Unbound Granular Materials

DTp Type 1 Granular sub-base

The name refers to Dept. of Transport specifications. The full title is "Type 1 granular sub-base to the Department of Transport Specification for Highway Works" It is still referred to as MOT1 (the old Ministry of Transport) or even the more inaccurate "40mm to dust". In Ireland it is usually known as "804", a name derived from the fact that the full specification is to be found in clause 804 of the National Roads Authority's Specification for Road Works - can you spot a trend here? 

There is a slight difference between the Spec used in Britain and that used in Ireland that results in DTp1 not being quite the same as 804 Type A, but we are talking about hundredths of a millimetre. For anyone wanting to know more, a PowerPoint Training presentation comparing and contrasting is available on request. 
This is a material from a certified source and should have the correct proportion of 'fines' (dusty material) and solids ('lumps'). The solids should not exceed 37.5mm in size. The mixture of solids and fines is designed to ensure that there are no voids in the sub-base material once compacted, that the aggregate forms an acceptable level of interlock between the angular particles, and that the compacted sub-base will allow any ground water to drain through. 

This material is typically crushed stone, crushed slag, crushed concrete or non-plastic well-burnt shale. It can be obtained from most Builders' Merchants in multiples of 1 tonne loads or direct from quarries in full wagon loads (usually 16 or 20 tonne loads).  
Grading range for DTp1 material 
1 tonne of DTp 1 covers approx 5 m² at 100mm compacted thickness, and approx. 3.5m² at 150mm compacted thickness.