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How Accurate Is It?

By Greg Koontz, PE, LS, President, Koontz-Bryant, P.C.

  1. I have worked in site engineering for quite a long time – longer than I would like to admit! When I went through high school we all had slide rules. One of my favorite teachers was my physics teacher – he had a monster slide rule hooked to the black board and could get an answer faster than anyone in the class. I can still remember him counting out the decimal places. In the mid to late 70’s calculators just started to hit the scene. My dad was a practicing engineer and let me play with his first one, a TI that added, subtracted, multiplied and divided. Technology started to come along pretty fast at that time, and I was actually required to have a calculator for my first year of engineering. Also while in college, there were main frame computers, Fortran punch cards for writing programs and “dumb” terminals. In the mid 80’s we all started to use PC’s and in the 90’s cell phones. In 2000’s we fell in love with smart phones with so much more computing power than any of the early mainframes.

  2. With all this technology, it seems everything we can now design on a site should be rock solid, exact down to the nth degree. Even with all this these improvements, many of the basic design elements, issues and assumptions encountered in a site design have not changed, just some of the tools we use to calculate the answer. Drainage and earthwork are two sitework areas where basic methods are the same while great advances in the calculations have been made - but are the answers really any more accurate?

  3. Drainage

  4. Today we typically use CAD to determine drainage areas where we once used “planimeters” followed by programs or spreadsheets where we used graphs. Is this really more accurate than it was 30 years ago? Much of the methodology used to calculate drainage (Rational Method) is exactly the same; however, the understanding of significant digits appears to be lost in the technology and everyone looks at the results as exact solutions. Some localities require the Soil Conservation Service (SCS) method for all drainage calculations. An interesting exercise can be to compare the SCS method to the Rational Method on drainage areas less than 200 acres. Typically the drainage results will be 20% to 50% greater, requiring much larger pipes. It is also interesting that some of the parameters accepted by VDOT were reduced several years ago, increasing the disparity between the two accepted methods.

  5. Earthwork

  6. Earthwork has continued to be an area requiring extra diligence. Early hand computation methods of “average end areas” and “borrow pit or grid” provided results used for years. Early computer programs used these same methods, mainly with manual entry of the same information and then calculated by a program. Today there have been some super software advances with programs using Triangulated Irregular Networks “TINS” and “surfaces”, and allowing adjustments that can balance a site within several Cubic Yards. Even with these precise calculations, topography used is only an approximation. Who really knows precisely how much topsoil will be stripped?

  7. Each sitework design is unique in that there are major design elements based on approximations for both drainage and topography. How accurate are these results, when the methods and assumptions used can dramatically change the results? There is no denying that to achieve acceptable results, both of these areasrequire great care during the design process. Design engineers, approval agencies, and owners all must realize that there are no exact answers for these critical design elements, just approximations based on the engineer’s input. As a business owner, I believe we serve our clients and the public best when our site engineers exercise a balance of technology, experience and good old common sense both in their design and approach to problem solving.

  1. Contact Greg, at 804-740-9200 for more information.