Trenches collapse for numerous reasons and excavations should therefore be shored for safety reasons, a trench does not have to be in excess of 1200mm deep to be dangerous particularly if you are bending down or crouching in the excavation in order to joint a pipe or pick up the tape measure you have dropped.
Loose and granular soil is naturally unstable as are most ground types when affected by ground water or inclement weather, trenches that cross disturbed or infill ground are also prone to collapse and vibration from site plant and vehicles can also disturb previously stable ground.
It is therefore necessary to use a proprietary trenching system such as hydraulic walling frames, trench sheets, trench boxes and drag boxes
Other techniques used for trench safety include battering back the sides of the excavation to a safe working angle and stepping the sides of the dig, though these techniques are usually used on larger projects.
(This article comes from draindomain.com editor released)