Excavation Safety

Ground collapse is one of the major risks when carrying out excavation work.


The sudden collapse, especially of a trench deeper than 1.5 meters, can occur in the blink of an eye and without warning. A mate, working in a trench, has no time to escape.

Not surprisingly, the deeper you go, the greater the risk.


Depending on the conditions, excavations down to 1.5 meters depth that can be shored, benched or battered are generally okay. You’ll need to carry out a job safety assessment (JSA) before starting, locate any buried services and have some ‘competency’ with excavations but limited excavation is usually within the realms of most tradies. It isn’t, however, a job for DIYers or weekend warriors. Things can still go wrong.


Start to go beyond the 1.5-meter depth and the requirements become more rigorous. You need to take additional safety steps AND show a higher level of competency such as a drain layers certificate or a civil engineering qualification.


Which brings us to the mysterious zone of influence; the area of an excavation most likely to fail. It’s defined as the edge of the trench angling in at 45° to the bottom.  Thinking of it geometrically, it extends as far back from the edge as the depth. The best thing to do is keep well away from it. That don’t drive the excavator track within this zone and always debris well away from the excavated area. The zone of influence gets a bit more complex if the ground is sloping or if you are excavating next to a building or retaining wall. And it’s usually a lot less stable if you are dealing with uncompacted material.


Worksafe NZ have produced some decent information on excavation safety, available at https://www.worksafe.govt.nz/dmsdocument/17-excavation-safety. The Hire Industry Association also produce a one-pager which we have on our website at https://www.ultraquip.co.nz/wp-content/uploads/2020/06/190811-04_EXCAVATOR-ESDS-HIANZ-V2.jpg


It’s worth a quick read and maybe an item for discussion at a toolbox meeting.