Materials used to build a path base layer are generally well graded granular sub base materials which help to spread the weight and forces of different path users in to formation layer below. The base layer can be built with one type of material or in combination with another type. For example, the lower half of the base layer can be built with a lower quality material such as crushed concrete (demolition waste), whilst a higher quality material like Type 1 granular sub base is used on the top half to remove all irregularities before laying the surface layer.
Well graded granular sub base materials can be:
- Natural aggregates
- Recycled aggregates
- Secondary aggregates
The Department of Transport ‘Specification of Highway Works’ (SHW) defines a number of well graded aggregates based on the composition of particle sizes contained in the commercially produced mixture. Clause 803 defines ‘Department of Transport (DTp) Type 1 granular sub base’ and Clause 804 describes ‘DTp Type 2 granular sub base’. These aggregates are more commonly referred to as ‘Type 1’ and ‘Type 2’.
This is the most common well graded granular sub base material used to construct a path’s base layer. It is a well tried and tested component of lowland paths that has been used for many years as the default base layer material. Commercial quarries supply DTp Type 1 granular sub base consisting of crushed rock graded to a specification that has a defined proportion of stone particle sizes. Well graded Type 1 contains a good mix of angular aggregate sized between 63mm and ‘fines’ (sand sized particles), however, most of the aggregate content is less than 32mm in size – the European Norm standard for Type 1 is 0/32. The solid stone particles should not exceed 63mm in size and less than 9% of fines. This ensures the material has an acceptable level of natural interlock between the angular aggregate particles and no voids once compacted. Type 1 is subject to regional variations based on 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.
Like Type 1, this is crushed rock less than 32mm in size with less than 9% ‘fines’ but with no specified grading. It usually contains finer material than Type 1, being composed of fewer angular aggregate sizes. Type 2 is not as strong as Type 1, so for that reason, it is not generally used on its own to build a base layer. It would usually be used to form the lower part of a base layer with Type 1 laid on top.
Another crushed aggregate with a lower ‘fines’ component than Type 1 or Type 2 – there should be less than 5% ‘fines’ and over half of the aggregate particles are greater than 4mm in size. This uniformly graded aggregate material is suitable for a free draining base layer for porous surfaced paths.
This granular sub base is a waste material from quarry stone processing. It is crushed material that comes straight off the rock crusher after the first crushing stage, which is not carried forward to sieving stage. It is similar to scalpings, its strength is variable, and is available in different maximum particle sizes, based on the screen sizes used on the rock crusher, which means there is no guarantee that the waste material is well graded. Providing that the rock crushed is not too soft, crusher run can be used as an alternative to Type 1 or Type 2.
This is a waste material from quarry explosions which is removed (sieved out) before the larger rock is crushed. It is often referred to as quarry waste, its strength can be variable and there is no guarantee that its well graded. It is often much cheaper to purchase than the other granular sub base materials because of quality issues. Particles range in size from 100mm – 40mm to dust. Scalpings can be used as fill material to make up the formation level where it has been excavated deeper than originally specified to deal with soft spots. It can also be laid as the lower part of a base layer with Type 1 on top. If carefully selected, scalpings may make a reasonably good base layer when used on their own, but avoid wet material with high clay content. It can cause problems.
This granular sub base material should consist of free draining, naturally occurring sands and gravels with sufficient clay content to bind the material together when compacted. It is sometimes referred to as ‘hoggin’. Such materials are usually glacial tills found mainly in glaciated areas and river valleys. ‘As dug’ material is taken straight out of the ground from small scale borrow pits alongside the route being built, and has not been crushed or graded to any specification. It can also be imported to site from one large borrow pit where onsite materials are not suitable for path construction.
‘As dug’ materials can be used to provide the bulk of a base layer, which can then be overlaid by a regulating layer of higher quality Type 1 on which the path surface can be laid. To improve the quality of the ‘as dug’ material it can be graded on site using a portable non-vibrating screener attached to the dumper or a larger mobile vibrating screener. The latter is suitable where good access is available.
‘As dug’ materials may be the only source of stone in some areas where it is impractical or prohibitively expensive to import commercially quarried aggregates. These situations could include small scale upgrades, or particularly remote locations. However, if the range and composition of particle sizes would result in a poor quality path, such as too much clay or soft rock, it is worth investigating other options before settling for ‘as dug’ material.
In recent years there has been an increase in the availability and use of recycled aggregates and re-useable by-products from industries. They tend to be locally available and the quality can vary greatly, but recycled and secondary aggregate quality standards are now available. In many cases it will be necessary to use RSA along with conventional path construction materials. A useful information resource about RSA is available onAggRegain – a complete online guide to sustainable aggregates.
Typical recycled aggregates are road planings (from road re-construction or maintenance) and crushed concrete (from demolition waste). All recycled and secondary aggregate products should be chosen carefully. Industrial by-products that do not form a specified standard aggregate mixture design must be individually assessed, because some materials are prone to leaching or just not suitable for construction.
Recycled Type 1, Type 2 can be produced from a variety of materials arising from demolition waste (crushed concrete, brick, hardcore) from buildings or other structures. However, quality can be variable so check the specification with the supplier before purchasing the material – check for a well graded size distribution. Recycled demolition wastes can make good bottoming or lower half of the base layer with Type 1 laid on top. Some path base layers have been successfully built using just recycled Type 1 where the material has been well graded to the specified quality standard.
These are crushed or milled bitmac or asphalt arisings produced in road or pavement reconstruction or maintenance work. This material has similar properties to well graded aggregate. If rolled hard in hot weather, the bitumen binder can soften and then re-bind the bitumen coated stone to form a hard surface. For this reason, planings have been mainly used for path surfacing, but they can be used to create a base layer. The planings can be either part of the base layer with higher quality granular sub base laid on top, or as a whole base layer.
The price of planings can vary considerably. Unscreened planings purchased straight from a site may be cheaper, but the quality will be variable. Well graded screened material bought from a processing centre will probably be processed to a material specification to produce a recycled Type 1 granular sub base material.
If you plan to use road planings for path construction, check where the material has come from before accepting it. It is possible that materials from deep planing of old roads may contain tar products, which generally should be treated as hazardous / special waste and not recycled. However, in some situations tar bound planings may be reprocessed in agreement with Scottish Environment Protection Agency (SEPA). It is important therefore to seek their advice before purchasing and using road planings.
This industrial waste by-product of colliery spoils and spent oil shale bings is reprocessed as a secondary aggregate and is available in some areas of Central Scotland, particularly West Lothian. Blaes from colliery spoil is either black or red. Black material contains coal dust that is combustible. Red material has been burnt, and can be re-used for other construction purposes. Blaes from spent oil shale is pinkish, and similar in nature to red burnt colliery spoil.
Blaes consists of particles of variable sizes making quality very variable. It can provide a well bound base layer material that is suitable as bottoming or the lower part of the base layer, with a higher quality granular sub base laid on top. It generally needs to be laid to greater thickness than other granular sub base materials to make a strong layer.
Care must be taken when using near to watercourses as some materials can be highly toxic. If you plan to use blaes for path construction, seek the advice of SEPA before using it, especially if the path is to be built near a watercourse.