Contents

  • Cover
  • Front matter
  • Preface
  • Scope, purpose and use
  • 1. Terminology, economic analysis, risk management
    • 1.1 Terminology
    • 1.2 Economic analysis
    • 1.3 Understanding risk
  • 2. Regulations, consents and approvals
    • 2.1 National Environmental Standards for Plantation Forestry (NES-PF)
    • 2.2 Heritage New Zealand Pouhere Taonga
    • 2.3 The Health and Safety at Work Act
    • 2.4 NZ Transport Agency approval for access onto state highways
    • 2.5 District council approval for access onto council roads
  • 3. Planning for roads
    • 3.1 Road classes
    • 3.2 Arterial roads
    • 3.3 Secondary roads
    • 3.4 Spur roads
    • 3.5 Establishment tracks
    • 3.6 Spatial information
    • 3.7 Initial field work
    • 3.8 Manual design method: Stepping out a roadline on a topo
    • 3.9 Running a grade line in the field
    • 3.10 Full road design
    • 3.11 Working with road survey data
    • 3.12 Geometric road design
    • 3.13 Curve widening
    • 3.14 Horizontal alignment
    • 3.15 Vertical alignment
    • 3.16 Calculating the safe stopping distance
    • 3.17 Setting out the roadline
  • 4. Planning for landings
    • 4.1 Common landing layouts
    • 4.2 Landing planning considerations
  • 5. Road and landing construction
    • 5.1 Soil and rock properties
    • 5.2 Managing adverse environmental effects
    • 5.3 Marking clearing widths
    • 5.4 Roadline salvage
    • 5.5 Daylighting
    • 5.6 Road formation
    • 5.7 Drainage control during earthwork construction
    • 5.8 Earthwork machinery
    • 5.9 Estimating machinery production
    • 5.10 Stabilising cut and fill slopes during construction
  • 6. Pavement design, subgrade preparation, pavement construction
    • 6.1 Traffic loading
    • 6.2 Evaluating subgrade properties
    • 6.3 Determining pavement depth
    • 6.4 Pavement material properties
    • 6.5 Compaction of subgrade and pavement
    • 6.6 Compaction equipment
    • 6.7 Pavement construction
    • 6.8 Weak subgrades
    • 6.9 Chemical stabilisation of pavement or subgrade
  • 7. Erosion, sediment and slash control structures
    • 7.1 Ditches
    • 7.2 Cut-outs
    • 7.3 Berms
    • 7.4 Drainage culverts
    • 7.5 Flumes
    • 7.6 Sediment traps and soak holes
    • 7.7 Silt fences
    • 7.8 Sediment retention ponds
    • 7.9 Debris traps
  • 8. River crossings
    • 8.1 Fish passage
    • 8.2 Selecting the location and crossing type
    • 8.3 Fords
    • 8.4 Temporary river crossings
    • 8.5 Single culvert river crossings
    • 8.6 Battery culvert river crossings
    • 8.7 Drift deck river crossings
    • 8.8 Single span bridge river crossings
    • 8.9 Prediction of flood flows, and sizing culverts
  • 9. Road maintenance, repairs and upgrades
    • 9.1 Maintenance programme
    • 9.2 Economic evaluation of road maintenance projects
    • 9.3 Managing maintenance requirements
    • 9.4 Commonly used maintenance machinery
    • 9.5 Road surface maintenance
    • 9.6 Road foundation maintenance
    • 9.7 Landing rehabilitation and decommissioning
    • 9.8 Roadside vegetation maintenance
    • 9.9 Erosion and sediment control structure maintenance
    • 9.10 River crossing maintenance
  • Forest road engineering terminology
  • References
  • Websites, resources, databases
  • Appendix: Forest Roads For High Productivity Motor Vehicles (HPMV) with Two Drive Axles Log Trucks

NZ Forest Road Engineering Manual

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  2. 6. Pavement design, subgrade preparation, pavement construction ›
  3. 6.3 Determining pavement depth
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6.3 Determining pavement depth

A thicker pavement attributes stress over a larger area

Knowing both the designed traffic loading, in terms of equivalent standard axles (ESA), and the Californian bearing ratio value for the subgrade, means that design charts can be used to provide an indication of pavement depth requirements. A thicker pavement layer distributes stress over a larger area, enabling the subgrade to withstand larger loads.

The standard chart used for public road design has two shortcomings for forest roads. It is based on very high volumes of traffic in excess of 10,000 ESA, requires a pavement with minimum CBR of 80%, and provides a 90% confidence that the road will not fail in its design life (ARRB 2010). Traffic will be less for most forest roads and the pavement material will be weaker (CBR approximately 40%). Also, 80% confidence is considered fit for purpose for forest roads. Small failures, such as rutting, can be readily remedied through maintenance and will not stop typical forestry traffic. As such, the chart presented in the ARPG report (1998) is most suitable for forest road design.

Example: CBR design chart

A subgrade soil for a new forest road has been assessed to have a saturated strength of CBR=5. Saturated strength has been used as the road is planned for use during the wet season. The design ESA is 35,000 (from previous example). Enter the graph at ESA=3.5 x 104 and draw a line vertically until the CBR=5 line is intercepted. Draw a second line horizontally from this point to intercept with the horizontal axis. The resulting value on the horizontal axis is the required pavement depth. For this example, a minimum compacted pavement depth of 200 mm would be required.

CBR versus pavement thicknessSource: Guide to the design of new pavements for light traffic, APRG Report 21, 1998

Central tyre inflation (CTI)

Central tyre inflation on the drive wheelsCentral tyre inflation (CTI) assists truck performance on forest roads through improved gradient, traction and handling. CTI also helps to maintain the pavement. In critical areas with steep adverse roads or poor pavement properties, roads may need to be designed solely for CTI trucks.

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