Excavation: How to estimate for concrete (footings, walls, slabs, sidewalks, etc.)?

Question

How to estimate the excavation for concrete footings (or slabs, sidewalks, etc.)? In other words, let's say you have a 2ft. wide by 1ft high spread footing, at a min. of 3ft. below grade; let's also say the foundation wall is 12 inches wide by 2.5ft. high. running the length of the footing. My question is, how much excavation do you figure given these dimensions, or any dimensions for that matter?

Do you automatically figure to dig down to the depth of the bottom of the footing? How wide do you figure the dig will be (past the width of the footing)? At what depth must you start to slope in from the side when digging to prevent cave-in's?

Answer 1

Call an expert, this is nothing for a weekend do-it-yourselfer to mess with.  Way too heavy and way to dangerous.