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Sep 14, 2015
Soil Types & Phase Relationships
What is a soil?Soil is the accumulation of sediments and mineral particles, typically non-homogenous but not always, influenced by change in moisture content. Differentiated mainly by grain size. Shape/size increase hydraulic and mechanic soil parameters.
General Definitions: Residual Soil: weathered soil, remaining at original placeAlluvial: transported by waterGlacial: Transported by glaciersLoess: transported by wind Marine: deposited in salt/brackish waterExpansive: large volume changes with addition of moistureDispersive: loss of cohesion in waterGranular: No cohesionREV: Representative Elementary Volume. The sample size which has a size big enough to represent the sample accurately cant be too small, the bigger the sample size the better. Course-grained samples must be 10x bigger. Within the REV scale, the soil behaviour can be described based on phase relationship parameters.Fine Grained SoilsOccurs due to weathering of parent rock (mineral), resulting in formation of groups of crystalline particles at colloidal sizeHigh specific surface area (high surface area to mass ratio)Surfaces of clay minerals carry residual negative charges, meaning they are less attracted to other particles and can be denserAttraction between clay particles happens because of van der waals bondsIncreasing ion concentration leads to net repulsionNet repulsion = face to face orientation, which makes it more denseNet Attraction = face to edge/edge to edge, meaning less close and less denseAbsorbed water is held around clay by hydrogen bonding & hydration of cations EquationsVoid Ratio[-]e = n/(1-n); e=Gw/SEffective Unit Weight [kn/m^3]
Porosity[-]n=1- (D/Gw)Dry Unit Weight[kn/m^3]
Moisture Content[%]Unit Weight of Solids [kn/m^3]
Degree of Saturation[%]Specific Gravity [kn/m^3]
Total Unit Weight[kn/m^3]
= 9.81(10)kn/m^3Saturated Unit Weight [kn/m^3]
Soil Characterisation & Soil States
Soil Tests: Moisture Tests:Oven Drying: soil sample taken & measured, then oven dried, measure again.MD = MCDS MCMw = MCMS MCDS w = (Mw/MD)x100%Sieving: soil is placed in sieves, shaken, each different size is measured & graphed on a PSD scaleAnalysis Uniformity Coefficient: Cu = D60/D10, Curvature Coefficient: (D30)2/(D10 + DD60)Hydrometer Method: wet dirt, put in tube of water, wait for it to settle, observe the layers of different soils, and take continual readings at different time intervals. Atterberg LimitsLiquid Limit: LL the minimum w at which soil flows (Liquid plastic)Plastic Limit: PL the minimum w at which soil deforms plastically (Plastic semi-solid)Shrinkage Limit: SL the w at which soil reduces volume (semi-solid-solid)Limit IndicesPlastic Index: PI IP = LL PLLiquid Index: LI IL = [w-PL]/IPConsistency Index: CI Ic = [LL w]/IPActivity: A = IP/[% clay by mass]4 = high activityAtterberg Limit TestsDetermine LL - Penetration: drop a machine pin into sample, measure penetration, analyse on log graph. 20 blowsDetermine SL Shrinkage: fill sample and measure, then dry sample and measure again, using the equation below:
Determine LL Casagrande Method: mix soil & water in dish, use a U shaped knife and spread/split the soil. Measure gap and see if it reforms, count number of blows delivered by the crank machine, usually at 2 drops/sec, till soil reforms. 25 blowsDetermine PL Ellipsoidal (Standard): mix dirt and water, roll into a ball and then roll onto the bench into an ellipsoidal mass until it breaks. Repeat at least two times and use w=PL to find plastic limit.
NOTE: PSD & Atterberg Limits are used to determine other properties; erosion, penetration (grouting), hydraulic conductivity, workability and more.
Soil Classification & Compaction
Undefined Soil Classification: G GravelS SandC ClayM SiltO Organic Soil P PeatW Well Graded P Poor Graded L Low Plasticity H High PlasticityFlow charts are used to sort samples of soil into certain categories, the following is an exampleThese classifications are related engineering parameters; strength, compressibility, hydraulic conductivity, workabilityApplied to dams & roads1 = highly desirable, 14 = highly undesirableThis is an internationally accepted classification system.CompactionIncreased density due to compaction leads to; Increased shear strength, Reduced compressibility, Decreased porosity, Resistance to shrinkageCompaction depends on soil types, size of crumbs, etc.Proctor Compaction Test: the standard test for compaction. Mix soil & water put in mould, compact the sample, weigh sample as well as mould. Then take out of mould, weigh it and determine the moisture content using moisture cans and ovens.Analysis: Plot dry unit weight on y-axis and moisture content on x-axisDraw a smooth connecting curveAlso draw a curve for complete saturation2 tests; standard compaction & modified compaction(larger compaction forces)-Should be noted that the size of crumbs affects the validity of results (40% dysfunctions/failures in embankments due to erosionSoil Stress & Principle Effective Stress
Force Pressure & Stress-Pressure & Stress are dependent on Area-Pressure & stress vary with space-Multiple components of stress, x, y & z-Internal pressure (P) = External Force(F)/Area(A)-Pressure ad stresses are better related to the material mechanical changes(damage, failure, stretching, ) rather than forcesEffective StressEffective Stress Principle: = - uw = total stress, the weight of everything above a certain point, including water, uw is the pre water pressure. Used for saturated soils-in dry soils = 0-change in leads to deformations and changes in strength-the soil grains and pore water are assumed to be incompressible-in a saturated soil, deformation on the application of stress is directly related to the expansion of water, which means its related to the hydraulic conductivitySand Shear Strength (): proportional to , = tan where; = internal angle of frictionClay Shear Strength (): = cu or cAlso proportional to , but the constant of proportionality is dependent on the over-consolidation ratio (OCR)(/ )NC = constant [typically = 0.25] NC = normally consolidated, OC = over consolidated m = a value experimentally found is equal to 0.8
Drained Behavior: In high permeability soils (sand and gravel) any excess pressure gained by an applied stress generally dissipates instantaneously, the applied stress transferring instantly to the soil skeleton-a drained situation has no real difference-quick conditions or liquefaction are an exception, the rate of stress application is faster than the drainage rate and the seepage stresses exceed the strength of the soil (pure water strength overrules)Short Term Undrained: similar to long term drained, in saturated soils of low permeability where any excess stress is taken as excess pressure and applied to the soil skeleton. (In a question add the extra pressure)-Loss of effective stresses can be caused by hydraulic forces-Critical hydraulic gradient-Seismic excitation (small vibrations in soil can cause increased pressure)-When liquefaction and earthquakes combine, safety issues can arise
Geostatic Stressv = total vertical stress, this is equal to the weight of everything above this point uw = hydrostatic pore water pressure, increases with depth under ground-effective vertical stress(v), this is the difference between total vertical stress and pore water pressurev = v uw H = k0 - vTotal vertical stress due to wet soil is equal to unit weight multiplied by depth at that pointv = Stress & Strain/Mohr Circle
General Consideration-Engineers use a rational approach to design considering continuum mechanics & differential equations to represent structural conditions leading to an initial boundary value problem (IBVP)-IVBPs are solved to assess safety of failure (collapse), safety of large deformations (serviceability) and safety of other problems such as water leakage.Assessing the following:-Seepage: need understanding of hydraulic conductivity and water flow-Slip lines: understanding of strength-Settlement: understanding of deformation due to loads & deformations in time due to pore water movement (consolidation)-To solve IVBPs all materials must be characterized in lab tests(morphologically, mechanically, hydraulically)-IVBPs are tests to represent conditions of future designs for big ass structuresRepresentations of Stress ConditionsStress state in soil is described by normal and shear stresses applied to the boundaries of the sampleStress states can be plotted 2 ways-Pair of coordinates (z , xy) and (x , -xy)-Mohrs circle of the effective principle stresses (1 and 3) Mohrs Circle-At an angle of 2 to the horizontal of the circle is a representation of the stress condition of a plane at an angle of to the direction of the minor principle stress, 3-The circle represents the stress states on all possible planes within the soil element-All info is then represented on a failure envelope-A stress condition represented by a point above the failure envelope is not possible (failure occurs with small shear stress)-Relationships between shear strength parameters & effective principle stress at failure can be found from shear stress (f) and normal stress (f) acting in the failure plane.f = 0.5(1 3)sin(2)f = 0.5(1 + 3) + 0.5(1 3)cos(2)
Where: = the theoretical angle between minor principle stress(3) and the failure plane, hence why: 2 = 90 + and = 45 + (/2)Mohr-Coulomb Criterion: this defines the relationship between principle stresses at failure and material parameters, and C(1 3) = (1 + 3)sin() + 2Ccos()(1) = 3tan2(45 + [/2]) + 2Ctan(45 + [/2])For a given state of stress it is apparent that because of ( = uw) that the Mohr circle will have to shift when dealing with effective stresse