On stony ground

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Type 1, the material used to build this path, is the commonest unbound granular material used to construct a path's sub base layer. Its full name is 'Type 1 granular sub base', and it must conform to a specification in Clause 803 of the Department of Transport Specification for Highway Works. The specification defines the maximum size of stone particles in the material (up to 37.5 mm), and the proportion of stone particles to fines, the smallest stones and dust. Fines are an important part of the material. They fill gaps between the larger stones, locking the sub base layer together, while still allowing water to drain through the material.

type 1 battleby

Type 1 granular sub base is typically made from crushed stone and can be obtained directly from commercial quarries in 16 or 20 tonne lorry loads. In Scotland, quarried Type 1 granular sub base varies depending on the geology – most quarries in central, west and south Scotland produce it from grey whinstone, a few quarries in eastern Scotland produce it from reddish whinstone, and further north it is produced from granite. It is also possible to get Type 1 made from recycled materials such as crushed concrete, made from demolition waste. As Type 1 is used to construct a path's sub base layer, which is not visible once the path is surfaced, you do not have to consider the colour of the material in terms of landscape fit.

A typical construction sequence to build a path's sub base layer with Type 1 granular sub base would be as follows:

  • Using a drag box, or with hand rakes, lay Type 1 granular sub base in a formation tray to the required depth and width to form a sub base layer with a camber or cross fall. Depending on the type of soil and the strength of the ground underneath (the sub grade), you may need to use a geotextile sheet to separate the soil from the sub base layer. Where the ground is soft, you may also need a geogrid on top of the geotextile as reinforcement.
  • With a heavy vibrating roller, compact to refusal with multiple passes, the sub base layer. The minimum number of passes needed will depend on the thickness of the sub base layer and the weight of the roller. Ride-on vibrating rollers give the best results. Where access is restricted or in remote areas you can use a walk-behind vibrating roller, or a vibrating plate, but you will need to go over the same surface area several times to compact the material sufficiently.

  • Once the sub base layer is compacted, check the level of the surface at regular intervals along the route. You need a consistent, even surface regularity, with a maximum gap of 10mm under a 3m straight edge laid along the compacted sub base surface. Any high spots or dips should be raked off or topped up with extra material and re-compacted to the correct levels.

  • Check that the compacted sub base surface is 'closed tight' with no exposed gaps or voids. If necessary, fill in any open voids with fines. The compacted surface should also be free of any roller marks before the path's surface is laid.

Proper compaction is vital in constructing a robust path – and it takes time! Compaction removes air and voids between stone particles, increasing the density and load-bearing capacity of the path's sub base layer. A properly compacted sub base layer should not show signs of loose material or roller marks – if any of these signs are visible the sub base layer has not been sufficiently compacted, and so further passes with the vibrating roller or vibrating plate will be needed.

You can find out about how many passes are needed to compact different sub base thicknesses with different weights of vibrating roller here or vibrating plate here.

The depth of sub base you need will depend on ground conditions, the type and amount of traffic you expect on the path, and the size of the granular sub base material to be used.

You can find out more about sub base depths for different ground conditions here.

The sub base is just one of the layers involved in constructing a typical path, although since it carries most of the load it is one of the most important. The next stop along the path, Is it accessible?, describes the other layers used to build a traditional Type 1 and dust path.

© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh