ABOUT of SOIL
WHAT IS SOIL MECHANICS AND WHY IS IT IMPORTANT?
Soils are natural, complex materials consisting of solids, liquids, and gases. To study soil behavior, we have to couple concepts in solid mechanics (e.g., statics) and fluid mechanics. However, these mechanics are insufficient to obtain a complete understanding of soil behavior because of the uncertainties of the applied loads, the vagaries of natural forces, and the intricate, natural distribution of different soil types. We have to utilize these mechanics with simplifying assumptions and call on experience to make decisions (judgment) on soil behavior. A good understanding of soil behavior is necessary for us to analyze and design support systems (foundations) for infrastructures (e.g., roads and highways, pipelines, bridges, tunnels, embankments), energy systems (e.g., hydroelectric power stations, wind turbines, solar supports, geothermal and nuclear plants) and environmental systems (e.g., solid waste disposal, reservoirs, water treatment and water distribution systems, flood protection systems). The stability and life of any of these systems depend on the stability, strength, and deformation of soils. If the soil fails, these systems founded on or within it will fail or be impaired, regardless of how well these systems are designed. Thus, successful civil engineering projects are heavily dependent on our understanding of soil behavior. The iconic structures shown in Figure 1 would not exist if soil mechanics was not applied successfully.
PURPOSES OF THIS CONTENT -:
This book is intended to provide the reader with a prefatory understanding of the properties and behavior of soils for later applications to foundation analysis and desi
LEARNING OUTCOMES-:
■ Describe soils and determine their physical characteristics such as grain size, water content, void ratio, and unit weight.
■ Classify soils.
■ Determine the compaction of soils and be able to specify and monitor field compaction.
■ Understand the importance of soil investigations and be able to plan and conduct a soil investigation. ■ Understand one- and two-dimensional flow of water through soils and be able to determine hydraulic conductivity, porewater pressures, and seepage stresses.
■ Understand the concept of effective stress and be able to calculate total and effective stresses, and porewater pressures.
■ Be able to determine consolidation parameters and calculate one-dimensional consolidation settlement.
■ Be able to discriminate between “drained” and “undrained” conditions.
■ Understand the stress–strain response of soils.
■ Determine soil strength parameters from soil tests, for example, the friction angle and undrained shear strength.
ASSESSMENT-:
Students will be assessed on how well they absorb and use the fundamentals of soil mechanics through problems at the end of the chapter. These problems assess concept understanding, critical thinking, and problem solving skills.
INTRODUCTION -:
1.1 The purpose of this chapter is to introduce you to the composition and particle sizes of soils. Soils are complex, natural materials, and soils vary widely. The composition and particle sizes of soils influence the load-bearing and settlement characteristics of soils. Learning outcomes When you complete this chapter, you should be able to do the following:
■ Understand and describe the formation of soils.
■ Understand and describe the composition of soils.
■ Determine particle size distribution of a soil mass.
■ Interpret grading curves.
DEFINITIONS OF KEY TERMS -:
1.2 Minerals are chemical elements that constitute rocks. Soils are materials that are derived from the weathering of rocks.
Effective particle size (D10) is the average particle diameter of the soil at the 10th percentile; that is, 10% of the particles are smaller than this size (diameter).
Uniformity coefficient (Cu) is a numerical measure of uniformity (majority of grains are approximately the same size).
Coefficient of curvature (CC) is a measure of the shape of the particle distribution curve (other terms used are the coefficient of gradation and the coefficient of concavity).
COMPOSITION OF SOIL-:
1.3.1 Soil formation Engineering soils are formed from the physical and chemical weathering of rocks. Soils may also contain organic matter from the decomposition of plants and animals. In this content, we will focus on soils that have insignificant amounts of organic content. The main agents responsible for this process are exfoliation, unloading, erosion, freezing, and thawingOften chemical and physical weathering takes place in concert. Alluvial soils, also called fluvial soils, are soils that were transported by rivers and streams. The profile of alluvial soils usually consists of layers of different soils.
1.3.2 Soil types Gravels, sands, silts, and clays are used to identify specific textures in soilsA few of these are listed below.
■ Alluvial soils are fine sediments that have been eroded from rock and transported by water, and have settled on river- and streambeds.
■ Calcareous soil contains calcium carbonate and effervesces when treated with hydrochloric acid.
■ Caliche consists of gravel, sand, and clay cemented together by calcium carbonate.
■ Collovial soils (collovium) are soils found at the base of mountains that have been eroded by the combination of water and gravity.
■ Eolian soils are sand-sized particles deposited by wind.
■ Expansive soils are clays that undergo large volume changes from cycles of wetting and drying.
■ Glacial till is a soil that consists mainly of coarse particles.
■ Glacial clays are soils that were deposited in ancient lakes and subsequently frozen. The thawing of these lakes has revealed soil profiles of neatly stratified silt and clay, sometimes called varved clay.
■ Gypsum is calcium sulfate formed under heat and pressure from sediments in ocean brine.
■ Lacustrine soils are mostly silts and clays deposited in glacial lake waters.
■ Lateritic 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 wind-blown, uniform, fine-grained soil.
■ Marine soils are sand, silts, and clays deposited in salt or brackish water.
■ Marl (marlstone) is a mud (see definition of mud below) cemented by calcium carbonate or lime.
■ Mud is clay and silt mixed with water into a viscous fluid.
1.3.3 Soil minerals
Quartz (a common mineral in rocks) is the principal mineral of coarse-grained soils. Quartz is hard and composed of silicon dioxide (SiO2) in colored, colorless, and transparent hexagonal crystals. The particles of coarse-grained soil are thus naturally angular. Weathering, especially by water, can alter the angular shape to a rounded one. Clay minerals are made up of phyllosilicates, which are parallel sheets of silicates. A central silica cation (positively charged ion) is surrounded by four oxygen anions (negatively charged ions), one at each corner of the tetrahedron .The charge on 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. Silica tetrahedrons combine to form sheets, called silicate sheets or laminae, which are thin layers of silica tetrahedrons in which three oxygen ions are shared between adjacent tetrahedrons . The mineral particles of fine-grained soils are platy. The main groups of crystalline materials that make up fine-grained soils, principally clays, are the minerals kaolinite, illite, and montmorillonite. These minerals are the products from weathering of feldspar and muscovite mica, families of rock-forming silicate minerals that are abundant on the Earth’s surfaceThe layers are held together by hydrogen bonds. Tightly stacked layers result from numerous hydrogen bonds. Kaolinite is common in clays in humid tropical regions.
This causes a charge inequity that is balanced by exchangeable cations Na+ or Ca2+ and oriented water .Additional water can easily enter the bond and separate the layers in montmorillonite, causing swelling. If the predominant exchangeable cation is Ca2+ (calcium smectite), there are two water layers, whereas if it is Na+ (sodium smectite), there is usually only one water layer. Sodium smectite can absorb enough water to cause the particles to fully separate. Worldwide, it is responsible for billions of dollars in damages to structures (on ground and below ground
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