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Chapter 1
General Geology
A. Introduction
There are several branches of science that enables the study of the earth’s features. Geology is the most important branch in the family of earth science. The different branches of geology deal with the history of earth’s origin, the materials in its interior, and the physical processes that take place within it, especially plate tectonics. Knowledge of geology is necessary to study fossils in rock formations and to reconstruct the plant and animal world of remote past. However, geology for civil engineering work primarily involves activities related to rocks and soil occurring in the top crustal layer of the earth (to a depth of only a few hundred metres). .
B. Input
The Association of Engineering Geologists includes in its 2000 Annual Report and Directory the following statement: “Engineering Geology is defined by the Association of Engineering Geologists as the discipline of applying geologic data, techniques, and principles to the study both of a) naturally occurring rock and soil materials, and surface and sub-surface fluids and b) the interaction of introduced materials and processes with the geologic environment, so that geologic factors affecting the planning, design, construction, operation and maintenance of engineering structures (fixed works) and the development, protection and remediation of ground-water resources are adequately recognised, interpreted and presented for use in engineering and related practice.”
The philosophy of engineering geology is based on three simple premises. These are:
- All engineering works are built in or on the ground.
- The ground will always, in some manner, react to the construction of the engineering work.
- The reaction of the ground (its “engineering behaviour ” ) to the particular engineering work must be accommodated by that work.
In general, engineering geology is concerned with the properties of materials such as soil and rocks used in engineering projects, including the quantitative assessment of their strength, permeability, and compactness. These properties help in the selection of sites for civil engineering structures. Engineering geology uses data, techniques, and principles of varied branches of geology to study the surface and subsurface materials for evaluating their origin, distribution, and effects on engineering constructions. ‘Engineering geological investigations of Geological Survey of India (GSI) involve application of geomorphology, economic geology, geohydrology, stratigraphy, structural geology, petrology and mineralogy besides geophysics’, observed Krishnaswamy (1972), the well-known engineering geologist and former Director General of GSI. In recent years, the field of engineering geology has expanded and encompasses rock mechanics, soil mechanics, satellite remote sensing, earthquake zoning, seismotectonics, and geohazard management.
I. Geology in Civil Engineering The scope of a geologist and that of a civil engineer while working on a project are the same; both work towards construction of safe structures for the benefit of human beings. The
Site model for a new building, illustrating some of the factors and hazards that need to be addressed by the engineering geologist.
rocks and the distribution of major and trace elements in rocks, rock suites, and minerals. This can lead to an understanding of how a particular rock has originated (petro genesis), and also, in the broadest sense, to a knowledge of the chemistry of the upper layers of the Earth. The distribution of rocks at the Earth’s surface is found by making a geological survey (that is, by geological mapping ) and is recorded on geological maps. This information about rocks is superimposed on a topographic base map. Knowledge of the nature and physical conditions of the deeper levels of the planet can be gained only by the special methods of geophysics, the twin science of geology; the term ‘Earth sciences’ embraces both. From the theory and methods of geophysics, a set of techniques (applied geophysics) has been evolved for exploring the distribution of rocks of shallower levels where the interests of geologists and geophysicists are most intertwined. Knowledge of the Earth at the present time raises questions about the processes that have formed it in the past: that is, about its history. The interpretation of rock layers as Earth history is called stratigraphy, and a study of the processes leading to the formation of sedimentary rocks is called sedimentology. The study of fossils (palaeontology) is closely linked to Earth history, and from both has come the understanding of the development of life on our planet. The insight thus gained, into expanses of time stretching back over thousands of millions of years, into the origins of life and into the evolution of man, is geology’s main contribution to scientific philosophy and to the ideas of educated men and women.
III. Earth Structure and Composition
The Earth is an oblate spheroid. It is composed of a number of different layers in spherical shells as determined by deep drilling and seismic evidence (Fig. 3.2). These layers are the following:
- The Earth can broadly be modeled as an outer solid silicate crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.
- The core is approximately 7000 km in diameter (3500 km in radius) and is located at the Earth’s center.
- The mantle surrounds the core and has a thickness of 2900 km.
- The crust floats on top of the mantle. It is composed of basalt-rich oceanic crust and granitic-rich continental crust.
FIGURE 3.2 (A) Schematic diagram of the Earth’s structure representing a three-dimensional perspective and (B) a sectional view portraying from central core to outer surface.
The continental and oceanic crusts have one common property. These tectonic plates have the ability to rise and sink. This phenomenon, known as isostasy, occurs because the crust floats on top of the mantle-like ice cubes in water. These tectonic plates deform and sink deeper into the mantle as and when the Earth’s crust gains weight due to mountain building or glaciations. The crust becomes more buoyant and floats higher in the mantle if the weight is removed. The Mohorovičić discontinuity (Moho) is the line between the Earth’s crust and the mantle. It separates oceanic crust and continental crust from the mantle. The Mohorovičić discontinuity named after Andrija Mohorovičić, a Croatian geophysicist, who has established it. The Mohorovičić discontinuity is 5-10 km (3-6 miles) below the ocean floor and 20-90 km (10- 60 miles) beneath the continents. The Gutenberg discontinuity , named after German scientist Bruno Gutenberg, is located at 2900 km depth beneath the Earth’s surface. The boundary is observed by the applications of seismic waves. This discontinuity is due to the differences between the acoustic impedances of the solid mantle and the molten outer core.
IV. Continental Drift and Plate Tectonics Continental drift is associated with the opening and extension of the ocean floor at the oceanic ridges. The temperatures of rocks near the centre of a ridge are higher than on either side of it, because material from the mantle rises towards the surface in the hotter central part of a ridge. The cause of this upward flow is believed to be the operation of slow-moving convention currents in the Earth's mantle. The currents rise towards the base of the lithosphere and spread out horizontally, passing the continental margins and descending again. The hotter rock-material in the rising current is less dense and possesses buoyancy, which is the driving force of the mechanism. Alfred Wegener of Germany first asserted the concept that light rocks of continents resting over the heavy crustal materials of deeper parts slowly drifted from their original positions through the long geological time. According to the modern accepted concept of earth scientists, the earth’s outer crust is made up of several huge fragments of continental dimensions, which are termed tectonic plates. These plates move relative to one another above a hotter, deeper, and more mobile zone at average rates of few centimetres per year due to convection current. According to this concept of continental drift , the earth was initially made up of a single continent called
Pangaea. Fossil evidence of Gondwanaland also proves that once there were land bridges connecting Australia, India, Africa, and South America. However, eventually Pangaea broke up to form the seven continents of the current time. Plate tectonic theory explains the processes responsible for global and regional earthquakes. Bolt (1999) has discussed in detail the theory of plate tectonics in relation to the earthquake processes. According to this theory, the lithosphere , which is the crust and outer mantle of the earth ranging from 80 km to 200 km in thickness, consists of several large and stable segments of rocks called plates. These plates are, in fact, oceanic and continental parts of huge dimensions. The plates have taken the current pattern by fragmentation of the ancient supercontinent Pangaea over the past 200 million years_._ Below the lithosphere is the asthenosphere composed of rocks very close to melting point. The asthenosphere occasionally rises to the lithosphere through some weak places as a hot spot. Each lithospheric plate moves horizontally relative to the neighbouring plate on the softer rock of asthenosphere in the lower part. The energy for the driving mechanisms of the drifting plates comes from the heat produced by the decay of the radioactive elements in rocks inside the earth. There are three main possibilities of movement of the plates. First, the plates move together causing ruptures of the crust at the edges of the plates. Second, the adjacent plates may move apart from each other creating a widening gap between them. Finally, the plates may slide alongside each other on transform faults. Thus, there are three environments in earthquakes caused by plate movements, namely compressional (or convergence), extensional (or divergence), and transform: i. At the convergent boundaries, the earthquakes are of the most severe types and originate from shallow depths to deep parts of the subduction zone , which is a very deep trench in the earth’s crust. There may be convergence of plates when the edge of a plate is moved down and forced into subduction. The subducting plate is pushed down into the asthenosphere, where it is heated and absorbed into the mantle rock. ii. At the divergent boundaries such as spreading ridges, the earthquakes originate at a shallow depth along the spreading line, showing the divergence mechanism. Spreading ridges or mid-oceanic ridges are formed due to the spreading apart of plates below the ocean.
an earthquake or a volcano. In mid-oceanic ridge , there is constant ejection of materials from the earth’s surface to form new crust. This material is deep seated on both sides of the plates and is responsible for breaking apart or unification of the plates. The processes arising from the growth of the plates produce earthquakes along the mid-oceanic ridges. Some related definitions are the following:
- Transform faults are the horizontal displacements, or spaces, between plates and also the main place of earthquakes that have shallow epicenter under the surface of the Earth.
- Oceanic ridges stand as boundaries between the divergent plates movement. In that process, magma is injected between plates or poured on the ocean floor forming growth of oceanic crust. The mid-oceanic ridges and growth of the oceanic crust represent in the form of effusive volcanic rocks.
- Subduction zone is a place where the Earth’s crust is broken down and consumes part of the oceanic crust or oceanic plate, which underscores (subduction) under a continent or island arc. Subduction zone is slope surface tilted in the direction of subduction, along which the main focus of earthquake takes place and along which remelting of rocks occur.
V. Earth Processes The wide range of conditions found in different parts of the globe at present reflects the variety of natural processes that operate to shape the surface of the Earth. Land areas are continually being reduced and their shape modified by weathering and erosion, and the general term for this is denudation. Rocks exposed to the atmosphere undergo weathering from atmospheric agents such as rain and frost. Chemical weathering, or decomposition, is the break-down of minerals into new compounds by the action of chemical agents; acids in the air, in rain and in river water, although they act slowly, produce noticeable effects especially in soluble rocks. Mechanical weathering, or disintegration, breaks down rocks into small particles by the action of temperature, by impact from raindrops and by abrasion from mineral particles carried in the wind. Biological weathering describes those mechanical and chemical changes of the ground that are directly associated with the activities of animals and plants. By these processes a covering layer of weathered rock is formed on a land surface. Normally the upper layers of this cover are continually removed, exposing the fresher material
beneath it to the influence of the weathering agents; in this way the work of denudation continues. In some circumstances the weathered material may remain in position as a residual deposit or soil that retains many characters of its parent rock and differs significantly in its mechanical properties from soils formed by the deposition of sediment. Agents of erosion - rivers, wind, moving ice, water waves - make a large contribution to the denudation of the land. They also transport the weathered material, the detritus, away from the areas where it is derived; the removal of material is called erosion. The deposition of detritus transported by erosion produces features such as deltas at the mouths of rivers, beaches on shore lines, screes at the base of slopes and sand dunes in deserts.
These are examples of the constructive or depositional aspect of surface processes.
Weathering Weathering is the process by which rock deteriorates until it eventually breaks down to a soil. It occurs close to the Earth’s surface and depends very much on climatic influences: rainfall and temperature.
Chemical weathering The processes most commonly involved in chemical weathering are listed in Table 3.1. Their rate of operation depends upon the presence of water and is greater in wet climates than in dry climates. Except where bare rock is exposed, the surface on which rain falls consists of the soil which forms the upper part of the weathered layer. This 'top-soil' ranges in depth from only a few centimetres to a metre or more, according to the climate and type of rock from which it has been derived. In temperate climates, in general, it is a mixture of inorganic particles and vegetable humus and has a high porosity, i.e. a large proportion of interstices in a given volume: it has a high compressibility and a low strength and for these reasons is removed from a site prior to construction.
the disintegration of the outer layers of rock. The loosened fragments fall and accumulate as heaps of scree or talus at lower levels, material which may later be consolidated into deposits known as breccia. By the removal of the fragments the surface of the rock is left open to further frost action, and the process continues.
Frost-heaving This occurs when the freezing of the soil results in the formation of layers of segregated ice at shallow depths. Each lens of ice is separated from the next by a layer of soil, whose water content freezes solid. The ice lenses vary in thickness from a few millimetres to about 30 mm. The total heave of the surface is approximately equal to the aggregate thickness of all the ice layers. Insolation In hot climates, when a rock surface is exposed to a considerable daily range of temperature, as in arid and semi-arid regions, the expansion that occurs during the day and the contraction at night, constantly repeated, weaken the structure of the rock. The outer heated layers tend to pull away from the cooler rock underneath and flakes and slabs split off, a process known as exfoliation. This weathering is called insolation. Under natural conditions, insolation of rock faces may result in the opening of many small cracks - some of hair-like fineness - into which water and dissolved salts enter; and thus both the decomposition of the rock and its disintegration are promoted.
Biological weathering Weathering effects which are small in themselves but noticeable in the aggregate can be attributed to plants and animals (biotic weathering). Plants retain moisture and any rock surface on which they grow is kept damp, thus promoting the solvent action of the water. The chemical decay of rock is also aided by the formation of vegetable humus, i.e. organic products derived from plants, and this is helped by the action of bacteria and fungi. Organic acids are thereby added to percolating rain-water and increase its solvent power. Bacteria species may live in the aerobic and anaerobic pore space of the weathering zone, and mobilize C, N, Fe, S and O, so assisting the process of weathering and sometimes attacking concrete and steel. Their mineral by-products can accumulate and cause expansion of the ground if not washed away by percolating water. The mechanical break-up of rocks is hastened when the roots of plants penetrate into cracks and wedge apart the walls of the crack.
Erosion and Deposition Four important agents of erosion and transport (wind, rivers, the sea at coasts, and glaciers) and the deposits associated with them are described in the following sections.
The work of wind (windborne) A strong wind blowing across rock debris or soil can lift and carry fine material as dust, and can move the larger sand grains by rolling them and making them bounce across the surface. This windborne movement of material occurs in areas with little or no vegetation and is typical of hot desert regions, although the process also operates in cold deserts and some coastal areas. Wind both transports and sorts the material. The finer, silt-sized fraction is carried in suspension by the wind, and may travel great distances before it is eventually deposited as loess. The coarser material that remains forms sand dunes. Both loess and sand dunes are liable to further erosion by wind unless their surface is stabilised by vegetation or another binding agent. Erosion and transport by wind are most important in desert regions, which lack a protective cover of vegetation and a skin of surface water to bind the grains together.
The works of sea The sea erodes by the pounding action of waves on the base of cliffs, and also by compressing air present in crevices and so causing further explosive impact within them. The cutting action of its currents as they sweep across the foreshore is aided by the rubbing of pebbles dragged along by the moving water. Limestone cliffs may also suffer some erosion by solution of the rock in water. As a result of these agents, a notch is cut at the foot of cliffs (Figs 3.9 & 10) undermining them and causing them to collapse and retreat from the sea. This leaves a wave-cut platform between the cliffs and low-water mark. Where the cliffs are formed of poorly consolidated rock and soils, erosion may proceed at rates that average 1–2 m per year. This erosion usually takes the form of sporadic and alarming inroads by cliff falls after storms. Along such coastlines, waves generated by wind action are more important than ‘tidal waves’. The size of waves generated in the open sea is dependent upon fetch, which is the distance over which the wind operates.
A change in the profile of equilibrium, or interference with the normal movement of sediment, produces compensatory erosion and deposition. If the slope of the foreshore were to become more gentle, there would be some erosion of the terrace as sand and shingle are carried landwards on to the beach. Part of this sand may be added temporarily to dunes, and later returned to the sea by offshore winds. Any loss of sediment from the offshore terrace and consequent steepening of the profile can increase beach and cliff erosion dramatically. So far, movement of beach material has been described in terms of a simple two dimensional model of a shore, where the striving for equilibrium moves sediment up or down a
profile at right angles to the shore. In real shores, this is complicated by movement of sediment along the coast, that is, by longshore drift. The direction in which waves meet a shore is controlled partly by its trend, partly by wind direction, and partly by the direction of the advancing flood tide. When the latter two factors operate, waves approach the shore obliquely (Fig. 3.12), and the sediment they carry is swept up the beach in the same direction. Backwash, however, carries the grains straight down the slope of the beach, so that they suffer a net lateral displacement along the shoreline. Cumulative movement of sediment by this process is controlled by the balance of the dominant wind and the strength of the advancing flood tide.
Longshore drift may be controlled by groynes built normal to the shore, which hinder sediment movement. A beach builds up at the groyne, but at the expense of erosion of the beach’s downdrift from it. Terminal erosion cannot be avoided unless the deficiency in longshore drift is counteracted by supplying material from off shore. For this reason, coastal protection should not be considered piecemeal. With planning it may be possible to arrange for terminal erosion to occur along an unimportant stretch of coast. Longshore drift is interfered with naturally at the mouths of rivers. On being swept into the deeper water of an estuary, the sand settles beyond the easy reach of wave action, and its accumulation produces a spit (Fig. 3.13).
Fig. 3.12 The path of a sand grain from A to B in an area where a long shore wind causes the waves to approach the beach obliquely. The uprush of the advancingwave carries the grain in the same direction (up), but the direction in which it is taken up by backwash (back) of the retreating wave is controlled by the slope of the beach.
Fig. 3.13 Longshore drift of sand is hindered by the groyne and sand
the Alpine-Himalayan belt, on mid-ocean ridges, at trenches and in young fold mountain belts. Seismic activity is of two types: shallow and deep earthquakes. The former are concentrated at ocean ridges and transform faults, and occur at depths of less than 30 km. Deep earthquakes occur in zones beneath the oceanic trenches, with the foci located at varying depths along the subduction zone, which extends downwards to depths of more than 250 km.
Mode of Occurrence During earthquakes, bodies of rock on opposite sides of faults move relative to each other. Both dip-slip and strike-slip displacements are recognized, as well as combinations of the two. During the Loma Prieta earthquake, for example, the relative strike-slip displacement between the Pacific and North American plates was 1.9 m. In addition, there was 1.3 m of uplift of the Pacific plate relative to the North American plate (Figure 8.35), despite the common perception of the San Andreas fault as a strike-slip fault. Vertical displacement may be due to a bend in the trend of this section of the fault.
Figure 8.35 Relative horizontal and vertical movement of plates during the Loma Prieta earthquake.
Figure 8.38 Elastic rebound theory. Line a-a’ is a real imaginary linear feature that crosses a fault (F- F’) such as a road of fence. Elastic strain produces distortion of the line to the form shown as line b-b’. At the time of rupture, block A slides rapidly past block B and the elastic strain energy is now represented by segments c-c’ and c’’-c’’’.
A longstanding theory in geology, formulated by the analysis of surface rupture after the 1906 earthquake, holds that rocks deform in an elastic manner until brittle failure takes place (Figu.re 8.36). Elastic strain that had been accumulating on either side of the fault is suddenly released, causing the rocks on either side of the fault to abruptly slide past each other. Elastic strain energy is released in the form of seismic waves, which radiate outward through the rocks in all directions from the Hypocenter or point of rupture.
Seismic prospecting The study of the propagation of mechanical waves in the Earth has created induced seismology, which is used to identify geologic structures in the subsurface and indirectly, the nature of the ground. The principle is identical to natural seismology, but it uses a man-made seismic source such as an explosion in a shallow borehole or a vibrational device. The signal is then captured by geophones distributed on the surface. Two principle methods can be distinguished: Seismic Refraction : This method is the most commonly used because it is inexpensive and easy to use. It is based on the fact that “rapid” ground at depth accelerates the wave propagation and sends it back to the surface. The wave arrival times are noted on each geophone. A time-distance curve (called a dromochronic plot) can be made showing the arrival time as a function of the distance between the geophone and the source. It is possible to deduce the speed of the waves in different media and the structure of the various media. This method is used to find shallow structures (bedrock under landslides, thickness of a filtering bed on top of aquifers etc.). Seismic Reflection : In this method, the surface devices record the arrival of waves that have been reflected at the interfaces of deep strata. As in the preceding method, the arrival times are recorded at each receptor, and then a time-distance curve is drawn to obtain the wave velocity in different media. This method is more complex to process but it gives excellent results for deep reflectors. It is used particularly in petroleum exploration to research tectonic structures likely to trap hydrocarbons. On a smaller scale, it makes it possible to prospect large alluvial valleys. In both cases, seismic profiles are obtained that make it possible to represent surfaces as a result of obvious seismic velocity contrasts between different media.
