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

NZ Forest Road Engineering Manual

  1.  ›
  2. 3. Planning for roads ›
  3. 3.14 Horizontal alignment
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3.14 Horizontal alignment

A consistent alignment which provides no ‘surprises’ for the driver is more important than an absolute design speed standard in terms of safety. A practical approach needs to be taken for alignment because the lay of the land and the earthworks balance dictates what can be done. The coordination of horizontal and vertical geometry is important to ensure:

  1. Safety and visibility
  2. Good traffic flow
  3. Acceptable level of visual impact of the road.

The diagram below shows how different curve radii and values of superelevation affect the design speed.

Alternatively, the minimum radius (Rmin) required for a given design speed and superelevation can be derived from the following formula:

Where:

v = design speed in km/hr
n = superelevation (m/m or %/100)
f = coefficient of friction (use 0.08 for gravel roads).

The design speed (v) is determined by the desired road standard and the terrain.

The superelevation (n) is the slope of the road surface and is expressed as a decimal. A maximum of 0.0626 (6.26%) is appropriate for logging trucks. For speeds of less than 26 km/hr, superelevation is not needed and normal crossfall is used.

The minimum road curve radius for highway trucks (even at very low speeds) should not be less than 18 m. At a lower radius, the trailer inside tracking becomes very significant, and the truck minimum turning circle may not be able to be accommodated. For off-highway trucks and stem trucks the minimum curve radius will be greater than 18 m and can typically be 40 m.

Curve radius, superelevation and design speed relationship

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