Structural stability during excavations
What you need to do
The law says you must prevent danger to workers in or near excavations. To maintain the required precautions, a competent person must inspect excavation supports or battering at the start of the working shift and at other specified times. No work should take place until the excavation is safe.
Commercial clients must provide certain information to contractors before work begins. This should include relevant information on:
- Ground conditions
- underground structures or water courses; and
- the location of existing services.
- This information should be used to during the planning and preparation for excavation work.
Key issues are:
What you need to know
Every year people are killed or seriously injured by collapses and falling materials while working in excavations. They are at risk from:
- Excavations collapsing and burying or injuring people working in them;
- material falling from the sides into any excavation; and
- people or plant falling into excavations.
- No ground can be relied upon to stand unsupported in all circumstances.
- Depending on conditions, a cubic metre of soil can weigh in excess of 1.5 tonnnes.
Trenchless techniques should always be considered at the design stage as they replace the need for major excavations.
Underground and overhead services may also present a fire, explosion, electrical or other hazard and will need to be assessed and managed.
Collapse of excavations
Temporary support – Before digging any trench pit, tunnel, or other excavations, decide what temporary support will be required and plan the precautions to be taken.
Make sure the equipment and precautions needed (trench sheets, props, baulks etc) are available on site before work starts.
Battering the excavation sides – Battering the excavation sides to a safe angle of repose may also make the excavation safer.
In granular soils, the angle of slope should be less than the natural angle of repose of the material being excavated. In wet ground a considerably flatter slope will be required.
Falling or dislodging material
Loose materials – may fall from spoil heaps into the excavation. Edge protection should include toeboards or other means, such as projecting trench sheets or box sides to protect against falling materials. Head protection should be worn.
Undermining other structures – Check that excavations do not undermine scaffold footings, buried services or the foundations of nearby buildings or walls. Decide if extra support for the structure is needed before you start. Surveys of the foundations and the advice of a structural engineer may be required.
Effect of plant and vehicles – Do not park plant and vehicles close to the sides of excavations. The extra loadings can make the sides of excavations more likely to collapse.
Falling into excavations
Prevent people from falling – Edges of excavations should be protected with substantial barriers where people are liable to fall into them.
To achieve this, use:
- Guard rails and toe boards inserted into the ground immediately next to the supported excavation side; or
- fabricated guard rail assemblies that connect to the sides of the trench box
- the support system itself, eg using trench box extensions or trench sheets longer than the trench depth.
A competent person who fully understands the dangers and necessary precautions should inspect the excavation at the start of each shift.
Excavations should also be inspected after any event that may have affected their strength or stability, or after a fall of rock or earth.
A record of the inspections will be required and any faults that are found should be corrected immediately.