Title: Role of geology in civil Engineering

By Prof. Devesh Rashinkar

Module 1

Introduction

Geology is the study of earth, the materials of which it is made, the structure of those materials and the effects of the natural forces acting upon them and is important to civil engineering because all work performed by civil engineers involves earth and its features. Fundamental understanding of geology is so important that it is a requirement in university level civil engineering programs.

Civil engineers design structures that are built on or in the ground. As such an understanding of how the ground behaves is fundamental to civil engineering design. Earth materials can pose significant problems that need to be predicted, planned and designed for. For a civil engineering project to be successful, the engineers must understand the land upon which the project rests.

Geologists study the land to determine whether it is stable enough to support the proposed project. They also study water patterns to determine if a particular site is prone to flooding. Some civil engineers use geologists to examine rocks for important metals, oil, natural gas and ground water.

Geological engineers play an important role in identifying and mitigating man-made and natural hazards that pose a threat to civil structures, infrastructure, or people. Their work includes performing site investigations for planned tunnels, dams, or roads; locating sites and designing facilities for nuclear waste disposal; developing and restoring groundwater resources; stabilizing rock and soil slopes for dams, highways, and property development; exploring and harvesting mineral and energy resources; and studying geologic hazards such as volcanoes, landslides, and earthquakes.

Optimization and reliability of civil engineering generates the need for more and more specified data about the ground-water environment and forecasts of possible changes. This chapter presents the meaning, scope (boundaries) and the competence of engineering geology in historical, formal and practical (real) terms. The following modules will be discussed in this chapter-

1. Basic concepts of geology and their application in Civil Engineering2. Importance of Geology for Civil Engineering Projects3. Geological structures and their significance in Civil Engineering projects

Module 2

Basic concepts of geology and their application in Civil Engineering

a) Plate Tectonics- Plate Tectonics is a geologic theory explaining the movements and forces in the Earth crust. The basic point in plate tectonic is, that earth's surface is broken into seven large and many small moving plates, hence the name. These plates move relative to one another an average of a few centimeters per year. Three types of movement are recognized at the boundaries between plates: convergent, divergent and transform-fault. This is a rather simple result of the the fact that earth is a sphere with more or less constant diameter. So if two plates move in different direction, they either collide or move away from each other. There are only two types of plates, continental plate, which is higher than sea level and oceanic plate, which is below sea level.

b) Uniformitarianism- In geology, uniformitarianism is the belief that Earth's physical structure is the result of currently existing forces that have operated uniformly (in the same way) since Earth formed roughly 4.5 billion years ago. The activities of the present are a key to those of the past. Uniformitarianism allows us to interpret the events of the past in rocks; it allows us to write the history of Earth. In addition to allowing the interpretation of the past, uniformitarianism allows for the prediction of the future. Understanding how and when rivers flood, what causes earthquakes and where they are likely to occur, or how and when a volcano will erupt can limit damage from these events. Although short-term prediction still eludes geologists, long-range forecasting of such disasters can ultimately saves lives and property.

c) Superposition- state that in a sequence of sedimentary rock layers, each layer of rock is older than the layer above it and younger than the rock layer below it. The Law of Superposition

also applied to other geologic events on the surface, such as lava flows and ash layers from volcanic eruptions. This is the basis of relative ages of all strata and their contained fossils.

d) The Geochemical Cycle- a geochemical cycle is the pathway that chemical elements take in the surface and crust of the Earth. The term "geochemical" tells us that geological and chemical factors are all included. The migration of heated and compressed chemical elements and compounds such as silicon, aluminium, and general alkali metals through the means of subduction and volcanism is known in the geological world as geochemical cycles.

e) Geological Time and the Stratigraphic Column- The geologic time scale (GTS) is a system of chronological measurement that relates stratigraphy to time, and is used by geologists, paleontologists, and other earth scientists to describe the timing and relationships between events that have occurred throughout Earth's history. Evidence from radiometric dating indicates that the Earth is about 4.54 billion years old. The geology or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by the absolute age.

f) Geomorphic Systems- Geomorphic Systems is the study of deep and shallow Earth processes that integrate through time to shape the landforms and landscapes that compose our physical environment. Once the link between process and landscape is understood, then we can read the landscape to interpret the present and past Earth processes active in a region. The societal applications for that knowledge include land-use planning, geologic hazard mapping, ecosystem restoration and predicting the effects of global climate change

g) Thresholds and Uniformity of Natural Systems- Natural physical systems operate under natural physical laws with the time element involved varying between uniformity and catastrophe. The dividing line between these is known as Threshold for a system, and may be defined a condition at which a process or system changes, some times abruptly, from a relatively simple to a relatively complex thing; from a relatively predictable to a relatively non predictable system; from a measurable to a non measurable system or one measured only with difficulty; from a uniform to a non uniform state.

Example- shearing strength of materialsPhysically beyond the shearing strength threshold, rocks lose cohesion, and potential energy (stored elastic energy) is converted into kinetic (particle motion) energy

h) Magnitude and Frequency of Forces in Geomorphic Processes- The relative importance in geomorphic processes of extreme or catastrophic events and more frequent events of smaller magnitude can be measured in terms of (1) The relative amounts of "work" done on the landscape and (2) in terms of the formation of specific features of the landscape.

For many processes, above the level of competence, the rate of movement of material can be expressed as a power function of some stress, as for example, shear stress. The frequency at which this maximum occurs provides a measure of the level at which the largest portion of the total work is accomplished.

Analysis of records of sediment transported by rivers indicates that the largest portion of the total load is carried by flows which occur on the average once or twice each year. As the variability of the flow increases, the size of the drainage basin decreases, and a larger percentage of the total load is carried by less frequent flows.

Closer observation of many geomorphic processes is required before the relative importance of different processes and of events of differing magnitude and frequency in the formation of given features of the landscape can be adequately evaluated.

i) Equilibrium of Geological Processes and their Disturbance by Engineering Activity-Balance in geological and geomorphic systems is a delicate condition which takes long period of time to establish. The balance is easily disturbed, by among others, civil engineering activity. It is of great importance to try to understand how geological systems and processes influencing those systems are likely to react to those disturbances.Example- Levels of in situ stress in geological materials result from more or less complex array of geological process ranging from over burden to episodes of tectonic deformation. The stresses may be disturbed in a number of ways by construction activity and processes permitting the redistribution of stresses often lead to situations which require careful control and monitoring

Module 3

Importance of Geology for Civil Engineering Projects

The importance of geology in civil engineering may briefly be outlines as follows:

Geology provides a systematic knowledge of construction material, its occurrence, composition, durability and other properties. Example of such construction materials is building stones, road metal, clay, limestones and laterite.

The knowledge of the geological work of natural agencies such as water, wind, ice and earthquakes helps in planning and carrying out major civil engineering works. For example the knowledge of erosion, transportation and deposition helps greatly in solving the expensive problems of river control, coastal and harbor work and soil conservation.

Ground water is the water which occurs in the subsurface rocks. The knowledge about its quantity and depth of occurrence is required in connection with water supply, irrigation, excavation and many other civil engineering works.

The foundation problems of dams, bridges and buildings are directly concerned with the geology of the area where they are to be built. In these works drilling is commonly undertaken to explore the ground conditions. Geology helps greatly in interpreting the drilling data.

In tunneling, constructing roads, canals, docks and in determining the stability of cuts and slopes, the knowledge about the nature and structure of rocks is very necessary.

Before staring a major engineering project at a place, a detailed geological report which is accompanied by geological maps and sections, is prepared. Such a report helps in planning and constructing the projects.

The stability of civil engineering structure is considerably increased if the geological feature like faults, joints, bedding planes, folding solution channels etc in the rock beds are properly located and suitably treated.

In the study of soil mechanics, it is necessary to know how the soil materials are formed in nature.

The cost of engineering works will considerably reduced of the geological survey of the area concerned is done before hand.

Role of geology in selection of sites

The geology of an area dictates the location and nature of any civil engineering structures.

A. Roads and Railways

Problems for a road or railway project may be caused by any of the following geological features:

• Faults

• Junctions between hard and soft formations boundaries between porous and impermeable formations

• Spring-lines

• Fractured granites

• weathered schists

• Landslip areas

• Areas where beds dip towards the road or railway, as shown in the adjacent diagram.

If the terrain and proposed route are such that these features cannot be avoided, construction of suitable safety features is required. Earthwork construction must include an embankment to stabilize areas of landslip. Lightweight material on a concrete raft may be needed where the road traverses deep, compressible deposits.

B. Dams

Geological investigations of a site proposed for construction of a dam must be complete and detailed. Features such as rock-types, geological structures, weathering, fractures and fissures must all be considered. The main considerations are that the material on which the dam rests must be able to carry the weight of the structure without failing. The geology upon which the dam is built must also be impervious to water. The abutments, (the rock faces to which the dam wall is attached) must also be impervious and strong enough to support the dam wall, especially in the case of an arch dam (where more force is transmitted to the abutments).

Failure of a dam can be due to many factors including:

• Earthquakes

• A sudden drop in water level

• Inadequate protection of the reservoir side of the dam from wave action

• Insufficient spillway capacity, so that water flows over the whole of the dam surface, with consequent erosion

C. Building Foundations

Since the type of rock and soil inevitably affects stability of buildings, the quality of the foundation rock must be investigated before construction commences. This rock must not be weak, crushed; water saturated or has been subjected to chemical weathering. The presence of fractures, faults, joints, cleavages, etc may indicate that the site is unsuitable for building. The possibility of soil-creep, slope movement, landslides etc must be borne in mind and factored into the design of any building foundation. Obviously, buildings should not be situated too close to the coast, especially where the sea level is rising relative to the land.

Rock and soil tests are taken before homes are built. For larger buildings, deep holes may be drilled to test the strength and stability of the rocks under the proposed building. The type and strength of foundations required are determined from the results of these tests.

D. SLOPE FAILURE

The term slope failure covers a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows.

Causes of Slope Failure:

1. Gravity

Although gravity acting on an over-steepened slope is the primary cause of a landslide, other contributing factors include:

• Earthquakes that create stresses causing weak slopes to fail.

• Volcanic eruptions that produce loose ash deposits and debris flows.

• Vibrations from machinery, traffic, blasting, and even atmospheric thunder that may trigger failure of very weak slopes.

• Excess weight from accumulation of rain, snow, the stockpiling of rock or ore, or from built structures that may stress weak slopes to failure.

2. Relief

Slope failure occurs in hilly or mountainous regions all over the world — essentially wherever there is any significant topographic relief.

3. Water

Rock and soil slopes are weakened through saturation by melting snow or heavy rain. Water filling the pores of permeable materials allows the grains to slide past each other with little friction. Water acts as a lubricant increasing the ease of movement of rock and soil particles (and therefore slope failure).

4. Undercutting

Undercutting is erosion of material at the foot of a cliff or steep bank — e.g. on the outside of a meander. Ultimately the overhang collapses and the process is repeated. Undercutting caused by rivers, glaciers, or ocean waves creates over-steepened slopes, which are prone to failure. Human activities, such as quarrying and road construction also result in undercutting.

5. Rock Types

In unconsolidated material, that is material not held together by cement or by a strong interlocking crystal structure, landslides start after a significant part of the whole rock mass is saturated with water and therefore lubricated. A single shock or vibration can trigger the down-slope movement of an entire unstable hillside. Any area of very weak or fractured materials resting on a steep slope will be likely to experience landslides.

6. Slope Angle

A pile of sand always assumes the same angle of slope, whether it is a few centimeters high, or a huge sand dune. The angle that the sand makes with the horizontal is called the angle of repose. It is about 37° for fine sand, and steeper for coarse sand and angular pebbles.

If a slope is steepened beyond this natural angle, for example for a road cutting, it then becomes unstable and the slightest vibration may lead to slope failure. The angle of repose is reduced if the sand or unconsolidated rock material becomes water-saturated. Moreover, the angle of repose is significantly reduced underwater.

Module 4

Geological structures and their significance in Civil Engineering projects

Geologic structures influence engineering projects in many ways. Folds and faults obviously have much to do with the selection of dam sites and even such seemingly unimportant matters as the spacing of joints may have vital bearing on uplifting pressure and safety of dams. Gushed and chemically altered rocks contiguous to originating along faults may damage or destroy engineering structures. The design of deep cuts in rocks is greatly influenced of geologic structures on circulation of the ground water. Some of the most common terms that are involved in the study of geologic structures are as follows:

I. Bedding Planes: The planes or surfaces which divide on bed from the other are called bedding Plane.

II. Dip: The dip of a bed in the angle between the bedding and the horizontal plane.III. Strike: It may be defined as the direction of line formed by the intersection of bedding and

horizontal plane.IV. Outcrop: The area of exposure of bed on the earth’s surface is called outcrop.

1. FOLDS

Perhaps the most common type of deformation is folding. As the name implies, folds are undulation, flexures waves which resembles to ocean waves. They are best displayed in stratified formation i.e. sedimentary rocks. But any layered or foliated rock such as banded gabbros or granite gnessis may display folds. Some folds are few miles away, the width of others to be measured in feet or inches or even in fraction of an inch.

On the basis of dip relationships two major types of folds can be distinguished:

a. Anticline: In which the strata on opposite flanks dip towards the axis in other words the folds that concave upwards.

b. Monoclines: Folds in which horizontal or gently dipping beds are modified by simple step like bends.

Over turned Fold: The axial plane is inclined and both limbs dip in the same direction but usually at different angles.

2. FAULTS

Faults are fractures in the earth’s crust along which slippage or displacement has occurred. As a result, formerly continuous beds have been dislocated in a direction parallel to fault’s surface. The displacement may vary from a few inches or less, to many miles.

When subjected to great pressure, the earth’s crust may have to withstand shear force in addition to direct compression. If the shear forces so induced become excessive, failure will result, movement will take place along the plane of failure until the unbalanced forces are equalized and a fault will be the result.

Types of Faults

The two common types of faults are normal faults and reversed fault. In a normal fault the hanging wall is displaced downward relative to the footwall. In the reversed faults the hanging wall is displaced upwards relative to footwall.

Active and Inactive Faults

Fractures that are known to have experienced dislocation within historic time are known as active faults. As they present hazard to construction, the differentiation of active and inactive faults is matter of considerable engineering importance, and it is quite unfortunate that frequently no very reliable decision can be made. The most direct and best evidence of activity is that furnished by seismographs and benchmarks. If the seismograph records show that earthquakes ocean along a fault it should, of course be regarded as active. Similarly accurately located bench mark exhibit horizontal in vertical displacement, and fault known to exist in the area should be regarded as active. If a fault is known to be overlain by younger strata that are not displaced, it is permissible to regard it as inactive.

3. JOINTS

Joints are planes or surface which intersect rocks, but along which there has been no appreciable displacement parallel to the joint surface. Joints result either from tension or shear stress acting on rock mass. The cause of stresses may be due to contraction, compression, unequal lift, subsidence, earthquake or other earth phenomena.

Tension joints arise, for instance by drying and resultant shrinkage of sedimentary deposits, or igneous rocks by contraction and cooling. Shear joints may arise from compression of sedimentary or igneous rocks.

Engineering Significance

Because of their almost universal presence, joints are of engineering importance, especially in excavation operations. It is desire able for joints to be spaced closely enough to reduce secondary plugging and blasting requirement to a minimum, but not so closely spaced as to impair stability of excavation slopes or increase breakage in tunnels. Needless to say, the ideal conditions are seldom encountered. Joints oriented approximately at right angles to the working face present the most unfavorable conditions, whereas joints oriented approximately parallel to the working face greatly facilitate blasting operations and ensure a fairly even and smooth break parallel to the face. Joint offer channels for underground water circulation and in working below the ground water table may greatly increase water problems. They also may exert an important influence on weathering.

Unconformities

An unconformity is a contact between two rock units in which the upper unit is usually much younger than the lower unit. Unconformities are typically buried erosion surfaces that can represent a break in the geologic record of hundreds of millions of years or more. For example, the contact between 400‐million‐year‐old sandstone that was deposited by a rising sea on a weathered bedrock surface that is 600 million years old is an unconformity that represents a time hiatus of 200 million years. The sediment and/or rock that was deposited directly on the bedrock during that 200‐million‐year span was eroded away, leaving the “basement” surface exposed. There are three kinds of unconformities: disconformities, nonconformities, and angular unconformities.

Objective:

For a civil engineering project to be successful, the engineers must understand the land upon which the project rests. Geologists study the land to determine whether it is stable enough to support the

proposed project. Optimization and reliability of civil engineering generates the need for more and more specified data about the ground-water environment and forecasts of possible changes. The objective of this chapter is to present the meaning, scope (boundaries) and the competence of engineering geology in historical, formal and practical (real) terms. The following modules will be discussed in this chapter-

1. Basic concepts of geology and their application in Civil Engineering2. Importance of Geology for Civil Engineering Projects3. Geological structures and their significance in Civil Engineering projects

Summary:

Geology provides necessary information about the site of construction materials used in the construction of buildings, dams, tunnels, tanks, reservoirs, highways and bridges. Geological information is most important in planning phase (stage), design phase and construction phase of an engineering project. Before construction begins, selection of site is important from the viewpoint of stability of foundation and availability of construction materials. Geology of area is important and rock-forming region, their physical nature, permeability, faults, joints, etc. Thus, geology is related to civil engineering in construction jobs with economy and success.