Engineering Classification of Soils

I. Overview

A. Two Systems of Classification

1. Pedological Classifications

(soil weathering, texture, chemistry, profile thickness, etc.)

2. Engineering Classifications– soil texture – degree of plasticity (Atterberg Limits)

WentworthScale

“Gravel”75-2 mm

“Sand”2-0.075 mm

“Silt and Clay”<0.075 mm

RemoveCobbles andBoulders from Analysis (>75mm)

Soil Texture

B. Overview of Mechanics

As water content increases,the shear strength decreases

It’s all about shear strength

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 alongthe bottom of a groove after 25 blowsof the liquid limit device.

animation

C. Liquid Limit

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

animation

C. Liquid Limit

D. Plastic Limit

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

C. Liquid Limit

D. Plastic Limit

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

In general….

PI Degree of Plasticity0 Nonplastic1-5 Slightly plastic5-10 Low plasticity10-20 Medium plasticity20-40 High plasticity40+ Very high plasticity(from Burmister, 1949)

Procedure for AASHTO Classification

(American Association of StateHighway and Transportation

Officials)

Developed in 1929 as the Public Road Administration Classification SystemModified by the Highway Research Board (1945)

Procedure for AASHTO Classification

• Determine the percentage of soil passing the #200 sieve

• Determine the subgroups– For coarse-grained soils (gravel and sand),

determine the percent passing the #10, 40, and 200 sieves, AND

– Determine the liquid limit and plasticity index– THEN, determine soil group or subgroup from

Table 9.1

– For coarse-grained soils (gravel and sand), determine the percent passing the #10, 40, and 200 sieves. Cobble

GravelVery Coarse to Med SandFine/Very Fine SandSilt/Clay

3 “#10#40

#200

#10

#40

#200

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

AASHTO Classification for Soils

• 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

fine

{e.g.: A-7-5(9)}

Example Problem

A-7-6(10)

II. Unified System

A. Overview

A. Arthur Casagrande (USAF) proposed for the construction of Airfields

B. Basis

-Over half of material retained on #200

sieve, use textural characteristics

-Over half of material passes the #200 sieve, use plasticity-compressibility characteristics

II. Unified System

B. The classification scheme

II. Unified System

C. The procedure

1. Determine the percent passing through the #200 sieve (boundary between sand and silt/clay).

1. If less than 50% passes, then it is a coarse grained soil (gravel and sand)

2. If greater than 50% passes, then it is a fine grained soil (silt and clay)

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

“clayey materials”

“silty materials”

 

Sieve Analysis -- % finer than        

Soil Sample #10 #40 #200 Liquid Lmt

Plastic Lmt

A 95 79 53 36 21B 100 95 78 65 26C 100 80 62 35 20D 90 55 45 28 20E 90 71 60 40 26

HOMEWORK:Classify the following soils by both the AASHTO and Unified Systems,and give the group index for the AASHTO system.

Alternate method for classifying soils using Unified Method…..(bonus information!)

For fine grained soils:

Where R = ‘retained’ F = ‘falling through’

For fine grained soils:

For fine grained soils:

• For coarse-grained soils:

See next slide

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

Fine grained soils

Fine grained soils

Gravelly soils

sandy soils