Soil Mechanics: Exam 1, Soil Mechanics Exam 2 100% correct answers

Soil Mechanics: Exam 1, Soil Mechanics Exam 2 100% correct answers Compaction quick, driving out air and water, under the dynamic loads Soil is a three-phase system consisting of what phases? 1. clay, silt, sand 2. solids, voids, water 3. water, solids, air 3. water, solids, air Void ratio is: 1. ratio of the volume of the voids to the total volume 2. ratio of the volume of the voids to the volume of the solids 3. ratio of the volume of water to the volume of the voids 2. ratio of the volume of the voids to the volume of the solids Select the incorrect statement. 1. Clay soils are unstable because they shrink and swell considerably with changes in moisture content. 2. Cohesive soils, such as silt, maintain their strength when unconfined. 3. Coarse-grained soils are more stable as a foundation material than silt or clay. 2. Cohesive soils, such as silt, maintain their strength when unconfined. Soil collected through test pits or test borings can be used to understand: 1. Extent of soil consolidation under loading 2. Permeability of soil 3. Water content of soil 4. All answers provided are correct 4. All answers provided are correct Which soil classification would be expected to have the highest bearing capacity? 1. Silt 2. Sand w/ fines 3. Clean gravel 4. Clay 3. Clean gravel Which of the following statements is incorrect? 1. Clay soils can be unstable because they may shrink and swell with changes in moisture content. 2. Clays tend to be impervious to fluids. 3. In general, bearing capacity decreases with increased soil density. 4. The water table is the level beneath which the soil is saturated with groundwater. 3. In general, bearing capacity decreases with increased soil density. Total stress at a point is 1. resisted by the soil skeleton and the water in the voids 2. resisted by the water in the voids 3. resisted by the soil skeleton 1. resisted by the soil skeleton and the water in the voids Total stress at a point is due to 1. the weight of everything acting above that point 2. the weight of the water in the voids above that point 3. the weight of the soil solids above that point 1. the weight of everything acting above that point effective stress is 1. the stress resisted by the water in the voids and the soil skeleton 2. the stress that is resisted by the soil skeleton 3. the stress resisted by the water in the voids 2. the stress that is resisted by the soil skeleton Pore water pressure 1. is the hydrostatic pressure 2. changes as the height of the soil column increases 3. is different in vertical and horizontal directions 1. is the hydrostatic pressure Moisture content and void volume is important to determine the unit weight Volume of Soil Solid particle distributed randomly with void spaces in between Void Spaces Continuous and occupied by water, air, or both Distribution of Stress along a given cross section of soil profile (Effective stress concept) Some fraction of the Normal stress at a given depth in a soil mass is carried by water in void space, other carried by the soil skeleton at points of contact of the soil particles Distribution of Stress along a given cross section of soil profile helps to analyze compressibility of soils, bearing capacity of foundation, stability of embankments, lateral pressure on earth-retaining structures Construction of the foundation increase in net stress (depends on load per unit area, depth at estimation of stress made, other factors) Pore Water Pressure (hydrostatic pressure) The portion of total stress carried by water in continuous void spaces (equal intensity in all direction) Effective Stress Rest of total stress carried by the soil grains at their points of contact. The sum of vertical components of the forces developed at the points of contact of the solid particles per unit cross-sectional area of the soil mass. total stress The total vertical stress acting at a point below the ground surface is due to the weight of everything lying above two broad classes of soils coarse grained soils and fine grained soil Coarse grained soil gravel and sand = relatively large particles (visible to naked eye) Fine Grained soil silt and clay = smaller particles ASTM Unified Soil Classification System Gravel (6.4-76.2mm), sands (0.05-6.4mm), silts (0.002-0.05mm) and clays (0.002mm) based on physical composition and characteristics The soil underlying a building site consist of superimposed layer (mix of soil types due to weathering or deposition) soil profile the diagram of vertical section of soil from the ground surface to the underlying material The integrity of building structure depends on stability and strength under loading of the soil underlying the foundation Suitability of soil as a foundation materials the stratification, composition, and density of the soil bed & variations in particle size, and the presence or absence of ground water A subsurface investigation includes the analysis and testing of soil disclosed by excavation of test pit up to 3m deep or deeper test boring A subsurface investigation is conducted to understand the structure of soil, its shear resistance and compressive strength, its water content and permeability, and the expected extent and rate of consolidation under loading The allowable bearing capacity of soil maximum unit pressure that foundation can withstand vertically or laterally on soil mass Density determine the bearing capacity of granular soils Compaction (rolling, tamping, or soaking) to achieve optimum moisture content, increase the density of soil bed the number of blows required by a hammer to advance a standard soil sampler measures the density of granular soils and the consistency of some clays at the bottom of a borehole (the Standard Penetration Test) Coarse Grained soils low percentage of void spaces, more stable as foundation materials than silt or clay, more permeable and drain better than fine grained soil, less susceptible to frost action Clay soils is unstable as it shrinks and swells with changes in moisture content. shearing strength of a soil measure of its ability to resist displacement when an external force is applied, due largely to the combined effects of cohesion and internal friction Sloped sites & excavation of a flat site unconfined soil displace laterally Cohesive soils clay - retain strength when unconfined Granular soils (gravel, sand, or some silts) require confining force for shear resistance. Have a shallow angle of repose The water table level beneath which the soil is saturated with groundwater foundation system groundwater should be removed to avoid reducing the bearing capacity of the soil and to minimize the possibility of water leaking into a basement Voids consists of water (bottom) and air (top) solids different size, shape, angularity As construction materials Bearing pressure = force/area If the soil cannot withstand the bearing pressure, the building will sink. (solution: drill down to bed rock for stronger supports) Soil Classification methods vary depending on grain size, huge difference in physical properties between soil types, classification helps defining behaviors soil classification systems AASHTO (roads), USCS - Unified Soil Classification System (structure) USCS classification based on 1. size of soil grain, 2. distribution or gradation of soil grains (well graded: well distribution of different soil grain size), 3. organic components (organic: behave weird - remove for the stable supports), 4. behavior of fine particles (sticky or not) Distribution or gradation of soil grains determine permeability and strength of the soil soil type cohesive (clay - resist some tension) vs noncohesive (sand and gravel) USCS soil classification based on grain size Boulder (12''), Cobbles(3"), Gravel(#4), Sand(#200), Fine (clay, silt) #200 sieve 200 opening per 1 linear inch sieve Classification of Coarse grained soil sieve analysis: separate particle by sizes - Distribution: plot % finer (% passing) vs grain size (log size) D60,D30,D10 Diameter associated with 60%,30%, and 10% respectively USCS gradation classification well graded, poor graded, gap graded USCS gradation- well graded wide range of sizes, can pack closely together USCS gradation - Poorly graded (uniformly graded) soil in narrow bands of sizes, lots of similar sizes USCS gradation- Gap graded Some sizes are not existed in the soil USCS fine classification 1. water content (moisture content), 2. Atterberg limits USCS fine classification - Water content liquid (High water content - flow), plastic (medium water content - sticks), solid or semi solid (low water content - crumbles) USCS fine classification - Atterberg limits 1. Liquid Limit: water content that separates liquid and plastic status, 2. plastic limit: water content that separates plastic and semi solid USCS fine classification - plastic index differences between liquid limit and plastic limit At the unsaturated zone (above water table) S 1, No hydrostatic pressure, resist only effective stress At the groundwater zone (below water table) S = 1, Vv = Vw, Hydrostatic pressure exists, resist both effective stress and hydrostatic pressure Depth and the stress As depth deepens, the weight above increases (stress increases). The resistance behavior at the top and the bottom is different Total Stress (vertical stress) stress imposed below the ground due to everything (including the soil above it) above the point of interest = sum of unit weight * depth +surface loads Total stress resisted by 1. soil skeleton (due to contact forces between soil particles (effective stress) and 2. pore water pressure (unit weight of water * height of water column) Effect of static water buoyancy force act on soil column displacing water Soil Strength = f = cohesion friction Cohesion chemical reaction between fine and water Friction force between 2 surfaces = normal force * coefficient of static friction Friction in soils 1. grain to grain contact, 2. assume grains do not crust or yield, 3. grains do slide passed each other, 4. friction strength = shear strength = Normal stress coefficient of static friction = Normal stress tan(soil property) The boiling typically occurs on the sand, (soil at depth can't support load (soil above + surcharge), typically soil with upward waterflow) The heaving typically occurs on the clay, (water under pressure, pushes soil upward, typically clay excavation) Similarities of the boiling and the heaving both is the phenomenon related to the water under the soil Seepage and total stress The downward seepage decreases the total stress since the effects on the porewater pressure and the effective stress cancel out each other Seepage and porewater pressure The downward seepage decreases the porewater pressure due to change in water height Seepage and Effective stress The downward seepage increases the effective stress due to change increase in pore water pressure The discharge velocity and seepage velocity discharge velocity is the rate of liquid flow at certain area while the seepage velocity is the velocity of the ground water (seepage velocity = different for each path - particle do not flow in straight line) Empirical Equation Equation created from the experience or observation, not the theories Effects of void ratio to permeability Void ratio is the ratio of the voids' volume to solid's volume. Higher the voids' volume, more space for fluids to move through High Permeable soil The highway for the drainage (remove water on the roads for the safety) Low permeable soil For the earth dam, storing water without any leaks The field permeability test for the hydraulic gradient is conducted 1. lab setting not same as actual setting (soil, density, structure, saturation) 2. the boundary of the soil at the lab= different than the soil at the site 3. size of sample different 4. the existence of nonlinear Darcy's law Flow net representation related to Laplace's equation of continuity Flow nets are the visual representation of the Laplace Equation (Darcy law). The Laplace equation in an isotropic soil = two orthogonal families of curves (flow lines and equipotential line). Determine the pore water pressure at different location, the uplift force....