Soil Mechanics −
Soil Classification Chih-Ping Lin National Chiao Tung Univ.
[email protected]
Outline 1. 2. 3.
4.
Purpose Classification Systems The Unified Soil Classification System (USCS) American Association of State Highway and Transportation Officials System (AASHTO)
Purpose 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. Communicate between engineers
Simple indices GSD, LL, PI
Classification system (Language)
Estimate engineering properties
Achieve engineering purposes
Use the accumulated experience
Classification Systems Two commonly used systems: Unified Soil Classification System (USCS).
American Association of State Highway and
Transportation Officials (AASHTO) System
Unified Soil Classification System 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 (Holtz and Kovacs, 1981). ASTM D2487 Four major divisions: (1) Coarse-grained (2) Fine-grained (3) Organic soils (4) Peat
Group Symbol – Group Name e.g. SM – Silty sand with gravel
Definition of Grain Size No specific grain size-use Atterberg limits
Gravel
Silt and
Sand
Boulders Cobbles
Clay Coarse
300 mm
Fine
75 mm 19 mm
Coarse
Medium
Fine
No.4
No.200
4.75 mm
0.075 mm
No.10
No.40
2.0 mm
0.425 mm
Group Symbols Soil symbols: G: Gravel S: Sand M: Silt C: Clay O: Organic Pt: Peat
Gradation symbols: Well − graded soil 1 < C c < 3 and C u ≥ 4 W: Well-graded (for gravels) P: Poorly-graded 1 < C c < 3 and C u ≥ 6 Fine Component (for sands) C: Clayey M: Silty Example: Liquid limit symbols: SW, Well-graded sand H: High LL (LL>50) SC, Clayey sand L: Low LL (LL 50%
Coarse-grained soil
P#200 < 50%
Highly Organic Soil
Primarily organic material
Dark brown, dark gray, or black color
Organic odor
Soft consistency
If P#200 > 5%, conduct Atterberg limits tests on the soil passing #40
Classification of fine-grained soils L
H
• The A-line generally separates the more claylike materials from silty materials, and the organics from the inorganics.
Plasticity Chart PI
• The U-line indicates the upper bound for general soils. Note: If the measured limits of soils are on the left of U-line, they should be rechecked.
LL
(Holtz and Kovacs, 1981)
Classification of coarse-grained soils P#200