Engineering Classification of Soils I A Overview Two

Engineering Classification of Soils

I. A. Overview Two Systems of Classification 1. Pedological Classifications (soil weathering, texture, chemistry, profile thickness, etc. ) 2. Engineering Classifications – – soil texture degree of plasticity (Atterberg Limits)

Soil Texture Remove Cobbles and Boulders from Analysis (>75 mm) “Gravel” 75 -2 mm “Sand” 2 -0. 075 mm “Silt and Clay” <0. 075 mm Wentworth Scale

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 along the bottom of a groove after 25 blows of 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 In general…. D. Plastic Limit PI Degree of Plasticity 0 Nonplastic 1 -5 Slightly plastic 5 -10 Low plasticity 10 -20 Medium plasticity 20 -40 High plasticity 40+ Very high plasticity (from Burmister, 1949) E. Plasticity Index (PI) • Difference between Liquid Limit and Plastic Limit • Important measure of plastic behavior

Procedure for AASHTO Classification (American Association of State Highway and Transportation Officials) Developed in 1929 as the Public Road Administration Classification System Modified 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, Cobble and 200 sieves. 3 “ Gravel #10 Very Coarse to Med Sand #40 Fine/Very Fine Sand #200 Silt/Clay #200 #40

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 silt/clay). 1. 2. If less than 50% passes, then it is a coarse grained soil (gravel and sand) If greater than 50% passes, then it is a fine grained soil (silt and clay)

Uniformity Coefficient = D 60/D 10, where we use the % finer by weight (% passing through) for the values

“clayey materials” “silty materials”

HOMEWORK: Classify the following soils by both the AASHTO and Unified Systems, and give the group index for the AASHTO system. Sieve Analysis -- % finer than Soil Sample A B C D E #10 95 100 90 90 #40 79 95 80 55 71 #200 Liquid Lmt Plastic Lmt 53 36 21 78 65 26 62 35 20 45 28 20 60 40 26

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 = D 60/D 10, where we use the % finer by weight (% passing through) for the values

Fine grained soils

Fine grained soils

Gravelly soils

sandy soils