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Each post should be 3 feet (0.91 m) away from each other—start by marking the first post location with chalk at one end of the wall. Now, move in 3 foot (0.91 m) increments along the line until you reach the end.  Use a taut string and stakes or a snapped chalk line to help you create a straight line. If you need to extend your wall with another post to create the final 3 feet (0.91 m) then do so. Space your posts 16 to 18 inches (41 to 46 cm) apart for load-bearing walls, which are walls that support something aside from their own weight such as a roof. Start by using a D-handle shovel to loosen the dirt along each side of the trench. Once it's loose enough, dig out the middle with a trench shovel. Continue loosening and digging into the soil with each of your shovels.  Call your local utility services number at least 3 to 4 days before digging to ensure it's legal and won't damage underground structures, such as power, sewer, water, or gas lines. Consider renting heavy equipment to dig your trench. Saw through large roots with a reciprocating saw or using the tip of your trench shovel. Use a steel bar to loosen rocks. for your posts. Thrust your shovel into the soil and jiggle it forward and backward and side to side. Once the soil is loosened, start digging down into the ground. Grip the middle of the shovel's handle with your non-dominant hand and the top of the handle with your dominant hand.  Use a steel bar to knock rocks loose. Remove large sections of loose soil with a clamshell digger. Saw through large roots by thrusting your shovel down into them or using a reciprocating saw.
Mark your post hole locations on the ground with chalk. Dig a 12 inch (30 cm) deep trench between your post locations. Dig 18 inch (46 cm) deep holes