|Construction Safety and Health |U.S. Department of Labor OSHA
Office of Training and Education An open excavation is an
unnatural situation. The average landscape shows no vertical or
near vertical slopes. Soil or dirt is a very heavy material. A
cubic foot can easily weigh 114 pounds and a cubic yard can be
as heavy as a pick-up truck. Consider a column of soil which is
one foot by one foot and several feet high. At one foot of depth
a horizontal plane one foot by one foot is carrying the cubic
foot which lies over it. The stress, or load per unit area is
100 lbs. per square foot (psf). At a depth of two feet the
horizontal plane is carrying two cubic feet or 200 psf. At a
depth of five feet the vertical stress is 500 psf, and so on.
The column described would soon collapse if not supported by
similar adjacent columns. Stresses are developed which act
horizontally on the column. These lateral stresses can be
considered ½ as large as the vertical stresses. At a depth of
five feet the vertical stress is 500 p