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)
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. Communicate between engineers Simple indices GSD, LL, PI Classification system (Language) Use the accumulated experience Estimate engineering properties Achieve engineering purposes
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).
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.
Grain Size Distribution Curves Cobbles or Boulders GRAVEL SAND FINES
Atterberg limits Atterberg limits are the limits of water content used to define soil behavior. The consistency of soils according to Atterberg limits gives the following diagram.
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)
Definition of Grain Size No specific grain size-use Atterberg limits Boulders Gravel Sand Silt-Clay Coarse Fine 75 mm No.4 No.200 4.75 mm No.40 0.075 mm 0.425 mm
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
Group Index GI 200 200 [ ] GI = ( F 35) 0.2 + 0.005( LL 40) + 0.01( F 15)( PI 10) (4.1) For Group A-2-6 and A-2-7 200 The first term is determined by the LL The second term is determined by the PI GI = 0.01( F 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.
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).
(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 A4 ~ A7 Granular Materials 35% pass No. 200 sieve Using LL and PI separates silty materials from clayey materials (only for A2 group) Silt-clay Materials 36% pass No. 200 sieve Using LL and PI separates silty materials from clayey materials The original purpose of this classification system is used for road construction (subgrade rating).
Example 4.1, Soil B Passing No.200 86% LL=70, PI=32 LL-30=40 > PI=32 GI = (F 200 + 0.01(F 35) 200 = 33.47 33 [ 0.2 + 0.005(LL 40) ] 15)(PI 10) Round off A-7-5(33)
This is the example of Das, Example 4.1 for the AASHTO system classification
USCS Classification System Originally developed for the United States Army Corps of Engineers (USACE) The method is standardized in ASTM D 2487 as Unified Soil Classification System (USCS) USCS is the most common soil classification system among geotechnical engineers
Unified Soil Classification System Origin of USCS: (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
Definition of Grain Size No specific grain sizeuse Atterberg limits Boulders Cobbles Gravel Sand Coarse Fine Coarse Medium Fine Silt and Clay 300 mm 75 mm No.4 4.75 mm 19 mm No.10 No.40 2.0 mm 0.425 mm No.200 0.075 mm
(Das, Table 4.2)
The Symbols Soil symbols: G: Gravel S: Sand M: Silt C: Clay O: Organic Pt: Peat Example: SW, Well-graded sand SC, Clayey sand SM, Silty sand, MH, Elastic silt Liquid limit symbols: H: High LL (LL>50) L: Low LL (LL<50) Gradation symbols: W: Well-graded P: Poorly-graded Well graded 1< 1< C C c < 3 (for gravels) c < 3 and and (for sands) soil C C u u 4 6
USCS System (cont.) A typical USCS classification would be: SM - Silty sand with gravel Group Symbol Group Name
Classification of Soils From sieve analysis and the grain-size distribution curve determine the percent passing as the following: > 3 inch Cobble or Boulders 3 inch - # 4 (76.2 4.75 mm) : Gravel # 4 - # 200 (4.75-0.075 mm) : Sand < # 200: Fines First, Find % passing # 200 If 5% or more of the soil passes the # 200 sieve, then conduct Atterberg Limits (LL & PL)
Classification of Soils If the soil is fine-grained ( 50% passes #200) follow the guidelines for fine-grained soils If the soil is coarse-grained (<50% passes #200) follow the guidelines for coarsegrained soils Find % Gravel & Sand Calculate Cu & Cc Calculate LL, PL and PI
General Guidance 50 % Coarse-grained soils: Fine-grained soils: 50% Gravel Sand NO. 4 4.75 mm NO.200 0.075 mm Silt Clay Grain size distribution C u PL, LL Plasticity chart LL>50 LL <50 C c Required tests: Sieve analysis Atterberg limit
Coarse-grained Soils
Fine-grained Soils
Example Soil A gravel sand fines Gravel 98-62 = 36% Sand 62-8 = 54% Fines = 8% Soil A: D 60 = 4.2 mm, D 30 = 0.6 mm, D 10 = 0.09 mm Cu = 46.67 Cc = 0.95
Grave = 36% Sand = 54% Fines = 8% Cu = 46.7 Cc = 0.95 Example Soil A (Cont.) LL = 42 PL = 31 PI = 42-31 = 11 GO TO Plasticity Chart
Example Soil A (Cont.) Soil A is then classified as SP-SM Poorly-grades sand with silt and gravel
Example Soil A (Cont.) LL = 42 PL = 31 PI = 42-31 = 11 ML
The Plasticity Chart PI 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)
Procedures for Classification Coarse-grained material Grain size distribution Fine-grained material LL, PI Highly (Santamarina et al., 2001)
Example Passing No.200 sieve 30 % Passing No.4 sieve 70 % LL= 33 PI= 12 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)
Organic Soils Highly organic soils- Peat (Group symbol PT) A sample composed primarily of vegetable tissue in various stages 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 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 (not oven dried) in the plasticity chart.
This is the Figure 4.7 of Das textbook, the scanning electron micrographs (SEM) for 4 peat samples.
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 poorly graded. The second symbol describe the contained fines. For example: SP-SM, poorly graded sand with silt.
Borderline Cases (Dual Symbols, cont.) 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: Silty, clayed sand. Soil contain similar fines and coarse-grained fractions. possible dual symbols GM-ML
Borderline Cases (Summary) (Holtz and Kovacs, 1981)
Reading Assignment: Das, Ch. 4 Homework: Problem 4.3