Soil Mechanics - Test 1 with verified questions and answers

Soil Mechanics 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 Coulomb Theories for soil strength Rankine Theories for soil stresses on retaining walls Terzaghi -Concept of effective strength, - Published the first book on soil mechanics - Considered to be the father of modern soil mechanincs Soil is formed by: Weathering & transportation of rock Weathering Process The process of breaking down rocks by mechanical and chemical processes Factors that effect soil structure -size, shape & Minerals that constitute soil particles - Consistency (interaction of soil with water) Mechanical Weathering Breaking down or disintegration of large rock pieces into smaller rock pieces WITHOUT changing the chemical composition (mainly temp & pressure changes) Types of mechanical weathering -Frost wedging -Exfoliation -Thermal expansion/contraction Chemical Weathering Breaking down or disintegration of large rock pieces into smaller rock pieces WITH changing the chemical composition. - Minerals are transformed into new minerals -The most important process because it forms important soil types Residual Soils Product soil from chemical weathering Transported Soil Product soil from mechanical weathering, a soil transported by water or wind or ice. Types of Transported soils: -Glacial soils: deposited by glaciers -Alluvial soils: transported by running water & deposited along streams -Aeolian soil: deposited by wing (colluvial: gravity , marine: sea, Lacustine: quiet lakes) "Clay" means: - clay minerals (complex Aluminum silicates) - clay size (0.002 mm USCS) Properties of clay minerals: -Flat, thin, plate-like shape -Carry a net negative charge on their surface -May have positive charges at particle ends Diffuse double layer -Water held to the surface of the clay -depends on surface charge & surface area -gives clay its plastic properties (more water causes soil to become expansive clay, affects soil strength) Large charge & large surface area = large volume change & low strength Smaller charge & smaller surface area = less volume change & higher strength Soils: Need to identify: -How much clay -What kind of soil (i.e. how will it respond to water) Atterburg Limit Tests -Consistency tests that indicate the effect water has on the clay particles Cohesive soils Silts & clays Cohensionless soils Sands & gravels Types of Cohesive soils -Dispersed (smaller void ratio, stack up) -Flocculated (larger void radio, honeycomb esque) Properties of Cohensionless soils - Large void ratio - Can carry normal static load - Collapse when subjected to heavy or shock loads - leads to large settlement Hydrometer - Indicates particle size & quantity, used for silts & clays - Measures the unit weight of a soil-water suspension Sieve Analysis For particle sizes larger than a #200 sieve, used for sand & gravel Poorly graded Soil contains particles which are primarily within a small band Well-graded Contains particles of all sizes in good distribution Gap-graded One size of particles is missing from the soil - the curve becomes horizontal Principle of Sedimentation - Different sizes of soil particles need different times to settle - Measures the size distribution of silt and clay Stoke's Law For a sphere falling at a constant velocity in a viscous fluid, the velocity depends on the grain diameter, the densities of the grains in suspension, and the viscosity of the fluid Liquid Limit Water content at which the soil passes from a plastic state to a liquid state Plastic Limit - Water content at which the soil passes from a semisolid state to a plastic state (or from liquid state to plastic state - decrease in water content) -The water content at which the soil begins to crumble when rolled in a thread 1/8 " in diameter Shrinkage Limit Water content at which soil becomes a solid and no longer shrinks in volume with decreasing water content Why classify soils? -Want to group similar soil types togetehr -Want to know properties of that soil (strength & compressibility) Classification: Coarse grained Grain size Classification: Fine grained Plasticity Compaction -Classification of a soil by mechanical energy -Expells air from the soil -Improves soil strength, reduces soil compressibility & reduces permeability -Measured in terms of dry unit weight -Results are presented in a graph as dry unit weight vs. moisture content Optimum Moisture Content -The water content at which a soil can be compacted to a maximum dry unit weight by a given compaction effort Compaction curve: low moisture Water works as a lubricant between soil particles, density increases as w% increases Compaction curve: high moisture Water starts to replace the volume of soil particles, density decreases as w% increases Factors that Affect Compaction -Water content -Compaction Effort -Soil Type Effect of Compaction on Soil Properties -Permeability (allowing liquids/gases to pass through) -Minimum hydraulic conductivity occurs at about 1-2% higher than the optimum permeability Field Compaction: Optimum dry unit weight -dry of optimum: better strength -wet of optimum: permeability is very small -wet of optimum is preferred, "the wetter the better" Factors that affect field compaction: -No. of passes (roller passes) -Thickness of soil layer Phreatic surface Ground water table -location changes depending on the season of the year -water below the phreatic surface creates a positive hydrostatic pressure (water pressure atmospheric pressure) -at the phreatic surface, the total air and water pressure is equal to the zero gage pressure (i.e. atmosphere) Vadose zone -Just above the water table -neg pressure (less than atmospheric pressure) -Divided into zone of capillary saturation zone of capillary fringe (water moves upward due to capillary forces) Water Flow Flows from a point of high energy to a point of low energy Energy of Water Flow -When water flows through soil, some energy is lost due to friction between water and soil particles Coefficient of Permeability (K) depends on: -Fluid viscosity -Void ratio -Connectivity of voids -Degree of Saturation -Roughness of mineral particles