SOILSIdentify Basic Soils Properties

TEC C03-A01

Why is Soil important ?

It is the foundation forall types of pavementsystems and building

structures.

What is Soil ...

is the entire unconsolidated earthen material that overliesand excludes bedrock; plusthe water, air, organic matter, and other substancesthat may be contained therein.

BASIC PHYSICAL PROPERTIES

GRADATION of sizes of the different particles

BEARING CAPACITY as reflected by soils density

PARTICLE SHAPES interlock according to shapes; this affects strength

Engineers evaluate soils by the following Basic Physical Properties:

An engineer will also consider the effect of water on various types of soil for the project.

PHYSICAL PROPERTIES

GRAIN SIZE PARTICAL SHAPE GRADATION DENSITY SPECIFIC GRAVITY MOISTURE CONSISTENCY ORGANIC SOIL

….a soil’s properties help determine the

engineering characteristic

s.

GRAIN / PARTICLE SIZE

COBBLES

GRAVELS

SANDS

FINES (CLAY OR SILT)

}+ 3”

- 3”+ #4

- #4+ #200

- #200

}

The shape of soil particles influencesthe strength and stability of a soil.

Gravels, sands and silt fall into the bulky shape ...relatively equal in all three dimensions. Bulky shapes are further subdivided based upon the effects of weathering. They may be angular, subangular, subrounded, or rounded.

Particles of clay exhibit a platy shape ... onedimension is very small compared to the othertwo.

BULKY

PLATY

GRAIN/PARTICLE SHAPE

WHAT ARE THE 2 TYPES?

The shape of soil particles influencesthe strength and stability of a soil.

bulky shapes

angular subangular subrounded rounded

platy shape

The distribution of the different sizeparticles in a soil mass is the gradation.

Well-graded soils have a good range of allrepresentative particle sizes between the largest and the smallest.

Poorly-graded soils contain either a narrow rangeof particle sizes or lack some intermediate sizes.

• Uniformly graded have a narrow range of sizes.• Gap, step, or skip graded have some

intermediate sizes missing or not wellrepresented.

GRADATION

WHAT ARE THE 2 TYPES?

Well-graded

Uniformly graded

Gap, step, or skip graded

GRADATION

• Density is how closely packedor loose the soil structure is.

• A density structure provides interlocking of larger particleswith smaller particles filling thevoids between larger particles.

DENSITY

• Ratio between the weight-per-unit volume ofa given soil sample and the weight-per-unit volume of a similar volume of water at a statedtemperature.

• Specific Gravity of water is 1, Gold is 18.

• If a soil has a Specific Gravity of 2.72, then it is 2.72 times heavier than water.

SPECIFIC GRAVITY

Moisture Content is often the most important factor affecting a soils behavior.

It is the proportion of the weight of water to the weight of the solid(dry) mineral grains in the soil.

MOISTURE

Moisture has its greatest effect on the behavior offine-grained soils such as silts and clays.

Coarse-grained soils with larger voids• Are less susceptible to capillary action• Hold less water• Tend to drain more freely

Clay soil particles retard movement of water• Vary from liquid to brick hard• Impervious to passage of free moisture

MOISTURE

Soil Moisture occurs in five types:

1) SURFACE WATER … water from precipitation or runoff.

… collected or held in pools or layers beneath surface by a restricting layer of soil or rock.

2) SUBSURFACE WATER

3) GRAVITATIONAL PULL

4) CAPILLARY ACTION

5) ABSORBED WATER& HYGROSCOPIC MOISTURE

… voids in soil may form tunnels & tubes and cause water to rise in the tubes

… seeks lower layer & moves through voids until it reaches a restriction.

… water that may be present as thin films.

• Plasticity is a property of fine-grainedsoils that allows it to be deformed withoutcracking or breaking.

• The Plasticity Index (PI) is used to determine weather soil is cohesive. Not allplastic soils are cohesive. (PURE SILT)

• Soil plasticity is determined by observingthe different physical states a soil passes through as the moisture content changes.

PLASTICITY & COHESION

Physical States and Atterberg Limits

In 1911, a Swedish soil scientist, Albert Atterberg described the effects of changing water content on the consistency of fine-grained soil. He also proposed a series of tests to determine soil properties, which define the water content ranges of soil states or phases. The change from one phase to the next is not observable as a precise boundary, but takes place as a gradual transition. Nevertheless arbitrary but specific boundaries have been established empirically and are universally recognized. The moisture contents at these boundaries are known as the Atterberg limits or consistency limits. In 1932, A. Casagrande designed a device, the cup in use today, to standardize the liquid limit test. He incorporated Atterberg limits in a procedure, the Unified Soil Classification System (USCS), to identify the soils used in construction.

CONSISTANCY

Consistency- four states of consistency for fined grained soils (liquid, plastic, semisolid and solid).

The dividing line between these states of consistency are called the Liquid Limit (LL) and the Plastic Limit (PL). All are quantified in terms of water content (w).

SOIL MOISTURE STATES

SOLID

STATE

SEMI-SOLID

STATE

PLASTIC

STATE

LIQUID

STATE

%W INCREASING%W %W

PL LL

PI

PI = LL - PL

LOW %W HIGH %W

PLASTICITY CHART

0

10

20

30

40

50

60

7

4

10 20 30 40 50 60 70 80 90LIQUID LIMIT (LL)

PLA

STIC

ITY

IN

DEX

(P

I)

“A” L

INE

“U”

LINE

MH OR OH

ML OR OL

CH OR OH

CL OR OL

CL - ML

16

PI =

0.9

(LL

- 8)

PI = 0.73 (L

L - 20)

LL-PL = PI

ORGANICS

Soil having a high content of organic material is described as organic soil.

Organics are typically very compressible and has poor load-maintaining properties.

Why classify Soil ...

purpose of a soil classificationsystem is to be able to predict the engineering properties of the soil.

WHAT SYSTEM DO

WE USE?

Unified Soil Classification System

The USCS looks at properties such as:

Percentages of gravel, sand, and fines.

Shape of the grain-size-distribution curve.

Plasticity and compressibility characteristics

Soil Classification gives agood indication of …

•Strength•Drainage•Resistance to Frost Action•Compressibility(Volume Change)

•Compaction(Constructability)

USCS Letter Symbols

Soils are divided into major groups and further subdivided by characteristics.

Soil Groups Soil CharacteristicsG Gravel W Well gradedS Sand P Poorly gradedM Silt H High compressibilityC Clay L Low compressibility

Pt Organic (Peat)O Organic (Silts and clays)L Liquid limit under 50H Liquid limit over 50

PI > 7 &Plots onor aboveA Line

PI < 4or PlotsbelowA Line

PIPlots onor aboveA Line

PIPlotsbelowA Line

LimitsPlot in

HatchedArea

USCSSoil Classification Chart

Determine %Passing #200

If 50 % or More of Total Sample

It is a Fine-grained Soil

If Less than 50 % of Total Sample

It is a Coarse-grained Soil

Determine %Passing #4

Plot AtterbergTest Data

Determine %Passing #200

Determine %Passing #200

Plot AtterbergTest Data

DetermineCu & Cc

Plot AtterbergTest Data

DetermineCu & Cc

If 50 % or More of Coarse Fraction

It is a Sand

If Less than 50 % of Coarse Fraction

It is a Gravel

If Less than 5%of Total Sample

If Between 5% & 12%of Total Sample

If More than 12%of Total Sample

If Less than 5%of Total Sample

If Between 5% & 12%of Total Sample

If More than 12%of Total Sample

Classify withDual Symbol

DetermineCu & Cc

Classify withDual Symbol

DetermineCu & Cc

Cu > 4&

1 < Cc < 3

Cu > 6&

1 < Cc < 3

Cu > 4&

1 < Cc < 3

YES NO

Classifyas

GW

ClassifyasGP

Cu > 6&

1 < Cc < 3

YES NO

Classifyas

SW

ClassifyasSP

Classifyas

GM

Classifyas

GC

Classifyas

GM-GC

Classifyas

SM

ClassifyasSC

Classifyas

SM-SC

Plot AtterbergTest Data

Plot AtterbergTest Data

Plot AtterbergTest Data

Plot AtterbergTest Data

Classifyas

GW-GM-GC

Classifyas

GW-GM

Classifyas

GW-GC

Classifyas

GP-GM

Classifyas

GP-GC

Classifyas

GP-GM-GC

Classifyas

SW-SM-SC

Classifyas

SW-SM

Classifyas

SW-SC

Classifyas

SP-SM

Classifyas

SP-SC

Classifyas

SP-SM-SC

ML or MHFines

ML or MHFines

ML or MHFines

CL or CHFines

CL or CHFines

CL or CHFines

ML or MHFines

CL or CHFines

CL-MLFines

CL-MLFines

CL-MLFines

CL-MLFines

YES NO YES NO

ML or MHFines

CL or CHFines

CL-MLFines

ML or MHFines

CL or CHFines

CL-MLFines

Classifyas

ML

Classifyas

CL-ML

Classifyas

CH

Classifyas

MH

ClassifyasCL

Liquid Limit< 50

Liquid Limit> 50

Classifyas

OH

Classifyas

OL

Liquid Limit (oven dried)Liquid Limit (not dried)

< 0.75If

or

PLASTICITY CHART

0

10

20

30

40

50

60

74

LIQUID LIMIT (LL)

PL

AS

TIC

ITY

IN

DE

X (

PI)

“A” L

INE

“U” L

INE

MH OR OH

ML OR OL

CH OR OH

CL OR OL

CL - ML

10 20 30 40 50 60 70 80 9016

PI = 0.

9 (LL -

8)

PI = 0.73 (L

L - 20)

Any Questions?