There is no safe depth for an excavation. Too often, our advisors are being told that an excavation does not need supporting unless it is 1.2m or greater in depth. This is out of date practice and has been superseded by a simple statement that advises that there is no safe depth, that all excavations no matter what depth should be assessed to take into account the type of ground, the conditions of the ground, the water table, and adverse weather conditions and of course, the length, width and depth of the excavation.
Other considerations on the excavation stability may include the proximity of the spoil heap created from the soil extraction, the movement of heavy plant and machinery near to the excavation or unloading into the excavation and the proximity of existing structures, all of which may present additional ground bearing pressures through the walls of any unsupported excavation. A competent experienced person considering these factors will be able to determine the appropriate supporting requirements of an excavation and ensure they are used.
Also, don’t forget; an excavation requires a written record of inspection at the start of every shift before entry. Any obvious signs of instability such as earth movement cracks and/or deposits in the bottom of the excavation should be noted and entry prohibited until advice from a competent engineer is sought and appropriate action is taken in-line with their advice.