5. Pavement Design

5.1 Preamble

• empirical methods (e.g. based on the California Bearing Ratio strength test for soils), and
• theoretical methods (e.g. based on layered elastic material theory).

5.2 Terminology

• Annual Average Daily Traffic (AADT) - The total yearly traffic volume divided by 365.
• Base - The portion of the pavement which supports the surfacing.
• Commercial Vehicle - A vehicle having at least one axle with dual wheels and/or having more than two axles.
• Course - One or more layers of the same material within a pavement structure.
• Deflection - The vertical elastic (recoverable) deformation of a pavement surface.
• Design Period - The period over which the pavement is expected to perform without significant deformation.
• Design Traffic - The equivalent number of standard axles in one direction over the design period.
• Full Depth Asphalt Pavement - A pavement consisting entirely of asphalt placed directly on the prepared subgrade.
• Pavement - The portion of the road which is placed above the subgrade to support and form a running surface for vehicular traffic.
• Shoulder - A portion of the road continuous and flush with the pavement, and to be used for the standing of vehicles.
• Standard Axle - A single axle with dual wheels loaded to a total mass of 8.2 tonnes.
• Sub-base - The portion of the pavement between the base and the subgrade.
• Subgrade - The prepared formation on which the pavement is constructed. (Fill or in-situ material).
• Surfacing or Wearing Surface - That portion of the pavement in direct contact with vehicular traffic.

5.3 Introduction

Although economics will always be a major factor in the choice between rigid and flexible pavements, and between different pavement designs, other factors will also influence the final design chosen. For example:

• the expertise of the construction organisation,
• presence of public utility services, and
• drainage conditions.

5.4 Design Considerations

• the traffic to be carried;
• the design life of the pavement;
• the serviceability condition at the design life;
• the properties of the foundation soil;
• the pavement environment; and
• construction conditions.

5.5 Design Methods

• empirical methods, and
• theoretical methods.

Theoretical methods attempt to combine structural theory (usually theory of elastic behaviour) with a knowledge of the behaviour of road materials and foundation soils under repeated loading, to develop a pavement thickness by analysis. The theoretical approach uses laboratory and field testing to determine material properties which are then used in the analysis process. The theoretical approach has limitations in the material behaviour theories which must be used (e.g. analysis of any system other than a multi-layered elastic structure is very difficult, and yet a multi-layered elastic system does not mirror the real world), and the testing available to determine material properties (e.g. for accurate analysis an elastic modulus established under dynamic repeat loading with variable load amplitudes is required, but such testing is extremely expensive). The design approach adopted by the Department of Main Roads, Queensland, for pavements utilising a variety of material types, is an example of a theoretical design method.

5.6 CBR Method of Pavement Design

5.6.1 CBR Test

The California Bearing Ratio (CBR) test was initially developed by the California Highways Department to assess the quality of fine crushed rock base material, by relating the load versus penetration curve to that of a selected good quality fine crushed rock. Later, when the test was used for assessment of subgrades this method of expressing the results was continued. The standard fine crushed rock was allocated a CBR of 100. The CBR of other materials is expressed as a percentage of this standard material. Poor subgrade materials such as heavy clays are very weak compared to the standard crushed rock and will therefore register CBRs in the area of 1 to 5. Pavement materials used for base layers would be expected to have strengths similar to crushed rock and the specification for base materials will therefore require a minimum CBR of perhaps 60 or 80. It is possible to obtain materials with CBRs higher than 100 and all this means is that the materials are stronger than the original standard crushed rock.

The CBR value is determined by forcing a cylindical plunger (50mm diameter) into a soil sample at the rate of 1 mm/min. The loads required to cause 2.5 and 5.0 mm of penetration are recorded and expressed as percentages of the loads to cause the same penetrations in the standard crushed rock material (13.3 and 20 kN respectively).

Although it is possible to determine the CBR of subgrade soils in-situ by jacking a plunger against a rigid frame (e.g. a loaded truck), it is more normal for the test to be used to determine the strength of remoulded material in a laboratory. If the soil is remoulded for laboratory testing it is compacted into a cylindrical mould to provide a sample 127 mm high and 152 mm in diameter. The sample may be soaked in water for four (4) days before testing to determine the strength for a worse case scenario (e.g. the road cover by floodwaters for 4 days).

5.6.2 CBR Pavement Design

The original CBR based design method was based on tests made on a variety of existing crushed stone pavements judged to have reached a critical structural condition. Two design curves were developed, one for 'light' traffic and one for 'heavy' traffic. The curves originally developed by the UK Road Research Laboratory (RRL, now the TRRL) were modified for Australian road conditions by Australian road authorities.

The CBR method of pavement design was developed for multi-layer pavements of granular materials and should only be used for such pavements having a thin bituminous seal coat surfacing.

5.7 Department of Main Roads (Qld.) Method of Pavement Design

5.7.1 General

Details of the Department of Main Roads (Queensland) method of pavement design are contained within their 'Pavement Design Manual'. The Manual provides detailed guidelines for each of the principal phases of the design process. Design charts are presented for a wide range of pavement types.

5.7.2.Design Theory

The design method is based on elastic response of the pavement to traffic stresses (i.e. each of the materials in the pavement structure behaves in an elastic manner). The materials in the pavement are characterised by parameters whose values are determined from field and laboratory testing. The method assumes that failure will not occur as a result of permanent deformation of granular or bound materials (and this assumption will be valid as long as good construction procedures are followed, and the pavement is not subjected to very high wheel loads such as can be caused by a very heavily overloaded vehicle). The method also assumes that loss of pavement serviceability can occur due to:
• fatigue of bitumen bound or cemented layers due to repetitions of tensile strains at the bottom of such layers; and/or
• permanent deformation of the subgrade due to repeated vertical compressive strains induced in the subgrade.
The critical locations for pavement failure are therefore the bottom of bitumen bound or cemented layers (where tensile strains occur) and the top of the subgrade (where compressive strains occur).

5.7.3. Design Period

The design period for a pavement depends on the type of road, its location, and the intended usage during and after the design period. Generally the more heavily trafficked the road the more difficult it becomes to perform maintenance or reconstruction.

Typical design periods are as follows:
 

Design Period (years)	Type of Road and Location
20	Rural roads other than freeways.
25	Major urban roads and freeways.
20	Minor urban roads.

5.7.4. Design Traffic

The design traffic is expressed interms of the equivalent number of standard axles (ESA) predicted during the design period. Design traffic is dependent mainly on commercial vehicles with car traffic playing only a very minor role in pavement life.

Calculation of design traffic involves calculating a daily ESA value for the pavement based on current traffic conditions. This value is then converted to a yearly value, and finally a design life value, taking into account anticipated changes to the traffic volume over the design life.

As an example, a road may have a current traffic volume of 1000 vehicles per day, of which 5% are commercial vehicles, i.e. 50 commercial vehicles per day. Each commercial vehicle may be an average of 1.2 ESA (the conversion figure being derived from extensive traffic surveys based on the type of road). This means that the current traffic loading is 60 ESA. Over a whole year this would be approximately 22 000 ESA. If the projected life for the pavement is 20 years and traffic doesn't grow in volume, this equates to 22 000 x 20 = 440 000 ESA. However if traffic is growing at 3% per annum, the figure increases to 620 000 ESA.

Design traffic figures will generally be in the order of 100 000 to 5 000 000 ESA, although lower and higher design values occur for lightly trafficked and very heavily trafficked roads respectively.

5.7.5 Subgrade Properties and Strength

As the pavement design method is based on layered elastic theory it would be beneficial to use an elastic modulus for each pavement material and the subgrade. However pavement materials and natural soils do not behave in a perfectly elastic manner and the determination of elastic moduli is difficult. For these reasons the subgrade strength is not specified in terms of elastic modulus but in terms of the more readily determined CBR value. As CBR is dependent on the nature of the soil, its density, and its moisture content, it is important that the determination of CBR is made at conditions under which the material is likely to perform in service.

5.7.6 Drainage

Pavement and subgrade moisture conditions exert a major influence on the performance of roads. In pavement design it is important to be able to recognise ways by which moisture may enter the pavement or subgrade and to determine measures needed to control moisture movement.

Moisture changes usually result from one or more of the following effects:

• seepage from higher ground near the road pavement;
• fluctuations in water table level;
• infiltration of water through the surface of the road pavement and shoulders; and
• transfer of moisture in liquid or vapour states.

5.7.7 Determination of Alternative Designs

The Pavement Design Manual contains numerous design charts. Each chart presents a unique solution for a particular design case (traffic, pavement type, and subgrade CBR).

For example, Chart 38 is for a pavement structure of 100 mm of asphalt, over cemented material category 1, over subgrade. The chart has traffic loading (ESA) along the horizontal axis, and cemented layer thickness along the vertical axis. On the chart are a series of diagonal lines (curves), with each line representing a different subgrade CBR. The chart is used by finding the known traffic loading (ESA) on the horizontal axis, moving vertically upward from this point to the curve of the subgrade CBR, then moving horizontally to the vertical axis to read off the required cemented material thickness. This particular chart applies to only one asphalt type in a particular climatic zone, and other climate zones require different design charts.

5.7.8. Selection of Optimum Solution

The use of appropriate charts will give different design solutions for the same problem. The optimum solution is that which satisfies the design requirements at minimum cost while allowing for the following additional constraints:
• Design considerations which cannot be readily incorporated into the design charts (e.g. the need for boxed construction in urban environments).
• The requirements of road users and future maintenance problems.
• Availability of plant and materials.