When Digging Beats Scraping

In recent years, many landfills have begun to use hydraulic excavators and trucks to excavate and haul soil. This is in contrast to the traditional method of using scrapers. There are many reasons for the increased popularity of excavators as a mass-excavation tool.

In many situations, an excavator and trucks can move dirt faster and at less cost than scrapers, especially as haul distances become longer and also when the material being loaded is wet, sticky, and/or otherwise difficult for a scraper to load (e.g., sand, cobbles, saturated soil).

Economics. In concept, a truck is a more efficient soil-transport vehicle than a scraper because it is not carrying any extra parts such as a paddlewheel, an auger, or cutting edges. While a scraper is a combination machine—having the ability to excavate, haul, and grade—a truck is built for one thing only: hauling. Therefore, once the cost of loading has been amortized (e.g., across a long-haul distance or over a large excavation project), a truck will eventually become a less expensive hauler than a scraper (see Figure 1).

Productivity. Depending on the type of material being excavated, the haul-road configuration, and the size of the job, a scraper might be able to function as a “work-alone” machine. In this situation, a scraper is hard to beat in terms of productivity and cost efficiency. However, if a large quantity of soil (e.g., several-hundred-thousand cubic yards) has to be moved, it might be preferable to use an excavator and trucks for the task.

Versatility. It’s a mistake to say that an excavator/truck system is more versatile than a scraper. In many cases, a scraper’s ability to excavate, haul, spread, and grade makes it the ideal machine. However, when it comes to working in saturated soils, loose (noncohesive) soils, or shot rock, the excavator/truck system is often preferable.

In addition to excavating, hauling, and spreading soil, the scraper can often perform a reasonable job of road grading, particularly if the scraper does not have a dropped center bit. Keep in mind that when working in hard material, the scraper might need additional support equipment for ripping and/or push loading. Similarly, when working in wet soils, a scraper might not be able to operate at all.

By contrast, the excavator/truck system is much more versatile when it comes to working in adverse soil conditions. For example, when working in hard material, an excavator can often load the material as is, without ripping. Similarly, when working in wet soils, the excavator has the ability to excavate saturated soils, in some cases even in standing water, while the trucks safely operate on a higher/drier level.

In some cases the best combination may include both an excavator and scrapers. This setup could work well when the material is too wet or hard for the scrapers to load but haul distances are relatively short. With this combination, the excavator loads material directly into the scrapers and the scrapers then haul and spread the material (no dozer required). Thus, you are able to take advantage of the benefits of both systems.

Balancing the Truck/Excavator System

The concept of balancing is based on the fact that by fully utilizing (or amortizing) the excavator, we can minimize its unit costs (e.g., $/yd.). For example, let’s assume that the excavator costs $180/hr. Its productivity costs are determined by dividing its hourly cost by the number of yards it loads per hour. Since the cost is fixed, it makes sense to get as much work as possible out of the excavator. As with any type of construction project, the truck/excavator system must be balanced if it is to be cost-efficient. In other words, the number of trucks must match the capabilities of the excavator(s) in order for the system to operate as efficiently as possible. We’ll go through a simple example so that you understand the concept of balancing. To do this, we balance the system by focusing on two factors.

First, we must determine how long it takes the excavator to load one truck. The loading time is affected by many factors, such as bucket size, type of soil, digging depth, swing time, but for the purposes of this discussion we’ll assume that the excavator can load a truck in 1.5 minutes.

Second, we need to know how long it takes the truck to make a roundtrip cycle. You can determine this from field observations or use a computerized vehicle-simulation model similar to the one used for scrapers. For this example we’ll assume that the truck cycle time is 6 minutes (including the 1.5-minute loading time).

Using this information, our goal is to balance the system so that the excavator and the trucks are all working at full capacity. If we have only one truck, the excavator will spend 1.5 minutes working and 4.5 minutes waiting for the truck to return. To keep the excavator busy, we need more than one truck. To determine how many trucks are needed, divide the total cycle time (6 minutes) by the load time (1.5 minutes):

Cycle Time (6 min.) ÷ Load Time (1.5 min.) = 4 trucks

Four trucks balance the system so that neither the trucks nor the excavator have any wait time (see Table 1). Because a balanced operation provides full utilization of all the equipment, it is the most economical in terms of the cost per yard.

In our example, the system balanced at exactly four trucks. More often the options are not so clearly defined. In many cases, the balanced fleet calls for some number of trucks plus a fraction (e.g., 4.4 trucks). Obviously, trucks aren’t available in fractions; you must round-off the balance to a whole number of trucks. The decision of whether to round up to the next whole number (thus creating some wait time for the trucks) or round down (thus creating some wait time for the excavator) should be based on economics. Usually the economics are measured in terms of cost per yard of soil hauled.

To determine the cost per yard for various equipment combinations, you’ll want to produce a table showing the hourly cost and the hourly productivity of various options. For example, if we assume that the trucks can carry a 22-yd. payload, the cost breakdown would resemble Table 2. Here we assume that the excavator costs $180/hr. and the trucks cost $80/hr.

Table 1. Excavator/Truck Productivity (all times in minutes)

Table 2. Cost per Cubic Yard

In Table 2, the minimum cost is achieved when four trucks are used. As noted above, sometimes the number of trucks will not come out to a whole number. However, when you calculate the costs, you find that there is usually a certain combination that is the most economical.

Increasing the Productivity of Trucks

Achieving a balanced operation is a big step toward having an efficient truck/excavator system. However, in terms of truck productivity, there are many other important factors that you’ll want to consider, including establishing good haul roads (e.g., proper slope, width, distance, surface) and paying particular attention to such things as sharp turns and sight distance. Remember, unlike scrapers, trucks do not have the ability to stop quickly by dropping the bowl. Also, if you use trucks, you’ll have to provide a dozer in the dump area to keep the piles knocked down and graded. In addition, truck-bed shape, truck-bed size, tire size, loading position, and dumping pattern will also affect the productivity of the trucks.

Truck-Bed Shape. If possible, the shape of the truck’s bed should match the type of material being hauled. A square-cornered bed generally provides a greater payload and improves the truck’s stability by keeping the center of mass lower. On the downside, a square-cornered bed will not clean out as well as a rounded bed, particularly when hauling wet, cohesive soil (e.g., clay, mud, silt). If excess soil is allowed to build up inside the bed of the truck, it can greatly decrease the productivity. A rounded bed might be slightly less stable because much of the load is carried higher on the truck. But when hauling wet or cohesive soil, a rounded bed might be preferable, as it is much less likely to allow soil buildup.

Truck-Bed Size. The size of the truck bed is obviously a major factor in productivity. The larger the bed, the larger the payload. However, the greater the payload, the slower the truck’s travel time. There is an optimum payload for any truck, just as there is an optimum payload for scrapers. Ideally the truck-bed size would be just large enough to carry the optimum payload. If the truck bed is much larger than necessary, you’ll be wasting some of the truck’s effort hauling the excess steel instead of dirt.

Tire Size. The tire size will also impact a truck’s productivity. The larger the tire, the greater the ground clearance and the greater the potential speed. Larger tires (width and/or height) will also provide better traction. On the other hand, smaller tires will provide greater power (more torque) and are superior where heavy loads and/or steep grades are common.

Loading Position. A common error made on many excavation jobs is the improper positioning of the trucks for loading. A good rule of thumb is to have the trucks go to the excavator. Often the excavator (or other loading machine) will hold a loaded bucket in position directly above where the truck should park for loading.

Dumping Pattern. Establishing a practical dumping pattern can improve the productivity of the trucks. Important factors include keeping the piles consolidated in as small an area as possible to minimize the work required for a dozer to knock down the piles. If the material is very wet and sticky, you may want to construct a tipping deck (using rubble or gravel). Loads would be dumped on the edge of this deck and pushed off with the dozer. Make sure that the deck is large enough to accommodate the trucks without causing any wait time.

It is a good idea to consider the factors that increase the excavator’s productivity when deciding where to position the trucks for loading. Such things as excavator swing time and depth of cut should be taken into account.

Increasing the Productivity of Excavators

In most cases, the productivity of an excavator/truck system is limited by how fast the excavator can load trucks. For our above example, suppose that we could somehow increase the excavator’s productivity so that its load time was only 1 minute per truck. Doing so would increase the production to 60 loads per hour. This would require six trucks to balance but would also decrease the cost to $0.50/yd.

Let’s discuss some simple ways that you can increase the excavator’s productivity. That these are general guidelines that might need to be modified to fit specific job conditions.

Bucket Size. Selecting the proper bucket size is a vital part of increasing the excavator’s productivity. The larger the bucket, the fewer bucket loads required to load a truck. However, if the material being excavated is hard, a large bucket might not be able to work well as a result of the reduced breakout force.

Similarly, if the soil is dense (heavy), a large bucket could overload the excavator, causing machine damage or instability. A large bucket might work well when the material is loose and fairly lightweight (e.g., sand, wood chips, compost). As a general rule of thumb, it is best to use the largest bucket that the excavator can easily handle. You can use a larger bucket if you select an excavator with a shorter boom and stick and keep the bucket close to the machine. Figure 2 illustrates the maximum safe bucket loads for a specific excavator, based on how far the bucket is from the machine.

Bucket Shape. Excavator buckets come in a wide array of shapes. For most mass-excavation projects, you will want to select a wide, square-faced bucket, which provides lots of capacity. Keep in mind that the width of the bucket will affect the breakout force. Thus, the harder the material, the narrower the bucket. In order to maintain a lot of capacity, some buckets are quite deep. The drawback of having a deep bucket is that with some types of material (clay, mud, etc.), the bucket might not clean out when dumped. If the soil builds up, it will decrease the bucket’s capacity.

To prevent soil buildup, some buckets might have three or four links of heavy chain (maybe with a weight) welded inside the bucket to help loosen soil that tries to stick to the bucket. In other cases, the bucket may be lined with plastic tiles or other slick material. In extreme cases, the bucket may be equipped with a hydraulically powered plate (an ejector) that pushes dirt out of the bucket.

Some buckets are built with a staggered tooth arrangement or rippers. These buckets are used to excavate very hard material (e.g., fractured rock or frozen soil). Again, these buckets are built more for strength and breakout power than for capacity. Thus, if you must use a rock bucket to excavate hard material, you should not expect to achieve the same productivity as you would with a mass-excavation bucket.

Positioning. Allow the excavator to obtain a full bucket of material (see Figure 3). If the bank is too low, the excavator might have to crowd too hard into the bank to get a full bucket. This not only slows production, but it is hard on the machine as well. The other option with a low bank is to obtain only a partial bucket of material. Either way, productivity is reduced. On the other hand, if the bank is higher than necessary, the excavator will waste time dipping down and raising material back up.

Swing Time. The excavator’s swing time is the time it takes to swing from the load position to the truck. You should always work to minimize the amount of swing that the excavator has to make. Usually this is accomplished by providing good access for the trucks and properly sequencing the excavation.

Another situation where the excavation depth can affect productivity is when you are digging a deep ditch. In this situation, it is not efficient to obtain a full bucket throughout the depth of the ditch (i.e., vertical cuts). If you do, every load will require the bucket to travel all the way to the bottom of the ditch (see Figure 4).

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