Chapter 16 Subgrade Materials

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Chapter 16 Subgrade Materials
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Components and Functions
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Subgrade

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

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

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

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Classification

• Gravel gt2.0 mm
• Sand 2.0 mm to 0.06 mm
• Silt 0.06 mm to 0.002 mm
• Clay lt0.002 mm
• Unified System Based on Grain Size and
  Plasticity

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Unified Soil Classification System

• Ggravel
• Ssand
• Msilt
• Cclay
• Oorganic soil
• Ptpeat

• Wwell-graded and clean
• Ppoorly graded, relatively clean
• Fpoorly graded, dirty
• Hhigh plasticity
• Llow plasticity

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Index Properties
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Atterberg Limits
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Strength Resist deformation and be stable under
loads

• Granular function of shearing strength and
  cohesion.
• Fine Grained Stability is measured by qu

S
S p tan F
F
p
C qu/2 Ult. Bearing cap. (qd) 5c 2.5
qu Pc 20 psi
F0
Qu/2
S
C
qu
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Soil Surveys obtain information about the
surface and subsurface soils

• Ideal Soil Properties Uniformity
• Free of excess moisture
• High internal friction
• High cohesion?
• High density
• Low compressibility
• Low capillary
• Low elasticity
• High compaction

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• Location and Selection
• Move alignment
• Improve drainage
• Borrow materials
• Stabilize
• Waterproof

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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

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