Subgrade to sub- b
Subgrade to sub- base interface
In this case geotextile acts in a very similar way to that of a geotextile in an unpaved road.
A geotextile may be placed at this level in the road structure to either control rutting , possibly enabling a reduction in the sub -base thickness, or to control contamination of the sub- base by fines from the subgrade soil. (9)
The construction technique
The construction technique is summarized pictorially in figure (I.13),which shows six stages in the operation . Firstly , ground level is reduced to formation level,where the formation is susceptible to rutting , only tracked vehicles should be employed in the final strip of soil down to formation level figure (I.13 a). The geotextile is rolled into position figure (I.13b) , a lap joint being used at the end of each roll length figure (I.13 c) . Sub- base or capping layer material is then placed over the geotextile by end tipping figure(I.13 d) .Under no circumstances should construction plant be allowed to traffic directly on the geotextile. Next, the sub- base is spread and levelled ready for compaction figure (I.13 e) .At the end tipping stage , the pile of sub- base material should not be allowed to become so high that it causes bearing capacity failure in the formation . Once the material has been spread and levelled, it is compacted figure (I.13 f) and should then be protected by construction of the bound pavement layers as soon as possible (18)