Chapter 16 Subgrade Materials. Components and Functions.
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Transcript of Chapter 16 Subgrade Materials. Components and Functions.
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/GeotechnicalTechnology
Very ImportantTTehnology
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 0.002 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 qu
Φ
S
p
S = p tan Φ
qu
SC = qu/2Ult. Bearing cap. (qd) = 5c = 2.5 qu
Pc = 20 psi
Φ=0Qu/2
C
Soil Surveys – obtain information about the surface and subsurface soils
Ideal Soil Properties – Uniformity1. 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 Pa max is 65 psi on ballast
Pc = 16.8 Pa / h1.25 (Talbot)
Where,
Pc ≤ 2.5 qu or ≤ 1.67 qu
qd 50% fs