Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 316

(A) (B)

FIGURE 12.1 Sloped excavation.

dirt has accomplished this at one time or another—nobody said soil science has to be boring). The
rolled piece is placed on a flat surface and then picked up. If a sample holds together for 2 inches, it
is considered cohesive.
The ribbon test is used as a backup for the thread test. It also determines cohesion. A represen-
tative soil sample is rolled out (using the palms of your hands) to a 3/4-inch diameter and several
inches in length. The sample is then squeezed between the thumb and forefinger into a flat unbroken
ribbon 1/8 to 1/4 inch thick that is allowed to fall freely over the fingers. If the ribbon does not break
off before several inches are squeezed out, the soil is considered cohesive.
Once soil has been properly classified, the correct protective system can be chosen. This choice is
based on both soil classification and site restrictions. The two main types of protective systems are
(1) sloping or benching , and (2) shoring or shielding . Sloping and benching are excavation protective
measures that cut the walls of an excavation back at an angle to its floor. Examples of a sloped or
angled cut and a bench system with one or more steps carved into the soil are shown in Figure 12.1.
The angle used for sloping or benching is a ratio based on soil classification and site restrictions.
In both systems, the flatter the angle, the greater the protection for workers. Reasonably safe side
slopes for each of these soil types are presented in Table 12.3.
Shoring and shielding are two protective measures that add support to an existing excavation;
they are generally used in excavations with vertical sides but can be used with sloped or benched
soil. Shoring is a system designed to prevent cave-ins by supporting walls with vertical shores
called uprights or sheeting . Wales are horizontal members along the sides of a shoring structure.
Cross braces are supports placed horizontally between trench walls. Shielding is a system that
employs a trench box or trench shield. These can be premanufactured or built onsite under the
supervision of a licensed engineer. Shields are usually portable steel structures placed in the
trench by heavy equipment. For deep excavations, trench boxes can be stacked and attached to
each other with stacking lugs.

TABLE 12.3
Maximum Safe Side Slopes in Excavations

Soil Type Side Slope (Vertical to Horizontal) Side Slope (Degrees from Horizontal)

A 75:1 53°

B 1:1 45°

C 1.5:1 34°

Source: OSHA Excavation Standard 29 CFR 1926.650-652.