Common questions about excavation and trenching safety on construction sites.
How deep does a trench need to be before it requires shoring?
There is no specific depth in UK law that triggers an automatic requirement for shoring. The legal duty under the Construction (Design and Management) Regulations 2015 is to prevent danger from collapse at any depth. In practice, the risk of collapse increases significantly once an excavation exceeds 1.2 metres in depth, and most competent persons will require some form of support — shoring, trench boxes, or battering back the sides — at or beyond this point. However, even shallow trenches can collapse in unstable ground, waterlogged conditions, or where there is surcharge loading from nearby vehicles or material stockpiles. The HSE has investigated fatal collapses in trenches as shallow as one metre. The correct approach is to carry out a risk assessment for every excavation, regardless of depth, considering the soil type, groundwater conditions, nearby loads, weather, and duration of the work. If there is any risk of collapse, you must provide adequate support. Relying on a fixed depth threshold rather than a proper assessment is exactly the kind of shortcut that leads to fatalities. When in doubt, shore it — the cost of trench sheets or a box is negligible compared to a life.
What does HSG47 say about locating underground services?
HSG47, ‘Avoiding Danger from Underground Services’, is the HSE’s key guidance document on this topic. It sets out a clear process that every contractor should follow before breaking ground. First, you must obtain up-to-date service plans from the asset owners — gas, electricity, water, telecoms, and any other utilities that may be present. You can request these through services like the LSBUD (Line Search Before U Dig) system, which is free. However, HSG47 makes it very clear that plans alone are not sufficient. Service positions on drawings can be inaccurate by a metre or more, and some services may not appear on any plan at all. The second step is to use a cable avoidance tool (CAT) and signal generator (Genny) to scan the ground and locate services. The person using the CAT must be trained and competent — an untrained operator can miss services or misinterpret the signals. The third step is to use safe digging practices when working near located services. This means hand digging or vacuum excavation within 500mm of a known service, never using mechanical excavators to dig directly over a service, and supporting any exposed services to prevent damage from movement. HSG47 also stresses the importance of a permit to dig system to ensure all these steps are followed before any excavation begins.
Who is the competent person for excavation inspections?
Under the Construction (Design and Management) Regulations 2015, every excavation must be inspected by a competent person at the start of each shift before work begins, after any event likely to have affected the stability of the excavation (such as a fall of material, heavy rain, or vibration from nearby plant), and after any accidental fall of rock, earth, or other material. The competent person must have a combination of practical and theoretical knowledge and experience of the type of work being carried out. For excavation inspections, this means someone who understands soil mechanics, the effects of water and loading on excavation stability, the correct installation of shoring and support systems, and the signs of potential collapse. In practice, this is usually an experienced site supervisor, site manager, or a specifically appointed temporary works coordinator. There is no single qualification that automatically makes someone competent — it is a combination of training, experience, and knowledge relevant to the specific excavation. The competent person must record the results of each inspection in a written report, and any defects or risks identified must be dealt with before work in the excavation is allowed to continue. These inspection records should be kept on site and available for review by the HSE.
Do I need a permit to dig on every construction site?
A permit to dig is not a specific legal requirement under UK health and safety law — there is no regulation that says you must have one. However, the HSE strongly recommends permit to dig systems as best practice, and in reality, any well-managed site will use one. The permit to dig is a formal, documented process that ensures all the necessary checks are completed before any excavation begins. A typical permit requires confirmation that service plans have been obtained and reviewed, that a CAT and Genny survey has been carried out, that the excavation has been risk assessed, that the method of excavation and support has been agreed, and that the relevant people have been briefed. On sites where a principal contractor is in control, the permit to dig will usually be part of the site management system, and no subcontractor will be allowed to break ground without a signed permit. On smaller sites where you are the only contractor, you might not have a formal permit system, but you still need to demonstrate that you have followed the HSG47 process and assessed the risks. If you strike a gas main or an electric cable and the HSE investigates, the first thing they will ask for is your permit to dig and your service plans. Not having them will make it very difficult to argue that you took all reasonably practicable precautions. In short, while not strictly mandatory, a permit to dig is effectively essential on any site where you are excavating near potential underground services.
What should I do if an excavation starts to collapse?
If you see signs of an impending collapse — cracking along the edges, bulging of the sides, soil falling into the excavation, or movement of the shoring — the immediate priority is to get everyone out of the excavation and away from the edges. Do not attempt to shore up a failing excavation while people are still inside it. Evacuate first, then assess. Once everyone is clear, establish an exclusion zone around the excavation. The extent of the zone depends on the depth and conditions, but as a minimum, keep everyone back at least as far from the edge as the excavation is deep. Do not allow anyone back into or near the excavation until a competent person has inspected it, identified the cause of the instability, and confirmed that adequate measures have been put in place to make it safe. This may involve installing additional shoring, dewatering, reducing surcharge loads, or battering back the sides to a safe angle. If someone is trapped in a collapse, call the emergency services immediately. Do not attempt to dig them out by hand or with machinery unless you are trained in trench rescue — secondary collapses are common and can bury rescuers as well. The fire service has specialist trench rescue teams trained and equipped for this exact scenario. After any collapse or near-miss, report the incident under RIDDOR if it meets the criteria, investigate the root cause, and review your risk assessment and method statement before work resumes.