Introduction to Estimating Earth-Moving Quantities

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Cut Uphi

Figure 15.2-a Contours 101 through 107 have been repositioned uphill, thereby creating a cut slope

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Showing Fi

Figure 15.2-B Contours 101 through 107 have been repositioned downhill, thereby creating a fill slope

The work done by a contractor for site grading—referred to as earthwork in technical specifications and contract documents—can represent a substantial proportion of a construction budget. For example, earthwork would represent the major expense of a construction budget for constructing a set of soccer fields or other sport facilities, including a golf course. In order to estimate the cost for earthwork, several calculations are required to determine the amount of cut and fill required to implement the grading plan. There are several commonly used methods for calculating the volume of cut and fill, representing varying degrees of accuracy and amounts of time required to make the calculations. Many offices use AutoCAD, Land CAD, and other computer software to prepare their grading plans. Most software applications have an earthwork calculation function that makes determining the amount of earthwork fairly easy as one becomes proficient with the software. In this chapter we will consider a graphic means for estimating cut and fill. With practice accuracy will increase, as will speed.

When going through the process of calculating earthwork quantities, keep in mind that there is a factor that often contributes to over- or underestimating these quantities. The factor has to do with shrinkswell. The undisturbed soils on a project site are generally dense, having been in place for hundreds, thousands, and more years, going through daily and seasonal cycles of rain and sun, giving the soils ample time to consolidate and become dense. One only has to take a shovel and begin digging a hole to understand how the existing soils can be somewhat difficult to penetrate. Some soils are more dense and thus more difficult than other soil types. Next, after digging a hole, shovel the removed soil back into the hole, and you will find there is an excess of soil after the hole has been filled. By compacting the soil as you shovel it in, you can replace most of the removed soil, but often an amount on the order of 10 percent is left, with no more room in the hole for any more soil. This same concept is experienced when doing earthwork on a site. Even after making careful cut and fill calculations, the landscape architect may find an excess amount of soil remaining after the contractor has completed most of the earthwork. As one goes about the process of calculating the quantities of cut and fill of a site grading plan, an excess of 10–25 percent may occur during the actual earthwork operations. Consider 10 percent for sand and sandy soils, and as much as 25 percent for common 69 68 earth. Rock removed may result in as much as 65 to 70 percent overage—that is, rock 67 that is removed can produce a quantity 165 66 percent of the quantity calculated from the 65 site-grading plan unless it is crushed into 64 finer material. To start, consider the grading diagrams 63 in Figures 15.2-A and 15.2-B. Dashed contour 62 61 60 lines represent the existing topography, and the solid lines represent the proposed contours. The solid proposed contours that move downhill from the original contour location represent fill. Note that an upper

Fill Cut 66.5 Level Area 20 Scale Figure 15.3 Grading plan diagram for creating a level shelf area