# airport pavement design

TRANSCRIPT

Airport Pavement DesignAirport Classification, and

Runway Pavement Design using

Canadian Department of Transport Method

Dr. M. A. FarooqiAssociate Professor, Department of Civil

Engineering, AMU, Aligarh

Difference in Design Approaches between Airfield and Highway Pavements

Other distinctive features between the design of highway and airfield pavements are as outlinesbelow:

1. Magnitude of applied load,2. Tire pressure,3. Geometric section of the pavement,4. Number of repetitions of load applied to the pavement during the design life of the pavement.

There are many principles that are common to the design of both airfield and highway pavements.However, there are several distinctive features. The most important is in the design philosophy:

The priority of highway-design engineers is to understand or assess the effect of vehicular traffic upon the pavement. While, airfield designers do consider the effects of aircrafts on pavements, but the priority of design involves the effect of the pavement upon the aircraft

Magnitude of Loads Considered

• In highways, maximum axle load of 8170kg or equivalent single wheel load of

4085kg is considered

• Airport pavements are designed for Equivalent Single Wheel Load (ESWL) of the

critical aircraft that is supposed to use the runway. For example:

• Boeing 747 has a total weight of 353,000kg and its ESWL =37,000kg

• Since wheel arrangement (known as Landing gear) of different aircrafts is

different, ESWL is to be separately calculated.

Tire Pressure Values

• It plays an important role in the design of pavements:

• For highway pavements: 7kg/cm2

• For aircrafts, it varies for different aircrafts. For example for Boeing

747 it is 12.04kg/cm2

Geometric Section of Pavement

• Runway width ranges from: 18m to 45m

• Since shoulders are unpaved and running engines can suck in any loose material.

So the design consideration is that the farthest of engines (on the wings) should

remain over the pavement.

• So for large aircrafts the farthest engine is 13.5m (45ft) away from the

longitudinal axis of plane.

• On both sides it becomes 13.5 x 2 =27m. Hence, 45m is enough width of runway

pavement.

Considerations in the Airfield Pavement Design• The rapid development of jet aircraft in recent years has had a profound effect on pavement-

design concepts.

• Jet engines are easily damaged by debris sucked into the air intake. Hence, design of shouldersand adjacent safety area and extended runway length considers to make them resistant toerosion from jet blast.

• The pavement must be resistant to the effects of fuel spillage and heat. Since spillage may causesoftening of asphalt binder.

• Type of load is another factor which affects the design of pavements for jet aircraft.

• Some jets are equipped with tires having pressures up to 400 psi. Such pressures necessitatehigh-quality pavement to withstand the "punching" effect of the tire.

• The bicycle landing gear has resulted in channelized traffic.

Type of Pavement

Rigid Pavement:

• A rigid pavement comprises either wholly or partly concrete construction which can be plain, reinforced orprestressed and which distributes the aircraft loading to the subgrade by means of its high flexural stiffness.

Flexible Pavement:

• A flexible pavement is composed of bound or unbound granular materials. It distributes the aircraft loadprimarily through the shear strength of the paving material. Cement-bound granular bases beneathbituminous surfacings make pavements quite rigid in their early years. This type of construction is alsotreated as a flexible pavement for design and evaluation purposes.

Composite Pavements:

• These the generally the result of various strengthening and maintenance overlays. They may comprise of:

• Flexible-on-rigid construction

• Rigid-on rigid: Multiple concrete slab construction

Choice Between Rigid / Flexible Pavement

Choice of Rigid Pavement:

• In general, concrete is preferred where there is likely to be venting of fuel, spillage of lubricating oils and hydraulic fluids, jet efflux gases from slow moving high performance jet engines, or areas subject to locked wheel turns.

Choice of Flexible Pavement:

• The absence of joints in flexible pavements gives them a better riding surface for high speed operations than most types of rigid pavement.

• On an unpredictable subgrade which is liable to long-term shrinkage or heave, a flexible pavement will generally be the best option. This is because a flexible pavement can cope with greater movement and remain serviceable; it can also be more cheaply and quickly overlaid to rectify the loss of shape.

Methods of Design of Flexible Airfield Pavement

• Corps of Engineers-CBR Method

• The Federal Aviation Administration(FAA)method,

• The Canadian Department of Transportation (CDOT) procedure, and

• The Asphalt Institute method

• ACN-PCN method (as given in the 1983 edition of the Aerodrome Design Manual, Part 3

published by the ICAO) [ACN=Aircraft Classification Number; PCN=Pavement

Classification Number]

ICAO Airport ClassificationAirport

TypeBasic Runway

Length (m)Width of Runway

Pavement (m)

Max Longitudinal

Grade (%)

Max Min

A >2100 2100 45 1.5

B 2099 1500 45 1.5

C 1499 900 30 1.5

D 899 750 22.5 2.0

E 749 600 18 2.0

Code No

Equivalent Single Wheel

Load (kg)

Tire Pressure (kg/cm2)

1 45000 8.5

2 34000 7.0

3 27000 7.0

4 20000 7.0

5 13000 6.0

6 7000 5.0

7 2000 2.5

Example:

An airport B-3 would have basic runway length

ranging between 1500-2099m. Single wheel load

capacity of 27000 with a tire pressure of 7 kg/cm2

Canadian Department of Transportation

• To determine the thickness of flexible airfield pavement a method was developed by Canadian Department of Transportation.

• It correlated the load carrying capacity of runway pavement with plate-load test.

• This investigation was done by N. W. McLeod

• An empirical equation was developed as shown below:

𝑇 = 𝑘 log𝑃

𝑆

In which,𝑇 is the thickness of gravel material.𝐾 is the Base course constant, read from Figure 1𝑃 is the gross wheel load (kg)𝑆 is the subgrade support value as obtained using Figure 2.

Design steps:

𝑎 =𝑊

𝑝 𝜋

𝑃

𝐴=

𝜋𝑑

𝜋𝑑2/4=4

𝑑

𝑆 = 𝑠 ×𝜋𝑑2

4

Step 1: Compute the radius of contact of the wheel

Step 2: Compute the perimeter over area ratio:

Step 3: Compute the value of Unit Subgrade Support (𝑠) for the given diameter of plate and yield stress at the given value of deflection (using Figure 1):

Step 4: Compute the value of total support:

Step 5: Compute the value of thickness (After plugging the value of k as read from Figure 2:

𝑇 = 𝑘 log𝑃

𝑆

Design ExampleDesign a runway pavement for a wheel load of 27,000kg with a tire pressure of 11kg/sqcm. In site investigation, a plate-load test has been conducted with a plate of 75cm diameter, that showed a bearing pressure of 2kg/cm2 at a deflection of 0.5cm, after 10 load repetitions.

𝑎 =𝑊

𝑝 𝜋Step 1: Compute the radius of contact of the wheel

𝑎 =27000

11 × 3.14= 28.5cm

𝑃

𝐴=

𝜋𝑑

𝜋𝑑2/4=4

𝑑Step 2: Compute the perimeter over area ratio:

𝑃

𝐴=

4

2 × 28.5= 0.07

Step 3: Compute the value of Unit Subgrade Support (𝒔) for the given diameter of plate and yield stress at the given value of deflection (using Figure 2):

𝑆 = 𝑠 ×𝜋𝑑2

4Step 4: Compute the value of total support:

𝑆 = 4.3 ×3.14 × 572

4= 11500kg

Step 5: Compute the value of thickness:

𝑇 = 𝑘 log𝑃

𝑆

From the above Figure, the base course constant for 57cm diameter is read out as 138, so the thickness may be computed as:

𝑇 = 138 log27000

11500= 48.5cm

In which,𝑇 is the thickness of gravel material.𝐾 is the Base course constant.𝑃 is the gross wheel load (kg).𝑆 is the subgrade support value.