Outline. Soil Classification. Soil Mechanics (USCS) Transportation Officials System (AASHTO)

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1 Soil Mechanics Soil Classification Chih-Ping Lin National Chiao Tung Univ. Outline 1. Purpose 2. Classification Systems 3. The Unified Soil Classification System (USCS) 4. American Association of State Highway and Transportation Officials System (AASHTO)

2 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. Simple indices GSD, LL, PI Communicate between engineers Classification system (Language) Use the accumulated experience Estimate engineering properties Achieve engineering purposes Classification Systems Two commonly used systems: Unified Unified Soil Classification System (USCS). American American Association of State Highway and Transportation Officials (AASHTO) System

3 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 Boulders Cobbles Gravel Sand Silt and Coarse Fine Coarse Medium Fine Clay 300 mm 75 mm No.4 No mm 4.75 mm No.10 No mm 2.0 mm mm

4 Group Symbols Soil symbols: G: Gravel S: Sand M: Silt C: Clay O: Organic Pt: Peat Gradation symbols: W: Well-graded P: Poorly-graded Fine Component C: Clayey M: Silty Liquid limit symbols: H: High LL (LL>50) L: Low LL (LL<50) Well graded soil 1< C 1< C c < 3 and (for gravels) c < 3 and C C 4 6 (for sands) Example: SW, Well-graded sand SC, Clayey sand SM, Silty sand, MH, Elastic silt u u Group Names The group symbol was enhanced by addition of several group names for each group symbol. e.g. SM Silty sand with gravel Adjective Noun with. Noun = Primary component Adjective = Secondary component or further explanation of first component with =Tertiary component

5 General Guidance 1. Major Component 50 % Coarse-grained soils: 50% Gravel Sand NO. 4 NO mm mm Fine-grained soils: Silt Clay 2. Gradation or Fine component Grain size distribution (C u and C c ) 2. Compressibility PL, LL (Plasticity chart) Required tests: Sieve analysis and Atterberg limits (P#40) Initial Classification Conduct sieve analysis test Fine-grained soil P#200 > 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

6 PI Classification of fine-grained soils Plasticity Chart L H The A-line generally separates the more claylike materials from silty materials, and the organics from the inorganics. The U-line indicates the upper bound for general soils. LL Note: If the measured limits of soils are on the left of U-line, they should be rechecked. (Holtz and Kovacs, 1981) Classification of coarse-grained soils P#200<50% 1 < C (for gravels) < 3 and (for sands) G(P#4<50%) P#200 <5% 5-12% >12% GSD (W,P) Well graded soil 1 < C c c < 3 and C C u u 4 6 Both Plasticity Chart (M,C) S(P#4<50%) P#200 <5% 5-12% >12% GSD (W,P) Both Plasticity Chart (M,C)

7 Organic Soils Highly organic soils- Peat (Group symbol PT) A sample composed primarily of vegetable tissue in various stages s of decomposition and has a fibrous to amorphous texture, a dark-brown to black color, and an organic odor should be designated as a highly organic soil and shall be classified as peat, PT. Organic clay or silt( group symbol OL or OH): The soil s s liquid limit (LL) after oven drying is less than 75 % of its liquid limit before oven drying. If the above statement is true, then the first symbol is O. The second symbol is obtained by locating the values of PI and LL L (not oven dried) in the plasticity chart. Summary of Soil Classification Coarse-grained material Grain size distribution Fine-grained material LL, PI Highly (Santamarina et al., 2001)

8 Example Passing No.200 sieve 30 % Passing No.4 sieve 70 % LL= 33 PI= 12 PI= 0.73(LL-20), A-line PI=0.73(33-20)=9.49 SC ( 15% gravel) Clayey sand with gravel Highly (Santamarina et al., 2001) Borderline Cases (Dual Symbols) For the following three conditions, a dual symbol should be used. Coarse-grained soils with 5% - 12% fines. About 7 % fines can change the hydraulic conductivity of the coarse-grained media by orders of magnitude. The first symbol indicates whether the coarse fraction is well or o poorly graded. The second symbol describe the contained fines. For F example: SP-SM, SM, poorly graded sand with silt. Fine-grained soils with limits within the shaded zone. (PI between 4 and 7 and LL between about 12 and 25). It is hard to distinguish between the silty and more claylike materials. CL-ML: Silty clay, SC-SM: SM: Silty, clayed sand. Soil Soil contain similar fines and coarse-grained fractions. possible dual symbols GM-ML ML

9 Borderline Cases (Summary) (Holtz and Kovacs, 1981) American Association of State Highway and Transportation Officials system 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 (Holtz and Kovacs, 1981) Rated soils according to their suitability for support roadway pavement Assign a group classification and a group index to the soil A-1 A best soil worst soil good soil poor soil

10 Definition of Grain Size No specific grain size-use Atterberg limits Boulders Gravel Sand Silt-Clay Coarse Fine 75 mm No mm No mm No mm General Guidance The The required tests are sieve analysis and Atterberg limits. 8 8 major groups: A1~ A7 (with several subgroups) and organic soils A8 A1 ~ A3 A4 ~ A7 Granular Materials 35% pass No. 200 sieve Silt-clay Materials 36% pass No. 200 sieve Using LL and PI separates silty materials from clayey materials (only for A2 group) Using LL and PI separates silty materials from clayey materials The The group index, an empirical formula, is used to further evaluate soils within a group (subgroups). The The original purpose of this classification system is used for road construction (subgrade( rating).

11 Classification Das, 1998 Classification (Cont.) Note: The first group from the left to fit the test data is the correct AASHTO classification. Das, 1998

12 Group Index GI = (F (F 35) 200 [ (LL 40) ] 15)(PI 10) For Group A-2-6 and A-2-7 GI = 0.01(F )(PI 10) The first term is determined by the LL The second term is determined by the PI 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. Example Passing No % LL=70, PI=32 LL-30=40 > PI=32 GI = (F (F 35) 200 = [ (LL 40) ] 15)(PI 10) Round off A-7-5(33)

LABORATORY CLASSIFICATION OF SOILS FOR ENGINEERING PURPOSES

Test Procedure for LABORATORY CLASSIFICATION OF SOILS FOR ENGINEERING PURPOSES TxDOT Designation: Tex-142-E Effective Date: August 1999 1. SCOPE 1.1 This method is a system for classifying disturbed and