Problematic Soil & Rocks • Some natural soil-rock inherits weakness from the parent material

such deposits include:

1. Shales & Weathered rocks 2. Residual soils3. Expansive clays4. Organic soils 5. Karst Region6. Loess 7. Dispersive clays8. Laterite soil9. Alluvial Sediments10. Saline & Alkaline soil11. Bentonite

Shale & Weathered Rocks•  A fine-grained, sedimentary rock composed

of mud

• flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite are present

• Types of Shales :• Soil-like Shales: Bonding material is water

• Rock-like Shales: Particulate components are cemented together by calcareous, silicious, gypsiferous, phosphatic material

Shale & Weathered Rocks

• Characteristics –Porosity : 3 – 52 %Void Ratio: 0.03 -1.07Moisture Content : 5 -35 %Colour: Gray

Residual Soils

• Produced by in-situ decomposition of underlying rock & action of factors like microclimate, flora, fauna & geometric formations

• Breakdown is active in hot & humid regions of tropical climate – Because these are favorable conditions for chemical weathering

Residual soils

• Zones in Residual Soils :1. Upper layer – high degree of weathering

& removal of material2. Intermediate Layer – some degree of

weathering in top portions3. Bottom Layer – partially weathered zone

where there is a transition from weathered material to unweathered parent rock

Expansive Clays

• They exhibit greatest volume change from dry to wet state

• Possesses a considerable percentage of montmorillonite clay

• They do not have a continuous granular skeleton

Expansive Clays• Expansive clays are made up of small negatively

charged particles

• These clay particles attract water molecules, which are slightly polarized

• Montmorillonite clay have a very huge surface area

• This results in massive change in volume

Expansive Clays

Expansive Clays – Why are they dangerous?

Expansive Clays

• Mitigation Measures –Extend building foundations beneath the zone of

water content fluctuation zoneCollect surface runoff and to limit surface infiltration

during the rainy winter months Remove expansive soil & replace it with non-

expansive fillApplication of hydrated lime

Expansive Clays

• Black Cotton Soil –Found in central & southern IndiaColour: BlackExhibits excessive swelling & shrinkageDeep & wide cracksHighly problematic for foundations

Organic Soils

• Organic soils are those whose solid constituents consist predominantly of vegetable mater in various stages of decomposition

• Parent Material : Native Vegetation

• Form in wetlands

Organic Soils

Peat

Fibrous material are

clearly visible

Muck

Plant residues are

fully decomposed

Organic soils

• Characteristics –Specific Gravity: 1.1 - 2.5 pH: Acidic - 4 to 7High void ratiosHighly compressible Do NOT receive proper compaction

Karst Region

• a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock

• Most commonly develop on limestone, dolomite, gypsum formations

• Formation of Karst region takes thousands of years

Karst Region

• As rain water falls down, CO2 gets dissolved in it forming carbonic acid

H2O + CO2 → H2CO3

• As water moves underground, through tiny fractures in the limestone bedrocks, the rock slowly gets dissolved away by weak acids

Karst Region

Sink Hole Development

Sink Hole Development

Karst Region

Loess/Aeolian Deposits

• Sediment formed by the accumulation of wind-blown silt

• Loosely cemented by calcium carbonate

• Found in arid regions and deserts

• Loess mainly consists of quartz, feldspar and mica grains

Loess/Aeolian Deposits

• Characteristics –Homogeneous and porous structureAvg. particle size: 25 – 35 μmCarbonates : up to 15-18 %Quartz grains up to 70 %Colour : Yellowish or Brown

Loess/Aeolian Deposits

Dispersive Clays

• Dispersive soils are usually found in flood plains and lake bed deposits

• Soils often disperse when they are sodic

• These soils are highly erodible because of the low inter-particle attraction

Dispersive Clays

• Clay particles have a negative charge

• this charge is balanced by positively charged cations, such as Ca2+, Mg2+, K+ and Na+

• Dispersive soils contain large amount Na+ ions compared to other ions

• So the clay particles are less tightly bound to each other and the soil aggregates easily disperse

Dispersive Clays

Laterite Soil

• Laterites are soil types rich in iron and aluminum

• formed in hot and humid tropical areas

• all laterite soils are red due to the presence of iron oxides

Laterite Soil

Alluvial Sediments

• Alluvial deposits loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting

• fine particles of silt and clay and larger particles of sand and gravel

Alluvial Sediments

Alluvial Sediments

• Colour: brownish - black

• pH: Slightly acidic

• Depth : up to 2 -3 m

Alluvial Sediments

Alkaline/Saline Soils

• Alkaline soils are soils with a high pH (> 9) and a poor soil structure and a low infiltration capacity.

• Found in North India & Central Europe

Alkaline/Saline Soils

• Causes –

1.Natural:the presence of soil minerals producing

sodium carbonate

2. Man – Made:the application of irrigation water (surface or

ground water) containing a relatively high proportion of sodium bicarbonates

Alkaline/Saline Soils