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Procedure to Protect Excavation Systems

EXCAVATION PROTECTION SYSTEMS:

There are three basic protective systems for an excavation and trenches:
a-                 Sloping and Benching Systems
b-                 Shoring Systems
c-                 Shields Systems
The protective systems shall have the capacity to resist without failure all loads that are intended or could reasonably be expected to be applied to or transmitted to the system.

SLOPING & BENCHING SYSTEMS:

Sloping Systems:
Maximum allowable slopes for excavations less than 20 feet (6.09 m) based on soil type and angle to the horizontal are as follows:
Soil Type
Height/Depth ratio
Slope Angle
Stable Rock
Vertical
90 deg.
Type A
¾ : 1
53 deg.
Type B
 1 : 1
45 deg.
Type C
1½ : 1
34 deg.


Benching Systems:
There are two types of benching, simple and multiple. The type of soil determines the horizontal to vertical ratio of the benched side.
As a general rule, the bottom vertical height of the trench must not exceed 4 feet (1.2 m) for the first bench. Subsequent benches may be up to a maximum of 5 feet (1.5 m) vertical in Type A soil and 4 feet (1.2 m) in Type B soil to a total trench depth of 20 feet (6.0 m).


Shoring Systems:
Shoring is the provision of a support system for trench faces used to prevent movement of soil, underground utilities, roadways, and foundations. Shoring is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring systems consist of posts, wales, struts, and sheeting. There are two basic types of sharing, timber and aluminum hydraulic.

Hydraulic Shoring:


Hydraulic Shoring is a prefabricated strut and/or wale system manufactured of aluminum or steel. Hydraulic shoring provide a critical safety advantage over timber shoring because workers do not have to enter the trench to install or remove hydraulic shoring.
All shoring should be installed from top down and removed from bottom up.


Pneumatic Shoring:
Works in a manner similar to hydraulic shoring. The primary difference is that pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneumatic shoring is that an air compressor must be on site.
Shielding Systems:
A- Trench Boxes:
Are different from shoring because, instead of shoring up or otherwise supporting the trench face, they are intended primarily to protect workers from cave-ins and similar incidents. The space between the outside of the trench box and the face of the trench should be as small as possible. The space between the trench boxes the excavation side are backfilled to prevent lateral movement of the box.
Combined Use:
Trench boxes are generally used in open areas, but they also may be used in combination with sloping and benching.
The box should extend at least 18 inch (0.45 m) above the surrounding area if there is sloping toward excavation. This can be accomplished by providing a benched areas adjacent to the box.
Spoil:


Temporary spoil must be placed no closer than 2 ft. (0.61 m) from the surface edge of the excavation

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