Contents

  • Cover
  • Front matter
  • Introduction
  • Identifying and managing risk
    • Risk overview
    • Working smart and efficiently
    • Starting the job – be prepared
    • The job is not finished until it is signed-off
  • Basics
    • Roading terms
    • Basics about soil
  • Good construction
    • A well-constructed road
    • A well-constructed landing
  • Earthworks
    • The right machine for the task
    • Clearing and stripping
    • Cut and side cast construction
    • Cut and bench fill construction
    • Full bench construction with end-haul
    • Earthworks compaction
    • Landing construction
    • Forming road corners and in-bends
    • Final grading before metalling
    • Stabilising cut/fill slopes
  • Water control
    • Water control overview
    • Ditches
    • Road drainage culverts
    • Berms and cut-outs
    • Flumes
    • Silt traps and soak holes
    • Silt fences
    • Single culvert river crossings
    • Ford crossings
    • Bridges
  • Applying aggregate (metalling)
  • Repairs and maintenance
    • R&M overview
    • R&M common to new construction
    • R&M of road formation
    • R&M during harvesting operations
    • R&M of river crossings
  • Assisting loggers and harvesting rehab
    • Installing deadmen
    • Installing debris traps
    • Harvesting track rehabilitation (rehab)
    • Harvesting track rehabilitation – cut-outs
    • Landing rehab
  • Want to learn more?
  • Glossary

NZ Forest Road Engineering Manual: Operators Guide

  1.  ›
  2. Basics ›
  3. Basics about soil
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Basics about soil

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

The goal is to use soil to your best advantage

  • Understand the soil and where water will drain
  • Adjust your construction methods if required
  • Don’t build when the soil is too wet. You’ll make a big mess
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