GEOTECHNICAL ENGINEERING - I

 Overview of Geotechnical Engineering

1.1 Definition of Soil

The term 'soil' has different meanings for different professional groups and is originated from a Latin word ‘Solum' To a Civil Engineer it means all the inorganic matter produced by weathering of rocks which may or may not contain admixture of organic matter and consist of mineral grains.

1.2 Formation of Soil

Soil is formed by disintegration and decomposition of rock and minerals through the actions of natural, chemical, mechanical agents into small to smaller particles. The factors of weathering are atmospheric i.e. changes in temperature and pressure, erosion and transportation by wind, water, glaciers; chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water. Soils formed by mechanical weathering have a similarity to the minerals of parent rock since chemical action does not take place here which could destroy their identity. Soils are disintegrated more by the process of chemical action than mechanical action in which some minerals disappear partially or fully and new compounds are formed. 95% of earth, crust consists of igneous rock and remaining 5% consist of sedimentary rocks.

1.3 General types of Soil

The soil formed at a place may be transported to other places by the agents of transportation such as water, wind, ice and gravity.

 1) Water Transported Soils: Water is an important factor of transportation of soil. Swift running water carries a large quantity of soil either in suspension or in rolling. The size of soil particle carried by the water depends on the velocity of water. All types of soils

 carried by water are known as alluvial deposits. Deposits made in lakes are called lacustrine deposits.

2) Wind Transported Soil: 

Soil particles which are transported by wind depends upon the velocity of wind. Soils deposited by wind are known as Aeolian deposits. Large sand dunes are formed by winds. Loess is a silt deposit made by wind. a

3) Glacier Deposited Soil: 

Glaciers are large masses of ice formed by compaction of snow. Soils get mixed with the ice and transported far away from their original position. Drift is a general term for deposits made by glaciers directly or indirectly. Deposits directly made by melting of glacier' are known as till.

4) Gravity Deposited Soil: 

Soils which are transported through a short distance under the action of gravity is known as gravity deposited soil. Celluvial soils, such as talus, have been deposited by gravity.

5) Soil transported by combined action:

 Soils transported by two ore more agents of transportation act jointly. A soil may fall under gravity and may be carried by wind after.

1.4 Importance of soil in Civil Engineering as construction material in Civil Engineering structures and as foundation bed for structures 

Every civil engineering structures, such as building, bridge, railway, road pavement, embankment, dam, tunnel has to be founded on soil and thus shall transmit dead and live loads to the soil stratum. Soil is the ultimate foundation which supports the structure. Here, foundation is a substructure that distributes the load to ultimate foundation means the soil.

From ancient items to modern days man has used soil for the construction of tombs, monuments, dwellings, barrages for storing water, building dams, pavements for highways, airfields in an important aspect of civil engineering. In the design and construction of underground structures such as tunnels , conduits, powerhouses, bracings for excavation, earth retaining structures, the soil play a crucial role.

1.5 Field application of geotechnical engineering 

Soil mechanics is the application of laws of mechanics and hydraulics in engineering problems.

1 Foundation Design and Construction: 

Every structure building, tunnel, pavement, dam etc is founded on the surface of earth. So, it is very important to know the bearing capacity of soil, the pattern of stress distribution in the soil beneath the loaded area, the settlement of foundation effect of ground water and effect of vibration etc. The stability of various types of foundation i.e. spread, pile, pier, well depend upon the soil strata on which the foundation is situated. 

2) Pavement Design: 

The thickness of pavement and its components depend upon the characteristics of soil, sub base, sub grade, base which should be determined before the design for the improvement of soil such as strength, stability of soil.

3) Design of underground and earth retaining structure: 

Soil plays a crucial role in the design of underground structures, such as tunnels, underground buildings, drainage structures, pipelines and earth retaining walls such as gravity retaining wall, anchored bulk head, coffers dam. 

4) Design of embankments and excavation:

 The possibility of seeping ground water reducing the strength of soil while excavating must be taken into account. Deep excavation may require lateral braces and sheet walls to prevent caving in.

5) Slope stability: 

Slope requires knowledge of whole f soil mechanics:

1.6 Historical development of Geotechnical Engineering 

In prehistoric times man built earthen mounds for burial, for protection against floods, for religious purposes and even for building caves to live in. In ancient times and in Roman period, the use of soil was appreciated in the construction of roads, canals and bridges. The large public buildings, harbours, aqueducts, bridges, roads, sanitary works of Romans certainly indicate some knowledge of engineering behavior of soil. B.C. Mansar and Viswakarma in India, wrote books on "Construction Science during the medieval period. The Leaning Tower of Pisa, Italy, built between 1174 and 1350 A.D. is glaring example of a lack of sufficient knowledge of the behavior of compressible soil. Coulomb published his wedge theory on earth pressure in 1776. He was first to introduce the concept of shearing resistance composed of two components: (1) Cohesion (2) Internal Friction. Poncelet, culmann and Rebhann where the other men who extended the work of coulomb. D'Arcy and Stokes were notable for their laws of the flow of water through soil and settlement of a solid. Rankine gave his theory in 1857. Boussinesq, in 1885, gave his theory of stress distribution in an elastic medium under a point load. Mohr, in 1871,

gave a graphical representation of state of stress in 1871. Atterberg gave the concept of consistency limits' of soil in 1911. Prandtl gave his theory of “plastic equilibrium in 1920. Terzaghi gave his theory of consolidation in 1923. R.R. Proctor and A. Casagrande were responsible for the development of this subject. Fifteen International Conferences have been held till now at Harvard, Rotterdam, Zurich, London, Tokyo etc.