IntroductionTracks are essential on a hill country farm. A well-constructed track, in the right place, speeds vehicleaccess and stock movement. If it is badlyconstructed or wrongly placed, it will cost money tokeep open, and may damage the environment bytriggering landslides or discharging sediment intowaterways.

Farmers and contractors sometimes do not realise:• the need to avoid constructing tracks on too

steep a grade• the importance of water-tables and cut-offs• correct design of stream crossings.

In the Taranaki hill country, many tracks can beseen where these aspects of construction have beenneglected. Rectifying them is worth-while.

Figure 1: Eroding farm track

Farmers and contractors know how to constructtracks with a tractor and blade or bulldozer. It is notthe Council’s intention to say how to do the job,which is why it has not written any rules about farmtracks into the Regional Soil Plan.

The Regional Fresh Water Plan has two rules (24 and25) classing discharges from soil disturbanceactivities to water or onto land in circumstanceswhere the discharge may enter water, as permittedprovided that the conditions in the rules are met.The conditions require adherence to industry-recognised guidelines. Current guidelines aresummarised in the Council’s Guidelines forEarthworks in the Taranaki Region. Much of thedocument relates to earthworks in general, so the

Council has prepared this information sheet as asummary of the parts that relate to farm tracks inparticular.

RouteFarm tracks are generally routed along terraces orfootslopes as long as possible, then up stable spursand ridges where country becomes steep. Even so,many tracks are cut by slips during winter rain.

This can usually be avoided by shifting the routejust a few metres when a track is cut or repaired.Places to avoid are:• Outside bends on creeks - the banks here will be

scoured by floodwater• Steep-gradient watercourses on slopes - they

gully during heavy rain• Damp hollows on slopes - they are potential

slips

• Places where soil, although currently vegetated,has been rucked-up - a sign that slips orearthflows may re-activate.

GradeGrade is a track’s upward slope. For instance a trackcut at a grade of 1 in 6 slopes 1 m up in 6 m length.If a track is cut at a grade steeper than 1 in 6, itssurface will generally rill during heavy rain. Thismeans that annual blading, to maintain a passablesurface for vehicles, will be necessary. Keep trackgrade as low as possible, and try not to exceed 1 in 6.

Camber is a track’s slope sideways. A track cut at acamber of 1 in 25 slopes 4 cm sideways every 1mwidth. A camber of at least 1 in 25 is needed todivert rainwater, so that it doesn’t run down thetrack and rill its surface.

Farm track constructionPrinciples and practices

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Side-cutsA side-cut is the bank formed where a track is cutup a hillside. Collapses are triggered if a side-cut isso high that the force of gravity exceeds the bankmaterial’s residual strength. This is particularlylikely where :

• Side-cuts are in soil or weathered rock, asopposed to bedrock

• Water pressure builds up behind the bank afterrain.

It is good practice to avoid excessive height. TheEarthworks Guidelines allow up to 20 metres on a25% slope on construction sites, but such heights aregenerally unnecessary - and dangerous - ifattempted on farm tracks. Here it would be unwiseto exceed :

Loose soil - 1.5 metresCompact soil - 3 metresWeathered or shattered surface rock - 5 metres

At points where water seeps from a side-cut,pressure can be dropped by installing horizontaldrains - small, perforated PVC pipes drilled severalmetres into the bank. This has to be done by acontractor with specialised drilling equipment, butmay not add greatly to total cost of trackconstruction.

Side-castSide-cast is material excavated in the course of trackconstruction. It is generally tipped on the track’sdownhill side where it spills downslope. This isgenerally not a problem, as in Taranaki’s wetclimate, side-cast quickly revegetates.

However, there is one circumstance where side-castshould be avoided - where a watercourse lies a shortdistance downslope, side-cast will be washed into it.Apart from polluting the water, the side-cast islikely to be deposited on top of good pasturesomewhere downstream.

It is usually impractical to truck soil from a trackunder construction, to somewhere elsewhere on thefarm. Perhaps the only way to avoid this problem, isto route new tracks well away from watercourses.

Water-tablesA water-table is a shallow drain cut alongside atrack, to intercept rainwater running off adjacentland. A lot of farm tracks are constructed withoutany water-tables at all, so runoff makes them greasyor boggy. Others have water-tables installed, butthey often scour during the next few winters,

reducing the width of track passable by four-wheeled vehicles.

Install a water-table where-ever a track is side-cut,so that runoff from the bank doesn’t flow across thetrack and rill it. For farm tracks, a shallow water-table about 0.5 metres wide and 0.2 metres deep willsuffice on gentle grades of less than 1 in 10. Atgrades between 1 in 10 and 1 in 6, runoff will scoura water-table. This can be averted by lining steepgrades with small-diameter rock (about 0.1 m);worth doing on an otherwise unmetalled track.Another option is to place sand-bags at intervals ofabout 5 metres, to act as sediment traps.

Cut-offsCut-offs are the places where runoff is dischargeddownslope from water-tables. Cut-offs are typicallysmall, open drains curving outwards away from atrack, where there are gaps in its side-cut. They maydischarge into hillslope watercourses, or scrubbyhollows, or simply onto grassed slopes.

Tracks do not always have cut-offs installed in theright places. In the wrong place, a cut-off may scourback across a track, or trigger a drop-out. Vehicleaccess is cut, and major earthworks accompanied bydrainage and stabilisation measures may be neededto restore it.

On grades exceeding 1 in 10, install cut-offs every 40to 80 metres. At grades less than 1 in 10, cut-offs willbe needed every 50 to 120 metres. Cut-offs shouldbe no more than 1 metre deep; on farm tracks, 0.5metres will generally suffice. Where there is nonatural gap in the side-cut, run the cut-off through asmall-diameter pipe (200 mm) to the outside of thetrack.

Do not discharge cut-offs into hollows next to thetrack - these turn wet from natural runoff duringheavy rain, and extra water may trigger a gully orslip. Straight even slopes, where the water candisperse through vegetation, are better. It is a goodidea to place some small-diameter rock (100 mm) ora sand-bag at the cut-off’s outlet. This dissipateswater energy, and reduces the risk of scour. Ageotextile mat (about 1m2) is another option, as isheavy polythene, but both must be secured byburying the edges.

On steep slopes, timber or corrugated iron flumesare sometimes used to conduct runoff downslopefrom cut-offs, particularly where loose soil has beenside-cast from a track. If installing one of these,secure it with stakes or fencing standards every 2metres, and place small-diameter rock, geotextilemat, or heavy polythene where it discharges.

Small culvertsWhere a track winds up a hillside, it usually crossesa few hillslope watercourses. These are usuallypassed under the track in small culvert pipes. If theculvert outlet is not armoured with small rocks or aconcrete sill, it scours, undermines the pipe, andtakes out part of the track.

If a concrete sill is infeasible, spread medium-diameter rock (200-300 mm) over an area of at least1m2 in the channel under the pipe outlet. Geotextileor polythene may also be used, but should be well-secured e.g. by steel pegs with earth over the textileedges.

Figure 4 : Cut-off with outlet protected

Headwalls around the culvert inlet and outlet are agood idea, to prevent floodwater from scouring thetrack. Inexpensive headwalls can be constructedfrom H4 treated timber planks held in place withiron fencing standards. Alternatively use sandbags.

Remember to use a pipe size big enough to takefloodwater. The Earthworks Guidelines recommend :

Pipe diameter (mm) Catchment area (ha)150 0.05300 0.2450 0.6500 1.0600 >1.0

Figure 5: Culvert with headwalls and energydissipater

Stream crossingsIn valley bottoms, tracks generally ford smallstreams. This results in considerable disturbance ofthe bed by vehicles and stock. The disturbedsediment is more easily eroded by floods, andannual maintenance work is needed to keep thecrossing passable.

A concrete sill is a good way to minimise beddisturbance, without going to the expense of abridge or culvert. Other options are to :

• Armour the crossing with a layer of small-diameter rock (100 mm)

• Place heavy-duty geotextile, held in place bysteel pegs, with the edges folded down andburied in the

• streambed.

Figure 6: Concrete or gravel stream crossing

Bridges and large culvertsLarge streams are frequently bridged, or culvertedwith large-diameter pipes. It is not unknown forthese structures to deflect floodwater against banksimmediately downstream, triggering bankcollapses. Another problem is the lodgement ofdebris on upstream side of the bridge or culvert,causing scour through one or other abutment, andcutting the approach to the structure.

Many old farm bridges and culverts are 'existinguses' which pre-date the 1967 Water and SoilConservation Act. Since that date, legislation hasrequired landowners to obtain a consent beforedisturbing the bed of a stream with earthworks, orerecting any structure in the channel. The TaranakiRegional Council will issue a permit so long as thebridge or culvert has been properly designed, to astandard that the Council considers will not impedefloodwater or cause bank erosion. The Council'sRiver Control Officer is able to provide advice tolandholders undertaking these activities.

Figure 7: Stream bridge or culvert

Other things landowners can do to minimisetrack erosionRevegetated - side-cuts, side-cast fill, and the tracksurface itself can be oversown with grass-seed.Spreading mulch e.g. old hay-bales or pine slashwill help protect soil from surface erosion while thegrass emerges through it.

Light metal - on parts of the track that are prone torill or sheetwash, light metalling at a rate of 1 cubicmetre per 10 metres length will improve thesituation.

Stabilise - where tracks are cut through ground thatis obviously unstable, plant poplar or willow poleseither side of the track. As the trees grow, their rootswill help to hold the track.

Figure 8: Revegetated track with tree plantingsalongside

For further advice or information contact:

The Land Management Section, orTaranaki Regional Council,Private Bag 713StratfordPh: 06 765 7127 Fax: 06 765 5097www.taranakiplants.net.nz