Introduction to Pavement Design

There are four types of pavement currently in use in the United kingdom at present:

Flexible - pavements with a bitumen bonded surfacing and roadbase. Flexible Composite - The surfacing and upper road base are bituminous on

a lower roadbase of cement bound material Rigid - Pavements with a concrete surface slab which can be un-reinforced,

joint reinforced or continuously reinforced. Rigid Composite - continuously reinforced concrete slab with a bituminous

overlay.

Although there has been considerable advance in the theoretical design of pavements, most of the current work is based on empirical methods and design charts. For this reason only these methods will be covered in these pages.

Normally a design is carried out for each of the alternatives and then the most economical chosen. However, if for environmental or technical reasons one is impractical then it may be omitted.

The general form of the different types of pavement is shown below. It can be seen that common to all types of construction is the Sub-base and subgrade. It is this aspect of the design which is treated first. However, it is first necessary to decide on a failure criteria on which to base the design.

Failure Criteria

This has been the subject of extensive research and is the result not of theory but of full scale tests and observations of real life situations.Croney1 states that permanent deformation in the near side wheel tracks of a flexible pavement of 20mm, when measured from a 1.8m straight rule, was normally accepted as the point at which reconstruction was considered necessary.

It was also noted that at this point the surface was beginning to crack. This led to water ingress and subsequently rapid deterioration.

The critical condition was much harder to define although it was found generally that overlays were being provided between deformations of 10 and 20mm.

1 1) D. Croney, "Failure Criteria for Flexible Pavements", Proceedings Third International Conference on Structural Design of Asphalt Pavements, University of Michigan, 1972.

Cracking at these points was generally limited to longitudinal cracks in the wheel tracks.

The advantage of this method is that it is easily measured by visual inspection and a straight edge.

Methods used in the United States including asking passing motorists to rate the trafficability between 0 and 5 and a slope variance device.Salter2 contains more information on both these methods as well as other relevant ones.

The roadbase and sub-base are generally only divided into upper and lower for highly trafficked roads.

The nest step is the foundation design. This can be found by following the link below.

2 2) R. J. Salter, "Highway Design and Construction", MacMillan Education, London, 1988.

Pavement Design - Foundation DesignThe Purpose of the foundation is to transfer the loading from the road to the soil or subgrade. The foundation for the purpose of these pages is defined as the sub-base and any associated strengthening materials used. The procedure described in this page is that in the Design Manual for Roads and Bridges, Volume 7

Introduction

The main purpose of the foundation is to distribute the applied loads to the underlying subgrade, without causing distress to either the foundations, the overlying layers or the subgrade.

The critical condition for a pavement foundation is whilst carrying the construction traffic as, whilst the repetitions are low, the stresses are applied direct to the foundation. Standard foundation design is thus to design for the construction traffic. That is the method described below.

Assessment Methods

The first step in the design of the foundations depends on the situation that is causing the design.

If the road is new then the first step is the assessment of the subgrade on which the road is to be built. This is described in the next section and is essential to the design. If this is not carried out correctly then the information on which the designs are based may be incorrect.

If the road is a not a new road then the first step is the assessment of the existing foundations. This is described later on in the chapter and is the same procedure as the compliance testing for a new road.

Subgrade Strength

The strength of the subgrade (soil below the pavement) is assessed using a test known as the California Bearing Ratio test. This was developed in California in the 1930's and makes no attempt to determine any of the standard soil properties such as density. It is merely a value and it is integral to the process of road design. Nearly all design charts for the road foundations are based on the CBR value for the subgrade.

California Bearing Ratio

The California Bearing Ratio (CBR) is a measure of the supporting value of the subgrade. It is not unique and other tests such as the R-Value test and the Triaxial are used occasionally. It is however by far the most commonly used in Pavement Design. The CBR test should be used with soil at the calculated equilibrium moisture content (see below) although in the United States it is usual for samples to be soaked for 4 days prior to testing.

To determine the CBR for a soil the designer has two options, they can either use the California Bearing Ratio (CBR) test if equipment is available or they can use the table below to estimate the CBR.

The CBR should be carried out in accordance with BS 13772

The test involves the equipment shown below. The plunger is then seated into the soil using a force of 50N for an expected CBR below 30% or 250N for greater than 30%. The plunger is then penetrated into the soil at a constant rate of 1mm/min and the forces recorded at penetration intervals of 0.25mm. The total penetration should not exceed 7.5mm. These results are then compared to a standard curve for a value of 100% CBR. The forces on the standard curve are 13.2kN at 2.5mm penetration and 20.0kN at 5.0mm penetration. The CBR is then a simple ratio of

the corresponding values and where a difference between the value at 2.5mm and 5mm occurs, the higher value is taken. Annular weights are sometimes used to represent a surcharge.

The first factor affecting the performance of the subgrade is the moisture content. Unfortunately this is normally extremely variable as water can come from many sources such as rainfall, capillary action, seasonal movement of the water table and ingress.

The importance of the moisture content is demonstrated by the variation of the CBR values as shown below. This is a plot of a typical soil sample values of CBR against moisture content. Thus the soil sample should have the moisture content re assessed after the test is performed and compared to the desired value.

Figure 3 shows how the CBR value of a soil varies with the moisture content.

Croney and Bulman1 used thermodynamic principles to define three categories into which the subgrade moisture conditions can be placed. These allow the relevant value of moisture content to be used when assessing the soils under laboratory conditions.

Under conditions of rainfall and evaporation such that the water table forms within 5m of the surface. Edge effects are small and the situation is achieved by a combination of drainage and capillary effects. This allowed the estimation of the water content for different water table levels. This is typical of a temperate climate with moderate evaporation or on the flood pains of rivers. The United Kingdom generally falls into this category.

The water table is greater than 5m but for several months of the year rainfall exceeds moisture loss by evaporation and transpiration. The moisture content is thus cyclical and depends on the permeability of the soil. This is typical of a hot climate with high seasonal rainfall.

Dry climates where the rainfall has little effect on the water table and the moisture content may be assumed to be close to that in the surrounding uncovered soil.

This is however a subjective method and not widely used. In America, the standard practice is to assume a saturated sample and test as such. As can be seen from Figure 3, this has a tendency to underestimate the CBR and thus to overdesign the foundation.

Standard practice in the United Kingdom is to assess the soil at the optimum moisture content. This is done using the procedure below.

Plot the dry density of the soil against moisture content. This will give you the optimum moisture content, that is the peak of the

curve. Plot the CBR against moisture content graph The CBR value carried forward to the design is that at the optimum

moisture content.

An example of this can be found in the problems page below.

Capping & Sub-base Design

Once the CBR value has been determined, it is then possible to design the actual capping and Sub-base layers. Capping is used to protect weak subgrades by using a relatively cheap material between the subgrade and sub-base

It is not practical to build on a layer whose CBR value is less than 15%. It is therefore necessary to improve this value either by capping or increasing the thickness of the sub-base. The chart below shows the two alternatives, each of which are equally effective and which is used depends on cost and construction restrictions.

In exceptional circumstances where the CBR value falls below 2%, a value below which the subgrade would deform under construction traffic, there are several options open to the designer:

The material can be removed and replaced with a more suitable material. The thickness replaced is typically between 0.5m and 1.0m. Irrespective of the quality of the new material a CBR value of just under 2% should be assumed for the subgrade.

For cohesive soils, it may be possible to treat the soils using lime. The sub-base and capping is again designed assuming a subgrade CBR of just under 2%.

For a reasonably permeable soil the drainage system may be lowered and the results monitored. The main foundation should then be designed assuming the achievable conditions.

For subgrades with CBR values of 15% and above the sub-base should have a standard thickness of 150mm, a value determined as the minimum practical for spreading and compaction.

For subgrades with CBR values in excess of 30% and a low water table or hard rock subgrades then the sub-base may be omitted.

When designing a road of some length, it is not advisable to frequently vary the foundation thickness but rather select an appropriate value for each significant change in the subgrade properties.

Drainage and Frost Protection

It is vitally important to keep moisture out of the sub-base, capping and subgrade both during the construction and during the life of the pavement. This is achieved by excluding incoming water and providing a drainage path for water already in the foundation.

This is achieved by placing a granular aggregate drain or drainage blanket. Where a drain is used, it should be below the bottom of the capping. The finished design should always slope towards the drain. There are circumstances where drains are not necessary and reference should be made to the Design Manual for Roads and Bridges if this is a possibility

All materials used should be non-frost-susceptible. More information on the susceptibility to frost can come from the Design Manual for Roads and Bridges and from the Meteorological Office.

Compliance Testing

Of you are designing the repairs to an existing road then this is the start point for you design. If you are designing the foundation for a new road then you will need to test the foundation to show that it will perform as desired. The tests used all comply with BS 1377 and reference should be made to this manual before any tests are carried out.

In Situ Testing

The following tests should be used in determining the compliance of the foundation to the required standard or for testing the existing road surface. Failure of these tests may indicate either that the specified method of construction was not followed, that the materials used were sub-standard, that damage has occurred or that abnormal conditions were encountered and not allowed for.

Moisture Condition Value (MCV)- This test involves compaction of soil or fine aggregates using a hand held device. The compactive effort can be compared to that needed at the optimum moisture content. This enables a comparison with the actual moisture content of the soil.

Density Testing - This can be either sand replacement or Nuclear Density. They are both time consuming but allow comparisons with actual density and Laboratory values to be made.

California Bearing Ratio (CBR) - This is explained in more detail in the section above.

Clegg Hammer - This is sometimes referred to as a dynamic CBR and is an extremely rapid test. A comparison has been made to the CBR values and as such is useful in finding weak spots in the paving or foundation.

Cone Penetrometers - Again enable rapid assessment of approximate CBR values to within 5 to 6%.

Plate Bearing Test - This equipment enables the user to determine the Elastic Modulus of the pavement or foundation. It's use is discussed in the Specification of Highway Works document3.