SESSION 3SESSION 3

SubgradeSubgrade

Subgrade

The foundation upon which the pavement and base are constructed

Embankment

Natural soil

Rigid layer} Subgrade

Concrete slabBase

Objectives

• Characterize subgrade for concrete pavement design purposes

• Select appropriate subgrade preparation methods

• Identify subgrade remediation measures for protection against frost heave and soil swelling

Subgrade Models

Dense liquid ( k ) model

Elastic solid ( E ) model

Real soil

Soil Behavior

• Elastic response (k or E)

• Plastic (permanent) deformation

• Time-dependent response

• Standardized tests have been developed to differentiate the elastic response from the plastic and time-dependent components

Static vs. Dynamic k

• Static k: the elastic portion of a soil’s response to a static load

• Dynamic k: the elastic response to a dynamic load

- a fast-moving wheel load

- an FWD load

K value steps,1986/1993 AASHTO

Guide

• K of unprotected subgrade soil

• Composite (top-of-the-base) k

• Adjustment for rigid layer

• Seasonal adjustment

• Loss-of-support adjustment

K value steps,1998 AASHTO Supplement

• K value methods

correlation with soil type and properties

backcalculation

plate bearing tests

• Adjustment for fill and/or rigid layer

• Seasonal adjustment

Plate Bearing Tests

Direct measurement of static elastic k value

new alignment

on subgrade soil

on test embankment

existing alignment

remove slab and base

Plate Bearing Tests

• Repetitive loading test

ASTM D 1195, AASHTO T221 k = slope of pressure to elastic deformation

760-mm (30 in) plate required

Pla

te p

ress

ure,

p

Deflection,

k = mean p / e

p e

Plate Bearing Tests

• Nonrepetitive loading test

ASTM D 1196, AASHTO T222 k = pressure/deformation ratio at 1.25 mm (0.05 in) 760-mm (30 in) plate required

Pla

te p

ress

ure,

p

Deflection,

= 1.25 mm (0.05 in)

k = p /

Correlation of k to Soil Properties

Soil Class Density CBR k

A-1-a, well graded 125 - 140 60 - 80 300 - 450

A-1-a, poorly graded 120 - 130 35 - 60 300 - 400

… … … ...

A-2-4 or 5, gravelly 130 - 145 40 - 80 300 - 500

A-2-4 or 5, sandy 120 - 135 20 - 40 300 - 400

… … … …

A-4, silt 90 - 105 4 - 8 25 - 165

A-4, mix 100 - 125 5 - 15 40 - 220

… … … …

Degree of Saturation Affectsk of Fine-Grained Soils

0

50

100

150

200

250

50 60 70 80 90 100

Degree of saturation (percent)

Su

bg

rad

e k

valu

e (p

si/i

n) A-6

A-7-6A-7-5A-5A-4

Dynamic Cone Penetrometer(DCP)

Backcalculation of k

Falling Weight Deflectometer (FWD)

existing pavement

new alignment on similar soil

Backcalculation of k

Backcalculation of k

Westergaard’s interior deflection equation:

P

= ----- { ( a / ) } k 2

= radius of relative stiffness:

E h3

= 12 ( 1 - 2 ) k

4

Backcalculation of kD

efle

ctio

n,

Load, P

radius, a

AREA = f (), for given sensor configuration

Adjustments toBackcalculated k Value

• Slab size adjustment usually needed

• Static k value needed for design:

approximately = dynamic k / 2

• Different backcalculation equations for deflections measured on AC-overlaid PCC

• Variations in embankment thickness and/or rigid layer depth affect k

> 10 ft

< 10 ft

246

8

10

12 Thickness of fill (ft) Density of fill (lb/cu ft)

Depth torigid layer

200400600psi/in

200 400 600 psi/in

90 100 110 120 130 140 150

200

400

psi/in

Enter with k fornatural subgrade

Adjusted k value

1 ft = 0.305 m,1 psi/in = 0.27 kPa/mm,1 lb/cu ft = 159 N/cu m

Embankment and/or Rigid Layer

Seasonal Adjustment

• 1998 AASHTO Supplement:

- seasonal movement of water table

- seasonal precipitation levels

- winter frost depths

- freeze-thaw cycles

- frost protection

• 1986/1993 AASHTO Guide

- annual average, or springtime?

Subgrade Preparation

• Foundation must provide:

– Assumed stiffness

– Uniformity

– Long-term stability

– Stable construction platform

• Has significant influence on smoothness

• Typically achieved by monitoring density and moisture content during compaction

Subgrade Improvement

• Excavation and recompaction with moisture density control

• Mechanical improvement (mixing in coarser material)

• Excavation and replacement with select fill

• Stabilization (with lime, cement, lime-flyash, asphalt)

• Reinforcement with geosynthetics

Frost Heave

• Formation of ice lenses in frost-susceptible soils

- fine sands and silts

- low-plasticity clays

• Both winter frost heave and subsequent spring thaw can cause pavement cracking

Frost Protection

• Replacing frost-susceptible soil with non-frost-susceptible within depth of frost penetration

• Covering frost-susceptible soil with sufficient thickness of non-frost-susceptible soil

• Factors to consider: drainage, change of grade, side slopes and ditches

Swelling Soils

• Some clays and shales are susceptible to swelling (significant volume increase) when sufficient moisture is available, especially when an overburden pressure is removed

- southern and western US

- dry climates, low soil moisture contents

- pavement inhibits evaporation from soil

- excavation reduces overburden

• Swelling causes heaving and cracking

Swelling Protection

• Avoid cut sections in soils with known swelling potential

• Avoid overcompaction on dry side of optimum moisture content

• Lime stabilization to adequate depth may be useful

• Minimize moisture variation (moisture barriers or geomembranes may help)

Collapsing Soils

• Soils experiencing large decrease in volume with increases in water content

• Treatment methods

– Modest depths: compaction with rollers, wetting or inundation, and overexcavation and recompaction (with lime or cement)

– Thicker deposits: ponding, flooding, dynamic compaction

Summary

• Foundation: soil, embankment, rigid layer

• k value model works well for concrete pavements

• Real soils exhibit some shear strength, elastic and plastic behavior, time-dependent response

• Various methods for determining design k

• Prepare subgrade to achieve stiffness, uniformity, long-term stability, stable construction platform, protection against frost and swelling