A road can be built above almost any (in-situ) material - but if that material is particularly weak (deforms easily when a load is applied), or the truck especially heavy, then a much thicker layerworks would be required to protect the in-situ material from the wheel load of the truck. Similarly, if the material we use to build the layerworks itself were weak - a thicker series of progressively stronger layers would also be required.
In a structural design specification, three broad materials types are generally considered:
■ the in-situ (sub-grade) material on which the road is built, and if required, often the fill above in-situ;
■ the sub-base and base layers (or as one, combined in a layer of selected blasted waste rock); and
■ the wearing course layer, which is placed on top of the base layer.
What are the layer characteristics and what would make a good or poor material?
in-situ materials
These could be any of the following:
■ soils;
■ weathered overburden;
■ loose blasted hard overburden; or
■ solid hard overburden.
When planning a new road, the first task is to find out how hard or soft is the material we are going to build on.
A Dynamic Cone Penetrometer (DCP) and/or centre-line surveys and soil classification systems or materials sampling and laboratory testing can be used to establish the engineering characteristics and strength of the in-situ or sub- grade material on which the road is built.
There is not much choice in mining about what we build the road on. A road must connect two points, and often the shortest distance, or the most logical from a planning perspective, is the cheapest option. The mine block model often dictates where a road will be built, how much space it occupies, and what would be the effect on waste mining costs were a road to be planned in any location than the ‘minimum cost’ location.
If the in-situ is hard, solid overburden rock, then we usually have no problem. This is strong and does not need much protection from the wheel loads of the truck. We will still place selected fill on top of the in-situ to get the road profile and alignment correct - and critically, to allow water to drain through this layer and not the wearing course (the
selected blasted waste rock fill is blocky and as such its strength not effected by water). Similarly, for a good strength blasted overburden, it is often only necessary to shape and compact the upper 300mm of the material before placing the wearing course. If the in-situ is weathered overburden, it will be much softer, have typically higher clay content and thus require more protection - or a thicker sub-base and base layer(s) above it. Occasionally, the material is so soft that we have to remove it. This is because we want to reduce the thickness of the base, so we need a stronger in-situ on which to build. If the in-situ is soil or clay, or not trafficable (California Bearing Ratio (CBR)<2%) then this will be removed completely to a depth where stronger material is found. Also, if the material is very wet, the layer will also be removed and/or drainage installed, since without, it will make road building very expensive.
When a reasonably strong in-situ material is exposed, this can be ripped and compacted to provide an anvil for the compaction of the layers above. Without this anvil, compacting the layer above is difficult, time consuming and expensive. In both cases, the next type of material, the blasted rock base or fill replaces the material taken out. How much we use depends on the strength of the in- situ, applied wheel loads and design life of the road.
sub-base and Base layers
Using a mechanistic structural design methodology, where a good quality (non-weathered) selected blasted overburden / waste is available, this material can be used as the combined base and sub-base. It is important that the blast block chosen as a source for this layer does not contain weathered rock, clay or soil, since for this layer we need blocky, hard material, with just a little (less than 20%) fine material. The largest block size is ideally 2/3 of the design layer thickness, which is usually between 200-300mm maximum. Any larger, and it is difficult to compact these boulders and they form a high spot in the layer surrounded by a ring of soft uncompacted material. It also makes shaping the road difficult when large blocks protrude out of the layer. If such material is unavailable, then layers are constructed from selected excavated materials that offer a high strength on compaction. The choice of materials will depend on the quality, comparative cost and local availability. With these material types, stabilisation may be an option when used as a base layer.
Wearing course layer
This layer is made from a single or mix of materials. In the specifications introduced later, two recommended selection areas are given. When the wearing course parameters are determined, the result (of a single or mix of materials) should lie within these recommendations. If it does not, the specifications also give an indication of what ‘defects’ would normally occur as a result. The recommended limits for the selection of this material are
established both in terms of performance and minimised road surface degeneration (degeneration equates to increased rolling resistance).
To achieve good layer strength, the material(s) for this layer must be carefully selected. Highly weathered rock will make for too much fine material, which will give very poor results and too soft a layer. Remember, we need hard, wear- and erosion-resistant materials for the large trucks to operate on. For this layer, a strength of greater than 80% CBR (California Bearing Ratio) is needed. This value is determined from laboratory tests of the material or by DCP Probing. If the mix is not correct, it can be too fine and will be slippery and dusty, or too unbound, when it will produce loose stones, corrugations, ravelling and, in both cases, rapidly increasing rolling resistance and operational risks once the road is trafficked.