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 during the planning and preparation for excavation work.

Key issues are:

• Collapse of excavations
• Falling or dislodging material
• Falling into excavations
• Undermining nearby structures
• Underground and overhead services
• Inflow of ground and surface water
• Damage to trees
• Other aspects of excavation safety
• Inspection

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.

Remember:

• 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 tonnes.

Trenchless techniques should always be considered at the design stage as they replace the need for major excavations.

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.

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.

Undermining nearby structures

Make sure excavations do not undermine the scaffold footings, buried services or the foundations of nearby buildings or walls.

Many garden or boundary walls have very shallow foundations which are easily undermined by even small trenches, causing the wall to collapse onto those working in the trench.

Before digging starts, decide if extra support for the structure is needed.

Surveys of the foundations and the advice of a structural engineer may be required.

Underground and overhead services

Many serious accidents have occurred when buried services have been damaged during excavation work.

Contact with any electricity cables can result in explosion and burns to those in the vicinity. Escaping gas which ignites can cause serious injury and/or property damage as a result of fire and explosion.

Excavation work should not start until steps have been taken to identify and prevent any risk of injury arising from underground services

Burns and electrocution can result if raised tipper truck bodies or excavators touch or come close enough to overhead power lines to cause arcing.

There is a risk to all those close to the item of plant which becomes live, as well as to the operator.

The need to undertake excavation work close to or below such lines should be very carefully considered and avoided where possible.

Inflow of ground and surface water

Depending on the permeability of the ground, water may flow into any excavation below the natural groundwater level.

The supports to the side of the excavation should be designed to control the entry of groundwater and the design should take any additional water loading into account.

Particular attention should be given to areas close to lakes, rivers and the sea.

Water entering the excavation needs to be channelled to sumps from where it can be pumped out; however, the effect of pumping from sumps on the stability of the excavation should be considered.

Alternative techniques for de-watering (such as ground freezing and grout injection) could also be used. Designers will need to consider these issues.

Damage to trees

Damage to trees, including root severance and root damage, can result in harm from trees falling over. Guidance to assist in reducing risks has been published by the National Joint Utilities Group (NJUG).

Other aspects of excavation safety

Provide a safe means of getting into and out of an excavation. If a risk assessment identifies that ladders are a reasonable means of access and egress from an excavation, they must be suitable and of sufficient strength for the purpose.

They must be on a firm level base, secured to prevent slipping and, unless a suitable alternative handhold is provided, extend to a height of at least 1 m above the landing place.

Consider hazardous fumes - do not use petrol or diesel engines in excavations without arranging for the fumes to be ducted safely away or providing for forced ventilation. Do not site petrol or diesel-engine equipment (such as generators or compressors) in or near the edge of an excavation; exhaust gases can collect and accumulate.

Inspection

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.

A written report should be made following most inspections and should contain the following information:

• name and address of the person the inspection was carried out for;
• location and description of the place of work or work equipment inspected;
• date and time of the inspection;
• details of:
  • any matter identified that could give rise to a risk to the health or safety of any person;
  • any action taken as a result of any matter identified;
  • any further action considered necessary; and
• name and position of the person making the report.