Unstable cut and fill leads to high maintenance costs, environmental incidents and road closures.
To improve the stability of cut and fill:
- Construct cut and fill angles correctly.
- Bench and compact fills.
- Construct good water control.
- Use slash, mulch and grass to protect soil from rain erosion.
- A slash bund at the bottom of the fill helps reduce sediment movement.
- The fill is contained and stable.
- The fill has been compacted to reduce the risk of failure on steep, unstable slopes.
- Grass will help protect the fill and reduce rill erosion.
- Poor water control at the top of the fill has led to rill erosion.
- The fill has not been protected.
- This will keep getting worse.
Grass cover is an excellent way to help prevent erosion but it can be difficult to establish:
- Autumn and spring are the best times for sowing grass.
- Grass prefers warm and moist conditions; summer drought or winter frost kills germinating seed.
- Most compacted fill has poor fertility as there is no topsoil to help growth.
- Grassing is usually unsuccessful on cut banks and is often ineffective in water tables because of seed being washed away after rain.
- The fill slope has been protected from erosion.
- Water is directed off the fill by a sock.
- Hay is being used to assist re-vegetation.
Grassing the berm and the fill slope has been effective
The cut bank is impracticable to grass due to the soil material
Types of vegetative stabilisation available
|Slash||Good||Free||Machinery able to place material correctly|
|Hay||Fair||Moderately expensive||Wind, short lifespan|
|Hand seed||Fair||Cheap||Can have low strike or can die off on steep or hard sites|
|Hydroseed||Fair||Expensive||Same as hand seed|
|Hydromulch||Fair||Expensive||Same as hand seed|
The advantages and disadvantages of steep cut banks
|Fewer earthworks are needed for the cut. This reduces the amount of fill.||More chance of water tables filling with debris.|
|Less land is taken up with the road.||More risk of major bank failure and slumping in some materials.|
|Cut banks are less likely to slump or erode in some materials.||Difficult to re-vegetate.|
- Correctly cut angles were used for the material so the batters are stable.
- Pumice batters can be near vertical to reduce rill erosion.
- Slash spread on the track has reduced surface erosion and sediment generation.
- The fill is protected from rain directly hitting and eroding it.
- The slash filters sediment and slows the speed of the run-off.
- Slash helps grass and vegetation grow by providing cover.
- The narrow ridge road has been poorly built.
- Radiata logs used to stabilise the road will rapidly rot.
- Some water is still running onto the fill.
- The fill is too steep to be stable and will continue to cause issues.
- It is difficult to fix something after a failure.
- Cuts/fill are being maintained until they fully stabilise.
- The slope is too steep to hold the fill and it has spilled 60 m down the slope.
- The fill is eroding and sediment is in the gully.
- Water will flow in the gully in heavy rainfall. This will affect water quality and stream life.