Routine pavement design is based on an elastic calculation, with a resilient modulus. The design criterion is, typically, a limit placed on the maximum vertical strain. More elaborated models take into account the irreversible behaviour, e. g.:
• The Chazallon-Hornych model is based on the Hujeux multi-mechanism yield surface improved by a kinematical hardening; and
• The Suiker and Mayoraz elasto-visco-plastic models evaluate the irreversible strains on the basis of an overstress (Perzyna theory) which is the distance between the stress level and a visco-plastic potential.
Each of these elaborated models is then based on a yield surface, a potential surface, a limit surface, in all cases a surface typical of the granular soil mechanics, with a frictional mechanism, possibly a cap contractive mechanism, a dependency not only on the shear/von Mise’s stress but also on the mean stress (p — q plane), and on the Lode angle.
How can we adapt these models to take into account the suction variation effects? For routine pavement design, only the elastic moduli need to be adapted. For the higher-level models, the yield surface and hardening mechanism also need to be adapted.
During the two last decades a number of models for partly saturated soils have been proposed (forareview, see e. g. Laloui etal., 2001). Most of them are based on the suction as an additional variable, with the same status as the stress tensor.
The so-called Barcelona Basic Model — BBM, proposed by Alonso et al (1990) is probably one of the best known. It is now the reference for most new developments in mechanics of geomaterials under partial saturation.
The BBM is based on the well-known CamClay model. It is written within the framework of the independent stresses state variables p — q — s defined in Section 9.5.
The BBM yield surface depends not only on p — q stresses but also on the independent stresses state variables p — q — s. Two lines are added with respect with the modified Cam-Clay model. On a wetting path (a loading path along which the suction decreases), the Loading-Collapse, LC, line allows a normally consolidated material to support irreversible plastic strains and hardening, and the plastic slope to depend on the suction level (as there will be an increase of stiffness with suction). For low stress level, the cohesion only depends on the suction level. For the case illustrated in Fig. 9.10, a capillary cohesion is postulated, which depends linearly on the suction. Eventually, under very high suction (a consequence of the drying process) irreversible strains may also occur. This is at the plane SI in the figure — the suction increase surface.
However, neither do the BBM, nor the other published models, introduce any suction dependency into the elastic moduli formulations.
From this illustrative model, it appears that building a coupled model for repeated loading and suction variation on granular soil material needs the following developments: [23]
• Using the generalised effective stress or the net stress approach allows developing coupled moisture — mechanics models to be developed.
• For any development an experimental basis will be needed to calibrate and validate the models.
This chapter deals with the constitutive modelling of the effects of water on the mechanical behaviour of pavements. It has been shown that routine pavement design is based on an elastic calculation, with a resilient modulus. The design criterion is, typically, a limitation of the maximum resilient vertical strain. Such design approaches do not model in a realistic manner the observed irreversible behaviour seen as rutting and as other forms of distress. To achieve a design approach that can replicate more closely the observed behaviour is likely to require use of the concepts of elasto plasticity and visco-plasticity. More elaborate models of soil and granular material will be needed to take into account these concepts and several approaches have been reviewed that attempt to do this.
At present few of these newer approaches explicitly include the effect of water pressures and suctions within the soil or aggregate pores, so the chapter has discussed how the available constitutive models could be improved to take into account suction and suction variation effects. Some research topics have, also, been suggested to enable further development.
