11.4.1 Modelling of Moisture Movements
Alonso (1998) presents, on the basis of in-situ measurements, relevant aspects of the water content development of pavement layers and its effect on the mechanical characteristics of granular bases and subgrades. A general model for the coupled analysis of transfer processes (water, heat) and stress-strain behaviour of unsaturated compacted soils is then presented. A review of some representative properties of compacted soils has been carried out from the perspective of modern concepts of unsaturated soil mechanics. As an application of the methods described a full simulation of a modern pavement structure under the effects of a Mediterranean climate has been developed. The last section of Alonso’s paper is devoted to collapse and swelling phenomena of subgrades. Collapse is found in some natural lightly cemented soils but it is more common in embankments compacted on the dry side of the optimum water content. A real case involving severe collapse deformations and the role of described models to analyse the problem is presented. Finally, the available techniques to design and analyse the behaviour of highways on expansive soils are presented.
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Alonso et al. (2002) also presented an analysis of the optimum position and depth of longitudinal drains in pavements. An analysis of different climates on the overall pavement behaviour is then given. Three climates have been defined: Tropical, Mediterranean and Sub-alpine (see Chapter 1, Section 1.11), which were defined on the basis of actual data involving rainfall, temperature and relative humidity records. Five years of climate were simulated and the reaction of the selected pavement structure and drainage position were computed and discussed. Figure 11.4 shows some of the computed results. It can be seen that the addition of longitudinal drains has a profound effect on the granular base and sub-base saturation over time, whereas their effect on the subgrade, Fig. 11.5, was found to be more limited. Longitudinal
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Fig. 11.5 Distribution of degree of saturation for a pavement in a Mediterranean climate. Evaporation through pavement allowed. (a) 1st July, (b) 1st December, adapted from Alonso et al. (2002)
drains are capable of maintaining a fairly well drained subgrade platform under a Mediterranean climate (as illustrated) whereas their effect was found to be more limited under sub-alpine or tropical climates.
