Plotting Proctor curves
Plotting Proctor curves
(OP)
I recently ran into a situation where we performed modified Proctor testing and the best fit parabolic curve as determined from gINT was less than the maximum point that was plotted. It was probably 3 or 4 pcf less than what the maximum point on the best fit curve was. What are some tolerances or standard rules of thumb for this situation.





RE: Plotting Proctor curves
If the material has very few fines (on the edge of where the Proctor test applies), you may not get a distinct peak. In this case, the fitted parabola may bear little resemblance to the data. Even more reason to have a human draw the curve.
RE: Plotting Proctor curves
Matter of fact, most Proctor curves have a right side parallelling the zero voids theoretical curve. That bends the other way, opposite from a parabola's right side line.
Modern stuff can't match mother nature too well.
RE: Plotting Proctor curves
Never believe what a software package is telling you if you "just know" it is not right.
RE: Plotting Proctor curves
My point is, you don't have to do it by hand, but if you try to do it mathematically, you need to know what you are doing.
RE: Plotting Proctor curves
Do it on Autocad and see what it does for curves. Probably like Panars so nicely explained. I use Autocad for contours and they work out well hitting all the points.
RE: Plotting Proctor curves
RE: Plotting Proctor curves
By DEFINITION in ASTM D1557, the curve is to be plotted as a "smooth curve connecting the points". The maximum dry density and optimum moisture are graphically determined points from the plotted curve. In software, the curve is a mathematical "best fit" with the point of inflection (1st derivative of the slope) being the MDD.
In hand plotted curves, the MDD is usually higher than any of the plotted points, because we try to bracket the optimum moisture, not hit it directly, in the test.
I'll take my trusty little 6-inch pointy french curve over the software every time.
You won't generally see that kind of variation you mentioned between two different people doing the test on a split sample (in the same lab).