Soil Mechanics PDF

Soil mechanics is the branch of science that deals with the study of the physical properties of soil and the behavior of soil masses subjected to various types of forces. Soils engineering is the application of the principles of soil mechanics to practical problems. Geotechnical engineering is the s...

Soil mechanics is the branch of science that deals with the study of the physical properties of soil and the behavior of soil masses subjected to various types of forces. Soils engineering is the application of the principles of soil mechanics to practical problems. Geotechnical engineering is the subdiscipline of civil engineering that involves natural materials found close to the surface of the earth. It includes the application of the principles of soil mechanics and rock mechanics to the design of foundations, retaining structures, and earth structures. 5  HISTORY Beginning of use of soil in pre-historic time.. 6  One of the most famous examples of problems related to soil bearing capacity and foundations in the construction of structures prior to 18th century is the Leaning Tower of Pisa in Italy. The construction of the Tower began in 1173 and last over 200 years.  The tower has tilted in the past to the east, north, west and, finally, to the south.  Recent investigations showed that a weak clay layer exists at a depth of about 11 m below the ground surface compression, which caused the tower to tilt. 10 HISTORY OF SOIL ENGINEERING.  1930: Soil Mechanics was established as branch of Civil Engineering  Began: 18th century since then, the science of Soil mechanics has evolved through 4 phases…  Pre-classical (1700 A.D. to 1776 A.D.)  Classical phase-1 (1776 A.D. to 1856 A.D.)  Classical phase-2 (1856 A.D. to 1910 A.D.)  Modern (1910 A.D. to present) 9  PRE-CLASSICAL PERIOD OF SOIL MECHANICS  This period concentrated on studies relating to natural slope and unit weights of various types of soils, as well as the semi empirical earth pressure theories. Henri Gautier (1660–1737) studied the natural slopes of soils when tipped in a heap for formulating the design procedures of retaining walls(1717). 10  Bernard Forest de Belidor (1671– 1761 French engg.) proposed a theory for lateral earth pressure on retaining walls and specified a soil classification system(1729). 11 Francois Gadroy (1705– 1759)Reported the first laboratory test results on a 76mm high retaining wall built with sand backfill(1746). Jean Rodolphe Perronet(1708- 1794) studied about slope stability(1769). CLASSICAL SOIL MECHANICS — Phase I (1776–1856)  Classical Soil Mechanics began in 1776 with Charles Coulomb’s (a physicist, 1736– 1806) used the principles of calculus for maxima and minima to determine the true position of the sliding surface in soil behind a retaining wall. 13 Classical Soil Mechanics—Phase I (1776–1856) In 1790, the distinguished French civil engineer, Gaspard Clair Marie Riche de Prony (1755–1839) included Coulomb’s theory in his leading textbook, Nouvelle Architecture Hydraulique (Vol. 1). In 1820, special cases of Coulomb’s work were studied by French engineer Jacques Frederic Francais (1775–1833) and by French applied mechanics professor Claude Louis Marie Henri Navier (1785–1836). These special cases related to inclined backfills and backfills supporting surcharge. In 1840, Jean Victor Poncelet (1788–1867), an army engineer and professor of mechanics, extended Coulomb’s theory by providing a graphical method for determining the magnitude of lateral earth pressure on vertical and inclined retaining walls with arbitrarily broken polygonal ground surfaces. In 1846 Alexandre Collin (1808–1890), an engineer, provided the details for deep slips in clay slopes, cutting, and embankments. Classical Soil Mechanics—Phase II (1856– 1910) Henry Philibert Gaspard Darcy(1803-1858) defined the term coefficient of permeability (or hydraulic conductivity) of soil, a very useful parameter in geotechnical engineering to this day. Sir George Howard Darwin (1845–1912), a professor of astronomy, conducted laboratory tests to determine the overturning moment on a hinged wall retaining sand in loose and dense states of compaction. Joseph Boussinesq, a mathematician and physicist (1842–1929), developed the theory of stress distribution(1885). In 1887, Osborne Reynolds (1842–1912) demonstrated the phenomenon of dilatancy in sand. Other notable studies duringthis period are those by John Clibborn (1847–1938) and John Stuart Beresford(1845–1925) relating to the flow of water through sand bed and uplift pressure. MODERN SOIL MECHANICS  This period was marked by a series of important studies and publications related to the mechanic behavior of clays.  Albert Atterberg (1846– 1916), a Swedish chemist and soil scientist, explained the consistency of cohesive soils by defining liquid, plastic, and shrinkage limits. 16  Arthur Bell (1874–1956), a civil engineer from England, developed relationships for lateral pressure and resistance in clay as well as bearing capacity of shallow foundations in clay.  Wolmar Fellenius (1876– 1957), an engineer from Sweden, developed the stability analysis of saturated clay slopes.  Karl Terzaghi (1883–1963), a civil engineer and geologist from Austria, developed the theory of consolidation for clays as Wolmar Fellenius we know today.. 17  The development of modern Geotechnical Engineering as a branch of Civil Engineering is absolutely impacted by one single professional individual – Karl Terzaghi.  Generally recognized as the father of modern soil mechanics and geotechnical engineering.  He started modern soil mechanics with his theories of consolidation, lateral earth pressures, bearing capacity, and stability.  His contribution has spread to almost every topic in soil mechanics and geotechnical engineering covered by the text book: Effective stress; Elastic stress distribution; Consolidation settlement; Shear strength; in situ testing 18 IMPORTANCE OF SOIL MECHANICS 1.) Foundation All foundations for any structure that a civil engineer constructs are bound to rest on the soil. The bigger the building or structure, the bigger its foundation and consequently the more important it is for a civil engineer to take into consideration the soil mechanics of the site. The foundation is where the load the structure bears is transferred hence understanding the soil is crucial to building a strong structure. Hard soil with sufficient strength allows an engineer to use shallow foundations, and the alternative is also true. Weak soil will need deep foundations to provide robust support for the structure being put up.  2.) Earthen Dams There are so many earthen dams constructed to retain the water. The soil to be used for the construction of these earthen dams must be suitable enough to use it in its construction. Various properties of the soil, like it permeability, strength, and density are checked on regular basis to know if the soil compacted to required density or not. The earthen dams are costly structure and also they have a high risk of getting failed, so they must be constructed with great care, so it is very important to study the properties of the soil.  3.) Embankments Embankments are usually constructed to raise the level of a road, railway or land above ground level. There are usually several reasons embankments are constructed. One of them is to raise the structure above flooding level. Anything that is built on the flat land is prone to flooding that can destroy the structure. Constructing the structure on an embankment is, therefore, a way of mitigating this. Embankments are also constructed to minimize or reduce the change in level due to a terrain’s profile. The embankment helps ensure the road/railway/structure is on the same level all through.  4. Retaining wall and other underground structures A retaining wall is designed to hold in place a mass of earth or the like, such as the edge of a terrace or excavation. The structure is constructed to resist the lateral pressure of soil when there is a desired change in ground elevation that exceeds the angle of repose of the soil.  WHAT IS “SOIL”?  Soil is the mixture of minerals, organic matter, gases, liquids, and the countless organisms that together support life on Earth.  Soil is considered to be the "skin of the Earth.  It includes widely different materials like sand, gravels, clays and silts.  The type of soil depend upon the size of particles i.e. sandy soil, loamy soil, etc and on the color of soil i.e. yellow soil, black soil, 4 etc. WHAT IS “SOIL”? A soil mass is commonly considered to consists of solid particles, enclosed voids or interspaces. Thus, there are two constituents of soil: 1.) Soil or solid particles 2.) Voids SOIL On the basis of constituents: 1.) Dry Soil 2.) Saturated Soil 3.) Partially Saturated/Moist Soil Formation of soil Weathering is the breakdown and decomposition of earth material, namely rocks. 1.) Mechanical Weathering Frost wedging Thermal expansion and contraction Alternate wetting and drying 2.) Chemical Weathering Oxidation Hydrolysis Hydration Carbonic acid action SOIL PARTICLE SIZE Soils generally are called gravel, sand, silt, or clay, depending on the predominant size of particles within the soil. To describe soils by their particle size, several organizations have developed particle-size classifications.  SOIL PARTICLE SIZE Gravels are pieces of rocks with occasional particles of quartz, feldspar, and other minerals. Sand particles are made of mostly quartz and feldspar. Other mineral grains also may be present at times. Silts are the microscopic soil fractions that consist of very fine quartz grains and some flake-shaped particles that are fragments of micaceous minerals. Clays are mostly flake-shaped microscopic and submicroscopic particles of mica, clay minerals, and other minerals. IDENTIFICATION OF SOIL 1.) Visual Examination 2.) Feel test 3.) Rolling Test 4.) Dry Strength Test 2.) Feel test 3.) Rolling Test 4.) Dry Strength Test WEIGHT-VOLUME RELATIONSHIP Volume relationship The volume relationships commonly used for the three phases in a soil element are void ratio, porosity, and degree of saturation. Void ratio (e) is defined as the ratio of the volume of voids to the volume of solids. Porosity (n) is defined as the ratio of the volume of voids to the total volume, The degree of saturation (S) is defined as the ratio of the volume of water to the volume of voids, The relationship between void ratio and porosity can be derived from equation 1, 2 and 3 Weight Relationship Moisture content Moisture content (w) is also referred to as water content and is defined as the ratio of the weight of water to the weight of solids in a given volume of soil. Unit weight Unit weight (g) is the weight of soil per unit volume. Note: Soils engineers sometimes refer to the unit weight as the moist unit weight Weight Relationship Dry Unit Weight Dry unit weight per unit volume of soil, excluding water. the relationship of unit weight, dry unit weight, and moisture content can be given as Weight-Volume Relationship Density and Dry Density Specific gravity of soil solids(Gs) can be expressed as: ɣ Unit weight in terms of e, w and Gs To obtain a relationship among unit weight (or density), void ratio, and moisture content, let us consider a volume of soil in which the volume of the soil solids is one, as shown in the figure. If the volume of the soil solids is 1, then the volume of voids is numerically equal to the void ratio, e. Unit weight in terms of e, w and Gs Degree of Saturation Because the weight of water for the soil element under consideration is , the volume occupied by water is Hence, from the definition of degree of saturation This equation is useful for solving problems involving three-phase relationships. Saturated Unit Weight If the soil sample is saturated—that is, the void spaces are completely filled with water Density Submerged Unit weight/Buoyant Unit Weight and Dry Unit Weight at Zero Air Voids Submerged Unit Weight is the effective unit weight per unit volume when the soil is submerged below standing water or below ground water table. Dry unit weight at Zero air voids is the weight of solids per unit volume of a saturated mass. Sample Problems: 1. Sample Problems: 2. RELATIVE DENSITY RELATIVE DENSITY RELATIVE DENSITY