Construction methods, and erosion and sediment controls, vary with the soil type. Soil types can change as you dig down, or within short distances along a road. Some can make construction very difficult.
What does this soil mean to you?
Some soils are highly erodible and can cause severe sedimentation
- Fine soil particles are slow to settle out of water. They can severely impact water quality and stream life.
- Pumice can easily erode even on gentle slopes. This can cause sedimentation problems.
The weaker the soil, the harder it is to build a good road. This is made worse on:
- Steeper ground.
- Ground with poor drainage.
- Soil that can easily slip off the bedrock.
- This soil is the one profiled at the top of this page.
- Cut banks erode even in relatively gentle country.
- Water tables and culverts can easily block with sediment or pumice.
- The soil is in a wet location which makes construction more difficult.
- The grey-blue soil indicates poor drainage.
- This mudstone’s bedding plane angle means the soil above can easily slide off.
- The soil has made construction and maintenance extremely challenging:
- Weak soils on steep country.
- Wet weather has added to poor soil drainage.
- Mudstone bedrock is prone to soil slip.
- Fine clays and silts often make poor fills.
- If fills are not stable and water control is poor then they may erode – they look like melting ice cream.
Leaning trees – warning sign
- Trees planted on unstable slopes often develop a lean, especially in earth flow country.
- As the ground moves under the growing trees they develop ‘J’ shape stems or lean up hill.
Good construction soils have:
- A thin topsoil layer which makes stripping easy.
- A good mix of material sizes:
- larger size (sand and gravel) gives strength.
- smaller sizes (clay and silt) bind everything together.
- Good drainage. Water can move down through the soil and bedrock.
Poor construction soils often have:
- Lots of topsoil. This has little strength and must be stripped.
- Fine soils (clays, loess and silts) are hard to compact and water erodes and weakens them.
- Poor drainage. A blue or grey colour with brown spots shows the soil is often wet.
- Just one dominant particle size and not a mix of rock fragments and finer soil.
- A hard bedrock layer underneath. Water doesn’t drain through it and soil can slide off.