6.1 Traffic loading

Resilient and permanent strainEquivalent Standard Axle (ESA)Standard 80 kN axle used for the equivalent standard axle (ESA) load methodAxles passing over a pavement generate stresses within the pavement that are transferred to the subgrade. These stresses cause two types of strain – resilient and permanent strain. Resilient strain deforms the pavement when the axle load is applied, but rebounds when the load is removed. However, with permanent strain, the deformation remains after the axle load has passed. While this might initially support compaction, over time the road starts to deform – that is, rut – and typically, the incremental damage will quickly accelerate with each passing truck.

The level of expected traffic and the various truck types must be estimated when designing a pavement. Forest roading is unusual in that roads are either sparsely used by light vehicles or, during harvest, have a high proportion of high axle loadings. Another aspect unique to forestry roading is the use of off-highway, oversize, heavy vehicles which can have axle loadings that exceed the legal highway axle loading.

The design traffic for a pavement is expressed in terms of the number of equivalent standard axles (ESA) which are expected to travel the pavement during the design period. The ESA number is a measure of the equivalent damaging effect to which the pavement will be subjected as a result of the passage of the total estimated traffic.

The design period is the time a pavement is required to be in service without requiring major rehabilitation or reconstruction. The design period is influenced by a number of factors, including the cost of capital expenditure, road class, intended usage, seasonality of use and location. For example, there is no need to design a secondary road to withstand the expected traffic loadings for 10-20 years, if it is only likely to be used for a few years of harvest, and then lie with minimal traffic until harvest of the next forest crop.

The standard axle used for the ESA design method is a single axle dual tyre configuration loaded to 80kN (8.15t). Each dual wheel load is taken as being applied to the pavement on two circular areas with centres 330 mm apart, and with a uniform contact pressure of 550 kPa.

Axle configurationsAxle loads causing equivalent damageA wide variety of axle spacing and configurations of tyres and axle sets are used for heavy vehicles working within a forest. To undertake pavement design, these configurations need to be simplified into a small group of standard axle sets and loadings. For design purposes, axle group loadings are generally simplified in terms of the following five types:

• Single axle with single tyres (SAST)
• Single axle with dual tyres (SADT)
• Tandem axles with single tyres (TAST)
• Tandem axles with dual tyres (TADT)
• Tri-axles all with dual tyres (TRDT)
• Quad axle with dual tyres (QADT).

Tandem axles, with wide axle spacing in excess of 2.4 m, can be considered to be single axles, with the total load on the spread tandem configuration being divided equally between the two single axles. Twin steer axles can be considered to be equivalent to tandem axles, both with dual wheels, which are loaded to 1.5 times the load on the twin steer axles.

Idealised load distributionThe idealised load distribution is where the stress area (circle of diameter B) of the subgrade is equal to the width of the tyre(s) (T) plus the pavement depth (D)The adjacent table shows the loads on each of these axle configurations that are considered to cause the same amount of pavement damage as the standard axle. For example, an 80kN load on a SADT configuration will cause the same damage to the pavement as a 181kN load on a TRDT configuration.

6.1.1 Axle load equivalencies – calculating ESA

The design traffic is given in terms of the number of equivalent standard axles (ESA), and is determined by:

• Present or expected traffic volumes
• Distribution of vehicles
• Axle types and loadings
• Traffic growth rates
• Design life of the road (before major rehabilitation is required).

When the number of passes of the configuration is equal to the number of ESA loads for which the pavement was designed, the pavement in theory will have deteriorated to the point where it is no longer useable and will need to be rehabilitated.

Axle groups operating at loads other than the standard loads shown above, need to be expressed in terms of the number of standard axles that would cause the same damage. Conversion to standard axles is achieved using the following equation:

For example, consider a logging truck operating at 44 tonnes and with the axle configurations shown in the top graphic below.The ESA for each axle group is individually calculated and summed to determine the ESA value for the vehicle. For this example, the logging truck will cause the same damage to the pavement as 3.17 standard axles.

Logging truck operating at 44 tonnes

Logging truck operating at 66 tonnes

Note that the conversion to standard axles uses an exponent of four. The consequence is that axle groups with actual loads greater than the standard load, will cause significantly greater pavement damage. For example, the same logging truck used in the previous example increases its GVM by 50% for off-highway operation. The resulting ESA would be 16.0 – a 500% increase in pavement damage.

The fourth power function means that off-highway trucks with very high payloads have correspondingly high ESA values. Pavements need to be designed for this. The higher road construction and maintenance costs need to be justified. Often the trade-off in increasing pavement construction and maintenance cost is the operational savings that occur with the use of high capacity trucks.

An estimate of the total ESA value for the design of a forest harvesting road can be calculated from the expected volume of harvest using the following steps:

• Estimate total volume of harvest (production forest area x yield per hectare)
• Estimate the number of logging truck loads (volume/average truck capacity)
• Calculate the ESA value for the typical log truck configuration that will be used for the operation (ESA per truck loaded plus ESA per truck empty)
• Multiply log truck ESA by the number of trucks required to complete the operation
• Add extra ESAs to allow for construction and service traffic (either estimate or increase wood flow ESA by 15%).

Example

ESA for logging truck round trip = 3.17+1.16 = 4.33A 140 ha area of forest with 550 m³/ha average yield is to be transported on a new forest road over a period of three years, following which the area will be replanted for the next rotation. The new road’s pavement design ESA is:

• Total volume of harvest: 360 ha x 550 tonne/ha = 198,000 tonnes
• Number of loads: 198,000 tonnes/28 tonnes/load = 7,070 loads
• ESA value for logging trucks:
  • Loaded ESA= 3.17 (from previous example)
  • Unloaded ESA = 1.16
• Total log truck ESA for operation: 7071 loads x 4.33 ESA/truck = 30,620 ESA
• Allowance for construction and service traffic (15%): 30,620 x 1.15 = 35,210 ESA

The pavement for this new forest road will need to be constructed to withstand the damage that will be caused by 35,000 standard axles.