Excavations and trenches

Why excavations are a priority risk

A cubic meter of earth weighs between one and two tons. A partial collapse of the wall of a two-meter-deep trench can bury a person in seconds, with fatal consequences from crushing or asphyxiation before rescue is possible. This is compounded by falls into the trench involving people and vehicles, ruptured gas or electrical lines leading to explosions and electrocutions, flooding from burst pipes or rain, and asphyxiating or toxic atmospheres at the bottom of deep excavations or those near sewers. Excavations consistently rank among the causes of fatal accidents in construction and utility services.

The safety of an excavation depends on decisions made before work begins: the geotechnical study and soil characterization, the location of buried utilities, the choice of protection system (slope, shoring, or shielding) according to depth and conditions, the planning of access and earth removal, and the organization of daily monitoring. Annex IV of Royal Decree 1627/1997 sets out the minimum requirements for earthworks, excavations, shafts, underground works, and tunnels, and Annex II includes excavations with a risk of burial among the works that involve special risks and require the presence of preventive resources and their specific treatment in the health and safety plan.

The INSST NTP 278 on trenches and prevention of landslides establishes criteria on depth from which shoring or sloping is mandatory (generally from 1.30 meters or in unstable terrain), types of shoring (light, semi-solid, solid), slopes according to the type of terrain, safety distances for stockpiles and machinery, and access.

Main preventive measures

• Preliminary study. Characterization of the terrain (nature, cohesion, water table level, nearby loads, adjacent buildings) and location of underground installations using company plans, detection and manual surveys.
• Wall protection system. Slopes with the appropriate gradient to the terrain, wooden or metal shoring, prefabricated shields or screens, dimensioned according to depth, terrain and surcharges; installed progressively during excavation and removed safely.
• Distances to the edge. Stockpiles of earth and materials at a sufficient distance from the edge (generally, no less than the depth of the trench or at least two meters), and vehicles and machinery kept away or with stops.
• Access and exits. Safe stairs or ramps at regular intervals, so that no person is more than a reasonable distance from an exit, and a prohibition on climbing on the shoring.
• Perimeter protection. Railings, fencing or marking of the excavation edge, protected walkways to cross trenches and nighttime signage on public roads.
• Water control. Pumping out, drainage and protection against heavy rain; inspection of walls after episodes of rain, frost or vibrations.
• Hazardous atmospheres. Measurement of oxygen, gases and vapors in deep excavations or near sanitation networks, ventilation and confined space procedures where applicable.
• Daily monitoring. Inspection of walls, shoring, access points and edges at the start of each day and after any circumstance that may affect stability, with a record.

Organization of the work

• Health and safety plan. Definition of the excavation method, the protection system and the specific measures in the plan approved by the health and safety coordinator .
• Preventive resource . Mandatory presence during work with risk of burial, in accordance with article 32 bis of Law 31/1995 and annex II of Royal Decree 1627/1997.
• Coordination with utility companies. Authorizations, releases, and safety distances for excavating near power, gas, water, or telecommunications lines.
• Machinery. Delimited work areas, signalmen, prohibition of people remaining within the machine’s operating radius and working under suspended loads.
• Training. Specific training for operators and trench workers on risks, protection systems, access and emergency response.
• Emergencies. Rescue plan in case of burial, media and coordination with emergency services.

Organizational application: how to plan and execute excavations safely

1. Gather the geotechnical study or characterize the terrain, and obtain information from the companies about buried services, with detection and test pits before excavating.
2. Define in the health and safety plan the wall protection system (slope, shoring or shielding), the access points, the distances and the measures against water and hazardous atmospheres.
3. Designate the preventive resource for work with risk of burial and train the excavation teams.
4. Carry out the excavation by installing the protection progressively, without any person accessing an unprotected trench from the defined depth.
5. Delineate and mark the perimeter, install walkways, and keep stockpiles and machinery at a distance from the edge.
6. Perform daily inspections and inspections after rain, frost or vibrations, with records, and suspend work if cracks or detachments are detected.
7. Have the rescue plan and the means of communication available, and coordinate with the service companies the work near their facilities.

Preventive management software allows recording daily excavation inspections, the presence of the preventive resource, authorizations from service companies, and atmospheric measurements, with evidence for the coordinator and the Inspection.

Limits and common mistakes

1. Excavating without locating buried services or relying solely on plans without test pits or detection.
2. Allowing access to trenches without shoring or slope because they are considered shallow or of short duration.
3. Stockpiling earth or placing machinery next to the edge, overloading the walls.
4. Lacking safe access and using the shoring as a ladder.
5. Do not check the excavation after rain, frost or prolonged stops.
6. Omit the measurement of atmospheres in deep excavations or near sanitation.

The technical criteria for slopes and shoring must be determined by competent personnel according to the terrain; this sheet is for informational purposes only.

Regulatory and reference framework

• Royal Decree 1627/1997, of October 24, Annexes II and IV . Minimum safety and health provisions in construction works: earthworks, excavations, wells and work with risk of burial.
• Law 31/1995, article 32 bis . Presence of preventive resources in dangerous activities.
• NTP 278: Trenches: prevention of landslides (INSST) . Criteria on shoring, slopes, distances and access.
• Technical guide for the assessment and prevention of risks related to construction works (INSST) . Criteria for the application of Royal Decree 1627/1997.
• Royal Decree 1215/1997, of July 18. Use of work equipment, including earthmoving machinery.

Royal Decree 171/2004 regulates the coordination of business activities applicable to works and work next to facilities of service companies; the UNE-EN 13331 standards regulate trench shoring systems.