No one earthmoving system is optimal for every jobsite. Consider the dilemma of choosing between an articulated truck paired with an excavator or a wheel tractor scraper.
The unique cut and fill conditions of the jobsite determine which system actually provides a better solution.
"Conventional wisdom suggests that the haul distance is the principal criteria for the right choice of equipment on a given project," says Govi Kannan, Volvo Construction Equipment. "In addition to the haul distance, there are several application-related and economic-related factors which influence the decision on equipment selection." These include the material, space constraints, weather, operator skill and owning and operating costs.
"When I go out to the jobsite, there are five major questions that I ask," says Scott Knoblauch, industry application consultant, Caterpillar Inc. "What kind of material is being moved? What are the underfoot conditions? How much material do I have to move? How far do I have to move it?
How long am I going to be here?" The answers to these questions help determine the most appropriate earthmoving system.
Material type is top criteria
"The first thing that I look at is material type," says Knoblauch. Certain materials favor the use of either the scraper or an articulated truck.
"Under the right conditions, it is virtually impossible to beat the production of a wheel tractor scraper. For example, consider the Caterpillar 627 push-pull wheel tractor scrapers. The main advantage is that you have four engines worth of horsepower on a single cutting edge in a push-pull configuration," says Knoblauch. "We have a little over 1,100 hp on one single cutting edge when they are working together. So the advantages are quicker loading times and increased density in the bowl."
The ideal conditions for a scraper would be easy loading material that spreads nicely. "Scrapers are really great in sandy loam soil," says Fred Casten, product manager, trucks and scrapers, Terex Construction.
"There is no cheaper way to move dirt than with a scraper when the conditions are right," he adds. "You can load a scraper very quickly in good conditions."
But scrapers are not as tolerant of wet, sticky clay that is more difficult to load, and rocky soils dont allow the scrapers to pull through as smoothly, Casten points out.
The amount of rock that a scraper can handle really depends on the type of scraper and the size of the rock. "If you are using a paddle-type scraper, we basically say no rock at all because of the potential damage to the flights and the chain," says Knoblauch. "The auger scraper has a broader material appetite than what an elevator has with fewer moving parts and less operational cost. If we can fit the rock underneath the apron, we can load it in the bowl. Auger scrapers can handle a rock up to the size of an average office chair with little or no trouble."
But rocky soils are not the best application for a scraper. "If I am loading shot rock, a scraper may do it, but it is not the best tool to be using," says Knoblauch.
Articulated trucks are top loaded. "Thanks to this configuration, the articulated haulers are not constrained by the type of material that is loaded on them," says Kannan. "Sometimes it may also be possible to carry semi-liquid, sludge-like material through the use of a tailgate."
Knoblauch agrees, noting, "With an articulated truck, if we can load the material (within reason) in the back, we can haul it."
Dealing with changing weather
Weather changes can dramatically alter jobsite soil conditions. "Articulated trucks can work in all seasons, year round, where your scrapers are usually limited as weather conditions cause the job to become wet and sloppy," says Michael Kennamann, senior sales and marketing consultant for articulated trucks, Caterpillar Inc.
Flexibility of the equipment to adapt to changing conditions is also important. "Articulated haulers are economical for a wide range of applications," says Kannan. "Their flexibility and versatility are best exhibited under adverse and difficult conditions."