Loading Efficiency For Sustainable Transport

In Ireland, timber and biomass haulage faces the challenge of transporting enough material within strict legal dimensions and gross vehicle weights restrictions for trucks and trailers. By understand the moisture content and wood specie, track hauliers can determine the optimum volume and weight of wood to be transported and ensure greater efficiency. By Amanda Sosa and Ger Dublin, University College Dublin, RadomirKlvac, Mendel University, and Enda Coates and Tom Kent, Waterford Institute of Technology


In 2007, forests covered 10 percent of Ireland’s land area, and this is projected to increase to 17 percent by 2030. It is also forecasted that roundwood volume will increase from 3.79 million cubic metres in 2011 to 6.41 million cubic metres in 2028. This volume increase will also have a direct correlation and impact on the logistics involved in the wood supply chain.

Several modes of transportation are used in the forestry sector worldwide and truck transportation constitutes an important part of the supply chain. Road haulage represents more than 75 percent of Europe’s freight transport activity and according to the European Conference of Ministers of Transport,freight transport will grow from 50 percent to 60 percent by 2015 from the 2001 values. 

Road transportation is and will remain the most important mode of timber transport in Ireland, forming a substantial part of the industry’s raw material cost and having a major influence on the sector’s overall economic performance and competitiveness.

The transport of wood from the forest to the industries is carried out by trucks of different makes and models. The difference is usually given by the number of axles, axle spacing, tare weight of the truck, and the engine position in relation to the front axle. 


Haulage Restrictions

All European countries impose haulage regulations related to the restriction on dimensions and weight of the trucks. The weight restriction is more complex due to the relationship between number of axles and the distance between them and how this changes the design gross vehicle weight (DGVW).

Ireland sets a maximum of 44,000 kg for trucks with 6 axles (to be increased to 46,000 kg), and 42,000 kg for 5-axled trucks (now proposed to be reduced to 40,000 kg). The truck’s weight is monitored at weighbridges and overloaded trucks incur penalties, normally of a financial nature or ban on transporting timber for a specified period of time.

Overweight trucks can cause problems such as deterioration of roads, short vehicle life, vehicle manoeuvre difficulty and safety issues. Over one third (178,000 ha) of the existing forest in Ireland are located on peatland soil and are served by basic road infrastructure of flexible road pavements. These pavements were severely deteriorated due to heavily loaded trucks. It was reported that even trucks with 3000 kg overloaded could reduce the roads serviceability.

Truck fuel consumption increases with the weight of the vehicle. Heavier trucks require more fuel to accelerate and to climb hills, and may reduce the amount of cargo that can be carried relative to the unladen (tare) weight of the truck. For every 10 percent drop in truck weight, fuel use is reduced by between five percent and 10 percent.

Regulation on the trucks weight and dimensions highlights the challenge facing truck operators to place enough material on a truck (and trailer) of fixed dimensions in an efficient manner. Carrying less than the legal maximum weight will incur opportunity costs.

In the case of wood biomass (chips and bundles), low density can decrease the productivity in transport with loads reaching the maximum legal dimensions of the truck and/or trailer before meeting the legal maximum weight. 

In the case of logs, the situation is different. The material has a higher bulk density in comparison to bundles and woodchips. Maximising the load implies attempting to reach the full load volume capacity without exceeding the legal maximum weight. A substantial net gain in average payloads can be achieved by eliminating both overloading and under loading. 


Gross Vehicle Weight Adjustment

Another way to potentially increase efficiency in transportation is to reduce the variability of the gross vehicle weight (GVW). GVW can be controlled by weighing devices—a truck scale is a common device for measuring the weight of a heavy vehicle. However, truck scales are usually installed at fixed locations, which are not convenient for patrols to check vehicle payload on the road.

It is considered practically impossible to handle the relationship of weight and volume of the wood accurately. The interactions between parameters as wood moisture content (MC), dry matter, solid and bulk density and truck payloads constraints are complex but need to be evaluated in order to deliver the material cheaply and efficiently. 

Using the interaction of these parameters,how the moisture content of wood affected the costs and operational planning of three biomass supply chains have been studied. Transportation decisions from the study included calculation of total number of truckloads with chips or logs to be transported to the plant by using MC as a determining factor.

In terms of weight and volume utilisation on trucks, it is natural that loads from the 100 trucks entering the weighbridge varied depending on factors such as the loader operator, MC and loading area conditions. The variation of underutilisation on volume (6.63 cubic metres to 27.5 cubic metres) and on weight (828 kg to 1913 kg) affect the transportation costs.The smaller the load, the higher hauling costs per unit of product supplied.

Improving loading efficiency improves hauling productivity (kg/day); to make as many trips as possible with the maximum legal weight will depend upon to a certain extent on the tare weight of the truck and trailer. The heavier the truck’s tare the lower the payload and this points out the importance of construction materials when choosing a truck, considering also that a crane increases the tare weight by an average of 2800 kg.

Studies in US present how weight reduction methods can increase payloads by one to four tonnes. Some strategies consist on the installation of a more modern and lighter crane of the same capabilities, the use of more modern materials for the construction of trailers/using lightweight components, using a fold-up pole trailer, and even, when possible, eliminating the sleeper-cab.

Another strategy to increase payload is by reducing GVW variability, a one percent decrease in GVW variability yielded 0.22 to 0.73 tonne increase in payload weight. A suggested potential savings of US$100 million annually in the southern US by fully loading trucks more consistently; so determine the load weight before arriving at the mill is important.

Weighing in-forest through an on-board system generally reduces the variability of GVW, and increases the payload by minimising the light payloads and reducing the possibilities of financial penalties due to overweight trucks. On the other hand, implementing on-board weighing systems constitute initial capital investment to install such a system together with the increase in managerial duties in order to effectively use the information it provides to the benefit of the haulage company.


Weighing Devices

In Ireland, most of the truckloads are weighed at the processing plant or weigh stations at fixed locations along the road network. Weighing devices on trucks are not common in Ireland, with only few hauliers installing basic systems based on pressure gauges to give an indication of payload weight. 

The UK Code of Practice on timber transport requires all timber haulage vehicles to operate with weighing devices. Load weighing devices were initially trialled experimentally in Ireland some 10 years ago, without any appreciable uptake, mainly as result of cost. In more recent years, individual hauliers have been experimenting with less expensive units. A more recent study that tested a new air weigh system fitted to the middle axle airbags of the trailer and back axle of the truck that gave a digital in-cab reading for the driver suggests an average accuracy to within 568 kg.

A possible method to control and improve the truck’s loading efficiency by developing a methodology where the maximum loading volume can be determined under legal weight restrictions through managing the MC of the wood in the field consists of truck hauliers determining the MC before loading the trucks. 

The load volume fraction (LVF) for specific species and products at different moisture content could be tabulated with the purpose of helping the haulier to determine the percentage (or side bar heights) of their trucks to be loaded, thus providing confidence to the hauliers that their load is under the maximum legal weight restrictions.

By optimising the load efficiency using the LVF, an aspect to consider is to choose the proper loading technique to help constitute a more uniform load. In Ireland it is common practice to load logs top to tail to ensure an even balance of the load. This loading method was observed to increase the payload for pulpwood loads by three percent and facilitate unloading without having the load fall apart.

While it is always desirable to maximise payload, there may be times when under loading is a more efficient strategy. The extra ton on a truck may actually be costing money if it required that the truck be pushed out of the woods by a skidder, run on lower range gears to make it over a mountain, use more fuel, and take longer to complete the trip.

Getting a better understanding of truck configurations and the volume and weight capacities is crucial to assessing the adequate type of truck to use. The use of articulated trucks without crane allow higher payload, followed by articulated trucks with crane, and ultimately rigid-trailer trucks presented the lowest LMP. 

Nevertheless, it has to be considered that rigid-trailer truck combinations give more accessibility to forest areas than the articulated counterparts. The authors acknowledge that further studies may be required to test and relate the variability of basic density for Sitka spruce in Ireland and is ultimately one of the factors that will affect the LMP in Ireland.

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