CE240LectW032Soilclassification

Classification Systems
• Two commonly used systems for soil engineers based on particle distribution and Atterberg limits: • American Association of State Highway and Transportation Officials (AASHTO) System (for state/county highway dept.) • Unified Soil Classification System (USCS) (preferred by geotechnical engineers).
Silt and Clay
No.200 No.10 2.0 mm No.40 0.425 mm 0.075 mm
(PI)
(LL)
Das, Figure 4.1
General Guidance
– – –
8 major groups: A1~ A7 (with several subgroups) and organic soils A8 The required tests are sieve analysis and Atterberg limits. The group index, an empirical formula, is used to further evaluate soils within a group (subgroups). A1 ~ A3
Group Index GI
The first term is determined by the LL
GI = ( F200 − 35) [ 0.2 + 0.005( LL − 40)] +0.01( F200 − 15)( PI − 10) (4.1)
The second term is determined by the PI
American Association of State Highway and Transportation Officials system (AASHTO)
Origin of AASHTO: (For road construction) This system was originally developed by Hogentogler and Terzaghi in 1929 as the Public Roads Classification System. Afterwards, there are several revisions. The present AASHTO (1978) system is primarily based on the version in 1945. (Holtz and Kovacs, 1981)
Communicate between engineers
Simple indices GSD, LL, PI
Classification system (Language)
Use the accumulated experience
Estimate engineering properties
Achieve engineering purposes
Definition of Grain Size
No specific grain sizeuse Atterberg limits
Gravel Boulders Cobbles
Coarse 300 mm 75 mm 19 mm Fine Coarse No.4 4.75 mm
Sand
Medium Fine
Definition of Grain Size
No specific grain size-use Atterberg limits
Boulders
Gravel
Coarse 75 mm No.4 4.75 mm
Sand
Fine
Silt-Clay
No.200 No.40 0.425 mm 0.075 mm
Soil particles The description of the grain size distribution of soil particles according to their texture (particle size, shape, and gradation). Major textural classes include, very roughly: gravel (>2 mm); sand (0.1 – 2 mm); silt (0.01 – 0.1 mm); clay (< 0.01 mm). Furthermore, gravel and sand can be roughly classified as coarse textured soils, wile silt and clay can be classified as fine textures soils.
CE 240 Soil Mechanics & Foundations Lecture 3.2
Engineering Classification of Soil (AASHTO and USCS) (Das, Ch. 4)
Outline of this Lecture
1. Particle distribution and Atterberg Limits 2. Soil classification systems based on particle distribution and Atterberg Limits 3. American Association of State Highway and Transportation Officials System (AASHTO) 4. The Unified Soil Classification System (USCS)
Using LL and PI separates silty materials from clayey materials (only for A2 group)
– The original purpose of this classification system is used for road construction (subgrade rating).
Some Explanations of Group Index GI
1, if Eq. 4.1 gives a negative value then GI=0; 2, round up the value calculated by Eq. 4.1 to an integer; 3, there is no upper limit for GI; 4, the GIs for soil groups A-1-a, A-1-b, A-2-4, A-2-5, and A-3 are always zero (0).
Objective
Classifying soils into groups with similar behavior, in terms of simple indices, can provide geotechnical engineers a general guidance about engineering properties of the soils through the accumulated experience.
Classification
(Proceeding from left to right against the columns)
Das, Table 4.1, 2006
Classification (Cont.)
Note: The first group from the left to fit the test data is the correct AASHTO classification. Das, Table 4.1, 2006
For Group A-2-6 and A-2-7 GI = 0.01( F200 − 15)( PI − 10)
(4.2) use the second term only
F200: percentage passing through the No.200 sieve In general, the rating for a pavement subgrade is inversely proportional to the group index, GI.
Unified Soil Classification System (USCS)
Origin of USCS: This system was first developed by Professor A. Casagrande (1948) for the purpose of airfield construction during World War II. Afterwards, it was modified by Professor Casagrande, the U.S. Bureau of Reclamation, and the U.S. Army Corps of Engineers to enable the system to be applicable to dams, foundations, and other construction. Four major divisions: (1) Coarse-grained (2) Fine-grained (3) Organic soils (4) Peat
Granular Materials ≤ 35% pass No. 200 sieve