Unstable cut and fill leads to high maintenance costs, environmental incidents and road closures.
Build it right first time
- 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 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
- The grass will help protect the fill and reduce erosion
- Poor water control and no fill compaction at the top of the fill has led to rill erosion
- The fill is not protected and erosion will likely get worse
- Water control and hydroseeding or grassing would help
What is a good fill slope angle?
Use the table below as a guide. Also drive around existing roads with similar material and see what angle provides the most stability.
What is a good cut slope angle?
For example, an average rock cut slope is steep. A 4 metre high cut bank only needs to be offset 1 metre horizontally
- Steep cut angles of about 2 metres vertical to 1 metre horizontal were appropriately used for papa, so the batters are more likely to be stable
- Pumice batters can be near vertical to reduce rill erosion
- Slash spread on the logging track has reduced surface erosion and sediment
- The fill is protected from rain directly hitting and eroding it
- The slash filters sediment and slows the speed of the runoff
- Slash helps grass and vegetation grow by providing cover
- Radiata logs should not be used as a permanent retaining structure as they will rapidly rot and leave the road section unstable
- 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
- A temporary structure, like radiata logs, should be removed after use
- Cuts and fill require regular maintenance until fully stabilised
- The slope is too steep to hold the fill and it has spilt 60 m down the hill
- 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
Establishing vegetation is an excellent way to help prevent erosion and to reduce sedimentation.
- Is easy to do by hand or with a handheld seed spreader
- Autumn and spring are the best times for sowing grass
- Grass prefers warm and moist conditions; summer drought or winter frost kills germinating seed
- The fill slope has been protected from erosion
- Water is directed off the fill by a sock
- Hay is being used to assist revegetation
- Grassing the berm and the fill slope has been effective
- The cut bank is impracticable to grass due to the soil material