The 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 (Available on-line at http://www.archive.official-documents.co.uk/document/ha/dmrb/index.htm).
The Map below is a clickable image.
 | The map alongside can be used as a guide to the process of Foundation Design. Clicking on the desired area of the map will take you to the desired section. When you have finished a particular section, if you wish to return to the map then use the back button on the browser or click on the map link. It is highly recommended that you follow the logical progression of the map |
| Figure 1 Foundation Design Process | |
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.
Return to top of page.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.
Return to top of page.The strength of the subgrade (soil below the pavement) is asssesed using a test known as the California Bearing Ratio test. This was developed in California in the 1930's and makes no attemp 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.
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.
| Type of soil |
Plasticity index (%) |
|
|||||||||||||||||
| Heavy clay |
| ||||||||||||||||||
| Silty clay | 30 | 5 | 3 | ||||||||||||||||
| Sandy clay |
|
||||||||||||||||||
| Silt | - | 2 | 1* | ||||||||||||||||
| Sand (poorly graded) | non-plastic | 20 | 10 | ||||||||||||||||
| Sand (well graded) | non-plastic | 40 | 15 | ||||||||||||||||
| Well graded sandy gravel | non-plastic | 60 | 20 | ||||||||||||||||
| Table 1 Estimation of CBR values | |||||||||||||||||||
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.
 |
| Figure 2 California Bearing Ratio Equipment |
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 2 shows how the CBR value of a soil varies with the moisture content.
 |
| Figure 3 California Bearing Ratio Vs 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.
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 tendancy 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.
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.
 |
| Figure 3 Capping and Sub-base Thickness Design |
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:
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.
Return to top of page.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.
Return to top of page.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.
Return to top of page.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.
An example of a foundation design problem can be found on the Thickness Design page, a link to which is at the bottom of this page.
Return to top of page.2) BS 1377: 1975 Methods of Tests for Soils for Civil Engineering purposes
3)Specification For Highway Works, MCHW.
e-mail: D.G.Toll@Durham.ac.uk.
Last Updated: 28 February 1997