15 AUGUST 2005
Support Systems. Excavation support systems commonly used are as
Trench Shield. A rigid prefabricated steel unit used in lieu of shoring,
which extends from the bottom of the excavation to within a few feet of the
top of the cut. Pipes are laid within the shield, which is pulled ahead, as
trenching proceeds, as illustrated in Figure 8-1.1 (from "Cave-In!" by
Petersen). Typically, this system is useful in loose granular or soft
cohesive soils where excavation depth does not exceed 3.5 m (12 ft).
Special shields have been used to depths of 9 m (30 ft).
Trench Timber Shoring. Table 8-1.3 illustrates the Occupational Safety
and Health Act's minimum requirements for trench shoring. Braces and
shoring of trench are carried along with the excavation. Braces and
diagonal shores of timber should not be subjected to compressive
stresses in excess of:
S = 1300 - 20 L/D
L = unsupported length (mm or inches)
D = least side of the timber (mm or inches)
S = allowable compressive stress in kilograms per square cm
(pounds per square inch) of cross section
Maximum Ratio L/D = 50
* Note: L/D units need to be consistent
Skeleton Shoring. Used in soils where cave-ins are expected. Applicable
to most soils to depth up to 9.1 m (20 ft). See Figure 8-1.2 (from "Cave-
In") for illustration and guidance for skeleton shoring. Structural
components should be designed to safely withstand earth pressures.
Close (Tight) Sheeting. Used in granular or other running soils, compared
to skeleton shoring, it is applicable to greater depths. See illustration in
Figure 8-1.3 (from "Cave-In")
Box Shoring. Applicable to trenching in any soil. Depth limited by
structural strength and size of timber. Usually limited to 18.2 m (40 ft).
See illustration in Figure 8-1.4 (from "Cave-In")
Telescopic Shoring. Used for excessively deep trenches. See illustration
in Figure 8-1.5 ("Cave-In").
Steel Sheeting and Bracing. Steel sheeting and bracing can be used in
lieu of timber shoring. Structural members should safely withstand water