5.6 Road formation

Balancing cuts and fills help minimise the overall earthworks required to construct a forest roadLandings can generate large volumes of earthworks. Benching is essential in anything other than the easiest country to help stabilise the fillThis section covers common road formation practices. It begins with clearing and stripping organic matter and vegetation, then discusses bulk earthwork construction techniques for road construction. These are side cast, cut and bench and end-haul construction. Some road segments will require solely fill, others cut, and the majority balanced cut and fill works. There is a continuum between a road fully constructed on fill material to one that is entirely cut out of the hillside (benched) where all material has been carted off-site (end-hauled). For many forestry roading applications, the formation methods fall somewhere between these extremes. Most earthworks are about how to balance cut and fill material in the best cost effective and environmental way. For example, on steeper slopes, a combination of cut and benched fill and end-haul should be considered. This can provide a practical and cost-effective solution.

Skill and expertise in identifying what approach to take in a given situation is essential. The approach varies depending on an array of on-site factors and social and environmental considerations. For example, topography, rainfall, soil, geology and the risk associated with these attributes help determine the approach.

One of the big issues with construction is what to do with the excavated material so that it is stable. On flat or rolling country this may not be a problem, as excavated material can be side cast and the road carriageway built partially on the fill. In moderate hill country, side casting is a risk for slope failure. The excavated material needs to be stable, and to help ensure this, a bench should be constructed below the road to contain it. On extremely steep or unstable areas, benching may become unacceptable, so the material is excavated and trucked away (end-hauled).

Another risk mitigation measure is to decommission non-essential roads after harvest where there is the possibility of off-site mobilisation of earthwork material. The fill material can be pulled back by excavator onto the road carriageway. The aim of the road rehabilitation is to reduce the likelihood of the fill slope failing and leading to sedimentation. Compaction, which is a critical construction step to stabilise fills, is discussed in detail in Chapter 6.

The NES-PF regulations have specific requirements for most aspects of earthworks. These are located throughout the regulations so make sure these are understood. Earthworks are an NES-PF activity, but general and ancillary rules also apply to aspects of road and landing construction. The NES-PF has earthworks threshold tests. This could include the erosion susceptibility classification (ESC), measurable limits like volume or area of earthworks, or secondary thresholds like sediment, fish spawning or setbacks in the worksite. Activities where you cannot meet the permitted rules will need consents. It is a fundamental aspect of forest engineering to understand the rules you need to work with to get the job done well.

Document and take photos of the location and construction of earthworks as they progress. For example, preparatory work like benching that is important to the integrity of the job but is hidden in the final formation.

5.6.1 Clearing and stripping

An excavator and dozer clearing and strippingStumps left resting on a hill slope amongst standing trees are a significant hazard to tree fallers and breaker outsClearing and stripping is the process of removing woody debris, organic soil and other unsuitable material – like surface vegetation of grass and scrub – before forming the road or landing. The reason why this material is removed is that organic material – such as tree stumps and roots, slash and branches – and topsoil are unable to be compacted. It decays over time and can be a point of water entry, resulting in weak and unstable fills that may collapse. Borrow pits and overburden dumps should also be cleared and stripped of vegetation and organic material.

Choose the right machinery size and combination for the terrain, stump size and soil type. Excavator/dozer combinations can work best. Place stumps and debris in a stable location where they will not interfere or cause safety issues for other forestry operations, or have adverse environmental effects. For example, put on flat stable ground or a secure bench or beyond the toe of fills. Keep stripped material away from water bodies or any restricted areas. Where there is no suitable placement option, cart to a safe disposal site.

Ensure the roadline salvage strip is wide enough and safe to construct the road or landing, and unencumbered by the surrounding trees. Earthworks should not commence if insufficient trees have been cleared, and there is a risk that the cut and fill batters will encroach into standing trees. If not wide enough remove or get more trees removed.

The NES-PF has rules around the maximum amount of wood and organic matter in the fill. A resource consent is required if there is more than 5% by volume.

Stumps have been placed on a bench, which is a safe location and one that will not cause environmental issues

5.6.2 Bulk earthworks – cut and side cast construction

Cut and fill side cast construction

Cut and side cast constructionThe cut and side cast method is used for most forest road construction in flat to rolling terrain. Some, or all, of the excavated material is placed as fill onto the down-hill slope. It is important to assess structural requirements of the fill during the planning phase to determine if side cast is suitable. Uncompacted fill may not support heavy logging traffic and collapse under loading.

There can be increased environmental risk if this method is not used in the right location. Loose uncompacted fill is prone to erosion, and can create sediment issues if the road and landing water control is not well managed. Cut and side cast should only be used where there is no risk of sediment being deposited in a water body. For example, do not use above a perennial river regardless of slope. Stabilise or vegetate side cast material immediately after construction to mitigate ongoing erosion and sedimentation risks.

5.6.3 Bulk earthworks – cut and bench fill construction

Bench cut to contain down slope fill material

Fill contained on visible benchCut and bench fill construction is a common method for constructing roads on moderate to steep hill country up to about 35 degrees, where fill side cast onto the natural slope cannot be retained in a stable state.

On moderate to steep slopes, a level bench (or multiple benches) should be constructed to provide the base for structural fill. The bench(es) need to be designed to balance cut and fill volumes, and have an appropriate batter slope; the finished road formation must meet width and safety specifications. At its simplest, this involves constructing a pilot roadway or track, on grade, but below the formation height, to create the bench. This keys in the compacted fill to the slope and provides a level surface on which layers of fill can be compacted.

Benches should be constructed wide enough for the safe and effective operation of compaction equipment. A typical cross section confirming cut and fill batter heights and slopes should be developed during the planning and design phase. Where it is anticipated that subsurface water will be encountered within the bench formations, special provision should be made for the installation of subsoil drainage.

Not all slopes are suitable for cut and bench construction, so the initial road location assessment and design should assess whether the slope is suitable for this type of construction. Cut and bench construction is only effective where the fill batter can safely be contained or hold at a slope steeper than the natural ground slope. Therefore, it is not suitable on slopes that are too steep for side cast and/or where side cast formation is required to support logging traffic so needs compaction.

With cut to fill construction, the fill will typically be part of the trafficked road formation (carriageway) and therefore needs to be structurally competent. It is good practice to compact, not track roll road fill. The fill should be spread and compacted in layers of uniform quality and thickness, parallel to the camber and grade for the full width of the cross section. The thickness of each layer should be limited to ensure that the specified compaction is achieved for the full depth of each layer. Refer to Chapter 6 for more information on this requirement.

On steep slopes, fill batters formed using cut and bench construction can produce significant areas of exposed soil. Bare earth should be stabilised or vegetated immediately after construction. Surface water controls need be established above the slope to direct storm water runoff away from the fill to prevent scour and rill erosion, and fill saturation and slumping. Downstream sediment controls should be installed to contain sediment generated from the fill batter and prevent discharge to a water body.

The recommended fill slope for most soils is 1:5 H to 1.0 V (33 degrees). Specialist advice should be sought if constructing fill slopes greater than this, on steep slopes.

Examples of poor practice. Roads above rivers or waterways need careful planning and operational management

5.6.4 Bulk earthworks – end-hauling (full bench) construction

End-haul (full bench) construction to eliminate fillFull bench or construction with no fill is necessary where slopes are too steep to contain fill or where, for example, there are unacceptable consequences of a fill failure. This means the fill needs to be end-hauled to a safe containment area. The road is literally constructed on solid in-situ material. On slopes greater than 35 degrees, end-hauling is a NES-PF requirement. Also, it is not possible to compact side cast material to an appropriate fill batter slope, whether benched or not. If the fill slope is either too steep for the material involved, or if it cannot be compacted, the integrity of the road is compromised. As well as being unstable, uncompacted fill slopes have insufficient strength to support vehicle wheel loads. There can be substantial adverse environmental impacts in this situation.

End-hauling involves removal of the entire excavated material to a disposal area. End-haul road construction typically cost several times the cost of side cast construction. To minimise the cost of end-haul construction, it is important to optimise the type and quantity of equipment as well as minimise the cartage distance to dump sites.

Full bench construction generates large volumes of material that needs to be disposed of. It is essential that dump sites are carefully located and treated as fill zones. Ideal locations for dump sites include shallow basins and areas of flat to gentle contour that are away from water bodies, and for operational efficiency, as close as possible to the work site. Avoid slip zones and visible earthflows, and areas above sensitive receiving environments. The fill area should be cleared and stripped of vegetation and logging debris prior to the placement of fill. Erosion and sediment controls also need to be established to prevent/contain sedimentation. The long-term stability of the fill must be considered and, where necessary, fill should be placed and compacted in lifts to prevent slumping. The fill should be contoured and revegetated after construction.

In some circumstances, end-haul construction may not be appropriate. For example, in unstable rock, especially soft sedimentary with unfavourable bedding planes, or deep soft clay soils, such as lacustrine or marine soils where it may induce rotational failures. Seek specialist advice if needed.

The layout of haul roads can be an oversight. However, it is important that they are designed, constructed and maintained to support the safe passage of dump trucks for the duration of the construction operation.

End-haul construction typically requires an excavator to form the road, and trucks to cart the excavated material to the dump site, or where fill is necessary for other construction areas. Each bucket of excavated material is loaded directly onto a truck for cartage and placement in the fill designated area.

End-haul road constructionTypical end-haul road construction with passing bays

End-haul road construction methodsThere are two methods of end-haul road construction – the ‘one pass’ and the ‘two pass’. With the one-pass method, an excavator clears the trees and stumps, and forms the road in one pass. Every bucket load of material is loaded directly onto a truck for transportation to a disposal site. This method is suited to the construction of narrow one-lane roads in very steep country. An additional benefit is less risk of sediment entering rivers than with other methods.

The two-pass method is ideal for wider formations or two-lane roads. A small pilot track is initially constructed at approximately the 3/4 mark. The excavator then constructs the final road, loading both the side cast and excavated material into trucks, while backing along the pilot track. This method is cheaper than the one-pass method because the swing time between excavation and loading is reduced. This method is only suitable if the excavated material will remain stable on the side of the hill while awaiting disposal.

The steps involved when end-haul constructing are:

End-haul construction: One-pass methodOne-pass method

• Remove any trees, stumps or other vegetation, and either take them from the site, or place them over the hillside
• Face the cut slope (in the direction of the road) with the excavator, and remove the material from the slope, swinging 180 degrees to load it into trucks
• Form the batter slope at the correct angle
• Shape the road camber and ditch to allow effective drainage.

End-haul construction: Two-pass methodTwo-pass method

• Place the pilot track at the 3/4 mark by stripping any trees or stumps, vegetation, top soil and side cast
• Facing back down the road, the excavator digs beneath itself backing along the pilot track. In this case, the excavator only swings 90 degrees to load the trucks
• Form the batter slope at the correct angle
• Shape the road camber and water table to allow effective drainage.

Reducing end-haul costs

End-hauling is expensive, and costs can rapidly escalate if not carefully planned and managed. Minimising the amount of earthworks, using the right equipment, providing passing bays, and reducing the distance to the dump site are ways of significantly reducing cost. The simplest method is to reduce the amount of material excavated. Excavator production can be considerably slowed by hard rock. Disposal site distance has the largest impact on costs, as cost increases directly with an increase in disposal site distance. The required number of trucks to keep up with the excavator will also increase, since it takes longer for trucks to return to the loading position.

Ideally dump sites should be flat – for example, abandoned landings or roads. More than one dump site may be required, and it is important to plan their location to avoid delays during construction. Installing passing bays at regular intervals along a one-lane road construction will increase production, since the excavator has less time to wait for the next truck to load. Usually bays can be constructed within naturally occurring areas where the terrain is flatter. Where no flat areas exist, a passing bay should still be constructed so that maximum turnout spacing does not exceed 300 m. Consideration could also be given to using specialised 6-wheel drive articulated dump trucks designed specifically for moving large quantities of earth in rough terrain. They typically have a capacity of 15 m³, and their all-wheel drive and large tyres allow them to travel quickly. They can also drive on soft and wet ground without the need for aggregate, which can be applied once the road formation has been completed. The effect on end-haul cost using other types of machinery can also be evaluated.