Preventing Collapse in Excavation and Trench Work

Excavation safety: before work starts

In utility lines, foundation excavations or channel digging, the greatest danger is the collapse of the side wall of the excavation. Even when collapsing soil only partly buries a worker, the pressure on the chest can stop them from breathing; rescue work also carries the risk of a second collapse. That is why excavation safety is planned before digging starts, not after.

The minimum requirements for excavation work are set out in Annex 4 (B), Section II of the Regulation on Occupational Health and Safety in Construction Work, under the heading "Excavation work, wells, underground work, tunnels and channel work". Before excavation starts, the Regulation (Art. 62) requires the following:

Adjacent structures: It is investigated whether the excavation will affect adjacent structures; if it may, the necessary measures are taken before excavation.
Underground services: The locations of underground cables, gas pipes, water, sewerage and other distribution systems are identified, and measures are taken to minimise the hazards they may cause.
Surroundings in built-up areas: The construction site is enclosed with a hoarding of sufficient height and strength, and warning arrangements are made.
Outside built-up areas: Warning tape is placed along the excavation edges and warning signs are posted.

Site preparation work (43.12.00) is in the very hazardous class in the 2025 hazard class list. The risk assessment for excavation work should not get lost in the general site assessment; ground, depth, duration and surrounding conditions should be addressed separately for each excavation.

Why does a trench collapse happen suddenly?

A trench collapse usually occurs through a combination of the following conditions:

Soil type: Sand and fill flow much more easily than cohesive clay; the ground can change from metre to metre in the same trench.
Water: Rain, groundwater or a burst pipe reduces the bearing capacity of the ground.
Surcharge: Spoil and materials piled at the excavation edge, and plant approaching the edge, put additional pressure on the side wall.
Vibration: Nearby traffic, compactors or breakers loosen the ground.
Time: A trench left open changes behaviour as it dries or gets wet; a face that looks sound in the morning may collapse in the afternoon.

The Regulation therefore requires inspection of excavation faces not as a one-off but as a cycle depending on conditions (Art. 64): a general inspection is carried out before each shift, after each blast if blasting is done, after unexpected falls of material, after significant damage to supports, and after heavy rain, frost and snow; work does not start until the area has been declared safe.

Excavation battering or a shoring system?

Art. 63/c of the Regulation sets the basic principle: in excavations, suitable batter angles are determined taking into account the ground structure, climatic conditions, nearby vibrations, surrounding water sources and excess loads, and/or suitable statically calculated supports and sheeting are used. Excavation faces and the slope and height of batters are chosen to suit the structure and strength of the ground and the working method.

The Regulation does not give a fixed table of angles or depths; the decision is made by engineering assessment according to the ground and the project. A comparison of the options:

Method	When is it suitable?	Points to watch
Excavation battering	Where there is enough space at the sides and the ground is suitable	The angle is set according to the ground; water and surcharge change the angle, and the batter face is checked separately
Benched excavation	In cohesive ground, as depth increases	Bench size is chosen according to the ground; may not be suitable in sandy ground
Trench box (prefabricated shoring)	In confined areas, for linear work such as pipe laying	The box must be sized to suit the excavation; no one should be inside during installation and removal
Timber or metal shoring, sheet piling	Deep excavations, near adjacent structures or roads	Requires static calculation; supports are not used as ladders

However strong a shoring system may be, Art. 70 of the Regulation prohibits the use of supports and sheeting for climbing in and out. The installation sequence must be planned in advance so that workers are not left in an unprotected section while shoring is being installed or removed.

Excavation edge loading: distance for materials and vehicles

On excavation edge loading, the wording of the Regulation is clear (Art. 68): sufficient distance is left between excavated spoil and the excavation edge, and if there is a risk of spoil sliding into the excavation, a suitable barrier is used; the necessary safety distance is left between the excavation edge and moving vehicles and other vehicles that may affect the stability of the excavation.

The Regulation does not give this distance in metres; the distance is determined according to the soil type, the excavation depth and the size of the load, and is taken into account in the shoring calculation. Rules to apply on site:

Determine and mark the area where spoil and materials such as pipes and concrete blocks will be stockpiled before excavation starts.
Determine the manoeuvring and parking areas and travel routes of construction plant (Art. 53); have reversing manoeuvres directed by a banksman and keep audible and visual warnings working (Art. 54).
Take measures such as chocks, barriers or stop blocks to prevent vehicles falling into the excavation (Art. 55).
Check the strength of the ground on which work equipment stands; take measures to prevent ground and machine slippage at trench edges (Art. 51).
Do not allow workers to enter the operating area of a machine while excavating by machine (Art. 71).

Underground services: electricity, gas and water

A power cable or natural gas pipe cut during excavation can have consequences as serious as a collapse. To apply Art. 62/b:

Before excavation, request information on services from the relevant utility companies and mark it on the drawings.
Verify the information on site with detection equipment and mark the route of the services on the ground.
In areas where services are known to be present, dig by hand and carefully, not with machinery.
Support or protect exposed services; do not place materials on top of them.
If an unexpected service is encountered, stop work and notify the relevant company.

For permit and isolation rules when working near power lines, see our articles on the permit to work system and the 5 rules of electrical safety.

Access and egress, rain and other rules

Safe access and egress: Safe routes are provided for entering and leaving the work area (Art. 63/b); ladders or ramps are positioned close to where workers are.
Supervision: Excavation work is carried out under the supervision of a competent person appointed by the employer (Art. 63/a).
Rain: No excavation work is done during rain (Art. 69); after rain, work does not resume until the excavation face has been checked.
Against falls: Suitable measures are taken against falling materials or objects, flooding and people falling into the excavation (Art. 63/ç).
Crossings: In built-up areas, crossings at least 80 centimetres wide with guardrails on both sides are used to pass over excavations (Art. 66).
Air and gas: Breathable air is provided at all workplaces; if substances such as hazardous gas or silica dust are detected, workers are evacuated immediately (Art. 63/d, 65).
Escape: Workers must be able to reach a safe place in situations such as collapse, flooding or fire (Art. 63/e).

For the risk of falling at the excavation edge, see our article what is working at height, and for general site order, our construction site OHS checklist. The health and safety plan and the duties of the coordinator are in our article on OHS in construction work.

In our risk assessment service, we carry out a separate assessment for each excavation according to ground, depth and surrounding conditions; during our OHS specialist visits, we monitor on site how excavation face checks and edge distances are applied. Other risks specific to the construction sector are on our construction sector page.

Start-of-shift excavation checklist

To record the inspection in Art. 64, use a short form to be completed by the competent person before each shift:

Are there cracks, bulging, flowing or falling material on the excavation side walls?
Are the batter angle and benches maintained; has rain caused any deterioration?
Are shoring elements, struts and the trench box in place and undamaged?
Is water collecting at the bottom of the excavation; is drainage or the pump working?
Are spoil and materials outside the set distance, and are the barriers in place?
Are plant parking and manoeuvring areas away from the excavation edge?
Is the access ladder or ramp in place and sound?
Are the warning tape, signs and guardrails around the excavation complete?
Are exposed underground services protected?

If any item is unsatisfactory, entry to the excavation is stopped until it has been corrected and the check repeated. Keep the form with the date, time and signature.

Frequently Asked Questions

At what depth is shoring required?

The Regulation on OHS in Construction Work does not give a specific depth; it requires a suitable batter angle to be determined, or statically calculated supports to be used, taking into account the ground structure, water, vibration and additional loads. The decision is made by engineering assessment according to the ground and the project.

How far from the excavation edge should spoil be placed?

The Regulation requires a "sufficient distance" and does not give a figure in metres. The distance should be determined according to the soil type, the excavation depth and the size of the load, taken into account in the shoring calculation and marked on site.

When should the excavation face be checked?

Before each shift, after blasting, after unexpected falls of material, after significant damage to supports, and after heavy rain, frost and snow. Work does not start until the check has declared the area safe.

Can work continue in a trench while it is raining?

No. The Regulation requires that no excavation work be done during rain. After rain, too, work should not resume until the excavation faces have been checked.