How deep should your path base be?

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It is unnecessary to carry out precise engineering calculations to determine the depth of a path sub base layer, but it is important that we estimate the strength of the underlying soil layer where the path sub base will be built.

Estimating the strength of the soil layer enables you to calculate the correct thickness of sub base required to transfer user loads from the path surface to the ground. It can also provide guidance as to whether, or not, to install a geotextile sheet and/or geogrid between the soil and sub base layers. There are two 'rule of thumb' methods that are perfectly adequate for estimating the strength of the soil layer, and determining the depth of a path sub base layer.

To estimate the strength of the soil layer, a simple field heel test, which involves digging your heel into the soil, can be used. For an illustrated, step-by-step guide to how to perform the field heel test, click here: icon The Field Heel Test.

field_heel_test

The 'rule of thumb' method for determining the depth of a path sub base layer is that it should always be at least twice the thickness of the largest stone particle size. This ensures that no 'rocking points' are created within a compacted sub base layer;meaning, no large lumps of stone will be in contact with both upper and lower surfaces - they will be 'cushioned' by finer material. This ensures that the sub base layer acts to spread the loads it carries, but also ensures uniformity. If the maximum stone particle size is 'X', then the minimum layer thickness will be '2X', see image below.

2x_diagram

For example, Type 1 granular sub base, the most commonly used material for a path sub base has a specified maximum stone particle size of 40mm. Therefore, the minimum layer thickness would be 40mm x 2 = 80mm. For Crusher Run, an alternative granular sub base material to Type 1, with a specified maximum stone particle size of 50mm - the minimum layer thickness would be 50mm × 2 = 100mm.

For more in-depth information about the path's base layer, click here.

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