Module 1 - Introduction.pdf

Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction Objectives: After studying these topics students will have: 1. Strong knowledge in Geotechnical Engineering Historical Development. 2. Appreciation of the importance of the subject. 3. Understand the significance of the causes and ef...

Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction Objectives: After studying these topics students will have: 1. Strong knowledge in Geotechnical Engineering Historical Development. 2. Appreciation of the importance of the subject. 3. Understand the significance of the causes and effects of the geotechnical engineering failures. Content: A. A Brief Geotechnical Engineering Historical Development Soil mechanics is the study of the response of soils to loads and these loads may come from human-made structures (buildings), gravity (earth pressures) and natural phenomena (earthquake). Soils are natural resources and complex materials consisting of solids (solid particles), liquids (water) and gases (air). A complete understanding of soil behavior is significant because of the uncertainties of the applied loads, the unexpected inexplicable and complexity of natural forces and the natural distribution of different soil types. A good understanding of soil behavior and responses is necessary for the analysis and design systems of structure foundations and environment systems. The serviceability life of any structures depend on the stability, strength and deformation of soils. Soils are the oldest and most complex engineering materials and used before as construction materials for flood protection and shelters. Western civilization credits the Roman Engineers for recognizing the importance of soils in the stability of structures. Coulumb (1773) is credited as the first person to use mechanics to solve soil problems. He is a member of French Royal Engineers who protected the old fortresses that fell easily from cannon fire and he postulated the mass of soils applies a lateral force to the fortress that cause it to slide away from the soil mass. Coulomb tacitly defined a failure criterion for the analysis of soil failures which is still prevail today. Karl Von Terzaghi (1883 – 1963) is the undisputed father of soil mechanics and the publication of his book “Erdbaudmechanik in 1925” laid the foundation for soil mechanics that brought recognition to the importance of soils in engineering activities. 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 of structures. Thus geotechnical engineering can provide solutions to soil problems and the work of geotechnical engineers do is often invincible once construction is completed and if the foundation pressure exceeded the load-bearing capacity these may cause extensive structural damage. Some of the most famous examples of problems related to soil-bearing capacity are the following: Leaning Tower of Pisa or Torre Pendenti di Pisa in Italian or simply the “Tower of Pisa” which is the campanile or free standing bell tower. It is located at Pisa, Italy and designed by Architect Bonanno Pisano last 1173 and completed 1372. The height of the tower is 55.86 meters from the ground on the low side and 56.67 meters on the high side. The tower began to lean during the construction in the 12th century due to soft ground which could not properly support the structure’s weight. By 1990 the tilt reached 5.5 degrees and the structure was stabilized by remedial work between 1993 and 2001 which reduced the tilt to 3.97 degrees. MBV Geotechnical Engineering 1 Page | 1 The Two Towers or Le due Torn in Italian are symbols of Bologna Italy and they are both leaning. The two towers are the most prominent Towers of Bologna which are located at the intersection of the roads that leads to the five gates of the old ring. The smaller one is called the Garisenda Tower while the taller one is called Asinelli Tower. The names of the towers were derived from the families which are traditionally credited and constructed the towers between 1109 and 1119. B. Foundation Engineering and Soil Mechanics 1. Soil is defined an uncemented aggregate of mineral grains and decayed organic matter (solid particles) with liquid and gas in the empty spaces between the solid particles, and it supports structural foundations. 2. 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. 3. Soils Engineering is the application of the principles of soil mechanics to practical problems. 4. Geotechnical Engineering is defined as sub discipline 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. Foundation Engineering is the branch of engineering which deals with design, construction and maintenance of shallow footings and deep foundations and other structural members which comprise foundation of buildings and other engineering structures. It also includes investigation of sites for foundation purposes. 6. Karl Terzaghi is known as the “Father of Soil Mechanics.” Karl Anton von Terzaghi Born on October 2, 1883 Died on October 25, 1963 He is a Civil Engineer who found the branch of Civil Engineering Science known as Soil Mechanics and the first to elaborate a comprehensive mechanics of soils with his publication of Erdbaumechanik in 1925. C. Definition of Soil and Rock a. Rock Cycle and the Origin of Soil Soil is defined as a natural aggregate of mineral grains, loose or moderately cohesive, inorganic in nature, that have the capacity of being separated by means of simple mechanical process, e.g. by agitation in water. This definition is not the same by the agriculturist or the geologist. To the agriculturist loose mantle at the surface of the earth which is capable of supporting plant life consists MBV Geotechnical Engineering 1 Page | 2 of soil; and similarly to the geologist, the soil is that material found in the relatively thin surface zone with in which roots occur. Rock is defined as hard and compact natural aggregates of mineral grains cemented by strong and more or less permanent bonds. 2. Soil Formation and Rock Soils are formed from the physical and chemical weathering of rocks. Physical weathering involves reduction of size without any change in the original composition of the parent rock. The main agents responsible for this process are exfoliation, unloading, erosion, freezing, and thawing. Chemical weathering causes both reduction in size and chemical alteration of the original parent rock. The main agents responsible for chemical weathering are hydration, carbonation, and oxidation. Often, chemical and physical weathering takes place in concert. Soils that remain at the site of weathering are called residual soils or transported soils. A civil engineer is concerned mainly with 10 meter to 15 meter top mantle of soil in dealing with small and medium sized projects. All soils are derived from igneous, secondary or metamorphic rocks. The rocks are weathered because of process of mechanical disintegration, chemical decomposition and solution. The process of rock weathering is affected by climatic and other conditions surrounding the rock undergoing alteration. Soil may also vary from large size boulders to small crystals of clay minerals. The sands (coarse particles), silts and clays (fine particles) resulting from the disintegration of rock may stay at the place of their formation. These are known as residual soils. If these soils are carried away by forces of gravity, water, wind and ice deposited at another location, they are known as transported soils. 3. Three (3) Basic Types of Rocks 1. Igneous rocks are formed by solidification of molten magma ejected from deep within the earth’s mantle. 2. Sedimentary rocks are deposits of gravel, sand, silt and clay formed by weathering may become compacted by overburden pressure and cemented by agents like iron oxide, calcite, dolomite and quartz. 3. Metamorphic rocks are either igneous or sedimentary rocks that have undergone considerable in their constitution, in their shape, structure and sometimes even in their mineral composition. Rock Cycle Process Weathering is the process of breaking down rocks by mechanical and chemical processes into smaller pieces. MBV Geotechnical Engineering 1 Page | 3 4. Two types of Weathering a. Mechanical weathering may be caused by the expansion and contraction of rocks from the continuous gain and loss of heat, which results in ultimate disintegration. b. Chemical weathering, the original rock minerals are transformed into new minerals by chemical reaction. 5. Products of Weathering or Residual Soils a. Glacial soils – formed by transportation and deposition of glaciers. b. Alluvial soils or Fluvial soils – transported by running water and deposited along streams. c. Lacustrine soils – formed by deposition in quiet lakes. d. Marine soils –formed by deposition in the seas. e. Aeolian soils – transported and deposited by wind. f. Colluvial soils – formed by movement of soil from its original place by gravity, such as during landslides. Metamorphism is the process of changing the composition and texture of rocks, without melting by heat and pressure. Mechanical analysis of soil is the determination of the range of particles present in a soil, expressed as a percentage of the total dry weight. Soil refers to all solid particles with or without organic constituents which are produced by the disintegration of rocks found overlying the solid rock crust of the earth. In general soil is composed of solid, liquid and gaseous matter. a. Solid phase maybe mineral organic or both. b. Liquid phase is usually the soil water that fills part or all of the open spaces between the solid particles. c. Gaseous phase usually air, occupies part of the space between particles not filled with water. 6. Major Division of Soil a. A soil is considered as coarse grained if its individual particle is visible to unaided eyes. b. A soil is said to be fine grained if its individual particle is not visible to unaided eyes. c. Organic soils are those which contain an amount of decayed animals and/ or plant matter. MBV Geotechnical Engineering 1 Page | 4 Principal Types of Soil, Description, and Average Grain Sizes According to Unified Soil Classification System (USCS) Particle Size Classification Description Gravel Rounded and/or angular bulky rock Sand Rounded and/or angular bulky hard rock Silt Clay Particles smaller than 0.075 mm, exhibit little or no strength when dried. Particles smaller than 0.002 mm exhibit significant strength when dried, water reduces strength Average Grain Size (mm) Coarse: 75mm19mm Fine: 19mm-4mm Coarse: 4mm-1.7 mm Medium:1.7mm0.380mm Fine: 0. 380mm-0. 075mm U.S. Standard Sieve Passing U.S. Standard Sieve Retained #8 #10 #10 #200 0. 075mm-0. 002mm Can’t be Separated by sized < 0.002 mm Sieving by wet Determine by analysis A soil is called gravel if its particles are bigger than 2 mm; the upper limiting diameter is usually 8 inches or 203 mm standard. But in highway engineering it is only 3 inches. All particles smaller than 0.005 mm is called clay those smaller than 0.001 mm is called colloids. 7. The following are the simple visual and manual tests used for the identification of fine grained soils in the field. a. Dry strength The wet soil sample is molded to any convenient shape and allowed to dry in the air or by heating. A small fragment of the dried sample is obtained and passed between the thumb and forefingers. b. Plasticity If a sample of moist soil can be molded and rolled into threads without breaking or crumbling the soil. A thin thread of clay or foot or more can supports its own weight or will not break when held. c. Water mobility A soil sample is mixed with water to the consistency of a thick paste, then it placed and shaken in palm of the hand. d. Dispersion A small quantity of soil is dispersed with water in a glass cylinder or tests tube and allowed to settle. The coarse particle will fall and the finer particles will remain in suspension, sand will settle 3 inches in about 10 seconds. MBV Geotechnical Engineering 1 Page | 5 8. Significant Properties of Soils The properties of soils are important for a project depend upon the nature of the project. The following properties are important for different types of engineering projects. a. Permeability is a measure of the ability of soil to let water pass through its pores. This property is of importance in earth dams and drainage problems. b. Consolidation and compressibility deal with changes in volume of pores in a soil under load. This property is made use of computing settlement of structures. c. Shear strength is a measure of the ability of soil to sustain stresses without failure. This property is of interest in computation of stability under load, stability of fills behind earth retaining structures and stability of earthen embankments. Other simple physical properties are Atterberg limits, moisture content, void ratio, relative density, grain size and sensitivity. Base Exchange is the property of the soil to bind exchangeable base from a solution onto its surface. This property is exhibited by some clay minerals. D. Mineralogical Composition and Structure of Soils The structure of soils means arrangements of soil particles and the electrical forces acting between adjacent particles. 1. Two (2) Methods of Particle Size Distribution The distribution of particle sizes or average grain diameter of coarse grained soils – gravels and sands – is obtained by screening a known weight of the soil through a stack of sieves of progressively finer mesh size. a. Sieve analysis consists of shaking the soil sample through a set of sieves that have progressing smaller openings. b. Hydrometer analysis is based on the principle of sedimentation in water. The particle shape can generally be divided into three major categories.  Bulky particles are mostly formed by mechanical weathering of rocks and minerals. Shapes: Angular, sub angular, rounded and sub rounded. MBV Geotechnical Engineering 1 Page | 6  Flaky particles have very low sphericity – usually 0.01 or less. These particles are predominantly clay minerals.  Needle – shape particles are much less common than the other two particle types. Examples of soils containing needle-shaped particles are some coral deposits and attapulgite clays. Stokes’s Law: v = s − w 18  D2 Where:  = velocity s = unit weight of soil particles  w = unit weight of water  = viscosity of water D = diameter of soil particles 2. Soil Types MBV Geotechnical Engineering 1 Page | 7 a. Sand and gravel are cohesive soils. These may be angular, sub - angular, sub-rounded, rounded and well-rounded in shape and are composed of usually unaltered mineral grains. b. Silt is a fine grained soil with little or no plasticity. The non-plastic variety consists usually equidimensional grains of quartz; it is sometimes called rock flour. The plastic variety of silt is composed of appreciable percentage of flakes taped particles. c. Organic silt is a fine grained, more or less plastic soil. It contains an admixture of finely divided particles of organic matter. Particles of partly decayed vegetable matter and shells may also be present. d. Clay is composed of microscopic and sub-microscopic particles of weathered rocks. e. Organic clay contains some finely divided organic particles and is highly compressible when saturated and their dry strength is very high. The color is usually gray or black and it may have a characteristics odor. f. Bentonite is clay with high percentage of clay mineral. Most of the bentonite is derived from chemical alteration of volcanic ash. g. Black cotton soils are inorganic in nature. These soils exhibit high compressibility and shrinkage and extremely high swelling characteristics under low loads. They are dark gray or black in color. h. Peat is composed of fibrous particles of decayed vegetable matter. It is light brown to dark in color. Peat is so compressible soil and is considered entirely to support any type of foundation. i. Varved clay is a particular type of lacustrine deposit consisting of alternating layers of medium gray inorganic silt and darker silty clay. The clay fractions being fine remain larger in suspension and settle during winter. The thickness of each is centimeter. j. Hard pan is any stratum of hard and cohesive soil which offers exceptional resistance to penetration by normal drilling tools used in practice. k. Glacial till is composed of material deposited by glacier and is a soil that consists mainly of coarse particles. The water did not have an opportunity to transport and sort out the material and it is a heterogeneous mixture of soil and rock particles. l. Boulder soils are mixture of boulders, large or small in size and matrix of soil. Depending upon the percentage of matrix (rock – main substance is crystal) in a boulder soil and its properties are affected. m. Calcareous soil contains calcium carbonate and effervesces when treated with hydrochloric acid. n. Caliche consists of gravel, sand and clay cemented together by calcium carbonate. o. Expansive soils are clays that undergo large volume changes from cycles of wetting and drying. p. Glacial soils are mixed soils consisting of rock debris, sand, silt, clays and boulders. q. Glacial clays are soils that were deposited in ancient lakes and subsequently frozen. The thawing of these lakes reveals a soil profile of neatly stratified silt and clay, sometimes called varved clay. The silt layer is light in color and was deposited during summer periods while the thinner, dark clay layer was deposited during winter periods. r. Gypsum is calcium sulphate formed under heat and pressure from sediments in ocean brine. s. Laterite soils are residual soils that are cemented with iron oxides and are found in tropical regions. t. Loam is a mixture of sand, silt and clay that may contain organic material. MBV Geotechnical Engineering 1 Page | 8 u. Loess is a windblown, uniform fine-grained soil. v. Mud is clay and silt mixed with water into a viscous fluid. Essentials points: 1. Soils are derived from weathering of rocks and are commonly described by textural terms such as gravels, sands, silts, and clays. 2. Particle size is used to distinguish various soil textures. 3. Clays are composed of three main types of mineral – kaolinite, illite, and montmorillonite. 4. The clay minerals consist of silica and alumina sheets that are combined to form layers. The bonds between layers play a very important role in the mechanical behavior of clays. The bond between the layers in montmorillonite is very weak compared with kaolinite and illite. Water can easily enter between the layers in montmorillonite, causing swelling. 5. A thin layer of water is bonded to the mineral surfaces of soils and significantly influences the physical and mechanical characteristics of fine-grained soils. 6. Fine-grained soils have much larger surface areas than coarse-grained soils and are responsible for the major physical and mechanical differences between coarse-grained and fine-grained soils. 7. The engineering properties of fine-grained soils depend mainly on mineralogical factors. References: 1. 2. 3. 4. 5. 6. 7. Images are Retrieved from https://www.google.com Geotechnical Engineering (Revised Third Edition) by C. Venkatramaiah, 2012 Principles of Geotechnical Engineering (Seventh Edition) by Braja M. Das, 2010 Soil Mechanics and Foundations (Third Edition) by Muni Budhu, 2011 Soil Mechanics 7th Edition, R.F. Craig, 2004 Basic Fundamentals of Geotechnical Engineering by Venancio L. Besavilla Jr., 1998 Fundamentals of Geotechnical Engineering by Diego Inocencio T. Gillesania, 2006 MBV Geotechnical Engineering 1 Page | 9