soil formation - university of waterloo

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Soil FormationDas Chapter 2

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Rock and SoilRock - mineral aggregate that is strongly bonded

Soil - mineral aggregate that is weakly or loosely held together

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When Does Rock Become Soil?Dividing line can be arbitraryContractors and engineers can have different definitions

Poor definition can result in contractor going out of business and significant project delays

For geotechnical applications:If you can sample it with a split spoon sampler then it is soil. If not then it is rockVery soft rock analyze as hard or stiff soil

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Standard Split-Spoon Sampler

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Configuration of Standard Penetration Test (SPT) hammers

(a) safety hammer;(b) donut hammer

(after Seed et al., 1985)

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Nvalue = number of blows (hits) it takes to drive a standard split spoon 300mm with a 623N hammer dropped a distance of 762mm

(ASTM D1586)

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How are Soils Formed?Degradation of rock into small pieces or weakly bonded mineral aggregates

The process that forms or deposits soil will influence the soil mass engineering properties

Compressibility, strength, ability to conduct fluid flow (permeability)

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Soil Formation ProcessWeathering

Chemical Physical

Vegetation growth

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Chemical WeatheringResults from reaction of mineral aggregates with oxygen, water, acids, salts, etc

Causes chemical decomposition of rock that can drastically change its physical and chemical properties

Decompose rock minerals by:Oxidation, reduction, carbonation, & other chemical process

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Physical WeatheringIncludes mechanical weathering

Freezing and thawingTemperature changes (expansion-contraction)Erosion

Activity by plants, animals (people)GlaciersWater transport

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Types of Soil DepositsOrganic - vegetation growth and decay

Very soft depositsHigh organic content due to decaying vegetationPeat or muck

Residual - remain in place of origin (weathered)

Transported soils - transported from place of origin and deposited else where

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Transported SoilsCommon transporting agents:

GravityWaterAir (wind)Glaciers

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Gravity Transported SoilsColluvium - gravity soil deposits

Develop from mud or land slides and debris flows - short transportation distance

Characteristics:Mixture of particle sizesLoosely compactedAngular particles

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Water Transported SoilsAlluvialLacustrineMarine

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Water Transported SoilsAlluvial soil - developed on flood plain

and deltaTransported by moving waterParticles can be transported large distancesSize of particles carried depends on water velocity: velocity particle size carried

Slow moving = silt and clayFast moving = sand and gravel

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Formation of point bar deposits and oxbow lake in a meandering stream

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Cross section of a braided-stream deposit

© 2004 Brooks/Cole Publishing / Thomson Learning™

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Water Transported Soils (con’t)Alluvial soil characteristics:

Sub-rounded to rounded particlesLayered deposits - seasonal changes

Spring fast moving waterSand and gravel

Summer slow moving waterClay and silt

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Water Transported Soils (con’t)Alluvial fans

River deposits where water velocity suddenly decreases: (river widening)Coarse soils, flat triangular deposits

DeltasSediment deposited at mouth of creeks, rivers, lakes, bays, or seasLoose deposits (compressible)

may contain organics

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Water Transported Soils (con’t)Lacustrine soil

Belonging to or produced by lakes or other bodies of water

CharacteristicsFine grained soils (silt and clay)varved - contains thin horizontal layers

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Levee and backswamp deposit

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Water Transported Soils (con’t)Marine soils – salt water deposits

Formed in the seaVarying particle sizes

Beach deposits - (high energy)Loose and one particle size

Bay and inlets - (low energy)Fine grained, may contain organics & shells

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Air Transported Soils


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Air Transported SoilsAeolin soil

Formed by windDunes, loess, volcanic dust (ash)

Characteristics:Well roundedOne particle size - silt or sandHigh dry strength, low wet strength

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Glacial Deposited SoilsResult from the action of glaciers

Erosion from ice sheetTill: mixture of particle sizes (clay to cobbles)

Well compactedAngular fragments

Soil deposits formed on ice sheetMoraine: mixture of particle sizes

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Glacial Deposited SoilsMelting of an ice sheet

Outwash deposits, kame and kame terraces, kettle, drumlin, esker

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Glacial Deposited Soils


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Expect the Unexpected

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Peck, Hanson and Thornburn

Foundation Engineering (Second Edition)1974 Wiley & Son

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Expect the UnexpectedSand deposits discovered during hand carving slabs of clay during construction of Chicago subway systemOrigin of formation unknown

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Expect the UnexpectedPermeability profile glacial deposit Chicopee, Mass USA (Terzaghi, 1929)Similar problem Oakridge moraine Markham, ON

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Expect the UnexpectedWaste fillClaySandGravel/cobble seamsBouldersChanging ground conditionsOrganics

DESIGN BASED ONLIMITED BOREHOLE DATA

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New York City

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Open fractures

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How Much Site Investigation?

More extensive site investigation & testing program

Cost

($)

Cost of site investigation & testing

Cost of construction

Total project cost

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How Much Site Investigation?

More extensive site investigation & testing program

Cost

($)

Cost of site investigation & testing

Total project cost

Minimum project cost

Optimum SI cost

soil formation - university of waterloo

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