SWIFT 2014 Workshop - Chris Olidis, P.Eng.

AIRFIELD SUBGRADE

CONSIDERATIONS

WHAT YOU NEED TO KNOW

PRESENTATION OUTLINE

Indentify subgrade design requirements

• Transport Canada pavement design methodology

• Pavement Load Rating (PLR)

How to evaluate

Subgrade construction

Other considerations

TRANSPORT CANADA METHODOLOGY

The Transport Canada pavement design

methodology is based on three key inputs:

• Airfield traffic loading

• Subgrade bearing strength

• Frost depth characterization

ASG 19 Manual of Pavement Structural Design

SUBGRADE BEARING STRENGTH

Subgrade bearing strength (S)

• Defined as the load (kN) required to produce a

deflection of 12.5 mm after 10 load repetitions

Subject to seasonal fluctuations resulting from

environmental conditions

Lower quartile strength used for design

Extensive database of testing from 1945 to 1995

STATIC REPETITIVE PLATE LOAD TEST

Procedure defined in AK-68-31 Pavement Bearing

Strength Measurement and Analysis

A full day is required to complete testing

Requires

• Load reaction frame

• Steel plate stack (750 mm diameter)

• Reference gauges

PLATE LOAD TEST

PLATE LOAD TEST RELATIONSHIPS

P = S * 10t/165 (flexible pavement)

• P = plate bearing value on pavement surface (kN)

• S = plate bearing value on subgrade surface (kN)

• t = equivalent granular thickness (cm)

k = 0.474 * S * 10t/165 (rigid pavement)

• k = bearing modulus (MPa/m)

TYPICAL ‘S’ VALUES

TYPICAL STRENGTH RELATIONSHIPS

Transport Canada Flexible Pavement

Design Curves

Transport Canada Rigid Pavement

Design Curves

HEAVY WEIGHT DEFLECTOMETER

HWD deflection data can be analyzed to provide

moduli of the pavement layers and subgrade

The HWD is a rapid, non-destructive method of

assessing pavement structural response.

HEAVY WEIGHT DEFLECTOMETER

Evaluated by Transport Canada as an alternative to

plate load testing

Relationships were developed based on maximum

deflection with a 450 mm Φ plate at a 250kN peak

impact load

Heavy Falling Weight Deflectometer vs Static Plate

Load Testing, Report Number AARME/C-02-01

Ps by HWD DEFLECTION

EFFECTS OF FROST

Frost is result of freezing within the pavement

structure

For both rigid and flexible pavements, from 20 to

50 percent of rainfall may infiltrate pavement

structure

Freezing and thawing greatly reduces pavement

support capacity

EFFECTS OF FROST

Frost heaving results from ice lenses

Ice lenses only develop when:

• Frost susceptible materials

• Freezing temperatures penetrate into susceptible

material

• Water supply is available

Eliminate a variable and frost heave is minimized

FROST DESIGN CONCEPTS

Two approaches to frost design • Complete prevention of subgrade freezing

• Limited frost penetration into subgrade

Complete protection generally required when: • Subgrade soil and moisture is extremely variable

• Subgrade has high silt content

• Differential heave will cause operations problems

Conditions for frost design consideration are: • Presence of frost susceptible soil

• Groundwater within 1.5 m of subgrade elevation

• Frost penetration greater than design thickness of pavement

FROST SUSCEPTIBILITY

FREEZING INDICES

TC MINIMUM THICKNESS

ADDITIONAL FROST THOUGHTS

Minimum granular thickness nomograph considers uniform subgrade soils

Provides only partial frost protection

Does not consider pockets of highly frost susceptible soils

• May require sub-excavation to depth of frost

Must consider subsurface drainage

SUBGRADE COMPACTION

As specified in

• ASG-20 Pavement Construction: Methods and Inspection

• ASG-6 Pavement Construction: Materials and Testing

Compaction based on modified Proctor density as determined in the laboratory

SUBGRADE COMPACTION

Layer Material Depth

(mm)

%

Compaction

Pavement

Embankment

Cohesive soil

Non-cohesive soil

90

95

Subgrade Cohesive soil

Non-cohesive soil

Lime modified

150

300

93

98

95

SUBGRADE COMPACTION

FAA AC 150/5320-6

Aircraft

Weight

kips

Non-cohesive, depth in inches Cohesive, depth in inches

100% 95% 90% 85% 95% 90% 85% 80%

Dual

50 12 12-28 28-38 38-50 6 6-10 10-17 17-22

100 17 17-30 30-42 42-55 6 6-12 12-19 19-25

150 19 19-32 32-46 46-60 7 7-14 14-21 21-28

200 21 21-37 37-53 53-69 9 8-16 16-24 24-32

Dual

Tandem

100 14 14-26 26-38 38-49 6 6-10 10-17 17-22

200 17 17-30 30-43 43-56 6 6-12 12-18 18-26

300 20 20-34 34-48 48-63 7 7-14 14-22 22-29

400 23 23-41 41-59 59-76 9 9-18 18-27 27-36

DC-10

L1011

B747

400 21 21-36 36-55 55-70 8 8-15 15-20 20-28

600 23 23-41 41-59 59-76 9 9-18 18-27 27-36

800 23 23-41 41-59 59-76 9 9-18 18-27 27-36

PROOF ROLLING

Specified in ASG-20 Pavement Construction: Methods and Inspection

Used to locate isolated soft spots in compacted soil

Ensure complete coverage across airfield

Proof Roller • Four large rubber tires, 740 mm spacing

• 45,360 kg total mass

• 0.6 MPa tire pressure

PROOF ROLLING EQUIPMENT

QUESTIONS