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 create road maintenance issues and 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 on the previous page
- Cut banks erode even on easy to moderate country
- Water tables and culverts can easily block with sediment or pumice
- A wet location makes construction more difficult
- The grey-blue earth 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, they can rapidly erode
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 uphill
Good construction soils have:
- A thin topsoil layer which makes stripping easy
- A good mix of material sizes:
- Larger sizes, like sand and gravel, gives strength
- Smaller sizes, like clay and silt, bind everything together
- Good drainage. Water can move down through the soil and bedrock
Poor construction soils often have:
- A lot of topsoil. This has low strength and must be stripped in almost all situations
- Just one dominant particle size, instead of a mix of rock fragments and finer soil
- Fine soils (clays, loess and silts) are hard to compact. Water erodes and weakens them
- Poor drainage. A blue or grey colour with brown spots shows the soil is often wet
- A hard bedrock layer underneath. Water doesn’t drain through it, and soil can slide off