Years ago earthmoving projects were won by the best guesstimator. Today they're won by the contractors who did their homework.
Earthmoving is a generic term used to describe the process or processes used to change, ready or reconfigure a job site in preparation for a construction project. It can include both excavating and grading techniques.
Typical earthmoving equipment includes track-type tractors, motor graders, backhoe loaders, hydraulic excavators, scrapers, off-highway trucks — rigid frame and/or articulated, wheel tractors, wheel loaders, tool carriers, and wheeled excavators.
The primary objective of an earthmoving project is to move the required amount of material as quickly and economically as possible. Due to the nature of Planet Earth, there are few sites that simply involve removing "dirt," most include a variety of other materials such as rock, clay, and even trash and debris. In addition to the original list of earthmoving equipment contractors often have to add drills, mass excavators and rippers.
Excavating is any digging operation involving the removal of earth, usually to create a hole, trench or depression; or to remove site material in preparation for a construction project, i.e. building, industrial facility, and road or bridge abutment.
Equipment specific to excavating includes backhoe loaders, hydraulic excavators, track loaders, wheeled excavators, trenchers, and off-highway haulers. Hydraulic shovels or mass excavators are used for extremely large projects.
Grading is the act of altering the ground surface to a desired grade, level, elevation, or contour by cutting, filling, leveling, and/or smoothing. It is usually the final phase of an earthmoving projects. Typical grading equipment includes motor graders and smaller finishing track-type tractors. Since the advent of GPS and machine control the track-type tractor has become a major tool in the grading process for a lot of contractors.
Since all earthmoving projects are about moving material, as quickly at the lowest possible cost, the selection of equipment is more critical than a lot of other construction projects. Machines that are too small will cost in lost efficiencies while equipment that is over sized will cost in wasted fuel consumption. Successful earthmoving contractors know and understand what works best in any given situation.
The key to successful earthmoving is doing your homework. Before you bid an earthmoving project know all that you need to know about the project: volume of material to be moved, type of material, condition of the site, travel distances for all phases of the job, access and egress, what was on the site before, and if nothing was there, know what's hidden below the surface. Many a contractor has found serious problems with the site after starting the removal process. Always prepare by soil sampling and testing the site. If you are going to be removing material from deep cuts or excavations, make certain you know what you will be pulling out of the hole — rocks, clay. What kind of rocks — granite, limestone, sandstone? If you're going to be facing serious rock you need to know how thick the seam is and how much of the site it covers.
If you're working a previously developed site then find out what was there before and how long it had been there. In many urban areas there are old sites that are loaded with debris or worse. Remember, it wasn't that long ago that the attitude about what got buried on an abandoned site was not one of concern. As a result there can be toxic waste or contaminated soil. You'll want to know this before you place your bid, not after you show up to start working. This is the kind of information that will help you know what equipment will work best for the project.
When considering equipment there are some basic guides that can help you make the right choices. Since much of what you will be doing is hauling the material, think in terms that machine travel gets costly once you've gone beyond a specific point. In today's economy fuel consumption has to be a major consideration. You want the most economical way possible to move the greatest volume of material in the least amount of time at the lowest possible price.
Excavators are a digging and loading tool in an earthmoving operation and as such have to be matched with a hauler, dump truck, articulated hauler, or rigid frame hauler. A very basic guideline is that you want to be able to fill the truck in three to five passes. Scale the excavator and hauler to the size of the project. For large projects you may want to look at mass excavators. Caterpillar, in its Performance Handbook, outlines six steps for selecting a mass excavator:
- Determine the material type and bucket fill factor
- Estimate the cycle time
- Calculate the effective cycle per hour
- Calculate the required bucket capacity
- Select mass excavator for required bucket size
- Select matching haulers — for a mass excavator the hauler fill should be in four to six passes; for a front shovel it should be in three to five passes. Typically off-highway haul trucks are best suited for this application.
Wheel loaders are another loading tool although they can be used to move material over a short distance. Wheel loaders will work best in a range from 200 feet to 450 feet, to 490 optimally and 500 feet as a maximum distance. Using them for shorter hauls can get just a costly as going beyond the recommended range. Again, you need to match the wheel loader with the haul truck.
Track loaders can be used as a loading tool or for some application like an excavating machine. They're great for digging swimming pools, basements, foundations, and the like. Because they are capable of navigating rough underfoot conditions they are a good choice for a lot of earthmoving projects. Because of their versatility and digging capabilities they can do a lot on an earthmoving project that can't be done with a wheel loader. Travel distances should be on the short side like a track-type tractor dozer. Dozers shouldn't move material beyond 325 feet optimally with 375 feet being a maximum haul distance.
Scrapers are available either as self-propelled machines or pull-type attachments. Self-propelled scrapers have the power, traction and speed for reliable output in a variety of earthmoving applications. In addition to the open-bowl type, there also elevating scrapers and tandem-powered scrapers. The latter have twin engines and are designed to handle steep grades and poor ground conditions. These machines can operate independently or with the help of a pusher dozer. Cat also offers an auger arrangement that also provides self-loading capability and dust control.
Scrapers can be equipped with a push-pull arrangement using a hydraulically actuated bail and cushion plate on the front of the tractor and a hook attached to the rear of the scraper. The arrangement allows a pair of scrapers to link together when loading, with the rear scraper pushing the front scraper through the cut. As the front scraper emerges from the cut, it pulls the rear scraper through the cut.
In recent years pull-type scrapers have become more popular as more manufacturers have entered the construction arena. There can be economic considerations. You can pull these with either a large agricultural or industrial tractor or a crawler tractor, depending on the terrain. Scraper trains of two pull-type scrapers have been used successfully on earthmoving projects.
Scrapers are available with bowl sizes ranging from 5 cubic yards to 26 cubic yards and cut widths from 84 inches to 192 inches. As with other earthmoving equipment there is an optimum distance for scrapers. Consult with your equipment supplier to find out what that distance is.
In addition to GPS and machine control systems there are software programs and computer-aided earthmoving systems on the market to help you with your pre-bid estimating and to make jobsite management easier, more accurate and more profitable. Years ago earthmoving projects were won by the best guesstimator. Today they're won by the contractors who did their homework.