levels. The net effect, however, is the estimates of regravelling frequency by each model are broadly similar and therefore use of the HDM-4 models provides a conservative result. Figure : Gravel loss trends for alternative gravel loss models Selection of optimal options The selected optimum economic treatment, based on an existing gravel surface of material 201 in poor condition in a moderate rainfall area, is shown in Table 52, Table 53 and Table 54. The results show that a gravel surface option is the best economic treatment for very low, low and medium traffic, whereas a concrete pavement option is preferred for high and very high traffic. This suggests that the break even traffic for upgrading to concrete lies towards the upper end of a nominal AADT range of 250 and 750, or an adjusted AADT of between 110 and 330. This is broadly consistent with the findings of international studies (SADC 2003). In the example shown the gravel option with grading of the surface at six monthly intervals is the best option for medium traffic. The challenge is whether this level of maintenance will take place, or whether a lower level of gravel road maintenance would occur in practice. A number of factors can influence this, and an indication of their effect on option selection can be sought by comparing the total transport costs of the different options, since the optimum option is simply the alternative which minimises total costs. Table : Selected optimum economic treatment option (*) for each representative section for low rainfall and initial poor condition Terrain Very low Low Medium High Very High 04 Upgrade to 201 02 Upgrade to 201 04 Upgrade to 201 06 Upgrade to 06 Upgrade to Rolling Gravel w RG @ Gravel w Min Maint Gravel w RG @ 50mm Concrete Concrete 50mm 04 Upgrade to 201 02 Upgrade to 201 04 Upgrade to 201 06 Upgrade to 06 Upgrade to Hilly Gravel w RG @ Gravel w Min Maint Gravel w RG @ 50mm Concrete Concrete 50mm 04 Upgrade to 201 02 Upgrade to 201 04 Upgrade to 201 06 Upgrade to 06 Upgrade to Mountainous Gravel w RG @ Gravel w Min Maint Gravel w RG @ 50mm Concrete Concrete 50mm Note: * see Table 4.8 for description of treatments
levels. The net effect, however, is the estimates of regravelling frequency by each model are broadly similar and therefore use of the HDM-4 models provides a conservative result. Figure : Gravel loss trends for alternative gravel loss models Selection of optimal options The selected optimum economic treatment, based on an existing gravel surface of material 201 in poor condition in a moderate rainfall area, is shown in Table 52, Table 53 and Table 54. The results show that a gravel surface option is the best economic treatment for very low, low and medium traffic, whereas a concrete pavement option is preferred for high and very high traffic. This suggests that the break even traffic for upgrading to concrete lies towards the upper end of a nominal AADT range of 250 and 750, or an adjusted AADT of between 110 and 330. This is broadly consistent with the findings of international studies (SADC 2003). In the example shown the gravel option with grading of the surface at six monthly intervals is the best option for medium traffic. The challenge is whether this level of maintenance will take place, or whether a lower level of gravel road maintenance would occur in practice. A number of factors can influence this, and an indication of their effect on option selection can be sought by comparing the total transport costs of the different options, since the optimum option is simply the alternative which minimises total costs. Table : Selected optimum economic treatment option (*) for each representative section for low rainfall and initial poor condition Terrain Very low Low Medium High Very High Rolling 02 Upgrade to 201 Gravel w Min Maint 04 Upgrade to 201 Gravel w RG @ 50mm 04 Upgrade to 201 Gravel w RG @ 50mm 06 Upgrade to Concrete 06 Upgrade to Concrete Hilly 02 Upgrade to 201 Gravel w Min Maint 04 Upgrade to 201 Gravel w RG @ 50mm 04 Upgrade to 201 Gravel w RG @ 50mm 06 Upgrade to Concrete 06 Upgrade to Concrete Mountainous 02 Upgrade to 201 Gravel w Min Maint 04 Upgrade to 201 Gravel w RG @ 50mm 04 Upgrade to 201 Gravel w RG @ 50mm 06 Upgrade to Concrete 06 Upgrade to Concrete Note: * see Table 4.8 for description of treatments