When it comes to mobile equipment operation in sanitary landfills, there are multiple choices in terms of machine types, sizes and options. It is important to provide guidance in these critical choices to landfill site managers by explaining how they can influence the final density and the lifetime of the landfill site.
Expanding a landfill’s life will be achieved not only when selecting a fleet of equipment but also by applying different machine operating techniques during the compaction process.
Proper technique for solid waste compaction consists of shredding the waste into small pieces, pushing it to mix it, placing it to fill voids and, finally, compacting it to maximize the tonnage of waste using a minimum of space in the landfill.
Oxygen is essential to combustion. If you control the presence of air in contact with combustible waste at the surface of the landfill, you reduce risks of fire. In the absence of compaction, more waste is in contact with air. If a fire starts, wind will increase combustion. Once a fire has started at a landfill, it is extremely difficult to stop combustion entirely. Smoke may contain toxic gases. Digging out the combusting material and covering it with dirt best controls fires within the waste.
The more compaction, the less infiltration of rain water inside the cell. Water remains close to the surface, and a higher percent evaporates instead of percolating through the landfill. This reduces the amount of leachate generated, reduces ground pollution risks and lowers leachate treatment costs.
Risks of uneven settlement and landslides are reduced once waste has been compacted. Landslides on a landfill generally start from the slopes of a landfill cell. These areas are more likely to receive less compaction and cover during cell development. However, they are critical areas where good compaction and surface stabilization must be achieved.
Compacting waste during the landfill’s active life reduces the amount of settlement that occurs after site closure. Uneven settlement creates various issues, such as the development of low areas where water can accumulate and leak through the cap cover of the cell, allowing leachate springs to run off the site. A limited degree of settlement allows the land to be reused as rural land or as a recreational park after the landfill’s closure.
Good compaction also provides a base for access roads for collection trucks during landfill operation.
HOW IT’S DONE
Most landfill cell dimensions should be kept to approximately 100 feet to 200 feet wide by 60 feet to 80 feet long on slopes of around 5 to 1. Lift height would be approximately 15 feet during working hours and would be brought in to 4 to 1 or 3 to 1 by the end of day so less cover soil is needed. A daily slope of 5 to 1 results in faster cycles, uses less fuel and requires less maintenance. Moving the slope to 4 to 1 or 3 to 1 by day’s end reduces exposure to rainwater, and machine operators can work in a manageable compaction area.
It is important to control collection truck traffic at a landfill. The landfill site spotter should control the collection truck drivers, allowing them to dump in designated areas. This way, the operator keeps the landfill cell to the above dimensions, reduces time needed to push waste from the dumping spot to the cell and improves safety on the site by allowing truck drivers on a dedicated access road only.
Once waste has been dumped, it needs to be pushed to the landfill cell.
A question worth mentioning is whether municipal and industrial solid waste, which comes in various shapes and sizes and often is enclosed in plastic bags, should be shredded, compressed and chopped during landfilling.
The answer is yes. When waste is compressed and reduced in size by shredding, it easily will find its place, bind together and reduce voids. Good compaction promotes mixing and blending of materials, creating a more even surface at the landfill.
On modern landfills, dedicated equipment tends to be used for each task required. Although mobile equipment often is designed to increase its versatility, it is important to take a closer look at machine configuration.
To effectively chop and shred waste, track-type tractors (dozers) or track loaders are recommended. These machines are fitted with steel tracks. Track grousers act as blades to chop and shred material. A track-type tractor or dozer is ideal for spreading waste on a slope. Its ability to push a load while precisely controlling the position of the blade ensures consistent layer thickness better than any other machine.
Landfill compactors are equipped with large capacity blades, enabling these machines to push large volumes of waste. However, when it comes to pushing and spreading waste, it is important to consider that a track-type tractor will have better traction force because its track design provides a better grip on loose refuse than a compactor.
Also, note that the more time spent pushing and spreading waste with a compactor, the less time for what it is designed for—compacting.
LAYER BY LAYER
Proper layering consists of pushing and spreading the waste in thin layers. Thin layers bind, compress and shred more easily than thick layers. Machines use less fuel, gain higher densities and have less maintenance issues with thin layers. Thick layers take away all of the above gains and result in only the top of the layer being compacted, leaving a spongy uncompacted area beneath it that increases rolling resistance during compaction runs.
Layer thickness depends on the weight of the equipment used for compaction. See the table to the right for layer thickness recommendations based on the compactor’s weight.
If the layer is thicker than these recommended values, the lower part of the layer will not receive enough pressure to reduce voids in the waste. The layer will act like a spring, and once the compactor has made a pass over the layer, it will return to its initial volume.
Dozers equipped with large capacity blades increase site productivity. The tracks provide good balance and flotation on loose waste and facilitate efficient spreading.
Alternatively, sites not equipped with dozers can use landfill compactors. Small sites can use track loaders equipped with large capacity buckets for compaction.
Mobile equipment applies a vertical force on the waste, which compresses the material and reduces voids. This force is linked to the pressure applied by the equipment, where pressure equals weight divided by contact surface.
A dozer has a large contact surface area of 43 square feet to 65 square feet for a 240-horsepower dozer. Therefore, by default, its compaction capability is far less than a landfill compactor equipped with steel wheels.
Evaluating the contact surface area of a machine equipped with wheels that is working on a loose surface is not easy; however, a good estimate is linear pressure, which takes into account the width of each wheel.
By further reducing wheel width, pressure and compaction on waste can be increased. It is, therefore, not recommended to use mobile equipment featuring two drums instead of four wheels as they increase the contact surface with the ground and decrease pressure when it comes to compaction.
Caterpillar, Peoria, Illinois, has carefully chosen the steel wheel size, as reducing wheel size does increase pressure but not necessarily compaction. For example, narrow wheels break free more easily and spin more easily: The sheer force of that wheel might be higher than the compacted waste, causing wheel spin.
Each pass increases final compaction. Experience shows that the economical balance between number of passes and operating cost of a landfill compactor is four passes for typical household waste. For industrial waste, more passes are required because of the different nature of the material. Industrial waste is generally dryer, and items are larger, requiring more shredding before compaction.
A pass is travel in one direction across the active cell. According to tests, once an operator does four passes on solid waste, the gain in density is minimal compared with the time and fuel spent.
Following a logical pattern—left to right or right to left—the compactor operator makes one pass forward and a second pass in the same track in reverse. He or she then moves over one wheel, makes two more passes (forward/reverse in same tracks). Moving over one more wheel width while making forward/reverse passes gains him or her four passes as he or she logically moves in this pattern across the waste.
The machine operator needs to make sure the machine travels straight on the face to keep waste in place and to compact forward and reverse in the same tracks. That way, both front and rear axles will compact waste on the active cell.
The compactor operator makes sure that he or she completes the passes forward and reverse until the wheel or wheels are completely off of the waste, ensuring the layer is completely compressed and shredded.
He or she also follows a pattern to ensure the whole face is uniformly compacted. Following a pattern helps to ensure the waste is shredded, compacted and covers the area uniformly.
By working without a pattern, the machine tends to roam on the face and not to be used efficiently. It may run more than four passes on some areas without significantly increasing compaction. In other instances, it may not have done four passes in some areas. This will cause differential compaction on the face.
Uniform compaction also helps to save fuel as it reduces rolling resistance on the surface.
Once four passes have been done on the working face, a final pass at 45 degrees from the pattern direction is suggested.
Caterpillar landfill compactors and track-type tractors are designed for optimal operation on waste, which has a low coefficient of traction and high rolling resistance. Their power trains include mechanical power shift transmissions.
To maintain excellent traction, landfill compactors are equipped with a no-spin differential, which provides a maximum traction effect by sending torque to both axles and to the slowest moving wheel.
When pushing straight, the no-spin differential is self-locking and sends equal torque to both wheels. Because of this, it is recommended to execute all operations requiring maximum pushing capacity in a straight line.
A PRACTICAL EXAMPLE
Cat took part in a landfill study where the tendency on-site was to push waste from the place where collection trucks had dumped it over an approximately 25-foot-high cliff. Then, at the end of the shift, the cliff would be remodeled to build a 3-to-1 slope. We recommended running compaction machines on a 4-to-1 slope permanently during their shifts and not only at the end of it, however. This way, the effect of compaction would be continuous throughout the shift.
Dozers were doing different tasks during the shift. They were not only pushing waste but also were doing some compaction work, working from the bottom of the face where, thanks to their better pushing capacity, they were able to push up the waste. They would then run over the waste to compact it.
Even if the site has a large reserve of airspace, improving landfill density improves safety on-site and for the neighborhood by reducing the risk of fire, odors and rodents.
We suggested operators should avoid compacting with dozers. Although it is possible to do it, the efficiency was less than that of a machine equipped with steel wheels. Operators should let dozers concentrate on tasks requiring pushing and/or spreading capacity, such as pushing piles of waste being dumped by collection trucks, spreading even layers of 15 inches and pushing waste down from the top of the face.
Landfill operators should use dozers for this task and relocate landfill compactors on the face itself to let them concentrate on four passes of compaction.
Cat spent time with operators on operating techniques for compaction. This essentially was about adopting an operating pattern, which would ensure that all of the face would be covered by parallel passes.
Proper operation would require reorganization of the face to let landfill compactor operators run their machines all the way down to the face.
To do so, Cat kept areas of the tipping floor free to the small businesses that came to dump their own waste and trained the spotters to ensure these small vehicles would be kept away from the landfill compactor and dozer operations. This ensured these machines had room to operate all the way down to the face without any risk of injuring personnel or visitors of the site.
The study site already was using a fleet of dedicated machines, such as 826 landfill compactors and D7R crawler tractors with waste handling arrangements. The remaining landfill life was estimated to be four years when the study was completed.
The results showed an 8 percent improvement in compaction using the new operating techniques, without the addition of any additional equipment. This resulted in an extension of the landfill cell life by more than three months.
The other important point to note was that the operator achieved these gains on compaction without reducing site production, such as how much solid waste he was able to treat per hour. By dedicating landfill compactors to compaction and dozers to pushing and spreading, operators increased the overall production of the fleet.
The measurements showed that overall production increased by 8 percent from 139 tons per hour to 150 tons per hour.
Cat measured a 14 percent increase in the combined fuel consumption of a landfill compactor and a dozer compared with the consumption of a landfill compactor used to push, spread and compact waste. However, when brought back to the cost savings in terms of landfill space, this was a cost increase the site was happy to bear.
The fleet organization as a team of machines, landfill compactors and track-type tractors also will bring more complete involvement of the fleet personnel, including spotters.
Even if the site has a large reserve of airspace, improving landfill density will improve safety on-site and for the neighborhood by reducing the risk of fire, odors and rodents.