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Technical paper: Stability and safe trenching

By P M Cashman, Sykes Construction Services

This paper was first published in GE’s April 1976 issue


Much of the day to day trenching for placement of underground services penetrates to depths of no more than a few metres below ground level. The volume of this type of activity undertaken each year is considerable but it is difficult to generate great interest since the depths are modest. However, there have been several serious accidents, some of them fatal, involving men working in quite shallow depth trenches due to lack of attention to ground stability of the open excavation.

Especially during the last decade there has been mounting pressure on all industries to give closer attention to safety, and rightly so. The enactment of the “Health and Safety at Work etc. Act” of 1974 is a very definite indication that safety considerations must be given even closer attention in the next decade.

The basic principles of safe trenching can be quite simply stated as “The sides of any trench must be made secure”. Depending on soil and ground water conditions at the time of trench excavation this may be achieved either by draining or drawing down the ground water such that trenching is carried out in dry ground, or by supporting the sides of the trench, or by a combination of both. Whilst for “deep trenches” there may be practical limitations or variations on the use of some of the techniques referred to in this paper the basic principles remain unaltered.

The trenchless techniques will not be dealt with in this paper. 

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