This is a layer of crushed stone used under paving to form, in effect, a stable foundation for it and is found between the bedding course and the subgrade. It is made of stone which has been crushed and sieved stone to end up with a mixture of sizes from normally about 40 mm down to dust. The proportions of the different sizes should be such that the smaller stones bind the larger ones together to stop them moving and these are bound together by the smaller onesl. The source of the stone varies widely according to what is the cheapest local supply and includes limestone, dolomite, and waste concrete and waste tarmac. It goes by various names; dolly, dolomite, crusher run, type 1, MOT type 1, road plannings, 40mm down and many others. Ministry of transport type 1 is a type of subbase produced to stringent standards which while making it perfectly suitable for garden use it is a degree of over engineering. There is also type 2 and when I asked a technician at a quarry the difference compared to type 1 he just said “very little”!
Dolomite has the disadvantage that it becomes saturated with water when you attempt to compact it there is a tendency for it to turn to something resembling plasticine in texture, although it hardens on drying out and road planning tend to become greasy when wet. Only hard stone should be used as even the hardest sandstone, for example, will rapidly breakdown to sand in use. Once spread subbase should be compacted with either a vibrating roller or plate BUT do not attempt to compact more than a 150mm deep layer at any one time.