Articulated dump truck/excavator earthmoving systems are a popular choice, especially in less than desirable underfoot conditions or where longer haul routes may be involved. But setting up an efficient earthmoving system doesn't happen by accident. It takes careful planning, expertise and the flexibility to change with jobsite conditions.
The place to start is with the excavator. "The loading tool is usually established first and then the number of trucks can be determined," says Steve Moore, Komatsu.
When choosing the excavator, consider not only the physical size of the machine, but also the bucket. "Matching the correct size articulated dump truck to the excavator is really going to affect your bottom line. But equally important is having the right size bucket on your excavator," says Chris Giorgianni, general manager of product marketing, JCB. "The general rule of thumb is with the right bucket and excavator, you should have a full dump truck in four to six passes."
"There are a number of factors that can limit the number of passes per load, such as the compaction of the material or the material itself," notes Doug Phillips, construction segment manager, Volvo Construction Equipment. "When possible, we try not to go over seven passes per load." Yet, this isn't always possible. "This all depends on the density of the material (or weight per cubic yard). The size bucket on the machine must match the breakout and lifting capacity of the excavator."
Advantages of ADTs
Articulated dump trucks are available in a variety of sizes from 14 to over 40 tons. "Most manufacturers focus on the 25-ton and above range," says Giorgianni.
This size range not only allows the flexibility to match up with a variety of excavator sizes, it offers solutions for varying jobsite sizes and underfoot conditions. Bigger articulated trucks can increase productivity on larger jobsites with good underfoot conditions, while smaller articulated trucks are more maneuverable and exert lower ground pressure.
"JCB's range really starts below [25 tons] and goes down to 14 tons," says Giorgianni. "The key attribute for our customers is to be able to negotiate soft ground conditions and tight jobsites that you might not be able to access with larger haul trucks."
The advantage of an articulated dump truck is that it can negotiate a variety of terrain. This can reduce the need to spend a lot of time preparing a well-groomed haul road. "With the flotation of the JCB trucks, you may be able to minimize the amount of work you put into the haul route up front," says Giorgianni. "You may not be as efficient from Point A to Point B, but you are not investing the money up front."
That said, a smooth haul road can be substantially more productive, even with articulated trucks. "Maintaining the haul roads can produce consistent cycle times by reducing wait times for the loading tool, and help the haul truck operator maintain a safe speed on the road," says Phillips. "A well-maintained haul road will also reduce damage to the trucks and fuel consumption."
"Not being able to get from point A to Point B at top speed is going to slow down the operation," Giorgianni agrees. "You are going to have to introduce more trucks into the process, which [impacts] the bottom line. So the more you can eliminate that, the better off you are."
Factors affecting cycle times
Travel and unloading times are key variables in determining the number of trucks needed for your earthmoving system.
Moore recommends dividing the truck cycle time by the time needed to load it to get a general idea how many vehicles you will require. However, he cautions that the cycle time can vary throughout the course of a project. "Truck cycle time will vary more than the loading tool, so you will have to consider the variance in truck cycle times before you decide on the final number of trucks," he states.