Preparation of Sub Base and Sub Grade

Preparation of Sub Base and Sub Grade - Essential preparation

The success and longevity of any paving project ultimately rests on the preparation of your bedding layers and the use of appropriate bedding materials. The purpose of the bedding layers is to support the paving surface on a stable and firm background. Successive layers are composed of materials of increasing particle size that have the effect of spreading the load placed on the paving surface over a larger area. The specification for the bedding layers is dependant on the load that they are expected to take. We will consider the specification for domestic patios and driveways.

Sub-grade is the description for the earth surface that the project will be laid on. As a minimum the topsoil should be removed to expose an undisturbed stable surface that has very little organic material present. The thickness of the topsoil can vary but is generally 100mm to 275mm thick. The exposed surface should be reasonably level and flat. If unconsolidated, loose areas are exposed they should be removed and filled in with with a denser material and compacted. Terram geotextile should be laid over the sub-grade to prevent the sub-base 'sinking' into the sub-grade.

The sub-base should be constructed using DTp type 1 (commonly known as MOT1), which is widely available from local builders merchants. DTp type 1 is made up of a mixture of aggregate sizes that when compacted form a dense stable surface with no voids. The use of other materials to form the sub-base should be avoided. Hard core (broken bricks and concrete mix), crushed concrete, sand/gravel mixes are all commonly used, however the particle sizes are often inconsistent so either voids form or the base is too loose. This is likely to lead to subsidence and potential failure of your paving surface. The sub-base should be laid to a minimum thickness of 75mm for areas subject to foot traffic, such as domestic patios and pathways or 100mm - 125mm thick for domestic driveways. The sub-base should be laid and compacted using a 'wacker plate' or vibrating drum roller. If the thickness is greater than 150mm then it should be laid and compacted in multiple layers (for example a 200mm thickness sub-base should be laid with two 100mm layers, both being fully compacted before the next layer). The finished surface of the sub-base should not deviate by more than +/- 10mm.

DTp type 1 coverage rate calculator:

Calculation:

(M x (T/1000)) x 2 = Approx. weight in tonnes

Key:

• Sq m (M)
• Thickness of base in mm (T)