Geometric road design involves the optimisation of the horizontal and vertical alignment of the proposed road by adjusting the position of road design points to find the best fit to the terrain for the given design parameters. While this process can be carried out manually, it is very time consuming, and road design software is now used to undertake this process and also assist in the design of curves, curve widening, ditches, and culverts.
The process of geometric road design for forests is more efficiently achieved using a specialised road design software package. Some examples are ROADENG, Civil3D, GEOCOMP, LUMBERJACK, and SDR Mapping and Design. These programs are fairly easily used by people who understand the basic principles of terrain modelling.
It is important to realise that computer road design software packages are only a tool to assist the designer. The design packages do not automatically produce a design which will be optimum. The final design is only as good as the user’s expertise in geometric road design. A good knowledge is required of how combinations of grade and curve radii restrict truck maneuverability.
Appropriate road cross sections need to be determined for use in the road design. The road cross section (or template) may be different for various sections of the road. It is essential that the site soil, geology, topography and other features are understood. For example, the cut slope angle may change depending on the soil type in the hillside, or additional widening may be required on corners to accommodate the inside tracking of trailing axles.
A major cause of road deterioration is water entering the pavement. This is commonly due to the road surface having insufficient side-slope (crossfall) or the ditches being too shallow to keep the subgrade soils free of saturation. On secondary or spur roads, a crossfall of 4-6% is needed for water to runoff and not to sit on the pavement, whereas on a higher standard arterial road a crossfall of 2-3% might be more appropriate to improve safety for higher speed traffic. The shedding of water will help prevent potholes from developing. For a two-lane road, the crossfall should always meet at the centre of the road, producing a crown. For a single-lane road, crossfall may be to both sides, or for maintenance purposes it may be easier to have the fall to one side only.
Avoid excessive crossfall and corner superelevation on steep gradients, because of the risk of roll-over and the tendency to remove weight from driven wheels on the high side of the road. The actual crossfall selected may need to be a compromise between that required for drainage, and that suitable for heavy vehicles.
For low speed forestry roads, superelevation is not normally used, but in some cases of climbing adverse loaded corners, negative super elevation is used to improve truck gradeability by providing an improved distribution of weight to the drive axles.
Road template with normal crossfallRoad template for left hand turn (following the survey direction)Road template for right hand turn (following the survey direction)
- Cut slope: l: c: The slope specified is less than or the same as the maximum stable slope which can be maintained. This is very dependent on soil type and local conditions. A good indicator is existing cut slopes on other well-established roads in similar soil types
- Fill slope: 1: f: The slope at which a fill can be constructed with reasonable confidence of stability. This is typically 1: 2 but can be as steep as 1: 1.5 in stable soils
- Ditch: Depth (D) is dependent on local conditions but is typically 0.4 m. Slope (1: d) is typically about 1:2
- Formation width: This is the full construction width from edge of ditch to edge of ditch, including any shoulder that may be provided beside the aggregate surfaced carriageway width
- Super elevation, S %: This is the slope of the whole road surface provided at curves, to oppose the sideways force on vehicles when cornering. For low speed forestry roads, super elevation is not usually used
- Cross-fall, C %: This is the cross slope applied on the road to assist with shedding water and should be applied to all roads.