1. INTRODUCTION 1.1 DEFINITIONS

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1 1. INTRODUCTION 1.1 DEFINITIONS The definition given to the word soil differs from one discipline to another. To a geologist, soil is the material found in the relatively thin surface region of the earth's crust where roots of plants occur. For the agriculturalist on the other hand, soil is the top thin layer of the earth where organic forces are prevalent and which supports plant life. Moreover, soil represents the products of past surface processes to a geologist while it denotes currently occurring physical and chemical processes to the agriculturalist. The engineering definitions of soil is however quite different from those given in geology and soil science. Eventhough there is still no official and conventional definition of the word soil in Civil Engineering the following have been suggested. Soil is the uncemented aggregate of mineral grains and decayed organic matter (solid particles) with liquid and gas in the empty spaces between the solid particles. Soil is the portion of the earth's crust which can be separated by gentle mechanical means and excavated without blasting. Soil is a natural aggregate of mineral grains, loose or moderately cohesive, inorganic or organic in nature that could be separated by means of simple mechanical processes and is thus distinguishable from a rock, which is a natural aggregate of minerals connected by strong and permanent cohesive forces. Soil is any uncemented or weakly-cemented accumulation of mineral particles formed by weathering of rocks, the void space between the particles containing water and/or air. Soil Mechanics is a branch of Civil Engineering that deals with the application of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulation of solid particles produce by the mechanical and chemical disintegration of rocks regardless of whether or not they contain and admixture of organic constituents. It is one of the youngest and fast developing fields of engineering that deals with the application of soil science, the laws of statics and dynamics and the principles of mechanics and hydraulics to understand the behavior of and use of soil as and engineering material. Structures such as buildings and bridges are eventually supported on soil. Soil is also used as a construction material as in the case of dam and highway embankments. Tunnels are burrowed - 1 -

2 through rocks and soil deposits. Prevention of slope failures and landslides is another problem of frequent encounter. There is thus an obvious need to study the engineering properties of soils to arrive at a safe and cost-efficient solution to civil engineering projects. 1.2 SOIL MECHANICS - A BRIEF HISTORICAL PERSPECTIVE Prior to the 18 th century, the practice of soil engineering was based only on past experiences through a succession of experimentation without any substantial scientific backing. After encountering several foundation-related problems during construction over past centuries, engineers and scientists began to address the properties and behavior of soils in a more methodical manner starting in the early part of the 18th century. Some defining moments in the development of soil mechanics are listed below. French scientist C.A. Coulomb determined the true position of the sliding surface in soil behind a retaining wall in In the year 1857, the British scientist W.J.M. Rankine suggested a notable theory on earth pressure and equilibrium of earth masses. The French engineer H.P.G. Darcy published a study on the permeability of sand filters which he applied to soil. The year was J.V. Boussinesq in 1885 forwarded the theory of stress distribution under loaded bearing areas in a homogeneous, semi-infinite, elastic, and isotropic medium. In the year 1911, A.M. Atterberg developed the concept of consistency limits for clays. W. Fellenius developed slip-circle analysis of saturated clay slopes in Karl Terzaghi gave birth to a new era in the development of soil mechanics with the publishing of his book on soil mechanics in Terzaghi's contributions in this field have been immense and he is known to many engineers as the father of modern soil mechanics

Photo taken from Construction failures by Jacob Feld, Kenneth L. Carper Transcona grain elevator of Manitoba, Canada failed in 1913. After 24 hours of lading, the grain elevator tilted and settled.

3 Photo taken from Construction failures by Jacob Feld, Kenneth L. Carper Transcona grain elevator of Manitoba, Canada failed in After 24 hours of lading, the grain elevator tilted and settled. No borings were done to identify the soils and to obtain information on their strength. Only an open pit of about 4m deep was made for the foundations and a plate was loaded to determine the bearing strength of the soils. Due to minimal structural damage the bins were later restored

The Leaning tower of Pisa - so called because it tilted significantly due to the presence of a non-uniform, sponge like saturated clay on which the foundation of the tower rests. 1.

4 The Leaning tower of Pisa - so called because it tilted significantly due to the presence of a non-uniform, sponge like saturated clay on which the foundation of the tower rests. 1.3 ORIGIN OF SOIL Based on origin, soil can be broadly classified as inorganic and organic. Inorganic soils are formed by the disintegration (weathering) of rocks. Organic soils are mixtures in which a significant part is derived from the growth and decay of plant life. Since most soil types fall under the inorganic category we shall focus our discussion towards this group. Weathering is the process of breaking down of rocks by mechanical (physical) and chemical processes. Mechanical weathering is caused by any or a combination of the following agents: Temperature fluctuations, Ice pressure, Wind, Impact and abrasive action of flowing water, Plant roots, - 4 -

5 Burrowing animals,...etc The final products of mechanical weathering are smaller pieces of the original parent rock without any change in the chemical composition. In chemical weathering however, the original rock minerals are transformed into new minerals by chemical reaction. Principal reactions involved in this type of weathering are: oxidation, hydrolysis, carbonation, and leaching. The surface of the earth is subject to continual processes that make up and disintegrate rocks. These processes are weathering, transportation, deposition and upheaval that form a cycle where each process is repeated on the end product. The products of weathering may stay in the same place or may be moved to other places by transportation agents such as water, ice, wind, and gravity. Soils remaining essentially at the places of their origin are known as Residual Soils while soils which have been removed from their original place of formation to other places of deposition are called Transported Soils. Transported soils may be classified into several groups, depending on their mode of transportation and deposition as: Alluvial soils - transported by running water and deposited along streams Aeolian soils - transported and deposited by wind Lacustrine soils - formed by deposition in quiet lakes Colluvial soils - deposited by movement of soil by gravity, such as during landslides Marine soils - formed by deposition in the seas Glacial soils - deposited as a result of glacial activities 1.4 COMMON SOIL TYPES Soil types that are of common occurrence in practice have been described in brief as follows. Sand, gravel, cobbles and boulders are coarse-grained cohesionless soils. Grain-size ranges are used to distinguish between them. Silts - microscopic soil fractions that consist of very fine quartz grains and some particles that are fragments of micaceous minerals fine grained soil with little or no plasticity. Grain sizes are in the range of 0.06 to 0.002mm - 5 -

Clays are microscopic particles of weathered rock. They have been defined as those particles which develop plasticity when mixed with a limited amount of water.

The main groups of crystalline materials that make up clays are the minerals: kaolinite, illite, and montmorillonite.

This is due to the abundance of these elements in the earth s crust. The most abundant element in the earth's crust is oxygen, making up 46.6% of the earth's mass.

6 Clays are microscopic particles of weathered rock. They have been defined as those particles which develop plasticity when mixed with a limited amount of water. They are also generally defined as particles smaller than 0.002mm. Inorganic varieties are generally more plastic than organic ones. The main groups of crystalline materials that make up clays are the minerals: kaolinite, illite, and montmorillonite. It is noteworthy to mention here that most minerals of interest to geotechnical engineers are composed of oxygen, silicon, and aluminum. This is due to the abundance of these elements in the earth s crust. The most abundant element in the earth's crust is oxygen, making up 46.6% of the earth's mass. Silicon is the second most abundant element (27.7%), followed by aluminum (8.1%). Silicon and oxygen ions form mineral groups called silicates. As shown below a central silica cation (positively charged ion) is surrounded by four oxygen anions (negatively charged ions), one at each corner forming a tetrahedron (4 sided polyhedron)

The net charge of a single tetrahedron is -4 and to achieve a neutral charge, cations must be added or

Silicate minerals are thus formed by addition of cations and interaction of tetrahedrons.

silica tetrahedrons in which three oxygen ions are shared between adjacent tetrahedrons.

7 The net charge of a single tetrahedron is -4 and to achieve a neutral charge, cations must be added or single tetrahedrons must be linked to each other sharing oxygen ions. Silicate minerals are thus formed by addition of cations and interaction of tetrahedrons. In such way silica tetrahedrons combine to form sheets, called silicate sheets, which are thin layers of silica tetrahedrons in which three oxygen ions are shared between adjacent tetrahedrons. Silicate sheets may contain other structural units such as alumina sheets. Alumina sheets are formed by combination of alumina minerals, which consist of an aluminium ion - 7 -

surrounded by six oxygen or hydroxyl (oxygen bonded with one

Note: You may visit http://virtual-museum.soils.wisc.

Kaolinite is formed by bondage of one silica sheet and one alumina

8 surrounded by six oxygen or hydroxyl (oxygen bonded with one hydrogen atom) atoms forming an octahedron (8-sided polyhedron). Note: You may visit for a graphic display of clay mineralogy. Kaolinite is formed by bondage of one silica sheet and one alumina sheet that are stacked repeatedly. The layers are held together by hydrogen bonds

Water can easily enter the bond and separate the layers in montmorillonite, causing swelling. Black cotton soil - dark or gray clay soil characterized by high expansive and shrinkage properties.

9 Illite consists of repeated layers of one alumina sheet sandwiched by two silicate sheets. The layers are held together by potassium ions. Montmorillonite has a structure similar to illite, but the layers are held together by weak van der Walls forces and exchangeable ions. Water can easily enter the bond and separate the layers in montmorillonite, causing swelling. Black cotton soil - dark or gray clay soil characterized by high expansive and shrinkage properties. This peculiar property calls for extra care during designing of structures to be erected on such soil. The mineral forming these types of clays is montmorillonite. Peat - highly organic soil composed entirely of decayed plant fibers. It is in generally highly compressible and thus not suitable for support unless no other option is available

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