Gravel as it occurs naturally is usually known as either "dug gravel" or "pit-run gravel". Such gravels often contain a considerable amount of clay or dirt (binder)‚ which if the material is required for road base or surface work‚ is an advantage. This material is known as Hoggin. Such materials when compacted can make a very stable surface.
Hoggin should generally contain no particle larger than 40mm and be well graded (i.e. different sizes of particles being evenly represented). The precise mix of clay‚ sand and gravel is essential if the material is to function effectively.
When finished a hoggin drive is only slightly permeable. Most rainfall will be shed from the surface. If the road is being laid across a slope remember to allow for drainage from one side of the road to the other or a large puddle may develop on the upper side.
The Hoggin should be spread evenly over a suitable base and compacted with an 8-tonne roller to the required thickness. Should this thickness be greater than 80mm then it should be laid and compacted in successive layers not exceeding 80mm. Usually a depth of around 80/100mm after compaction will be suitable for pedestrian / light vehicle use.
When the required depth of Hoggin has been laid‚ 50mm depth of a 20mm gravel should be rolled into the surface. This layer of gravel is bound by the Hoggin and provides the wearing course. With 100mm of Hoggin and 50mm of gravel‚ total build up will be about 130mm. It is important that the Hoggin does not sit in water. The sub-base must be laid to falls like the final surface.