Planning considerations and treatment options for soil rehabilitation¹

Soil rehabilitation treatments are expensive. Whenever the construction of temporary access is being considered, every attempt should be made to limit the amount of soil disturbance and the area that will require rehabilitation. In addition, consideration should be given to all future access requirements to avoid rehabilitating areas that will be required for access before the next harvest.

Rehabilitation of landings

Planning for landing rehabilitation

• Plan for and conserve topsoil during construction and replace it during rehabilitation. For large landings and unplanned rehabilitation, consider rehabilitating some portion of the landing in cases where there is insufficient topsoil to spread over the entire landing area, or where some access is still needed.
• Minimize the area occupied by the landing and the size of the running surface to reduce the amount of tillage needed.
• Use landing debris piles to help provide soil amendments (see "Soil amendments, fertilizers and mulches"). For example, spread the ash from burned debris piles over the landing (ash is rich in plant nutrients), along with stockpiled topsoil, and consider using fine organic debris (unburned) as a source of mulch or soil organic matter.
• For sites with cold soils, use local experience to determine the appropriate amount and type of organic matter to incorporate into the surface soil and deposit on the surface. The objective should be to restore the soil profile to conditions similar to those of the undisturbed forest floor and topsoil.
• When planning rehabilitation, make allowance for the effects of other uses of landings such as vehicle access, wildlife, cattle grazing and sheep corrals.

Drainage control

• Evaluate the need for drainage control on landings where converging trails increase surface water flow onto the landing area.
• Leave at least two permanent drainage outlets in case one outlet fails.

Tillage

• Expect to decompact almost all landings. Areas subjected to repeated vehicle traffic are usually more severely compacted than areas used primarily for decking logs.
• Although landings often have relatively uniform conditions over the disturbed area, variable exposure of subsoils occurs even on gentle terrain, as a result of leveling that occurs during construction. Identify changes in depth of compaction or depth to unfavorable subsoils and modify tillage techniques accordingly.

Revegetation

• Do not rely on natural revegetation from adjacent undisturbed areas. Seeding or planting will likely be required for erosion control and rehabilitation of soil productivity.
• Establish suitable conifers or commercial hardwoods.
• If two or three landings cover an area large enough to be considered a separate standards unit, consider ordering special stock and using special treatments within a prescription.
• For landings with good access, consider the broader range of equipment options available for soil rehabilitation and revegetation than are available for many other disturbed areas. For example, wet broadcast seeding and truck-mounted hydroseeders can be used.

Rehabilitation of roadsides

Planning for roadside rehabilitation

• Plan for and use proper topsoil handling during construction.
• During planning and construction, follow these basic principles of erosion and sediment control (Goldman et al. 1986):
  • fit development to the terrain
  • time construction to minimize duration of soil exposure
  • retain existing vegetation wherever possible
  • divert runoff away from denuded areas
  • minimize length and steepness of slopes
  • keep runoff velocities low
  • prepare drainageways and outlets to handle concentrated or increased runoff
  • trap sediment on site
  • revegetate denuded areas as soon as possible
  • inspect and maintain control structures.

Drainage and erosion control

• Maintain natural drainage patterns through proper placement, frequency and size of culverts.
• Construct and maintain adequate ditches (i.e., minimum 1.5% gradient, deep, flat bottom).
• Keep ditches and culverts clean.
• Keep water from accumulating in ditches, running onto road surfaces or draining into fills.
• Avoid concentration of water, especially in areas without existing channels.
• Inspect roads during wet weather to catch problem areas.
• Outslope the roads where ditches cannot be maintained or where concentration of drainage waters could destabilize slopes some distance below the road.
• Where necessary, stabilize waterways and discharge points with vegetation cover, riprap or flumes.
• Where sediment is a problem, control it temporarily by digging settling basins and placing straw bales, mulch blankets or geotextiles in ditches.

• pull culverts
• install cross-ditches and waterbars
• pull back unstable fill
• outslope roads
• improve ditchlines.

Restoring soil productivity

• remove berms
• round-off the top of fill slopes
• remove cut slope overhangs
• reduce slope angles
• bench long cut slopes (Figure 1, #5) unless the cut is in non-cohesive materials that will continually ravel
• enhance site productivity by conserving and respreading topsoil. You can also use end-haul materials or other suitable waste as a soil amendment for rubbly sidecast areas to enhance productivity.
• where possible, scatter slash on top of fill slopes to inhibit surface erosion.

Vegetation

• Use grass and legume seeding to control surface erosion, improve visual quality and enhance soil productivity. Roadside seeding may be done by either dry seeding or hydroseeding. Use the method that is most suitable for the soil materials, slope angle and local climate.
• Establish suitable conifer or hardwood species on fill slopes that can support tree growth.
• Manage plantings along mainline rights-of-way to maintain visibility. This requires either pruning and thinning coniferous plantations, planting low shrubs or seeding only grasses and legumes along critical sections.

Rehabilitation of roads

Planning for road rehabilitation

• Plan for and use proper topsoil handling procedures during construction.
• Do not use ballast for roads to be rehabilitated, or mineral soils for top-dressing and contouring of rehabilitated roads, when the rock ballast or mineral soil has the potential to generate acid-rock drainage.
• Schedule treatments to avoid traveling over treated (such as decompacted) areas.
• Where conditions change over the length of the road, be prepared to modify techniques.
• Build erosion control structures into sections with long, continuous gradients.
• On sites where organic matter additions are essential, consider mulching if the forest floor cannot be recovered.

Drainage and erosion control

• Restore drainage channels for all identifiable watercourses.
• Pull back unstable fill and outslope road surfaces on slopes where concentrations of water could lead to instability.
• On gentle terrain, cross-ditch to prevent road surface erosion.
• Seed with grasses and legumes, or use other measures to prevent surface erosion where gradients are long and continuous.

Tillage

• Till to alleviate compaction. Depth of decompaction depends on site features and depth of disturbance.
• Use measures to improve soil structure.

• Consider that tillage is difficult in extremely compacted areas, where it can result in excessively large clods or pulverized soil.

Revegetation

• Because of the linear nature of roads, carry out seeding and planting concurrently with tillage treatments or in a way that does not recompact soils.
• Establish suitable conifers or commercial hardwoods.
• Consider planting shrubs and hardwood or conifer tree mixes where erosion is not a problem, or after other measures have been implemented to control erosion.
• Evaluate the need for measures to protect planted trees from damage caused by wildlife, cattle or other users of the road.

Excavated and bladed trails

Planning for rehabilitation of excavated and bladed trails

• Use excavators to build trails.
• Where the forest floor is sufficiently thick, remove it separately from the topsoil and pile in mounds.
• On trails made largely of snow, carry out rehabilitation concurrent with the completion of harvesting operations.
• Schedule treatments to avoid traveling over treated (e.g., decompacted) areas.
• Where conditions change over the length of the trail, be prepared to modify techniques.

Drainage and erosion control

• Control drainage and erosion on excavated trails to reduce the likelihood of landslides and stream sedimentation.
• Restore drainage channels for all identifiable watercourses.
• For contour-built trails on steep slopes, pull back unstable fill, outslope trail surfaces to avoid collecting water, and decompact the running surface to a greater depth on the outer portion of the trail to avoid creating a subsurface water trap next to the cut.
• If there is a time lag between the end of a trail's use and its rehabilitation, install waterbars that cut through the running surface at specified intervals. Ensure that waterbars do not expose erodible subsoils.
• Pull back material from all gully crossings.

Restoring soil productivity

• Carry out decompaction with an excavator at the same time as sidecast is pulled back.
• Restore slopes to natural contours.
• Remove material used to construct corduroyed trails (puncheon).
• When topsoil has been piled on snow and respreading is planned before snow has melted, minimize the mixing of the snow with the topsoil.
• Where the forest floor has not been stockpiled separately, salvage the forest floor and topsoil as one layer; do not mix it with unfavorable subsoils.
• Minimize the mixing of woody debris with recovered soil.

Mulching

• Use slash and woody debris gleaned from areas adjacent to trails to cover the recontoured surface.
• Avoid stripping areas adjacent to trails of organic material.
• Avoid turning reclaimed trails into corridors of slash.

Revegetation

• Revegetate exposed mineral soil to prevent erosion and reduce visual impacts.
• Where the risk of erosion is low, seeding may not be required because of natural ingress of vegetation from adjacent areas.

• Establish suitable conifers on rehabilitated trails.

Compacted logging trails

Planning for rehabilitation of compacted logging trails

• When decompacting logging trails, do not mix subsoils and topsoils. Use a winged subsoiler with the wings set at an angle that achieves adequate shatter but does not cause unfavorable subsoils to be brought to the surface and mixed with productive topsoils and forest floor.
• Where conditions change over the length of the trail, be prepared to modify techniques.
• Schedule treatments to avoid traveling over treated (e.g., decompacted) areas.

Drainage and erosion control

• Consider grass and legume seeding on long sloping trails, such as those coming down fall lines.
• Consider using drainage to dry out areas for mechanical treatment, or excavator mounding to create raised planting spots where drainage is not possible.

Tillage

• Limit treatments to severely affected areas.
• Where possible, decompact using equipment (such as a winged subsoiler) that minimizes the inversion of soil profiles and the destruction of native vegetation.
• Assess the target depth for decompaction after considering the depth of disturbance, natural rooting depth, and the presence of root-restricting layers or unfavorable subsoils.
• Assess whether tillage is likely to cause damage to residual trees.

Mulching

• Mulching alone is a useful, low-risk treatment for areas like trails that are only moderately damaged (see "Mulches" in the section on "Soil amendments, fertilizers and mulches").
• If mulching is anticipated, look for good local sources of material such as hay or fine logging debris.

Revegetation

• Be sure that revegetation is needed. Many types of vegetation, including some grasses, are able to recover from roots after the tops have been scraped off.
• Consider transplanting shrubs from adjacent areas for extreme situations.
• Avoid using equipment for seeding that might cause further degradation.
• Establish suitable conifers or hardwoods.

Rehabilitation of ruts and areas of dispersed disturbance

• Ruts result from inappropriate use of machinery in the wetter portions of blocks, where options are limited for rehabilitation. Plan to spot treat them with low-ground pressure equipment after the soil has frozen or dried out.
• Pay careful attention to soil moisture conditions, and ensure that equipment used for rehabilitation does not cause additional damage to the soils.

Drainage control

• Where long ruts on slopes may channel water, restore natural drainage patterns.
• Loosen soil in the bottom of ruts to promote drainage.

Restoring soil productivity

• Use small excavators or work by hand to loosen the soil in the bottom of ruts, then fill them with soil from berms to provide a continuous soil layer for root exploration. Ensure that unfavorable subsoils are not exposed by this process. If the berms do not provide enough soil to fill the ruts, place woody debris in the bottom of the ruts before covering with soil.
• Create mounds for planting in very wet areas.
• Where other options are limited, try fertilizing at high rates (up to 100 kg N/ha) to encourage plant growth and restore soil structure.

Revegetation

• Use species that are tolerant of saturated conditions to rehabilitate raised portions of the area. Forest floor development will eventually improve rooting conditions in the bottom of ruts.

Rehabilitation of severely burned areas

Broadcast burns

• Exposed mineral soil and ash deposits are highly erodible. Act quickly to control erosion on severely burned slopes.
• Burned areas may become nutrient deficient (especially in nitrogen and sulfur) a few years after the burn. Fertilize if established vegetation shows signs of nutrient deficiency.
• Revegetate as soon as possible after the disturbance. Use grass and legume seeding to control surface erosion. To establish conifers in ash deposits, dig planting holes deep enough to reach the undamaged soil below.
• Establish nitrogen-fixing species to restore soil N status.
• Substitute burn-adapted tree species (e.g., substitute lodgepole pine for white spruce) during plantation establishment.

Slash accumulation burns

• Remove any logs or debris that obstruct natural drainage channels.
• Install ditches where appropriate to prevent ponding.
• Either till the ash layer into the underlying mineral soil, or dig individual planting holes through the ash.
• Revegetate as soon as possible. Use grasses and legumes to control surface erosion.
• Establish suitable conifers or commercial hardwoods.

Rehabilitation of gravel and rock pits

Planning for rehabilitation of gravel and rock pits

• For anticipated rehabilitation, plan for and conserve topsoil and overburden, recontour slopes and decompact working surfaces.
• Consider rehabilitating forest productivity only on those portions where the slopes are gentle. Practise erosion control on the rest.
• Identify additional sources of surface soil and organic matter.
• Rehabilitation and contouring should be designed to ensure that natural drainage patterns are restored, or appropriately re-routed, to prevent water from entering nearby streams directly from the pit.
• Consider and manage for public safety in the final rehabilitated pit design, and include measures such as large rock placement or water bars to deter public access to former pit areas. Pay special consideration to public access considerations in community watersheds.

Drainage and erosion control

• Where feasible, re-contour walls to slopes that are less than the angle of repose. Where practicable, bench or terrace long slopes.
• Install cross-ditches on slopes above and below the pit.
• Ditch or reslope the rehabilitated pit to prevent ponding.
• If necessary, install sediment basins to control surface erosion.
• Where feasible, rehabilitate the pit floor to ensure the base is above groundwater and slopes into the pit to prevent outflow of silt-laden drainage.

Soil productivity

• Decompact compacted areas using an excavator or conventional ripper.
• Consider using finer-textured soil materials or suitable waste from other sources (e.g., end hauls) to cap gravelly materials.
• Topsoil and overburden conservation and replacement are essential. Where limited amounts of topsoil are available, use it on gentle slopes where the chances of improving soil productivity are higher.
• Spread slash over the area if possible, but ensure the slash does not inhibit planting.

Revegetation

• Use grass and legume seeding with fertilization to control erosion. Wet broadcast seeding and truck-mounted hydroseeders can be used.
• Use dryland shrubs such as wolf willow and creeping juniper to establish in areas where soil moisture deficits are expected.
• Establish suitable conifer or hardwood species, especially early successional species tolerant of adverse conditions.

Rehabilitation of landslides

Conditions on landslides may be hazardous to other resources, to people and property in the vicinity and to those who will do the rehabilitation work. Seek advice from experienced terrain and soil specialists and/or engineers to develop rehabilitation plans for major landslides or where issues of public safety arise.

Figure 2 shows a typical landslide profile for an open-slope debris avalanche.

Planning for landslide rehabilitation

• Objectives: Set clear goals; evaluate each landslide individually.

  The primary objective of all landslide rehabilitation is to control further surface erosion and soil loss. Depending on site-specific concerns, one or more of the following may also be a major objective:

  • preventing or controlling sediment delivery to streams
  • restoring conifer production where sufficient mineral soil remains
  • reducing visual impact in visually sensitive areas.

• Timing: Do not delay treatment.
  • Most landslides occur during high rainfall events or during snow melt. Seeding should occur within six months.
  • Seed before fine soil particles get washed away and "pavement" forms that can hinder revegetation.

  Figure 2. Typical landslide profile and effects.

• Stability: Do not revegetate unstable surfaces.
  • Do not seed immediately after a slope failure because there may still be smaller slumps, soil movement or settling occurring.
  • While seeding controls surface erosion that may lead to rills or gullies, it does not solve deep-seated stability problems.
  • It is important to assess the need for resloping the ground, such as rounding off the top of road fill scarps, or using structural solutions before beginning to seed.

• Cost/benefit: Evaluate alternatives against the probability of success.
  • Do not use expensive methods for sites with little productive potential; do not use inadequate methods for sites with high productive potential.
  • Weigh the risks of less expensive methods that rely on "luck" for success versus more costly methods with a greater chance of success.

Slope stability and drainage control

• Restoring natural drainage patterns is essential for the long-term success of landslide rehabilitation. Figure 3 recommends approaches for restoring drainage when slides:
  • leave a steep "headscarp" at roads
  • cut across mid-slope roads
  • deposit debris onto a road
  • interrupt drainage patterns
  • deposit debris in a gully or creek.
• Route road drainage away from landslide tracks unless the landslide travels down a natural gully.
• Remove landslide debris from road drainage structures.
• Do not divert too much water to a single culvert.

Soil

Figure 3 recommends approaches to the following situations:

• Where slides have initiated in road fill, reduce the slope angle of the headscarp by re-sloping with an excavator. In most cases, this requires benching the road further into the slope. Do not deposit new material onto the headscarp.
• When slides are not associated with roads, it may be necessary to manually re-slope and/or terrace the middle (transport) zone. However, this is not usually feasible because of the risk of injury on steep slopes and the expense (i.e., very labor intensive). On problem sites, bioengineering techniques, such as wattling or brush layering, may be used to create terraces.

Revegetation

• Revegetation is the main rehabilitation technique for landslides.
• Use grasses and legumes to control erosion.
• Dry seeding is recommended for gently sloping deposition zones, for moderate slopes in areas with moist summers, for older slides where an erosion pavement has developed, and for areas with rocky surfaces. Dry seeding is advantageous for rocky sites because the seed settles into small crevices between rocks.

Figure 3. Slope preparation and drainage restoration associated with landslides and road systems.

• Hydroseeding is recommended for steep slopes to control erosion and to reduce seed wander. Depending on the access, you can use either ground-based or aerial hydroseeding.
• Because topsoil has usually been lost, soil productivity on landslide scars is usually poor. Fertilizer, mulches, soil binders or tackifiers and nitrogen-fixing plants should be used routinely.
• Plant shrubs or hardwood trees to achieve longer-term revegetation objectives and to create stronger root mats. Tables A2.4 and A2.5 list native shrubs and hardwood trees for use on coastal and interior sites. Trials with native shrubs and hardwoods show that Sitka alder, black cottonwood, paper birch, willow and snowberry can be established on landslides.
• Conifers can be planted anywhere sufficient mineral soil remains on a landslide. Early successional species capable of growing in disturbed soils are preferred.
• Revegetation serves to dry out slopes and contributes root strength to stabilize the surface. On sites where vegetation is a major contributor to surface slope stability, bioengineering techniques, such as wattling and brush layering, may provide interim stability to allow vegetation to re-establish.