Geological Information

Engineering  works may succeed or fail according to how well we understand the geological process having occurred at the area/site.

The topography, geology of the bedrock and surface deposits, and the hydrology are factors which are to be considered in planning, designing and construction of engineering structures. In this section, we will study the geological aspects that govern the site-selection process, such as, 

1. Ground stability and its relevance to the foundation design for various structures , 
2. Ease and cost of excavating different kinds of formations/ground 
3. Susceptibility of various kinds of pound toerosion 

Determining the geology of an area for engineering purposes involves more than merely locating and indicating the kind of surface deposits and rock formations. The relationships between these must be determined in order to learn about their geological history. 

The earth's crust began solidifying, some four and a half billions of years ago. As the crust developed into a solid formation, water vapour, nitrogen, carbon dioxide, and other gases gradually escaped from the earth's interior through vents in the slowly cooling crust. Water vapour formed clouds and when the rains did come some four billion years ago, water running off the high ground collected to form the first streams, lakes and seas. 

It was in the seas that life began and not until the photo-synthesizing organisms evolved did oxygen become a constituent of the atmosphere. The gradual evolution of life was of increasing diversity and complexity. In the course of the next three and a half billions of years, slabs of the earth's crust thickened and broadened to form the present continents. The evolution of higher forms of life and the development of the bio-chemical process, so important today in weathering and soil formation, took place in the last half years. Most of the surface deposits and soils which are of interest to the engineers, formed in the past few million years and many influences upon these materials date back to less than ten thousand years (Hunt, 1972). 

The loose deposits covering the bedrock are called surface deposits. They are composed largely of mineral matter, like the bedrock from which they are derived. Their thickness may vary from 3 to 30 meters. Most surface deposits are sediments weathered from bedrock in one area and transported by water, wind or ice to another area. Because of this, they are often unrelated mineralogical to the underlying bedrock. Some surface deposits are non-transported layers and they are, as such, called residual deposits. Still other non-transported deposits are formed by the accumulation of organic matter at the location; accumulation of plant material forms peat. 

Weathering of surface deposits causes the development of layers of different types of soil. Thus, at a building site, we may find surface deposits as well as weathered soils which are relatively modem. The engineering properties of the soil layers depend upon the nature of weathering, moisture content, degree of fragmentation and to a certain extent the chemical nature of salts present in them. Soil engineering is the subject which deals with such phenomena. Apart from this, the stability of large structures, such as dams, is influenced by geological features like faults in the earth's crust. Changes in the crust really occur on a gigantic scale resulting in earthquakes, eruption of volcanoes, faults and creation or disappearance of large land masses. The ground on which we build is, geologically speaking, really unstable, but then human knowledge is growing steadily to cater, to some extent, to such extraordinary forces of nature. 

Structure of the Rocks 

Bed rock, the parent material of surface deposits (and ultimately of soils), is classified according to mode of their formation: igneous, sedimentary and metamorphic. There are many varieties of each kind depending on the minerals in them, and their texture. 

Igneous rocks were once molten lava, granite is a typical example. 

Sedimentary rocks were once unconsolidated sediments and subsequently these became compacted and firmly adhered to each other to form a rock. Some such rocks are chemical precipitates like limestone and dolomite. Sand stones and shale are other examples. 

Metamorphic rocks are those that were once either igneous or sedimentary, but were subsequently altered or recrystallized by heat and or pressure at great depth in the earth's crust. Marble, the metamorphic equivalent of limestone or dolomite is widely used in building construction for ornamental purposes. 

Based on the structure of rocks certain physical behaviours can be predicted as under : 

1. Permeability - it is the degree to which water can enter and flow through the inter-stices within the rock. It is high in sandstone, at 10 percent. But it is 14w for igneous rocks such as granite. Water absorption by small specimens is an indirect test for permeability. Specimens of igneous rocks may absorb water only upto 1 to 2 percent of water of its weight. 
2. Fractures are structural characteristics of a rock that influence its weatherability. Fractures in massive igneous rocks (fissures and cracks) have preferred directions, and water which may carry dissolved acids seeps into such cracks and reacts with rock walls causing chemical alterations. Few sedimentary or metamorphic rocks are massive, and most of them are stratified. Water can seep along the bedding planes and open up further fractures and damage these rocks faster. 
3. Freezing and thawing of water and hydration of minerals are major factors in the weathering of all types of rocks. 
4. Erosion of rocks takes place by the action of wind and water. Shale is easily eroded, and sandstones are susceptible to erosion by rain. 

Ground Stability 

Ground stability is dependent on numerous variables, one of which is slope. Rocks in which bedding planes or fractures dip in the same direction as the slope of the hill are dangerously subject to landslides, particularly if water can enter these fractures. When a cutting is made in rocks for roads and foundations, horizontal bedding planes even in poorly frosted sandstone often give near-vertical faces,  which are stable. Heavy rain, especially after a drought, saturates the material forming the slope, and thus, increasing its mass and the gravitational pull; and the reduced friction between the joints, due to water thus, initiate a rock slide in hilly regions. 

Constructions at Rocky Sites 

Dams are meant to hold water and the rocks at the ground and those below them must be impervious. The consequences of increase in water table elevation and the deposition of sediments on the foundation rock must be studied. Fault zones filled with previous deposits as well as open joints may serve as paths for leakage to occur. 

In the case of tunneling, the feasibility, planning, design, the techniques used and the risk of serious accidents during construction are all dependent on the geology of the site. Location of quarries for production of building stones, crushed stone aggregates etc., also requires geological studies. However, a detailed study of geology is beyond the scope of this text. 

Engineering Geology 

The systematic study of geology, including testing of engineering properties of rocks and surface deposits, which fall within its scope (that covers areas between classical geology and older disciplines of Civil Engineering such as tunnelling, dam construction, bridge construction etc.) is called engineering geology. In a major engineering project, the following stages of geological investigations are essential : 

1. preliminary investigation using published information, such as geological maps, 
2. a details geological survey of the site, possibly with aerial photographs, 
3. applied geophysical surveys to provide information about the sub-surface geology. 
4. boring, drilling and excavation to provide confirmation of the results so far arrived at and quantitative detail at the critical points on the site, and 
5. testing of soils and rocks to assess their suitability, particularly their mechanical properties, either in situ or in the laboratory. 

These tasks are performed by experts but it is necessary to appreciate the situations wherein the advice of these experts is needed.