Seasonal Variation in Pavement Design and Analysis – Some National Examples
The primary objective of this annex is to present examples of how moisture condition is taken into account in pavement design and analysis.
The pavement design regarding the influence of water has significantly different objectives in different European countries. While in central and northern Europe the most important question is how to protect the pavement structure from frost action, in southern European countries it is more critical to control excessive water that suddenly penetrates the pavement after heavy rainfall. At the same time identical processes play significantly different roles in different climates. For example, the suction, that has negative effect in locations of freezing and thawing, has positive effect in southern countries by increasing the stiffness, and this influences the decision of which type of material is used in unbound base and sub-base. In northern countries the percentage of fines is limited typically to 5-8%, while the Mediterranean countries allow up to 15% fines.
In many parts of Europe freeze/thaw effects play a crucial role in pavement design. How these effects are taken into account in pavement design varies in complexity from using the frost index, or Stefan or modified Berggren equations (Aldrich & Paynter, 1956) to calculate frost depth to coupled heat/moisture flow models. Most of the European pavement design methods take into account phenomena related to freezing and thawing by using the frost index. This indicator is supposed to account for the “quantity of frost” to which a pavement was subjected for a given period. The frost depth is then calculated based on some empirical equation as a function of frost index. Some pavement design methods also consider the influence of sunshine on this phenomenon.
Many design methods take into account the loss of bearing capacity during thawing. This is typically done by adjusting the modulus of each of the unbound materials by a reduction factor that depends on the frost susceptibility of the material (COST 337, 2002).
The European project AMADEUS (Amadeus, 2000) identified the need for further research of deterioration mechanisms related to freezing and thawing and development of a long-term predictive model for freeze/thaw related deterioration.
The project also identified the need for more information on how the bearing capacity of different types of soils is affected by the drainage conditions.
The World Bank’s Highway Development and Management Model HDM-4 (ND Lea International, 1995) models the seasonal performance variation using a simple two season model (“wet” and “dry” seasons). The program uses the following parameters:
• mean monthly precipitation,
• drainage effectiveness,
• surface cracking,
• potholed area,
to calculate the ratio between “wet” and “dry” pavement adjusted structural numbers. Loizos et al. (2002) modified this approach in the Road Infrastructure Management System (RIMS) developed for the Greek government, to enable multiple season analysis, more suitable to a European context, by introducing the environmental function.
In the following, examples of how seasonal variations are handled in design systems are given.
In Finland the design and analysis of pavement structures is done by separate consideration of different criteria:
• Frost resistance (structure and subgrade)
• Traffic loading/Resistance to fatigue
• Resistance to settlement (deformation in structural layers and subgrade)
• Rutting due to studded tyres
Design of public roads in Finland still, commonly, uses a semi-empirical method based on acceptable calculated frost heave, which depends on
• road class;
• structural durability;
• how homogenous the subgrade is;
• the freezing index, F10, of the geographical location (583-1416 °C. days); and on
• an empirical factor of frost heave of the subgrade, which depends on the proportion of soil passing the 63 ^m and 2 mm sieves and whether it is a wet/dry location.
On homogenous clay soil, the measured frost heave values can also be used.
According to the results of the “Road Structures Research Programme” the control of frost behaviour of the road is divided into two parts: control of frost heave and control of the effects of thaw weakening. The frost heave of a road is estimated using the segregation potential (SP) concept, in which SP is the parameter that describes the frost susceptibility of the subgrade. The total thickness of the road structure is designed on the basis of frost susceptibility of the natural subgrade, and on the thermal conductivity of the used materials. If necessary, the structure is protected using insulation against frost, so that the permitted frost heave, set as the design criterion, is not exceeded. The procedure can be applied in designing new roads and improving old roads in all road classes. The program also calculates settlement profiles based on the investigations and identification of variations in the water content of soil layers along the road line.