphic origin of charnockites. Cbamockites are believed to have been formed out of recrysl31lization of igneous rocks under conditions of high temperature and pressure (i.e., of plutonic metamorphism).
Appearance of Charnockite in Hand Specimens Megascopically, i.e., in hand specimens, all chamockites, irrespective of their composition, are melanocratic, i.e., black coloured. All are phaneric coarse grained and equigranular with shining laths of feldspar. ln acidic varieties, greasy looking bluish grey quartz occurs.
Structure and Texture Like other plutonic rocks chamockites also are compact, massive, dense and non-porous (impervious). Texturally, these are phaneric coarse grained. Foliation and banding occur sometimes only for short lengths.
Availability Charnockites, which are Archaean in age, occur widely distributed in peninsular India and form a portion of the Nilgiri hills and southern parts of the Eastern and Western ghats. Tamil Nadu, Karnataka and, to some extent, Andhra Pradesh have large and good outcrops of charnockites.
Properties and Uses Chamockites are hard, strong and durable rocks with a high load-bearing capacity. Like other plutonic rocks these are also non-porous and impermeable. They do not have any weak planes like bedding, significant banding or foliation. They are suitable for all civil engineering works. However, their colour is blackish. Mahabalipuram temples (Tamil Nadu) were constructed many centuries ago from charnockites. The chamockites of peninsular India are claimed to be one amongst the strongest and the most durable stones of the world.
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162
Textbook of Engineering Geology
8.1 METAMORPHISM
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=
The word metamorphism means change of form (meta change; morpb form). In petrology, it indicates the effectof temperature, pressure and chemically active solutions over the texture, minerals and composition of parent rocks. Igneous and sedimentary rocks which serve as parent rocks are formed under a certain physicochemical environment, i.e., at the time of their formation, they were in equilibrium with their surroundings in terms of temperature, pressure and chemically active fluids. Subsequent to their formation, if any of these factors changes significantly, the equilibrium gets upset and necessary metamorphism, i.e., textural, compositional and mineralogical changes take place to create a new equilibrium. This means the constituent minerals, texture and composition of parent rocks by metamorphism change over to new minerals or new textures or new compositions which are more stable and suitable under new conditions. For example: As a result of metamorphism (i) granite, one of tile most abundant igneous rocks, changes to granite gneiss; (ii) periodotite, an ultrabasic igneous rock, changes to serpentine and talc schist; (iii) gabbro (or dolerite), an intermediate igneous rock, changes into hornblende schist. Among sedimentary rocks (iv) sandstone changes into quartzite; (v) limestone changes into marble; (vi) shale changes into slate and so on. The range of temperature and pressure which occurs in nature is very wide. The normal surface temperature and pressure affect the rocks by causing weathering. At the other extreme, intense heat in the subsurface (at great depths) melts the rocks and produces magma. As accepted by many, the term metamorphism does not include either weathering of rocks or magma formation. The extreme states of metamorphism are represented by palingenesis or ultrametamorphism or anatexis. In these the intensity of temperature and pressure will be so high that it involves partial melting and mixing of rocks. Since the identity of the altered rocks is not totally destroyed, in these states, they come under metamorphism. They are next only to the formation of new magma. 8.1.1 Metamorphic Agents The process of metamorphism occurs in rocks due to the effect of high temperature, pressure and chemically active fluids. These three are known as metamorphic agents. Generally, all these three act together and cause metamorphism. But, sometimes, any one or two of them may dominate and play an active role. The following are a few relevant details about metamorphic agents.
Temperature The source of temperature which is responsible for metamorphism is either due to depth or due to the contact with magma (i.e., magma chamber or magmatic intrusion). The metamorphic changes mainly take place in the temperature range of 350-850°C. The temperature rise also increases the chemical activity in rocks and facilitates reactions during metamorphism.
Pressure The pressure which causes metamorphism is of two different kinds, namely, uniform pressure and directed pressure. . Uniform pressure increases with depth (i.e., with increasing overburden), It acts vertically downwards and affects the volume of both liquids and solids. Naturally, its effect is significant 'only at great depths, but not at or near the surface. This also means that high temperatures will also be associated (due to the depth factor) with high uniform pressure. So, both of them act together and bring about metamorphism in rocks.
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