Basic Soil Mechanics Chapter 1
Basic Soil Mechanics Chapter 1:
Author: Eze-Odikwa Tochukwu Jed
Note: All articles posted here are accurate, up-to-date and drafted from real university curriculums. Proper references will be added at the bottom of this article upon its completion.
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1.1 Definition of Soil:
The term soil which has its origin from the Latin word ‘Solum’ has different meanings to different professionals. To an agriculturist, it means the loose material lying on earth’s surface, formed by the weathering of rocks, with an admixture of organic matter, which can support plant life. To the geologist, it means the disintegrated rock material overlying the parent bedrock. To the civil engineer, it means all the inorganic material on the earth’s surface, produced by weathering rocks, being either ‘residual’ or ‘transported’. It may or may not contain an admixture of organic matter. Both soil and rock consists of the mineral grains. But the bond between the mineral grains in soil is relatively weak compared to the strong bond between mineral grains in rock. Terzaghi (1929) based on this distinction defined soil as a natural aggregate of mineral grains which can be separated by such gentle mechanism means such as agitation in water.
1.2 Definition and Development of Soil Mechanics:
From the early twentieth century, the rapid growth of cities, industry and commerce required myriad building systems – for example, skyscrapers, large public buildings, dams or electric power generation, reservoirs for water supply and irrigation, tunnels, roads and railroads, port and harbor facilities, bridges, airports and runways, mining activities, hospitals, sanitation systems, drainage systems and towers for communication systems. These building systems require stable and economic sub-structures, thereby resulting to the paramount importance of having significantly organized way for the design and construction of structural foundations. For example, what is the state of stress in a soil mass, how much would a building settle, and what is the stability of structures on or within soil? We continue to ask these questions and to try to find answers as new issues confront us.
Some of these new issues include removing toxic compounds from soil and water, designing foundations and earth structures to mitigate damage from earthquakes and other geo-hazards, and designing systems to protect the environment and be sustainable.
To answer these questions we needed the help of some rational method, and, consequently, soil mechanics was conceived. Karl Terzaghi (1883-1963) is the undisputed father of soil mechanics. The publication of his book Erdbaumechanik in 1925 laid the foundation for soil mechanics and brought recognition to the importance of soils in engineering activities. Soil mechanics, also called geotechnique or geotechnics or geomechanics, is the application of engineering mechanics to the solution of problems dealing with soils as a foundation and as a construction material. Engineering mechanics is used to understand and interpret the properties, behavior, and performance of soils.
Soil mechanics is a subset of geotechnical engineering which involves the application of soil mechanics, geology, and hydraulics to the analysis and design of geotechnical systems such as dams, embankments, tunnels, canals and waterways, foundations for bridges, highways, buildings, solid waste and nuclear disposal systems. Every application of soil mechanics involves uncertainty because of the variability of soils – their stratification, composition, and engineering properties. Thus engineering mechanics can provide only partial solutions to soil problems. Experience and approximate calculations are essential for the successful application of soil mechanics to practical problems. Many of the calculations in the textbook are approximations.
Stability and economy are synergy tenets of engineering design. In geotechnical engineering, the uncertainties of the performance of soils, the uncertainties of the applied and dead loads, and the complexities of natural forces makes us to compromise between sophisticated and simple analyses or to use approximate methods. Stability should never be compromised for economy. An unstable structure compromised to save cost can result in death and destruction.
Nineteen International Conferences have been held till now under the auspices of the International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE) at Harvard (Massachusetts U.S.A) 1936, Rotterdam (Netherlands) 1948, Zurich (Switzerland) 1953, London (U.K) 1957, Paris (France) 1961, Montreal (Canada) 1965, Mexico City (Mexico) 1969, Moscow (U.S.S.R) 1973, Tokyo (Japan) 1977, Stockholm (Sweden) 1981, San Francisco (U.S.A) 1985, and Rio de janeiro (Brazil) 1989, The thirteenth was held in New Delhi (India) in 1994, the fourteenth in Hamburg (Germany) in 1997, the fifteenth in Istanbul (Turkey) in 2001, the sixteenth in Osaka (Japan) in 2005, seventeenth in Alexandra (Egypt) 2009, eighteenth was held again in Paris (France) in 2013. The nineteenth edition was held in Seoul (South Korea) between 17th -22nd September, 2017.
These conferences have given a big boost in research in the field of Soil Mechanics and Foundation Engineering.
1.3 Soil Formation:
Soil is formed by the process of weathering of rocks, that is, disintegration and decomposition of rocks and minerals at or near the earth’s surface through the actions of physical and chemical agents into smaller and smaller grains.
The factors of weathering may be atmospheric, such as changes in temperature and pressure; erosion and transportation by wind, water and glaciers; chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water, especially rainwater, with time or even by biological processes.
Obviously soils formed by mechanical (physical) weathering (that is, disintegration of rocks by the action of wind, water and glaciers) bear a similarity in certain properties to the minerals in the parent rock, since chemical changes which could destroy their identity do not take place.
It is to be noted that 95% of the earth’s crust consists of igneous rocks, and the remaining 5% consists of sedimentary and metamorphic rocks. However, sedimentary rocks are present on 80% of the earth’s surface area. Feldspars are the minerals abundantly present (60%) in igneous rocks. Amphiboles and pyroxenes, quartz and micas come next in that order.
Rocks are altered more by the process of chemical weathering than by mechanical weathering. In chemical weathering some minerals disappear partially or fully, and new compounds are formed.
The intensity of weathering depends upon the presence of water and temperature (magnitude and variations) and the dissolved minerals in water. Carbonic acid and oxygen are the most effective dissolved materials found in water which cause the weathering of rocks. Chemical weathering has the maximum intensity in humid and tropical climates.
Leaching is the process whereby water-soluble parts in the soil such as Calcium Carbonate, are dissolved and washed out from the soil by rainfall or percolating subsurface of water. Lateritic soil, the reddish brown soil, is formed by leaching .
Harder minerals will be more resistant to weathering action, for example, quartz present in igneous rocks. However prolonged chemical action may affect even such relatively stable minerals, resulting in the formation of secondary products of weathering, such as clay minerals – illite, kaolinite and montmorillonite. ‘Clay Mineralogy, has grown into a very complicated and broad subject.
1.4 Residual and transported Soils:
Soils which are formed by weathering of rocks may remain in position at the place of origin. In that case, these are ‘Residual Soils’. These may get transported from the place of origin by various agencies such as wind, water, ice, gravity. In that case, these are termed “Transported Soils”. Residual soils differ very much from transported soils in their characteristics and engineering behavior. The degree of disintegration may vary greatly throughout a residual soil mass and hence, only a gradual transition into rock is to be expected. An important characteristics of these soils is that the sizes of grains are not definite because of the partially disintegrated condition. The grains may break into smaller grains with the application of a little pressure.
The residual soil profile may be divided into three zones: (i) the upper zone in which there is a high degree of weathering and removal of material; (ii) the intermediate zone in which there is some degree of weathering in the top portion and some deposition in the bottom portion; and (iii) the partially weathered zone where there is the transition from the weathered material to the unweathered parent rock. Residual soils tend to be more abundant in humid and warm zones where conditions are favorable for chemical weathering of rocks and have sufficient vegetation to keep the products of weathering from being easily transported as sediments. Residual soils have not received much attention from geotechnical engineers because they are located primarily in undeveloped areas.
Transported soils may also be referred to as ‘sedimentary’ soils since the sediments, formed by weathered rocks, will be transported by agents such as wind and water to places far away from the place of origin and get deposited when favorable conditions like a decrease of velocity occur. A high degree of alteration of particle shape, size, and texture such as sorting the grains also occurs during transportation and deposition. A large range of grain sizes and a high degree of smoothness and fineness of individual grains are the typical characteristics of such soils. In some zones in South India, sedimentary soil deposits range from 8 to 15m in thickness.
Transported soils may be further subdivided, depending upon the transporting agent and the place of deposition as:
Alluvial soils: Soils transported by rivers and streams: sedimentary clays.
Aeoline soils: Soils transported by wind: loess
Glacial soils: Soils deposited in lake beds: Lacustrine silts and lacustrine clays.
Marine soils: Soils deposited in sea beds: Marine silts and marine clays.
Broad classification of soils may be:
- Coarse-grained soils, with average grain-size greater than 0,075mm, e.g., gravels and sands.
- Fine-grained soils, with average grain-size less than 0.075mm, e.g., silts and clays.
Further classification according to grain-size and other properties are given in later chapters.