Before you can build an office building, home, strip mall, parking lot or any other structure, the site has to be graded.
Estimating the cut and fill quantities required to grade the building site is one of the most involved and tedious processes in construction estimating. Calculating building materials, areas to be painted, number of doors and windows, etc. is child’s play compared to estimating cut and fill quantities for a building site. The reasons for this is that the unimproved site almost always has an irregular surface. The amount of dirt that must be removed from the unimproved site or brought in can be very difficult to determine because of this.
There are three main ways to calculate cut and fill for building site. The first two are “by-hand” and require the building site’s plan, a ruler, a pencil, a calculator and hours of time. The third method requires excavation software, a computer and a few computer skills.
The first “by-hand” method uses the grid method to calculate cut and fill for the buildings site. In this method, the site is divided into equal sized grid squares. The grid squares are typically 1 inch to 2 inches in size on the paper plan. For every corner of every grid square both the existing elevation and the proposed or final elevation elevation is calculated and noted on the plan. Once all of the elevations are determined, the difference between the existing elevation and the proposed elevation at each corner is noted on the plan as well. Then for each grid square, these elevation differences for each of the four corners are added together and averaged by dividing the sum by 4. If the average of the elevation differences for a grid square is negative, then this is a cut area. In cut areas, existing elevations need to be lowered down to the proposed elevations or “cut”. If the average elevation difference is positive, that means the existing elevations need to be raised to the proposed elevations or “filled”. Then all of the negative numbers are added together and totaled to determine the total of the cut depths. Then the positive numbers are added together and totaled as the total fill depths. Then each of these cut totals and fill totals is multiplied by the area of the grid squares in order to determine the number of cubic feet of cut and cubic feet of fill. And finally these cut or fill volumes are divided by 27 in order determine the site’s total number of cubic yards of cut and cubic yards of fill (excavators work in cubic yards rather than cubic feet). Finally, the difference between the total cut volume and total fill volume is calculated. If there is more cut than fill, then dirt must be removed from the building site and trucked away. If there is more fill than cut, then dirt must be imported onto the building site. The time required to calculate cut and fill with the grid method is about eight hours for a single sheet site plan of moderate complexity. The 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 at equally spaced horizontal lines across the site plan separated by 1 to 2 inches. Then for each cross-section line the estimator plots out, on graph paper, both the existing surface and the proposed surface vs. the distance along the cross-section. He then counts the number of grid squares where the existing surface is above the proposed surface. This is the cut area for that cross-section. He then counts up the number of grid squares where the existing surface is below the proposed surface. This is the fill area for the cross-section. He repeats these steps until all of the cross-sections for the site plan are completed. Then he averages the cut area between each pair of cross-sections and multiplies that by the distance between the cross-sections and divides that number by 27. This is the cut volume between the adjacent cross-sections. He then repeat these steps for the fill areas to determine the fill volume between all the adjacent cross-sections. He then sums up all of the cut volumes and all of the fill volumes in order to determine the total cut and fill for the building site. If there is more cut than fill, then dirt must be removed from the building site. If there is more fill than cut, the dirt must be imported onto the building site. The time required to calculate cut and fill with the cross-section method is about 20 to 30% longer than the grid method. The accuracy with this method is about +/-15%.
The third and final method to calculate cut and fill for building site is to use excavation software. In this method elevation data is either traced off of paper site plan using a large format digitizer, traced on screen using a PDF of the building site plan or imported directly from an AutoCAD file. The computer then analyzes the drawing in order to build both the existing and proposed surfaces. Then for hundreds of thousands of points across the site, the software calculates the difference between the existing elevations and the proposed elevations in order to determine the total cut or fill volumes for the building site. The time required to calculate cut and fill using excavation software is about one quarter of the time required for the grid method. The accuracy with this method is about +/-5%.
The advantages of both the grid method and the cross-section method to calculate cut and fill is that both use inexpensive and commonly available supplies. The disadvantage is that they both require many hours of work and have moderate accuracy.
The advantage of using excavation software to calculate cut and fill is much higher estimating speed and much higher accuracy. The disadvantages are cost which can run from several thousand dollars to several tens of thousands of dollars for the software and hardware and require that the estimator has some computer skills.
The appropriate method for your company to calculate cut and fill for your bids depends on the number of excavation take offs you need to perform each year. If you are only doing a few excavation takeoffs a year and you or your estimator are not terribly busy, then the “by-hand” methods are probably adequate. If you are an excavator or a general contractor who does a lot of cut and fill estimating, then purchasing the necessary hardware and excavation software will greatly increase your ability and accuracy in bidding for jobs that require excavation.