Before you can build an office building, a house, a shopping center, a parking lot, or any other structure, the site must be qualified.
Estimating the amounts of cut and fill required to grade the construction site is one of the most complicated and tedious processes in estimating construction. Calculation of construction materials, areas to be painted, number of doors and windows, etc. is child’s play compared to estimating cut and fill amounts for a construction site. The reason for this is that the unenhanced site almost always has an uneven surface. The amount of dirt to be removed from the unimproved site or introduced can be very difficult to determine because of this.
There are three main ways to calculate cut and fill for a construction site. The first two are “by hand” and require the construction site plan, a ruler, a pencil, a calculator, and hours of time. The third method requires excavation software, a computer, and some computer skills.
The first “by hand” method uses the grid method to calculate cut and fill for the building site. In this method, the site is divided into grids of equal size. Grid squares are typically 1 inch to 2 inches in size on the paper plane. For each corner of each grid, both the existing elevation and the proposed or final elevation are calculated and noted on the plan. Once all elevations are determined, the difference between the existing elevation and the proposed elevation at each corner is also noted on the plan. Then, for each square on the grid, these elevation differences for each of the four corners are added and averaged by dividing the sum by 4. If the average of the elevation differences for a square on the grid is negative, then this is a cutting area. In cut areas, existing elevations must be reduced to proposed elevations or “cut”. If the average elevation difference is positive, that means the existing elevations must be raised to the proposed elevations or “filled in.” Then all the negative numbers are added and added to determine the total depths of cut. Then the positive numbers are added and added as the total fill depths. Each of these cut and fill totals is then multiplied by the area of the squares on the grid to determine the number of cubic feet of cut and cubic feet of fill. And finally, these volumes of cut or fill are divided by 27 to determine the total number of cubic yards of waste and cubic yards of fill at the site (excavators work in cubic yards instead of cubic feet). Finally, the difference between the total cut volume and the total fill volume is calculated. If there is more cutting than filling, then the dirt must be removed from the construction site and trucked. If there is more fill than cut, then soil must be imported to the construction site. The time required to calculate cut and fill using the grid method is approximately eight hours for a single sheet site plan of moderate complexity. Accuracy with this method is +/- 20%.
The second method used to calculate cut and fill is called the cross section method. In this method, the estimator draws a set on equally spaced horizontal lines along the site plane 1 to 2 inches apart. Then, for each cross-section line, the estimator plots, on graph paper, both the existing surface and the proposed surface vs. the distance along the cross section. Then count the number of squares on the grid where the existing surface is above the proposed surface. This is the cut area for that cross section. Then count the number of squares on the grid where the existing surface is below the proposed surface. This is the fill area of the cross section. Repeat these steps until all cross sections of the site plan are completed. Then you average the cut area between each pair of cross sections and multiply it by the distance between the cross sections and divide that number by 27. This is the cut volume between the adjacent cross sections. Then repeat these steps for the infill areas to determine the infill volume between all adjacent cross sections. Then add up all the cut volumes and all fill volumes to determine the total cut and fill for the construction site. If there is more cut than fill, then construction site dirt must be removed. If there is more backfill than cut, the earth must be imported to the construction site. The time required to calculate cut and fill with the cross section method is approximately 20-30% longer than with the grid method. Accuracy with this method is approximately +/- 15%.
The third and final method of calculating cut and fill for a construction site is to use excavation software. In this method, elevation data is plotted from the paper site plan using a large format digitizer, plotted on screen using a PDF of the construction site plan, or imported directly from an AutoCAD file. The computer then analyzes the drawing to construct both the existing and proposed surfaces. Then, for hundreds of thousands of points throughout the site, the software calculates the difference between the existing elevations and the proposed elevations to determine the total volumes of cut or fill for the construction site. The time required to calculate cut and fill using the excavation software is approximately one quarter of the time required for the grid method. Accuracy with this method is approximately +/- 5%.
The advantages of both the grid method and the cross section method for calculating cut and fill is that they both use inexpensive and commonly available supplies. The downside is that they both require long hours of work and have moderate accuracy.
The advantage of using excavation software to calculate cut and fill is a much higher estimation speed and much higher precision. The downsides are cost, which can range from several thousand dollars to several tens of thousands of dollars for software and hardware and requires the estimator to have some computer skills.
The appropriate method for your business to calculate cut and fill for your bids depends on the number of excavation takeoffs you need to do each year. If you are only doing a few excavation takeoffs per year and you or your estimator are not very busy, then “manual” methods are probably adequate. If you are an excavator or general contractor who does a lot of cut and fill estimates, purchasing the necessary excavation hardware and software will greatly increase your ability and accuracy in bidding for jobs that require excavation.