Unsealed gravel (or aggregate) road pavements are the most common type of forest road construction. They are technically referred to as unsealed flexible pavements.
The pavement layers elastically deform beneath the vehicle load to disperse the stress to a level that the subgrade soil can bear. Like planning, correct pavement design is critical for a fit-for-purpose road that allows truck traffic to move effectively in all but the worst weather conditions, be safe and have minimal long-term environmental impacts.
The pavement can account for a large percentage of the total road cost, and subsequently, there can be substantial savings in construction and maintenance costs by getting the pavement design and construction right. This becomes increasingly critical as the distance from the aggregate source increases, because transporting aggregate gets increasingly expensive as the cart distance lengthens.
Sealed or paved surfaces are seldom used in most forest applications and are not discussed in this Manual.
Subgrade strength is an important factor influencing pavement design; a formal assessment at the planning stage can lead to more cost-effective road construction. Once the subgrade strength has been identified, the pavement thickness for a given traffic loading can be calculated, based on the axle loads being applied. A thicker pavement layer distributes stress over a larger area, enabling the subgrade to withstand larger loads or a higher number of repetitive loads. This load distribution by the pavement layer is very important, because most subgrade materials will not have sufficient strength to carry a large number of heavy axle loads applied directly. A weak subgrade will require a thicker pavement to distribute the applied traffic loads, or conversely, the subgrade can be strengthened to reduce the thickness of pavement.
The objective of pavement design and construction is to produce fit-for-purpose pavement which will perform the following functions with an acceptable level of maintenance:
- Distribute the pressure from the wheels, so that the load can be supported by the road foundations (subgrade)
- Protect or shield the subgrade from water, so the strength of the underlying soils is maintained or improved
- Provide a surface that has sufficient traction, so that vehicles can climb and brake safely.
The layer of material closest to the surface needs to be sufficiently strong to withstand the direct vehicle load, while successive layers beneath the surface can be of lower strength as the vehicle load disperses. Public road pavements are generally constructed using a traditional multiple layer design, whereas most forest roads will use a single improved layer design. Both methods work adequately in terms of being fit for purpose if constructed correctly; however, the multiple layer approach tends to be more expensive to construct, both in terms of material and cost.
The function of the surface course, also called the ‘running course’, is to provide low permeability with a smooth finish that is resistant to ravelling and scouring. These attributes help seal the road surface and provide a good running surface for traffic. Running course is not often applied on forestry roads, or if it is, it tends to be on steep adverse grade sections where a good surface is required. A well-graded fine aggregate, such as AP20, is typically used. Note that AP20 refers to a designed aggregate standard (AP) where the larger particles are 20 mm in length. The AP standards have a defined particle size distribution, to ensure the particles lock together when they are compacted.
The function of the base course is to withstand the high stress concentrations immediately below the traffic load, and to distribute the stress to the lower levels of the pavement and onto the subgrade. The material should be compacted to achieve a high binding effect. The base course should have good crushing strength, and a good proportion of broken faces to distribute load and take shock load. A well-graded AP65 is suited for use as a forest road base course.
The purpose of the sub-base is to distribute the load onto the subgrade. It is usually a lower quality, and therefore a lower cost, aggregate. For example, weathered rock or river run, since its position low down in the pavement means that it is subject to less stress. The sub-base material should have good compaction and a high proportion of large aggregate material to help spread the load across the subgrade.
Pavement design can be challenging. Design requirements will change among roads within and between forests as subgrade properties and vehicle loading change. Company standards on pavement design need to reflect this variability. Ideally, the pavement for each section of new road should be individually designed. The use of a standard design for all roads within a forest or estate runs the risk of either over or under engineering pavements. This will result in roads that are either excessively expensive to construct or fail and require extensive maintenance to repair.
The key factors in pavement design are:
- Traffic loading (maximum axle loads, tyre pressure and number of loads)
- Subgrade soil strength
- Pavement layer material properties
- Pavement thickness.
Using the fit-for-purpose design approach, it is highly likely that some segments might not meet expectations right after construction, from factors such as lower than expected subgrade strength or the improvement layer failing to compact adequately. A pragmatic method at time of construction is to see if the road deforms under a loaded metal truck and to add aggregate (or compaction effort) in those locations.