The design process outlined previously assumes a strong aggregate material being used for the improvement layers. In many forestry applications, the roads consist of just a single pavement layer, known as an improved layer. Aggregate is spread and compacted and, as long as the aggregate is well graded with an appropriate proportion of fines, can develop a good pavement that concurrently works as a base course and surface course. On steep adverse sections, where traction is critical, an additional layer of crushed running course may be spread.
Material properties and characteristics are the most important factors when determining which material to use for a pavement. Other factors which must also be considered are cost, availability and environmental factors. Often the material is selected as a compromise between achieving the desirable properties and acceptable overall costs. Ideally, the best aggregate is well graded, has numerous broken faces, is hard, and has a low water absorption rate.
Aggregate materialThe material property which has a significant influence on pavement strength, compaction and longevity is the particle size distribution. Particle size distribution is described in terms of standard particle size classifications. For example, gravel, sand, silt and clay or alternatively, the fractions which pass various sieve sizes. A soil-aggregate consisting of a well-graded gravel sand mixture with a 10-15% proportion of fines (particles smaller than 0.06 mm diameter) will usually be the most desirable material. The large gravel provides structural strength, the finer gravels and sands fill the voids between the larger particles to provide high density and to maximise frictional interfaces, while the fines combine with water to bind the aggregate together. Note that it is desirable for the fines to have some plasticity (Plastic Index between 3 and 10) to improve binding capacity.
Aggregate mix properties and workability
The table above shows the main characteristics and workability properties of three materials: A good particle size distribution material, a course material lacking in fines, and a material with excess fines. The coarse material lacking in fines will be porous and unstable, which makes it unsuitable for a base course or wearing course, but ideal as a sub-base material.
Aggregate material gains some of its strength from surface friction between the particles. An increase in the number of broken faces will increase the surface contact area between the aggregate and increase the load bearing capability of the material.
Soft material, which is easily broken, will quickly deteriorate under the constant motion of the traffic loadings. This results in degradation of the pavement material, which lowers the bearing capacity.
The environment can affect both the subgrade and pavement materials. Changes in moisture and temperature can influence the properties of a material, and, therefore, the overall performance and serviceability of a pavement structure. Pavement design must take into account the moisture and temperature, both averages and ranges, which the pavement and subgrade will encounter during service. The significance of environmental effects will depend on the materials selected for the pavement, and the period during which the heaviest loads will use the pavement.
Aggregate material, which absorbs water from the environment, will generate problems with shrinkage and swelling, as well as freezing and thawing during winter periods. Freezing causes not only a build-up of ice on the running surface of the pavement, but also expansion within the aggregate material. This causes material to fracture and break down into fine material with a decreased load bearing capacity.
Pavement material properties and characteristics
6.4.1 Aggregate selection
Several factors determine aggregate selection. These include:
- Material availability
- Cost of transportation
- Structural properties
- Royalty, purchase or extraction costs.
Selection and use of local pavement materials is a key factor in minimising the cost of forest roads. Often in forestry, the cost of transporting high quality aggregate may be prohibitive. It may be better to use a local, poorer quality material which requires less transportation. However, additional quantities of the poorer quality material will be needed to improve the design performance. Be careful, as substandard quality material requires careful consideration to determine pavement depth, load bearing and load distribution properties. Additional pavement depth of stabilised material may be required. An option is to use the higher cost material for the upper pavement section if required.
In some regions of New Zealand, there are multiple options for sourcing material. For example, in parts of Hawke’s Bay there is a choice between red metal (old river cobbles), weathered greywacke and limestone all within close proximity. Further north in the Gisborne region, many forests do not have an in-forest metal source, and metal is often carted long distances.
The following are sources of potentially suitable local pavement materials:
- Granular material exposed in cuttings during the formation of the road. This may consist of weathered rock that can be easily excavated or alluvial gravels
- Borrow pits established near the road formation that contain suitable granular material. Where deposits of suitable material are found, the establishment of borrow pits can significantly reduce the cost of pavement surfacing
- Extraction of gravels from rivers. This will usually require resource consents, and may have specific conditions. However, in some circumstances, removal of gravel from an aggrading river channel is beneficial to river management, and may provide a useful local source of pavement material
- Establishment of a forest quarry. This may require geological investigation and application for consents. However, a quarry producing suitable pavement material within a forest can significantly reduce the cost of forest roading
- Blending of materials from off-site with on-site materials. A granular material that is available on-site may be lacking in a particular particle size and could be improved by blending this with a complimentary material from off-site. For example, crushed bluestone aggregate imported from a quarry can be blended 50/50 with lime rock from local deposits to produce a road surfacing aggregate that will bind well. It will also have much better durability and strength than the lime rock would have if used without blending
- On-site crushing of suitable material. If suitable source material is available in a borrow pit, quarry or from extracted river gravels, the suitability of this can be significantly improved by arranging an on-site crushing operation using a mobile crushing plant
- In-situ crushing of material. A simple towed steel grid roller can be effective at crushing larger aggregate particles that have been placed on a road pavement sub-base layer from a weathered rock quarry face. Mobile in-situ crushing equipment can crush and blend aggregate that has been placed on the road pavement. This enables rock or gravel from local sources to be spread on the road pavement and then processed to a suitable particle size distribution for compaction into a pavement layer
- Other creative options that could include crushed concrete, but the reinforcing steel needs to be removed as it is a hazard.