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 stresses, the same diameter will be used it will just be movedOverview-Tests with soil (mechanical and hydraulic) are needed to quantify material parameters in order to calculate safety against failure, settlements, etc.-Mohrs circle is used to describe stress states of soil both before and after failure-Stress and strain invariants are used to show evolution of stresses for changing stress conditions or deformations (stress paths)Mechanical Behaviours

Mechanical BehavioursModes of failure in triaxial compression:-Shear plane failure (brittle)-Barrelling (plastic failure)-Combination of both (intermediate failure) -Elastic behavior (idealised) has deformation that is recovered after loading-Behavior can be linear or non linear-Can act both elastically or brittle (rigid)-Elasto-plastic(idealized) behavior: deformation is permanent after unloading-Behavior can be linear or non linear-Can act elastically or brittle-Can act with hardening or softening-Loose normal porous soils act in a ductile (plastic) manner-Dense compacted/consolidated materials act in a brittle way (sudden loss of strength)-In realistic soils there is always some deformation necessary to actuate frictionSoil Strength

Mechanical BehavioursSoil moduli is also known as the soil stiffness, it can be determines from stress-strain relationships

Summary of Strength-Total stress increases, with depth-Pore water pressure, uw, reduces the total stress to effective stress, -Normally horizontal stresses < vertical stresses-Stress states/conditions (both 2D and 3D) can be visualized using Mohrs circle-Difference in stresses at different locations leads to shear stresses -When shear stresses exceed shear strength, f failure occurs-Therefore there is a maximum limit of shear strength soil can withstand-The maximum shear strength is dependent on the normal stresses n acting in the shear plane-A failure envelope is found by a combination of points of maximum shear stresses and normal stresses-A linear relationship known as the mohr-coloumb criterion maps shear and normal stresses at failure-f = C + f tan()-C = effective cohesion = friction angle-Failure due to wall movements

-Changes in stress conditions is represented by sets of Mohrs circles

10.3 Summary of Mohrs Circle Rope Test

Week 12: Soil Testing

Direct Shear TestShear Box Test: relatively quick and simple -> soil is forced to shear at the interface between the two halves of the box-during shearing, the shear stress and normal stresses are measured-the effect of large shear displacement is obtained by reversing the shear box after initial displacements are measured then repeating a bunch of times to achieve a steady(residual) shear strengthRing Shear Test: pretty sure a sample of soil is just twisted until it fails-Effective parameters = C & -Effective residual parameters = Cr & r-Advantages of this test = this test can also be used for measuring angle of friction developed at an interface of soil and other material-Disadvantage = major assumption that stresses within sample are uniformly disturbedConventional Triaxial Test-Conventional triaxial test carried out in 2 stages1. Isotropic compression consolidation(in time) or not 0Consolidated(C) or unconsolidated(U)2. Loading(shearing) to failure with drainage or not > 0Drained(D) or undrained(U)-An unconsolidated and drained test is highly uncommonUnconfined Compression Test-This is a special case of a triaxial test, determining Unconfined Compression Strength (UCS)-Special case: r = o, therefore dimensionless-Only 1 stage, loading until failurec(quick test)-Samples are capable of being formed as self-supporting cylinders-Ultimate Strength

Unconsolidated Undrained Test (UU)-Only 1 stage, loading until failure-Drainage is prevented-Repeat test with multiple samples extracted from the same depth and position to give an average(line of best fit for horizontal line Su)-Mean pressure does not affect strength because the confining pressure only affects the pore water pressure-Disadvantages: unconfined strength is not intrinsic of material, varies from one point to another in the same soil mass, varies with depth-However C and the material parameters do not vary greatly with depth

Consolidated Undrained Test (CU)-2 stages, consolidation and loading to failure without drainage-During consolidation the volume change with respect to isotropic pressure can be determined-During loading there is no volume change cause valves are closed-If pore water pressure changes are measured, effective parameters can also be calibratedConsolidation Drained Test (CD)-2 stages, consolidation and loading to failure with drainage-During consolidation, volume change with respect to isotropic pressure can be determined-Pore water pressures are zero, total stresses = effective stresses-Only drained parameters can be foundOverview-Mohrs circle contains NO info about deformations-Shear parameters and stress strain relationships are dependent on density-During consolidation (phase 1) load deflection curves are measured which can be used to asses consolidation parameters-Residual parameters are measured by repeating the shear box test-Triaxial tests are differentiated by consolidation and hydraulic conditions (drained/undrained)-Drained conditions means pore water pressure changes = 0-Tests must be done very small to eliminate/reduce development of uw-Undrained conditions means no change in volume, but uw can change-During CU tests effective parameters can be determined

Soil Testing and consolidation12.1 Coefficients of Primary ConsolidationCompression Index: Gradient between initial state of normally consolidated clay and final stress condition for prediction of settlements Recompression /Expansion Index: Gradient between initial state of over consolidated clay and initial state of normally consolidated clayCoefficient of Volume Compressibility: Relates the vertical stress with the vertical strain by the following rate equation: (note: mv needs to b x 1000)

12.2 Oedometer Test-Test procedure for primary consolidation-Each dead weight is added and left for a period of time. During this time the vertical displacement is measured-For each increment of time, a pair () is obtained12.3 Summary

Material Behaviour.