6.3 Deep Excavations, Retaining Systems, Cofferdams & Caissons, Monitoring and Rescue

When excavations become engineered structures

Every excavation needs measures that prevent collapse (CDM regulation 24). Above a certain depth, or near structures, those measures must be designed by an engineer:

• Depth. DOSH's 2015 excavation bulletin (paragraph 68), restating the former BOWEC regulation 116, says that where a trench, cofferdam, caisson or tunnel exceeds 4 m in depth, its protection shall be built to the design and drawings of a Professional Engineer. BOWEC is repealed, but this remains DOSH's published guidance and is a sound trigger. CDM regulation 21(2) requires all temporary supports to be designed for foreseeable loads.
• Adjoining structures. Where the stability of nearby buildings or services is in question, support them by underpinning, sheet piling, shoring or bracing to a Professional Engineer's design (bulletin paragraph 26).
• Designers' duties. Under CDM regulations 11 and 12, the designers and principal designer must use the site investigation to eliminate or reduce excavation risks at the design stage, for example by choosing a retaining system that can be installed without people entering unsupported ground.

The Professional Engineer must be registered with the Board of Engineers Malaysia. The design pack (drawings, calculations, installation sequence, monitoring plan and trigger levels) is kept on site, and the excavation must be built to it.

Common earth-retaining systems

Other hazards of deep basements:

• base heave or boiling when the excavation is below the water table
• dewatering that causes settlement of neighbouring property
• falls into the excavation from its edges
• falls from the strut levels
• tremie and slurry work
• poor air in deep, confined corners (Section 6.4)

Cofferdams and caissons: CDM 2024 regulation 25

A cofferdam is a temporary enclosure that keeps water or soil out so work can be done inside, for example a bridge pier in a river. A caisson is defined in CDM 2024 as a structure sunk through ground or water to excavate and place work at a set depth, which then becomes part of the permanent works.

Regulation 25 requires every cofferdam or caisson:

• to be of suitable design and construction
• to be appropriately equipped so workers can shelter or escape if water or materials enter
• to be properly maintained
• not to be used for construction work unless a designated person has inspected it, and any plant and materials affecting its safety:
  • at the start of every shift
  • after any event likely to affect its strength or stability
• not to be used unless the designated person is satisfied that the work can be done safely

If the designated person is not satisfied, work does not proceed until the matter is put right. Inspection reports follow regulation 26 (Section 6.4).

The CDM Fourth Schedule lists, as work involving particular risks:

• work in wells, underground earthworks and tunnels
• work by divers with an air supply system
• work in caissons with a compressed-air atmosphere
• work exposing workers to drowning

The construction phase plan must contain specific measures for each of these.

DOSH's bulletin also advises:

• Never work alone in a trench, cofferdam, caisson or tunnel. Another person should be in the immediate vicinity at all times, and remote work needs a phone or two-way radio (paragraph 67).
• Treat the confines of deep trenches, cofferdams, caissons and tunnels as possible confined spaces, and apply the confined space ICOP where its definition is met (paragraph 71).
• Control dust and gases by ventilation so they do not injure health, obstruct vision or exceed safe levels (paragraph 72).

Monitoring deep excavations

Engineers specify instruments to warn of movement before failure. The SSS should know what they are and what the readings mean:

Monitoring plans usually set three trigger levels. The names vary, but a common scheme is:

Readings are useful only if they are taken on schedule, reviewed promptly and acted upon. The SSS should check that monitoring is on the inspection checklist and that the contingency actions are in the method statement.

Emergencies and rescue

A trench collapse is a time-critical emergency, and rescuers often become victims. A site's emergency arrangements must cover excavations (CDM regulation 32):

1. Raise the alarm and call emergency services (999). Give the exact location and say it is a trench collapse.
2. Stop nearby plant, piling and traffic to reduce vibration and further collapse, and keep vehicles and spoil away from the edges.
3. Do not enter an unsupported trench. Rescue only from protected positions: trench boxes, hastily installed support, or from the surface.
4. Locate the casualty. Uncover the head and chest first to allow breathing, using hands and hand tools, not machines.
5. Prepare for first aid for crush injury and suffocation. Hand over to the Fire and Rescue Department on arrival.
6. Preserve the scene and notify DOSH. A death or serious injury must be notified immediately under NADOPOD regulation 5. Collapse of a temporary support in a structure under construction is also a listed dangerous occurrence.

Scenario: a three-level basement beside an LRT line

A contractor plans a 12 m deep basement using secant piles and two levels of steel struts, next to an elevated rail line and a row of shops. The SSS's review:

1. Design: confirm a Professional Engineer's design, approved by the relevant authorities, with the installation and excavation sequence, the strut installation levels and the monitoring plan with trigger values.
2. Construction phase plan: specific measures for the particular risks (burial, falls from height into the excavation, work near the rail operator's assets).
3. Daily controls:
   • a designated person inspects at every shift start and after rain
   • the excavator is not allowed to dig more than the specified depth below a strut level before the struts are installed
   • strut-impact protection
   • edge protection and access stairs
4. Monitoring: readings reviewed daily by the engineer, with alarm-level actions briefed to supervisors.
5. Emergency: a rescue plan and an evacuation route from each basement level, tested in a drill (CDM regulation 32(3)).