CEGTEC30 Module 1 - Introduction to Soil Mechanics.pdf

MODULE 1 INTRODUCTION TO SOIL MECHANICS Introduction Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneo...

MODULE 1 INTRODUCTION TO SOIL MECHANICS Introduction Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids (usually air and water) and particles (usually clay, silt, sand, and gravel). Still, soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil or structures that are buried in soil. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as engineering geology, geophysical engineering, coastal engineering, agricultural engineering, hydrology, and soil physics. Foundation Engineering and Soil Mechanics Soil is defined as 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 forces. Soil Engineering is defined as a subdiscipline of civil engineering that involves natural materials found on 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 that deals with the design, construction, and maintenance of shallow footings and deep foundations, and other structural members that comprise the foundation of buildings and other engineering structures. It also includes an investigation of sites for foundation purposes. Fig. 1.1 Karl Terzaghi is known as the “Father of Soil Mechanics.” Rock Cycle and Origin of Soil Soil is defined as a natural aggregate of mineral grains, loose or moderately cohesive, inorganic in nature, that have the capacity to be separated by means of a simple mechanical process, e.g. by agitation in water. This definition is not the same as the agriculturist or the geologist. To the agriculturist, the loose mantle at the surface of the earth which is capable of supporting plant life consists of soil; similarly to the geologist, the soil is that material found in the relatively thin surface zone 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. Soil is formed from the physical and chemical weathering of rocks. Physical weathering involves the 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 reductions 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. All soils are derived from igneous, sedimentary, or metamorphic rocks. The rocks are weathered because of the 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 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. Three (3) Basic Types of Rocks Igneous rocks are formed by the 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 and 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 their mineral composition. Rock Cycle Process Weathering is the process of breaking down rocks by mechanical and chemical processes into smaller pieces. 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. In chemical weathering, the original rock minerals are transformed into new minerals by chemical reaction. Products of Weathering or Residual Soils Glacial soils – formed by transportation and deposition of glaciers. Alluvial soils or Fluvial soils – transported by running water and deposited along streams. Lacustrine soils – formed by deposition in quiet lakes. Marine soils – formed by deposition in the seas. Aeolian soils – transported and deposited by the wind. Colluvial soils – formed by movement of soil from its original place by gravity, such as during landslides. Major Division of Soil A soil is considered coarse-grained if its individual particles are visible to unaided eyes. A soil is considered fine-grained if its particles are not visible to unaided eyes. Organic soils are those which contain several decayed animals and/or plant matter. Principal Types of Soils Table 1-1 Principal Types of Soils Particle Size Classification Description Average Grain Size (mm) Rounded and/or angular Coarse: 75 mm to 19 mm Gravel bulky rock Fine: 19 mm to 4 mm Rounded and/or angular Coarse: 4 mm to 1.7 mm bulky hard rock Medium: 1.7 mm to 0.380 Sand mm Fine: 0.380 mm to 0.075 mm Particles smaller than 0.075 Silt mm, exhibit little or no 0.075 mm to 0.002 mm strength when dried. Particles smaller than 0.002 mm, exhibit significant Clay < 0.002 mm strength when dried, water reduces the strength Significant Properties of Soils 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 the volume of pores in soil under load. This property 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 the computation of stability under load, stability of fills behind earth retaining structures and stability of earthen embankments. 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 fine-grained soil with little or no plasticity. The non-plastic variety consists of usually equidimensional grains of quartz; it is sometimes called rock flour. The plastic variety of silt is composed of an appreciable percentage of flakes taped particles.  Organic silt is 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 characteristic odor. Bentonite is clay with a high percentage of clay minerals. Most bentonite is derived from the chemical alteration of volcanic ash. Loam is a mixture of sand, silt, and clay that may contain organic materials. Loess is windblown, uniform fine-grained soil. Mud is clay and silt mixed with water into a viscous fluid.