Soil Index Properties and Classification GLE/CEE 330 Lecture Notes Soil Mechanics William J. Likos, Ph.D. Department of Civil and Environmental Engineering University of Wisconsin-Madison

Purpose of Soil Classification Simple indices: 1) Grain Size 2) Atterberg Limits Classification system (Language) Estimate engineering properties Achieve engineering purposes Use the accumulated experience Communicate between engineers (after HKUST)

Classification Systems 1) USDA (US Department of Agriculture) Based on texture (grain size) Used primarily for land use mapping 2) AASHTO (American Association of State Highway and Transportation Officials) Used for roadways (subgrade) Designation: A1-A8 with subgroups Sieve analysis and Atterbergs *3) USCS (Unified Soil Classification System) Used for general geotechnical work 2-Symbol designation (e.g., SP, ML) Sieve analysis and Atterbergs

Tools for Soil Classification Coarse-Grained Gravel Sand Sieve Analysis Grain Size Distribution, Cc, Cu Percent Clay (% < 2mm) Plasticity Chart Fine-Grained Silt Clay Hydrometer Analysis PI Liquid Limit Plastic Limit LL (Photos: NRCS)

Coarse-Grained Soils (Sands/Gravels) d > 0.075 mm Well Rounded Poorly graded Medium to Fine Angular Poorly graded Medium to Fine (Mitchell, 1993)

Grain Shape How does soil formation influence grain shape/size? How does grain shape/size influence engineering behavior? (Mitchell, 1993)

Fine-Grained Soils (Silts/Clays) d < 0.075 mm Silt: 2 mm < d < 75 mm (glimmers in sun) Clay: d < 2 mm AND SPECIFIC MINERALOGY!!! (Mitchell, 1993)

1:1 Clay Minerals (e.g., Kaolinite) Non-swelling Surface area = 10-20 m2/g Gs = 2.60 used in ceramics (R.L. Kugler)

2:1 Clay Minerals (e.g., Smectite) Highly Swelling Surface area = 50 -120 m2/g (Primary) 700 – 800 m2/g (Secondary) Gs = 2.65 – 2.80 Used for adsorbents, drilling fluids “Expansive Soils” (Mitchell, 1993)

Expansive Soils In the US (after Tourtelot, 1973)

Tools for Soil Classification Coarse-Grained Gravel Sand Sieve Analysis Grain Size Distribution, Cc, Cu Percent Clay (% < 2mm) Plasticity Chart Fine-Grained Silt Clay Hydrometer Analysis PI Liquid Limit Plastic Limit LL (Photos: NRCS)

Laboratory Sieve Shaker Sieve Analysis Laboratory Sieve Shaker

Grain Size Distribution Curve (GSD) Poorly Graded (narrow size range) % Passing by Mass Well Graded (wide size range) Gap Graded Grain Diameter (mm) (LOG SCALE!)

Grain Size Coefficients, Cc and Cu (e.g., for well graded soil below, Cu = 400) Poorly Graded (narrow size range) % Passing by Mass Well Graded (wide size range) Gap Graded D10 = 0.02 mm D60 = 8 mm Grain Diameter (mm) (LOG SCALE!)

Hydrometer Analysis Stoke’s Law Fdrag = f(d) Fbouy = f(d) Sedimentation Technique Used for material passing #200 (75 mm) sieve Typically used to determine % Clay (d < 2 mm) dispersing agent Stoke’s Law Fdrag = f(d) Fbouy = f(d) m = viscosity rs = solids density rw = water density v = settling velocity Fgrav = f(d)

Atterberg Limits describe consistency of fine-grained soils (<75 mm) as function of water content solid semi-solid plastic liquid water content Shrinkage Limit (SL, %) Plastic Limit (PL, %) Liquid Limit (LL, %) Plastic Limit Plasticity Index PI = LL - PL used as classification tool used to assess swelling potential (“fat clays”) Activity (A) and Liquidity Index (LI) Liquid Limit

Liquid Limit Test (Casagrande Cup)

Liquid Limit Test (Fall Cone Method) LL = w @ 20 mm penetration PL = c(2)m

Unified Soil Classification System (USCS) Four major divisions: Coarse-grained Fine-grained Organic soils Peat Coarse Fine Gravel Sand Silt and Clay Boulders Cobbles Coarse Fine Coarse Medium Fine No.200 0.075 mm 300 mm 75 mm No.4 4.75 mm 19 mm No.10 2.0 mm No.40 0.425 mm

USCS Designation Soil symbols: G: Gravel S: Sand M: Silt C: Clay O: Organic Pt: Peat Liquid limit symbols: H: High Plasticity (LL>50) L: Low Plasticity (LL<50) Gradation symbols: W: Well-graded P: Poorly-graded Use GSD curve to define gradation

USCS Designation Examples 2-symbol designation First symbol is general soil type Second symbol is adjective describing that soil type GC = Clayey gravel SW = Well-graded sand SC = Clayey sand SM = Silty sand CH = High plasticity clay (aka “fat” clay) CL = Low plasticity clay (aka “lean” clay) GC-ML = Dual classification

Plasticity Chart (Holtz and Kovacs, 1981) LL PI H L The A-line separates clays from silts and the organics from the inorganics. LL = 50 separates low plasticity from high plasticity The U-line indicates the upper bound for general soils.

(Grain size distribution) USCS Procedures COARSE (Grain size distribution) FINE (LL, PI) (Santamarina et al., 2001)

Example Passing #200 = 30% (so 70% retained on #200) Passing #4 = 70% SC (15% gravel) Clayey sand with gravel LL= 33 PI= 12 (Santamarina et al., 2001) (after HKUST)

Organic Soils Highly organic soils - Peat (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 (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. (after HKUST)

Dual Classification For the following three conditions, a dual symbol should be used. 1) 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 describes the fines. For example: SP-SM, poorly graded sand with silt. 2) Fine-grained soils plotting within the shaded zone of plasticity chart CL- ML: Silty clay, SC-SM: Silty, clayey sand. 3) Soils containing similar fines and coarse-grained fractions. possible dual symbols GM-ML (after HKUST)