Module 1 - Introduction.pptx

Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction A. A Brief Geotechnical Engineering Historical Development Geotechnical engineering is an essential branch of civil engineering. It is an in-depth study of soil and rock properties and behavior to ensure the safety and s...

Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction A. A Brief Geotechnical Engineering Historical Development Geotechnical engineering is an essential branch of civil engineering. It is an in-depth study of soil and rock properties and behavior to ensure the safety and stability of structures. Geotechnical engineering played a fundamental and significant role in the history of the development of humanity from ancient civilizations to advances of modern technologies. Ancient civilizations flourished as recorded by history along the bank of the rivers, such as the Nile (Egypt), the Tigris and Euphrates (Mesopotamia), the Huang Ho (Yellow River, China), and the Indus (India). Dikes were built dating back to about 2000 B.C. in the basin of the Indus to protect the town of Mohenjo Dara which became Pakistan after 1947. Chan Dynasty in China built dikes during 1120 B.C. to 249 B.C. for irrigation purposes.  Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction A. A Brief Geotechnical Engineering Historical Development Kerisel (1985) stated that there is no measurement taken to stabilize the foundations erosions caused by floods. Isolated pad footings and strip-and-raft foundations for building structures were used by Ancient Greek civilization. The most important five pyramids were built in Egypt 2750 B.C. in a period of less than a century and these are Saqqarah, Meidum, Dashur South and North, and Cheops which gave rise to challenges regarding foundations, stability of slopes, and construction of underground chambers. During the Eastern Han dynasty 68 A.D. in the arrival of Buddhism thousands of pagodas were built which mostly constructed on silt and soft clay layers thereby caused extensive structural damage.  Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction A. A Brief Geotechnical Engineering Historical Development Modern geotechnical engineering came to importance during 1700 to 1900 in the publication of key theories on soil mechanics and the movement of water through soil by scientists such as Coulumb, Darcy and Atterberg. Through their theories the publication of “Theoretical Soil Mechanics” by the prominent civil engineer Karl Anton von Terzaghi 1925 set the course for modern geotechnics. Coulumb during 1773 was credited as the first person to use mechanics to solve soil problems and Karl Anton von Terzaghi was recognized as the undisputed father of soil mechanics which laid the very foundation of soil mechanics that brought the importance of soils in engineering activities. The historical evolution of geotechnical engineering demonstrated the continuous research of understanding the significance of the properties of soil and rock to ensure the safety and stability of structures in different environments.  Geotechnical Engineering 1 (Soil Mechanics) Module 1: Introduction A. A Brief Geotechnical Engineering Historical Development Examples of problems related to soil-bearing capacity structures prior to 18th century are the Leaning Tower of Pisa in Italy in 1773 A.D., the Tilting of Garisenda Tower and Asinelli Tower in Bologna, Italy between 1109 and 1119. B. Foundation Engineering and Soil Mechanics 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. 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 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. 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. Karl Terzaghi is known as the “Father of Soil Mechanics.” C. Definition of Soil and Rock 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 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 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 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 3. Three (3) Basic Types of Rocks Igneous rocks are formed by solidification of molten magma ejected from deep within the earth’s mantle. 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. 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. Two types of Weathering 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. 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; and chemical fluids. 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. Solid phase maybe mineral organic or both. Liquid phase is usually the soil water that fills part or all of the open spaces between the solid particles. Gaseous phase usually air, occupies part of the space between particles not filled with water. 6. Major Division of Soil A soil is considered as coarse grained if its individual particle is visible to unaided eyes. A soil is said to be fine grained if its individual particle is not visible to unaided eyes. Organic soils are those which contain an amount of decayed animals and/ or plant matter. Principal Types of Soil, Description, and Average Grain Sizes According to Particle Size Unified Soil Classification System Average Grain Size (USCS) U.S. Standard U.S. Standard Description Classification (mm) Sieve Passing Sieve Retained Rounded and/or angular Coarse: 75mm-19mm Gravel #8 #10 bulky rock Fine: 19mm-4mm Coarse: 4mm-1.7 mm Medium:1.7mm- Rounded and/or angular Sand 0.380mm #10 #200 bulky hard rock Fine: 0. 380mm-0. 075mm Particles smaller than Separated by Silt 0.075 mm, exhibit little or 0. 075mm-0. 002mm Can’t be sized no strength when dried. Particles smaller than 0.002 mm exhibit Determine by Clay significant strength when < 0.002 mm Sieving by wet analysis dried, water reduces strength 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. Principal Types of Soil 7. The following are the simple visual and manual tests used for the identification of fine grained soils in the field. 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. 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. 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. Dispersion. A small quantity of soil is dispersed with water in a glass cylinder or tests tube and allowed to settle. The coarse 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. 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. 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. 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.  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 Bulky Particles Flaky Particles Needle - Shape Particles Stokes’s Law: Where: = velocity s = unit weight of soil particles  w = unit weight of water = viscosity of water D = diameter of soil particles 2. Soil Types 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. 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. 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. Clay is composed of microscopic and sub-microscopic particles of weathered rocks. 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. Bentonite is clay with high percentage of clay mineral. Most of the bentonite is derived from chemical alteration of volcanic ash. 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. 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. 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. Hard pan is any stratum of hard and cohesive soil which offers exceptional resistance to penetration by normal drilling tools used in practice. 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. 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. Calcareous soil contains calcium carbonate and effervesces when treated with hydrochloric acid. Caliche consists of gravel, sand and clay cemented together by calcium carbonate. Expansive soils are clays that undergo large volume changes from cycles of wetting and drying. Glacial soils are mixed soils consisting of rock debris, sand, silt, clays and boulders. 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. Gypsum is calcium sulphate formed under heat and pressure from sediments in ocean brine. Laterite soils are residual soils that are cemented with iron oxides and are found in tropical regions. Loam is a mixture of sand, silt and clay that may contain organic material. Loess is a windblown, uniform fine-grained soil. Mud is clay and silt mixed with water into a viscous fluid. Essentials points: Soils are derived from weathering of rocks and are commonly described by textural terms such as gravels, sands, silts, and clays. Particle size is used to distinguish various soil textures. Clays are composed of three main types of mineral – kaolinite, illite, and montmorillonite. 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. 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. 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. The engineering properties of fine-grained soils depend mainly on mineralogical factors. References: 1. Images are Retrieved from https://www.google.com 2. Geotechnical Engineering (Revised Third Edition) by C. Venkatramaiah, 2012 3. Principles of Geotechnical Engineering (Seventh Edition) by Braja M. Das, 2010 4. Soil Mechanics and Foundations (Third Edition) by Muni Budhu, 2011 5. Soil Mechanics 7th Edition, R.F. Craig, 2004 6. Basic Fundamentals of Geotechnical Engineering by Venancio L. Besavilla Jr., 1998 7. Fundamentals of Geotechnical Engineering by Diego Inocencio T. Gillesania, 2006