Soil

-is the mixture of minerals, organic matter, gases, liquids, and myriad organisms that together support plant life.

Engineering definition: civil engineering : soil is the earth material that can be

disaggregated in water by gentle agitation.

Geologist: residual material formed in situ from weathering of a parent material either bedrock or sediment n

Soil scientist: organic rich material that supports plant growth.

Soil

gravels soil organic soil

Engineering Properties of SoilsMineral Soils

Soil profile

Horizons

Index properties of soil

Index properties

• Grain-size distribution• Atterberg limits• Density• Specific gravity• Consistency.

•Grain-size distribution. The grain-size distribution of soils is determined by means of sieves and/or a hydrometer analysis, and the results are expressed in the form of a cumulative semi-log plot of percentage finer versus grain

•Atterberg limits. The Atterberg limits indicate the range of water content over which a cohesive soil behaves plastically. The upper limit of this range is known as the liquid limit (LL); the lower, as the plastic limit (PL). The LL is the water content at which a soil will just begin to flow when slightly jarred in a prescribed manner. The PL is the water content at which the soil will just begin to crumble when rolled into threads 1/8 inch in diameter.

Shrinkage limit: Water content at which the soil changes from solid state to semi-solid state. Plastic limit: The moisture content at which the soil changes from semi solid state to plastic state. Liquid limit: At which a soil changes from plastic stage to liquid state.

•Density. The mass density of a soil material is its weight per unit volume. The dry density of a soil is defined as the weight of solids contained in the unit volume of the soil and is usually expressed in pounds per cubic foot.

•Specific gravity. The specific gravity of the solid constituents of a soil is the ratio of the unit weight of the solid constituents to the unit weight of water. For routine analyses, the specific gravity of sands and clayey soils may be taken as 2. 65 and 2. 70, respectively.

•Consistency. The consistency of an undisturbed cohesive soil may be expressed quantitatively by the unconfined compressive strength qu.

Weight Volume relationships of soils

Engineering Classification of Soils

I. OverviewA. Two Systems of Classification

1. Pedological Classifications (soil weathering, texture, chemistry, profile thickness, etc.)

2. Engineering Classificationssoil texture degree of plasticity (Atterberg Limits)

B. Overview of Mechanics

As water content increases,

the shear strength decreases

C. Liquid Limit

•Soil is practically a liquid•Shows minimal shear strength

•Defined as the moisture content required

to close a distance of 0.5 inch along

the bottom of a groove after 25 blows

of the liquid limit device

D. Plastic Limit•Water content at which the soil is a plastic•Less water content than liquid limit•Wide range of shear strengths at plastic limit•Defined as the moisture content % at which the soil begins to crumble when rolled into 1/8”

diameter threads

E. Plasticity Index (PI)•Difference between Liquid Limit and Plastic Limit• Important measure of plastic behavior

In General….

PI Degree of Plasticity 10-20 Medium plasticity

0 Nonplastic 20-40 High plasticity

1-5 Slightly plastic 40+ Very high plasticity

5-10 Low plasticity (from Burmister, 1949)

DEVELOPED IN 1929 AS THE PUBLIC ROAD ADMINISTRATION CLASSIFICATION SYSTEMMODIFIED BY THE HIGHWAY RESEARCH BOARD (1945)

Procedure for AASHTO Classification(American Association of StateHighway and Transportation Officials)

DETERMINE THE PERCENTAGE OF SOIL PASSING THE #200 SIEVEDETERMINE THE SUBGROUPSFOR COARSE-GRAINED SOILS (GRAVEL AND SAND), DETERMINE THE PERCENT PASSING THE #10, 40, AND 200 SIEVES, ANDDETERMINE THE LIQUID LIMIT AND PLASTICITY INDEXTHEN, DETERMINE SOIL GROUP OR SUBGROUP FROM TABLE 9.1

Procedure for AASHTO Classification

- For coarse-grained soils (gravel and sand), determine the

percent passing the #10, 40, and 200 sieves.

#10

#40

#200

3 “#10#40

#200

CobbleGravelVery Coarse to Med SandFine/Very Fine SandSilt/Clay

Procedure for AASHTO Classification

Determine the percentage of soil passing the #200 sieve

Determine the subgroups

For fine-grained soils (silt & clay), determine the liquid limit and plasticity index

Determine soil group or subgroup from Table 9.2

Determine the Group Index (usually reflects the relative strength of the material, where low values have the greatest shear strength)

Determine the group index

Determine the group index

II. Unified SystemOverviewA. Arthur Casagrande (USAF) proposed for the construction of AirfieldsB. Basis-Over half of material retained on #200sieve, use textural characteristics-Over half of material passes the #200 sieve, use plasticity-compressibility characteristics

Uniformity Coefficient = D60/D10, where we use the % finer by weight (% passing through) for the values

10% sand46% silt44% clay

Silt Clay

Example 1

42% sand20% clay38% silt

“Loam”

Sample 1

gravel sand silt clay 68% 28% 4% 0%

D60 = 3.5mm D10 = 0.23mm

Grain Size Analysis (cont.)

• Sorting (Grading) Parameters

• Uniformity coefficient (Cu)

Cu = D60 / D10

= 3.5mm / .23mm

= 1.5

• coefficient of concavity (Cc)

Cc = [ D30 ]2 / [ D10 x D60 ]

= [ 1.8 ]2/ [ 0.23 x 3.5 ]

= 4

http://www.soil-net.com/primary/ks2/topic6/topic6_factsheet.pdf

http://sosc.poly.edu/curriculum/urban-infrastructure/soil-engineering/

http://www4.hcmut.edu.vn/~cnan/Principles%20of%20geotechnical%20engineering%20%28Fifth%20Edition,%20Das%29/83-99.PDF

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