How should I level the concrete foundations for my fence's CMU block pillars?

Question

I'm getting ready to build a fence, consisting of CMU pillars spaced every 10 feet, with wooden pickets or latillas between them. For the block pillar foundations, I plan to dig holes in the ground, add gravel, and pour 4" pads of concrete for the CMU blocks to lay on top of. The grade is ever so slightly not perfectly flat.

How should I level the concrete foundations for the block pillars relative to one another? Both technically, and theoretically? Should I have the block pillar foundations precisely match the grade? Or keep them as level in an absolute sense as possible to keep the fence straight? And if I should level them... how to you level discontiguous pads of concrete 10 feet away from one another?

Answer 1

First off, your last question, which is most answerable:

how to you level discontiguous pads of concrete 10 feet away from one another?

Homeowner, these days, a laser level is easiest unless you happen to accumulate old survey equipment (guilty.) But you use the same techniques that work with a dumpy level or a transit - shoot a level line, and measure how far below it your surface is. Two things that are 42" below a level line are at the same level. If one is 44" below, it's 2 inches lower. 41-3/8" it's 5/8" higher. With CMU, adjust to the nearest 8 inches if using mortar, or the actual height of a block (no mortar) if using blockbond. Nails on the inside of the form is the usual level mark before pouring concrete in.

Whether you set the fence "level" or follow the grade is, well, up to you, people do it both ways, matter of opinion, also tends to vary in a practical sense with how far out the grade is (a level fence might be 4 feet high at one end and 20 at the other, which gets a little impractical and suggests following the slope more.)

You can use the level and measuring stick to determine exactly how far off level your grade actually is, if that would help you come to a conclusion.

Answer 2

I saw a crew installing a series of six manufactured units for temporary classrooms at a nearby school. Professional laser surveying equipment was used to level the site and for pouring concrete strips for foundations.

The units were supported on the concrete and in between with columns of cinder blocks (CMUs) topped with wood shims set to height with a long transparent hose connected to a liquid reservoir. The reservoir looked like a gallon plastic jug of auto windshield deicer (blue color) with a rough hole cut into the screw cap and the tubing going down into it.

The hole allows air to pass through around the tube (could be two holes). The tube would have been filled by suction (and siphon) and was small enough so that it could be filled with no bubbles left in it. Under these conditions if the open end of the tubing is kept higher than the level in the jug, then the level of the liquid at the open end of the tubing would be the same as the level in the jug. This would be true even if coils of the tubing would be higher than the level in the jug and the open end.

When the worker holding the end of the tube had to go downhill or uphill from the jug he would seal the end of the tube with his thumb, but small vice grips (or any suitable clamp) might be better for someone working alone.

To set the level of a unit, the jug would be placed at an elevation that matched the previous unit then the open end used to set the height of each pedestal in the current unit. In this way over a period of several days they installed six units so that when it was finished all six are at the same level.