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