The Danish design system deals with seasonal variations by adjusting the expected bearing capacity (E-modulus) of each pavement layer. In the design software MMOPP (Mathematical Model of Pavement Performance) (Ullidtz, 1993) the user can choose an advanced design procedure, where the performance of the road is simulated over (for example) 40 years. The program is given constant E-modulus values as material parameters for each pavement layer. These moduli are then varied over the seasons as shown in Table A.1. The constant E-values given as input represent the summer values. In wet seasons the E-moduli of unbound layers are reduced. In frost seasons the values are increased.
Table A.1 Coefficient multiplied to the E-value dependent on season and layer
When designing pavements in Sweden the different layers are given stiffness varying with the season of the year. The procedure is very similar to the one used in Denmark (see above). In Fig. A.2 the first column holds the thickness of each layer and the other six columns show how the stiffness of each layer is predicted to vary over the year.
Fig. A.2 Road design dependent on season and layer – Swedish case using PMSObjekt (Vagverket, 2005) showing moduli of pavement layers in 6 “seasons”
The Catalogue of Typical Flexible and Semi Rigid Pavement’s Construction, was developed for the Polish General Directorate of Public Roads. Typical structures were designed, based on analysis of stresses and deformations in pavement, using multilayer elastic and viscoelastic half space theory. According to this Catalogue and the Roads Design Guidelines (General Directorate of Public Roads, 1995), in pavement design process following factors should be taken into account:
• climatic and ground-water conditions,
• intensity and kind of traffic structure during whole designed life period (20
• values of allowable loads from vehicles (100 kN/axle),
• function of pavement.
Climatic conditions are freezing depth, average annual temperature and temperature differences.
Subgrade bearing capacity groups (G1-G4) depend on type of soil, water conditions and CBR value. The G1 is the best subgrade, mainly sandy soils of CBR > 10, G4 is the weakest subgrade, mainly cohesive soils of CBR < 3. Bearing capacity group has an impact on the necessity and kind of subgrade improvement. For typical structures taken from the Catalogue, the bearing capacity of the subgrade to be achieved is as follows. The secondary static modulus, E2, must be greater or equal to 100 or 120 MPa and the compaction ratio, Is, must be greater or equal to 1.00 or 1.03 depending on the traffic loading.
Water conditions are evaluated depending on ground water depth (z) from the bottom of the pavement structure. If subgrade drainage is required, a capping layer made from frost non-susceptible materials with a coefficient of permeability, K > 9.3 x 10-5 m/s should be used. The capping layer (at least 15 cm thick) should be placed across the whole width of road bed. For the situation where there is unimproved soil under the capping, a “tightness condition” is imposed for the layers:
^ < 5 (A.1)
D15 is the dimension of sieve, through which 15% of grains of separating layer or drainage layer will pass and d85 is the dimension of sieve, through which 85% of grains of the foundation soil will pass. In situations when the above layer tightness condition cannot be fulfilled, then between these layers a separating layer (of thickness at least 10 cm of suitably graded soil) should be arranged or a non-woven geosynthetic interlayer should be inserted.
In the case of frost susceptible subgrade soils, it is necessary to check if the total thickness of all layers (taken from the Catalogue) and any improved subgrade layer is sufficient to achieve frost resistance. In situations when this condition cannot be fulfilled, then the lowest layer of improved soil should be thickened.