Soilconsistence

Consistence and consistencyConsistence and consistency

Sadia Bibi Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan

2010

• Important from both soil science and engineering Important from both soil science and engineering point of viewpoint of view

• Three termsThree terms

• StrengthStrength

• ConsistenceConsistence

• ConsistencyConsistency

• Consistence term is used by soil scientists Consistence term is used by soil scientists consistency by engineersconsistency by engineers

WHAT IS ConsistenceWHAT IS Consistence

• Soil consistence is a physical property to describe the resistance of a soil to Soil consistence is a physical property to describe the resistance of a soil to mechanical stresses or manipulations at various moisture contentsmechanical stresses or manipulations at various moisture contents

OROR• Soil consistence provides a means of describing the degree and kind of cohesion and Soil consistence provides a means of describing the degree and kind of cohesion and

adhesion between the soil particles as related to the resistance of the soil to deform or adhesion between the soil particles as related to the resistance of the soil to deform or rupture. rupture.

• Since the consistency varies with moisture content and clay minerals, the consistence Since the consistency varies with moisture content and clay minerals, the consistence can be described as dry consistency, moist consistency, and wet consistency. can be described as dry consistency, moist consistency, and wet consistency.

• Consistency that is evaluated includes rupture resistance and stickiness. Consistency that is evaluated includes rupture resistance and stickiness.

• The rupture resistance is a field measure of the ability of the soil to withstand an The rupture resistance is a field measure of the ability of the soil to withstand an applied stress or pressure as applied using the thumb and forefinger.applied stress or pressure as applied using the thumb and forefinger.

• Consistence is commonly determined in the field by feeling and manipulating the soil by hand

• Observations are made on the amount of force needed to crush the clod and on the manner in which the soil responds to force

• The degree of cementation of the soil by such materials as silica, calcite, or iron is also considered in identifying soil consistence

• Cohesion is the attraction of one water Cohesion is the attraction of one water molecule to another resulting from hydrogen molecule to another resulting from hydrogen bonding (bonding (water-water bond).

• Adhesion is similar to cohesion except with Adhesion is similar to cohesion except with adhesion involves the attraction of a water adhesion involves the attraction of a water molecule to a non-water molecule molecule to a non-water molecule (water-solid bond).

Consistency

• Consistency is used to describe the degree to which a soil resists deformation when a force is applied

• Consistency is determined by the soil’s resistance to penetration by an object while the consistence describes resistance to rupture

• Instead of crushing a clod of soil, the engineers attempts to penetrate it with either the blunt end of pencil or a thumb nail

Soil strength

•Most imp. Property of a soil for engineering uses

•This is a measure of the capacity of soil mass to withstand stresses without giving away to those stresses by rupturing or becoming deformed

• Soil consistence in general terms can be described as:Soil consistence in general terms can be described as:

• Categories are associated with increasing moisture contentCategories are associated with increasing moisture content

Hard or harsh consistencyHard or harsh consistency

• At low moisture content soil is hard or harshAt low moisture content soil is hard or harsh

• If tilled clods formationIf tilled clods formation

Friable consistencyFriable consistency

• As moisture content increases friability (ease of crumbling of moist As moisture content increases friability (ease of crumbling of moist soils)soils)

• For tillage these are optimum conditions For tillage these are optimum conditions

• range from firm to looserange from firm to loose

• As moisture content increases still further stickiness, plasticity, viscousAs moisture content increases still further stickiness, plasticity, viscous

• But when plough under this condition puddlingBut when plough under this condition puddling

Air dry consistencyAir dry consistency

• In air dry state consistency is measured by soil’s resistance In air dry state consistency is measured by soil’s resistance to rupture or to fragmentation when squeezed. So it is to rupture or to fragmentation when squeezed. So it is characterized bycharacterized by

• RigidityRigidity

• BrittlenessBrittleness

• Max. resistanceMax. resistance

• Tendency to crush to powderTendency to crush to powder

• Inability of crushing material to cohere again when pressed Inability of crushing material to cohere again when pressed togethertogether

Air dry ConsistencyAir dry ConsistencyClass Description

Loose (sands)Separate from each other when soil is non coherent

Symbol L or LO or lo

Soft (<8 N) (loams) Soil crushes under very low pressure/ weakly coherent

Slightly Hard (8 to < 20 N) Soil material crushes- low pressure - little resistance

Mod Hard (20 to < 40 N) Soil material crushes- moderate pressure -resistance

Hard (40 to < 80 N) ( clay loam)Soil material crushes under strong pressure

Break b/w hands

Very Hard (80 to < 160 N) (clays)

Can not be crushed between thumb and forefinger but b/w hands with difficulty

Extremely HardPressure applied by foot with full body or with the help

of hammer so depends upon cementation

1 Newton (N) = 0.224 lb/ft

Wet ConsistencyWet Consistency

• Describe Stickiness- Describe Stickiness-

– The capacity of soil to adhere to other objectsThe capacity of soil to adhere to other objects

– Estimated at moisture content that displays maximum Estimated at moisture content that displays maximum adherence between thumb and fore fingeradherence between thumb and fore finger

• Describe PlasticityDescribe Plasticity

– Degree a soil can be molded or reworked causing Degree a soil can be molded or reworked causing deformation without rupturing. We take soil material in deformation without rupturing. We take soil material in wet condition and roll it b/w thumb and fore finger and wet condition and roll it b/w thumb and fore finger and then observe whether rod or wire is formedthen observe whether rod or wire is formed

Stickiness ClassesStickiness Classes

• Non-StickyNon-Sticky – little or no soil adheres to fingers after release of – little or no soil adheres to fingers after release of pressurepressure

• Slightly StickySlightly Sticky – soil adheres to one finger but other remain – soil adheres to one finger but other remain clean after release of pressure with little stretching on clean after release of pressure with little stretching on separation of fingersseparation of fingers

• Moderately Sticky – Moderately Sticky – soil adheres to both fingers after release soil adheres to both fingers after release of pressure with some stretching on separation of fingersof pressure with some stretching on separation of fingers

• Very Sticky - Very Sticky - soil adheres firmly to both finger and thumb soil adheres firmly to both finger and thumb after release of pressure with stretches greatly on separation of after release of pressure with stretches greatly on separation of fingersfingers

Non-StickyNon-Sticky

Very StickyVery Sticky

Moderately-Moderately-StickySticky

Why Plasticity?

• Water Content Significantly affects properties of Silty and Clayey soils (unlike sand and gravel).

• Plasticity property describes the response of a soil to change in moisture content.

– Strength decreases as water content increases – Soils swell-up when water content increases – Fine-grained soils at very high water content possess

properties similar to liquids – As the water content is reduced, the volume of the soil

decreases and the soils become plastic– If the water content is further reduced, the soil becomes semi-

solid when the volume does not change

Plasticity ClassesPlasticity Classes

• Non-PlasticNon-Plastic – will not form a 6 mm dia, 4 cm long wire, or if formed , can – will not form a 6 mm dia, 4 cm long wire, or if formed , can not support itself if held on handnot support itself if held on hand

• Slightly PlasticSlightly Plastic – 6 mm dia, 4 cm long wire supports itself, 4 mm dia, 4 – 6 mm dia, 4 cm long wire supports itself, 4 mm dia, 4 cm long wire does notcm long wire does not

• Moderately PlasticModerately Plastic – 4 mm dia, 4 cm long wire supports itself, 2 mm dia, – 4 mm dia, 4 cm long wire supports itself, 2 mm dia, 4 cm long wire wire does not4 cm long wire wire does not

• Very PlasticVery Plastic – 2 mm dia, 4 cm long wire supports itself – 2 mm dia, 4 cm long wire supports itself

Moist consistencyMoist consistency

• It is a midway b/w air dry consistency It is a midway b/w air dry consistency and field capacityand field capacity

• It is resistance to squeezing forces It is resistance to squeezing forces b/w thumb and fore finger b/w thumb and fore finger

Moist ConsistencyMoist Consistency

MoistMoist commentscomments StressStress

LooseLoose Non coherentNon coherent 00

Very FriableVery Friable

Soil material crushes very Soil material crushes very easily under very gentle easily under very gentle

pressurepressure < 8 N< 8 N

Friable (best)Friable (best)Soil material crushes under Soil material crushes under

gentle pressuregentle pressure 8 to < 20 N8 to < 20 N

FirmFirmUnder moderate pressure b/w Under moderate pressure b/w

thumb and forefingerthumb and forefinger 20 to < 40 N20 to < 40 N

Very FirmVery Firm Strong pressure is requiredStrong pressure is required 40 to < 80 N40 to < 80 N

Extremely FirmExtremely Firm Can’t crushCan’t crush 80 to < 160 N80 to < 160 N1 Newton (N) = 0.224 lb/ft

Atterberg limitsAtterberg limits

• Behavior of soil at different water contents is called Atterberg limitsBehavior of soil at different water contents is called Atterberg limits

• Before building roads and motorway is consideredBefore building roads and motorway is considered

• The state of consistency b/w solid and liquid range has been divided into a The state of consistency b/w solid and liquid range has been divided into a number of distinct stages by imposing limits to indicate the soil water content number of distinct stages by imposing limits to indicate the soil water content limits for various states of consistency.limits for various states of consistency.

• Depending on the water content of the soil, it may appear in four states:Depending on the water content of the soil, it may appear in four states:

• solidsolid

• semi-solidsemi-solid

• plastic plastic

• liquid.liquid.

• In each state the consistency and behavior of a soil is different and thus so are its In each state the consistency and behavior of a soil is different and thus so are its engineering properties. These limits were created by Albert Atterberg, a Swedish engineering properties. These limits were created by Albert Atterberg, a Swedish chemist.chemist. They were later refined by Arthur CasagrandeThey were later refined by Arthur Casagrande

Atterberg Limits

• Liquid Limit (LL) Upper limit of plasticity of soilis defined as the moisture content at which soil begins to behave

as a liquid material and begins to flow but posses a small shearing strength.

At this point water film becomes so thick that cohesion is decreased and soil mass flows under an applied stress

• Plastic Limit (PL) is the state of minimum water contents & defined as the moisture content at which soil begins to behave as a plastic material. At that state soil has the property of ploughing. Plastic limit is determined by finding the water contents at which the soil rolled into thread 3mm dia. Begins to break.

• Shrinkage Limit (SL) is defined as the moisture content at which no further volume change occurs with further reduction in moisture content. (SL represents the amount of water required to fully saturate the soil (100% saturation)

Atterberg limitsAtterberg limits

Plastic Index (PI)

• Plasticity Index (PI) is the difference between the liquid limit and plastic limit of a soil

By increasing the clay content and O.M. we can increase the plastic and liquid limit

Bigger surface area plasticity more

UsesUses

• The knowledge of the soil consistency is important in defining or classifying a soil type

• For farming operations e.g., tillage

• The soil consistency is a practical and an inexpensive way to distinguish between silts and clays

• Construction purposes

• On organic soils we cant make roads which have high plastic and liquid limits