How to Make a Sloped Grade Flat
I’ve got an interesting problem here at the ole homestead. How to set up a 12’x24′ prefab greenhouse in a temporary location without disturbing the natural slope of the grade too much. The greenhouse doesn’t really need to be plumb, but the base must be Planar, IOW, flat.
First a little discussion about the differences between level and flat, and, perpendicular and plumb. Webster’s 1913 dictionary says that perpendicular is “being at right angles to a given line or plane.” Plumb means being parallel to the force of gravity at any given place on the earth.
Flat just means being Planar. The top of a table is flat even when the table is sitting on its’ side. Level means being perpendicular to plumb. If a flat table top is perpendicular to a plumb bob hung over it, then it is flat and level.
Back to the greenhouse base. First I’ll set 8 batter boards to define the perimeter and set their elevations at an arbitrary 24″ above grade at each of their locations, and run the 4 string lines. I define the upper long side, (A,) and the lower short side, (B,) as temporary references. I then adjust the A end of the B stringline up or down until it just touches the A string. Then, I do the same thing for the A end of the D string, (upper short side,) and the D and B ends of the C string, (lower long side.) finally, I’ll go back and tweak the elevations of the C-D corner and the D-A corner.
This is what it now looks like.
Now I have the Perimeter rectangle strung, but I don’t yet know if it’s Planar because any pair of diagonal corner batter board sets could be high or low compared to the other pair. I’ll drive Stakes in the Lees of each corner batter board pair so that the strung lines will cross close to the perimeter corners. See illustration Diagonal Stakes and Strings below. I then pull stringlines diagonally across the area, setting their elevation to just touch the perimeter lines near the greenhouse corners. After I set the second end of each diagonal line, I have to go back and tweak the elevation of the first end.
Looking at where the diagonals cross, it turned out that the A-B and C-D corners were 1 1/8″ higher then the other two, so I’ll lower the C-D batter boards and stake 1 1/8″ and raise the B-D set 1 1/8″. Why both? Because moving the corner elevation only has 1/2 the effect at the center of the diagonals.First tweak the daigonal string elevations as needed to bring them into contact, then do the perimeter strings to take them to the diagonals.
Since the batter boards were kept level thru their changes in elevation, I can resquare the rectangle and check the grade to perimeter string elevations and decide what is the easiest way to excavate and fill to make the base a planar surface.
Edited 8/10/2007 1:08 pm by SamT
Edited 8/10/2007 1:46 pm by SamT