Chapter 16 Subgrade Materials

Components and Functions

Subgrade Supports and distributes the loadings Facilitates drainage Serves as a working platform Can be either foundation or embankment

Use Typical Soils/Geotechnical Technology Very Important TTehnology Very Important Subgrade

Subgrades Vary Must Evaluate Consider Stabilizing Top 2 Feet Important Stabilize ??? Top 2 feet important Subgrade

Classification Gravel>2.0 mm Sand 2.0 mm to 0.06 mm Silt 0.06 mm to mm Clay<0.002 mm Unified System – Based on Grain Size and Plasticity

Unified Soil Classification System G—gravel S—sand M—silt C—clay O—organic soil Pt—peat W—well-graded and clean P—poorly graded, relatively clean F—poorly graded, dirty H—high plasticity L—low plasticity

Index Properties

Atterberg Limits

Strength – Resist deformation and be stable under loads Granular – function of shearing strength and cohesion. Fine Grained – Stability is measured by q u Φ S p S = p tan Φ ququ S C = q u /2 Ult. Bearing cap. (q d ) = 5c = 2.5 q u P c = 20 psi Φ=0 Q u /2 C

Soil Surveys – obtain information about the surface and subsurface soils Ideal Soil Properties – Uniformity 1. Free of excess moisture 2. High internal friction 3. High cohesion? 4. High density 5. Low compressibility 6. Low capillary 7. Low elasticity 8. High compaction

Location and Selection 1. Move alignment 2. Improve drainage 3. Borrow materials 4. Stabilize 5. Waterproof

Track Design Assume P a max is 65 psi on ballast  P c = 16.8 P a / h 1.25 (Talbot) Where, P c ≤ 2.5 q u or ≤ 1.67 q u q d 50% fs