Excert
"A vast number of trench’s are constructed every day throughout the country with a low percentage of failures occurring. It is however important due to the complex nature of the potential rescues that we are correctly informed and prepared.
So what is a trench? The definition given by The Trenching Practice carried out on behalf of CIRIA is “an excavation forming a structure as a result of digging, lifting and removing earth, fill or other materials from the ground. The purpose of which may be to form foundations, install services, drainage etc”
A trench by law need not be supported as long as the depth does not exceed 1.2m, the ground is self supporting and a safe working method can be employed. Over 1.2m alternatives to trenching should be considered and a suitable support system designed by an experienced person should be
employed.

In a study of fatal accidents in trench’s in the period 1973 – 1980 it showed that:
63% of deaths were due to unsupported excavations
20% of deaths were due to working ahead of the support system
14% of deaths were due to inadequate support systems