General Historical structure of pavement

70 course. Under the base course is the subbase, and under the subbase is the subgrade natural soils. The type of material in each layer and the thickness of the layer will directly affect the strength of the pavement. Sections of pavement that have an identical surface course ADO, 1999. Figure 4.1 cross section and pavement layer thickness in secondary roads OECO, 2008

a. Subgrade course

The subgrade is the in situ material upon which the pavement structure is placed. Although there is a tendency to look at pavement performance is terms of pavement structure and mix design alone, the subgrade can often be the overriding factor in pavement performance and must be investigated OECO,2008. The experimental results obtained on the soils beneath the cracked pavement section, from these results the following observations were made Bashir 2006 see Table 4.1: 1- Soil contains low percentage of clay and in general a relatively high percentage of sand. 2- Soil low in plasticity. 25 cm thickness Base course CBR not less than 80 11 cm thickness surface course 00 cm thickness subbase course CBR must be more than 25 3.25 m 3.25 m 0.75 m 0.75 m Natural Subgrade 5 cm wearing course 6 cm Binder course 71 3- The relative compaction of the soil under the pavement was good for the required standards of pavement construction. Therefore according to AASHTO the soil is considered to be a good soil. Table 4.1 the summary of subgrade soil properties Bashir 2006. Property Unit Depth of sample m 0.75 Sieves analysis Percent of passing from sieve 10 Percent of passing from sieve 40 Percent of passing from sieve 200 100 100 53.7 Consistency limits Liquid limit LL Plastic Limit PL Plasticity PI 15.70 8.80 6.90 Compaction Optimum water content Max .dry density Mgm 3 10.00 1.865 Degree of compaction 97.3 Unified soil classification A4

b. Sub base course

The subbase course is between the base course and the subgrade. It functions primarily as structural support but it can also OECO, 2008 : 1- Minimize the intrusion of fines from the subgrade into the pavement structure.