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Engineering Properties of Soils -

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Soil Types
Soil – all unconsolidated material in the earth’s crustSoil includes –Mineral particles – sand and clayOrganic Materials – found in topsoil and marshAirWater
Mineral Soils
Result from weathering of rock that forms the solid crust of the earthPhysical weathering – due to the action of frost, water, wind, glaciers, landslides, plant and animal life, and other weathering agents – that break particles away from the bedrockParticles are often transported by wind, water , or iceRounds them and further reduces their sizeSoils created this way are referred granular soilsGrains or particles are similar to the original bedrock
Mineral Soils
Chemical weathering – occurs when water flows through rocks and leaches out some mineral componentsNew soil particles are formed from these mineralCalled claysClay particles are mineral crystals that have very different properties from those of the original bedrock
Types of Mineral Soils
GravelSandSiltClayCourse grained soils – gravel and sandFine grained – silt and clayCobbles – over 75mm or 3 inBoulders – over 200mm or 8 inClays are cohesive soils – bonded to each other
Gravels and Sand
Composed mainly of rounded or cubical grains that are supported by adjacent grainsCan carry significant loadsLoads are spread across many particles through frictionFairly easy to compactExcellent soils for construction
Clays and Silts
Clays are softerDo not carry loads very wellClay grains are small size and flat plate like shapeThe mass of the grain as a force is negligible when compared to the forces resulting from the surface properties of the grainClays have charges on surface – figure 1-1 page 3Result of these charges is clay can hold a lot of waterSurface charges attract water moleculesClays absorb or hold water – permanently unless conditions changeMay dry out due to evaporation – or squeeze water out when load is applied –Will absorb moisture quickly when re appliedThe plates of clay are attracted to opposites charges
Field Test to Identify Soil types
Large grains (sand and gravel) are easily to identifiedOrganics are also easySilts and clays – are not as easy cause grains are not visiblePage 5 table 1-2 differences between silt and clay
Mass-Volume Relations
Soil sample containsMineral – possibly organic particlesWaterAirMass and volume of each phase is usually calculatedVa= Volume of AirVw= Volume of waterVv = Volume of voids (=Va+Vw)Vs = Volume of dry soil solidsV = Total volume (=Va+Vw+Vs)Ma = Mass of air (=0 by definition)Mw = Mass of waterMs = Mass of dry soil solidsM = Total mass (=Mw +Ms)
Mass and Volume
Soil sample consisting of 10cm3 of air, 25cm3 of water (mass = 25g) and 65cm3 of soil solids (mass = 175g)Vv = 35cm3Va= 10cm3Vw= 25cm3Vs =65cm3V=100cm3Mw =25gMs=175gM=200g
Mass and Volume
Relationship between the mass and volumeWaterPw = Mw/VwWhere Pw = density of waterDensity of water is 1g/cm3 or 1000kg/m3Example 1-1 Pw = 25g/25cm3 = 1g/cm3Soil solidsPsoilsolids = Ms/VsWherePsoilsolids = densityfothe dry soil solidsRatio between soil solids and density of water is the relative density of the solids or specific gravity GsRD (relative density) =Psoilsolids/Pw = Ms/(Vs*Pw)Or RD =Ms/(Vs x Pw)Example 1-1 RD=175g/(65cm3 x 1g/cm3) = 2.69Most soils – RD is between 2.6 and 2.8
Properties Calculations
Density (P) P=m/vDry Density (Pd) Pp=Ms/VWater content (W) W=Mw/MsVoid ratio (E) e = Vv/VsDegree of saturation (S) S=Vw/VvPorosity(n) n=Vv/VProblem 1-1on page 8
Classification Tests
Two TestsGrain size – to measure grain sizesSieve analysis used for sands and gravelsGrain size distributiongraphExample 1-9 page17Hydrometer used for silts and claysSedimentation testRate at which particles settleStrokes law states –that particles in a suspension settle out at a rate that varies with their sizePlasticity – to measure grain types
Grain Size Distribution Curve
Used to help describe and classify a soilShape –Uniform soil –curve a on page 19Well graded – curve b on page 19Effective size10% size is considered effective size – page 19 – sample b .09mmUniformity coefficient –Value gives some indication of the shape of the curveCu=D60/D10Coefficient of curvatureCc=(d30)2/(d60xD10)
Textural Classification
American Society for Testing and MaterialsGravel – larger than 4.75 (no. 4)Sand 4.75mm to 0.075mm (no. 4 to No. 200)Silt .075mm to .005mm (No. 200 to .005mm)Clay – smaller than .005mm
Plasticity Test
Measures the amount of water that a soil adsorbsPlastic limit – soil is roll into a thread(Wp)Liquid limit (WL)Index of plasticity – range of water contents over which this soil is plasticIp=wl-wpAtterbergLimits test
Soil Water
Type of water found in soilFree water or gravitational water – found below groundwater – free to flow under the laws of gravityCapillary water – brought up through the soil pores – above the groundwater tableAttached water or held water – moisture film around soil grainsThe rate of water flow – or permeability of the soilDarcy’s lawQ=k (h/l)a – page 33Problem page 34
Soil Strength and Settlement
Two type of soil failureFailures due to shear – grains slide with respect to other grainsSettlement failures – where a layer of soil is compressed and becomes thinner under loadingForces on soilForces acting perpendicular to the plane are normal forcesForces acting parallel are shearShear strength in most clays is due to cohesionT=c were t is sheering pressure and c = cohesionShear strength in granular soils is due to frictionT=o tan o/





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Engineering Properties of Soils -