Transportation Engineering-II

Principles of Pavement Design

Pavement Design

What is a Pavement?

• Pavement is the upper part of roadway, airport or parking area structure

• It includes all layers resting on the original ground

• It consists of all structural elements or layers, including shoulders

What is Design?• Conceived/developed plan for something to

serve a specific function.

What is a pavement design?

• It is the process by which the structural components of a road segment are determined, taking into account the nature of the subgrade, density and traffic composition.

Pavement Design

Pavement Design

‘OR’• It is the process of developing the most

economical combination of pavement layers (in relation to both thickness & type of materials) to suit the soil foundation and the traffic to be carried, during the design life.

‘OR’• It is a process of selection of appropriate

pavement and surfacing materials to ensure that, the pavement performs adequately and requires minimal maintenance under the anticipated traffic loading for the design period adopted. This selection process involves adoption of material types, thicknesses and configurations of the pavement layers to meet the design objectives.

Pavement Design

What is the goal of pavementdesign?

• Provide the most cost-effective structure while optimizing the level of service provided to road users.

Pavement Design

Pavement Design Phases

Highway design consists of three stages• Geometric design

(route selection or alignment design)• Capacity design

(number of lanes to meet traffic demand)• Structural design

(to withstand loads and environment)

Structural design consists of 3 steps• Selection of materials (types of pavement)• Proportioning of materials• Layer thickness design

Pavement Design Principles

Design objectives are to:

• Provide safe and comfortable riding conditions to all road users, being motor vehicles, cyclists and pedestrians, optimized for the road’s intended functions and the level of use.

• Provide low cost of ownership (i.e. minimum whole of life cost) to the Government.

• Comply with the Standards and relevant State Road Authorities’ Guidelines and/or Standards.

Fundamental Parameters

Following are the fundamental parameters to be considered:

• Subgrade• Loads• Environment

Subgrade

Characterized by strength and/or stiffness

• California Bearing Ratio (CBR)

• Measures shearing resistance

• Units: percent

• Typical values: 0 to 20

• Resilient Modulus (MR)• Measures stress-strain relationship

• Units: psi or MPa

• Typical values:3,000to40,000 psi

Subgrade

Some Typical Values

Classification CBR MR (psi) Typical Description

Good ≥ 10 20,000

Gravels, crushed stone and sandy soils. GW, GP, GM, SW, SP, SM soils falls in this category.

Fair 5 – 9 10,000

Clayey gravel and clayey sand, fine silt soils.  GM, GC, SM, SC soils falls in this category.

Poor 3 – 5 5,000

Fine silty sands, clays, silts, organic soils.  CL, CH, ML, MH, CM, OL, OH soils falls in this category.

Loads

Load characterization

• Tire loads• Axle and tire configurations• Load repetition• Traffic distribution• Vehicle speed

Load Quantification

Equivalent Single Axle Load (ESAL)• Converts wheel loads of various magnitudes and

repetitions ("mixed traffic") to an equivalent number of "standard" or "equivalent" loads

• Based on the amount of damage they do to the pavement

• Commonly used standard load is the 18,000 lb. equivalent single axle load

Load Equivalency– Generalized fourth power approximation

factor damage relativelb. 000,18

load4

Typical LEFs

Notice that cars are insignificant and thus usually ignored in pavement design.

LEF Example

The standard axle weights for a standing-room-only loaded Metro articulated bus (60 ft. Flyer) are:

Axle Empty FullSteering 13,000 lb. 17,000 lb. Middle 15,000 lb. 20,000 lb.Rear 9,000 lb. 14,000 lb.

Using the 4th power approximation, determine the total equivalent damage caused by this bus in terms of ESALs when it is empty. How about when it is full?

Environment

• Temperature extremes

• Frost action• Frost heave• Thaw weakening

Secondary Parameters

Following are the secondary parameters for pavement design;

• Soil Classification• Soil horizons• Parent materials• Moisture-solid relationship

• Material Characterization• Soil and Base Stabilization• Sub-grades Strength Studies

• Compaction• Strength-Density-Moisture Considerations

• Base and Subbase Courses• Feasibility• Grading• Construction

• Surfaces• Functions• Types of materials• Skid qualities• Cracking

• Materials Considerations

Secondary Parameters

Design factors

General Framework of Pavement Design