Codes & Safety第 117 / 120 题
A trench must be excavated 22 ft deep in Type B soil to reach an existing sewer main. Which statement about the protective system and the excavated material is correct?
a.The standard sloping tables may be used at any depth, and the spoil may be piled at the trench edge to serve as a barrier
b.Sloping is prohibited over 20 ft so only a trench box may be used, and the spoil must be kept 1 ft back
c.The competent person may design the protective system on site at this depth, and the spoil must be kept 5 ft back
d.A protective system for an excavation deeper than 20 ft must be designed by a registered professional engineer, and excavated material must be set back at least 2 ft from the edge
解析
The tabulated sloping, benching and shoring options in the standard stop at 20 ft; anything deeper requires a protective system designed by a registered professional engineer. Spoil, tools and other surcharge loads must be kept at least 2 ft from the edge, or retained by a device that prevents material from falling in. Piling spoil at the edge adds load to the very wall that is most likely to fail, so it is never a barrier. A competent person inspects the excavation daily, before each shift and after rainstorms, but cannot substitute his own judgment for the engineered design a 22 ft trench requires.
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