In order of occurrence, soil in Area 1 at the base of the wall moves into the trench, and then is followed by the failure of Area 2. The failure of Areas 1 and 2 leave the remaining trench wall, Area 3, unsupported. Area 3 will break the cohesion and shear off the wall under its own weight and fall into the trench. Typically, time elapses between the failure of segments. It is the uncertainty of when the next failure will occur that makes rescue or recovery extremely hazardous.
Time is a major consideration.
The longer the trench is unsupported, the more potential there is for further trench collapse. Some employers and contractors believe that proper safety procedures waste valuable time and money, and that faster work creates larger profit margins. However, accidents that occur because safety precautions are not taken can be costly. In some cases, fines are also imposed. Six company employees were in a 12-foot deep trench when it collapsed, killing one person and injuring another.