Estimating the actual cost of performing earthwork requires careful consideration of the variables involved. This includes items such as type of soil, whether water will be encountered, dewatering, whether banks need bracing, disposal of excavated earth, and length of haul to fill or spoil sites, etc.
It is very important to consider what time of year the project is scheduled for completion. Bad weather can create large cost overruns from dewatering, site repair, and lost productivity from cold weather.
When figuring the slope areas of pit excavation, remember that the slope areas usually form a right triangle when viewed as a section. Since the area of two right triangles equals the area of one rectangle with equal base and height dimensions, the volume of the slope area will equal the depth of the pit (the height) times the cut back distance of the slope (the base) times half the perimeter of the pit.
Cut and Fill
If a project has a large quantity of cut or fill, consider inquiring whether the site grade can be raised or lowered to economize the earthwork portion of the estimate.
If the project requires a large quantity of fill, consider creating a borrow pit on the site. This can lower the estimate significantly, as the cost for suitable fill material can be quite high in some areas of the country. The borrow pit can also be helpful as a receptacle for on-site spoil or other excavated materials, in this case saving dump charges.
It is important to keep in mind the location and availability of dump sites for spoil while estimating earthwork . Unexpectedly, large cost can accrue if the only site open to receive spoil quite a distance away.
Clearing and Grubbing
Often one of the most expensive parts of clearing and grubbing is the disposal of tree stumps. It is a good idea to know how and where this task can be carried out before completing the estimate.
One of the most common mistakes made in site work estimating involves site access. Estimators should visit the site not only to review site conditions, but to trace the route that the general or subcontractors’ equipment will take to get to the site. If the equipment that is to be used on this project cannot get to the site, the estimate may be completely inaccurate.
Sewerage and Drainage
One should not automatically assume that the sewerage and drainage lines will go in at the early stages of the project. It may appear to make sense from a scheduling standpoint, but the inconvenience of having the site divided in half, with open trenches, loose pipe, and restricted access, may ultimately cost more in lost time.
Many companies, eager to get started on their projects, shortchange the site investigation process. For the relatively short times and small amount of money involved, it is not a good idea to skimp on this important item. The untimely discovery of even one subsurface “abnormality” can be a painful lesson. Investigate the site thoroughly!