This depends on the quality of the sub-grade material which is usually determined by geotechnical surveys and centerline investigation which will also tell us what soil indicators are present and whether or not mechanical modification with borrow material is required. In most cases we can achieve good strength on the sub-grade with a small percentage of cement and good compaction, provided that the sub-grade is not sitting on top of shifting clay soil layers, which will be difficult to deal with in any situation. Once that is done build the wearing course of 150mm.