6.4 Excavation Access, Edge Protection, Inspection Reports, Underground Services, Water and Air

Getting in and out

DOSH's 2015 excavation bulletin, restating the former BOWEC rules, advises:

• Stairways, ramps or runways should be the means of access to working levels below ground, except where the nature or progress of the work prevents them; then use ladders or other safe means (paragraph 50).
• In every excavation more than 1.2 m deep, provide ladders, stairways or ramps for safe access and egress, in sufficient numbers and locations to be readily accessible (paragraph 51).
• Where people must cross a trench, provide access-ways at least 600 mm wide with edge protection, and reduce crossings by limiting the length of trench left open (paragraph 52).

Ladders must also follow DOSH's falls guideline: set at 1:4, secured top and bottom, with stiles extending 1 m above the step-off point. Position ladders inside the protected zone of a trench box or shored section, never in an unsupported length of trench.

Preventing falls into excavations

CDM regulation 24(1)(d) makes preventing people, plant or material falling into an excavation a legal duty for every contractor.

Inspection and reports: CDM regulations 24 and 26

When to inspect

If supports or battering have been provided, no construction work may be carried out in the excavation unless the excavation, and any plant and materials that may affect its safety, have been inspected by a designated person (regulation 24(2)):

1. at the start of every shift in which work will be done
2. after any event likely to have affected its strength or stability, such as heavy rain, flooding, vibration, impact or a change in the support
3. after any material has unintentionally fallen or been dislodged

The designated person must also be satisfied that work can be done safely. If not, no work may be carried out in the excavation until the matter is put right (regulation 24(3)).

A designated person is someone who has completed an OSH training course and has adequate training and experience or knowledge of the work (regulation 3). DOSH's bulletin stresses that the excavation supervisor should be trained in geotechnical principles and senior enough not to be swayed by commercial pressure.

Reports (regulation 26)

If the inspection finds that work cannot be done safely, the designated person must:

1. inform the person for whom the inspection was done before the end of the shift
2. prepare a written report within 24 hours, containing:
   • the name and address of the person for whom the inspection was done
   • the location
   • a description of what was inspected, including plant and materials
   • the date and time
   • details of any matter that could give rise to risk
   • the action taken
   • any further action needed
   • the designated person's name and position

Other rules on reports:

• The person for whom the inspection was done keeps the report on site until the work is complete, and for three months afterwards, and provides copies or extracts to DOSH officers on request.
• Where start-of-shift inspections are made more than once in a seven-day period, only one report is needed for that period (regulation 26(4)).

A practical inspection checklist:

• the condition of supports or battered faces
• cracks at the edges
• water entry
• spoil and plant set-back
• access ladders
• edge barriers
• services exposed and supported
• the atmosphere where relevant
• lighting
• whether the work matches the method statement

Underground services

CDM regulation 27(4) says no construction work that could create a risk from underground services, or from damage to or disturbance of them, may be carried out unless suitable and sufficient measures have been taken to prevent the risk.

A permit-to-dig system typically requires:

1. Records: current drawings from the asset owners (TNB, water, gas, telecoms) and the client's as-built drawings. Treat them as indicative only.
2. Detection: a trained operator scans the area with a cable avoidance tool and signal generator before marking out, and repeats the scan as the dig deepens.
3. Trial holes: hand-dig with insulated tools to expose and confirm each service before machine excavation. Mark services on the ground.
4. Safe digging: machines stay back from located services. Expose services by hand or vacuum excavation. Support exposed services across the trench, and do not use them as steps or anchors.
5. Unknown cables: DOSH's bulletin (paragraph 81) says that where the exact location of underground power cables is unknown, workers using jack-hammers, bars or hand tools should have insulated protective gloves and footwear. The bulletin also says the employer should inform workers of the cable locations, hazards and protective measures, and de-energise circuits where practicable (paragraph 79).

If a cable is struck, keep clear, assume it is live, isolate the area, and inform the asset owner. If a gas main is damaged, evacuate, eliminate ignition sources and call the gas operator and the Fire and Rescue Department.

Overhead lines near excavations

Excavator booms and tipping trucks can contact overhead lines. CDM regulation 27(2) and (3) require lines to be diverted, isolated and earthed, or protected with warning notices, barriers and suspended protection. DOSH's bulletin recommends barriers at least 6 m horizontally from overhead lines, and red-and-white goalpost crossing points.

Water: dewatering and drowning

• Plan for groundwater and rain. Supports must be designed for the hydrostatic load and to control groundwater entry (bulletin paragraph 59). Provide dewatering and de-silting facilities so that floods and pollution do not reach drains, and consider the effect of sump pumping on stability: pumping fine sands can draw out soil and cause settlement (paragraph 60).
• Drowning (CDM regulation 28): where a person could fall into water or another liquid and drown, prevent falls so far as practicable, minimise the risk of drowning, and provide rescue equipment that is maintained and used promptly. Excavations next to rivers, ponds and flooded basements need lifebuoys and throw lines.
• Influx and silt entrapment: assess the risk from sudden water entry and trapping in mud (bulletin paragraph 58). Stop work and evacuate when water enters faster than it can be pumped.

Bad air in excavations

Trenches and pits can collect gases:

• carbon monoxide from petrol pumps, generators or vehicles running nearby
• hydrogen sulphide and methane from sewers and organic ground
• carbon dioxide from some soils
• solvent vapours from pipe jointing or contaminated ground

Many of these are heavier than air, so they collect at the bottom.

The confined space ICOP 2010 applies where a space is enclosed or partially enclosed, is not designed mainly as a place of work, could have harmful contaminants, oxygen deficiency or enrichment, or engulfment, and could have restricted entry or exit. Open-topped pits are among its examples. A deep, narrow shaft or a manhole being reconstructed can meet the definition, while a wide, well-ventilated trench usually does not. Decide by assessment, not by a fixed depth.

Where an excavation is, or may be, a confined space:

• An authorised gas tester tests from outside before entry: oxygen first, then flammables, then toxic gases.
• The atmosphere is hazardous if oxygen is below 19.5% or above 23.5%, flammable gas is above 10% of its LEL, or toxic gas is at or above its PEL.
• Use forced ventilation, keep engines and generators away from the opening, and follow the confined space permit system (Section 9.3).

Scenario: the toxic manhole

A crew is reconnecting a sewer at the bottom of a 3.5 m deep shored pit, using a petrol pump at the edge. One worker feels dizzy. The SSS:

1. Orders everyone out using the ladder in the shored section, and moves the pump well away and downwind.
2. Treats the pit as a confined space: an authorised gas tester tests the atmosphere from the surface and finds low oxygen and raised carbon monoxide.
3. Arranges forced ventilation and a permit to work, with a standby person and rescue tripod, before re-entry.
4. Records the near miss and has the designated person re-inspect the excavation, noting the event in the regulation 26 record.