Water, atmospheres and confined spaces

Two invisible or easily underestimated hazards share this lesson. Water undermines walls, adds pressure to supports, hides the trench floor and can flood a trench faster than workers can climb out. Gases and vapours can collect in a trench just as they do in a tank or manhole, and a trench can become a confined space. Both are controlled the same way: anticipate them in planning, monitor them during the work, and stop when conditions change.

Where the water comes from

Water reaches excavations from several sources:

• Groundwater, when the excavation extends below the water table or through a water-bearing layer of sand or gravel.
• Surface water from rain, snowmelt, irrigation or runoff from paved areas uphill of the trench.
• Broken or leaking water mains, sewers and services, sometimes damaged by the excavation itself.
• Nearby watercourses, ponds and ditches that feed water through permeable soil.
• Frost thaw in spring, which releases water held in frozen ground.

Water weakens soil walls by adding weight, reducing friction between grains and washing out fine particles. Granular soils below the water table can flow into the trench, a condition sometimes called running sand or quick conditions. In the floor of the trench, upward-flowing water can cause boiling or piping, where the floor loses all strength. Water in a trench also hides tripping hazards, makes footing slippery, increases the risk of electrical shock from tools and pumps, and causes hypothermia in cold weather.

Rules for water in excavations

Ontario's O. Reg. 213/91, section 230, requires an excavation in which a worker may be required to enter to be kept reasonably free of water. Section 235 permits timbering and shoring from the regulation's table only where there is no hydrostatic pressure; where groundwater pressure exists, an engineered system is needed. Alberta's soil categories treat soil that is wet, granular below the water table, or that exerts hydraulic pressure on supports as soft, sandy or loose, the weakest class.

Controlling water

• Divert surface water before it reaches the excavation, with berms, ditches, sandbags and grading. Do not let roof drains, pavement runoff or pump discharge flow back towards the trench.
• Pump accumulated water from sumps at the low end of the trench, with pumps sized for the inflow and someone responsible for keeping them running.
• For excavations below the water table, use dewatering systems such as well points or deep wells designed by an engineer, which lower the water table before digging rather than chasing water after it arrives.
• Use sheeting or engineered shoring that can resist water pressure where dewatering is not practical.
• Stop work and get out if water starts to rise faster than pumps can remove it, if the floor starts to boil, or if fine soil is being pumped out with the water, which shows that soil is being removed from behind the walls.
• Locate gasoline and diesel pumps on the surface, downwind and away from the trench, so that exhaust does not flow into it. Electric pumps need ground fault protection.

Pumped water can carry sediment and contaminants, and its discharge is regulated. In Ontario, construction dewatering above certain daily volumes must be registered on the Environmental Activity and Sector Registry or covered by a Permit to Take Water, and discharge to sewers is subject to municipal sewer-use bylaws. Check current thresholds and conditions with the Ministry of the Environment, Conservation and Parks and the municipality.

Hazardous atmospheres in excavations

An open trench feels like the outdoors, so workers rarely think of its air as a hazard. But a trench is a long, narrow pit with limited air movement, and gases heavier than air settle in it. Sources include:

• Leaking natural gas from damaged mains or services. Natural gas is lighter than air but can still collect under plates, in pockets and in soil before it disperses.
• Propane, gasoline vapour and other fuels from leaking tanks, contaminated soil or spills. These are heavier than air and pool in the bottom of a trench.
• Carbon monoxide from engine exhaust: excavators, compactors, generators, cut-off saws and gasoline pumps operating in or near the trench.
• Hydrogen sulphide from sewers, septic systems, organic soils and some industrial sites. It is heavier than air, toxic and deadens the sense of smell at higher concentrations. HSE-244 covers it in detail.
• Methane and carbon dioxide from landfills, organic soils, peat and decaying vegetation.
• Oxygen deficiency from displacement by other gases, or from oxidation of iron-bearing soils and decaying matter.
• Toxic vapours from contaminated soil at former industrial sites, gas stations and dry cleaners.

Testing the atmosphere

Atmospheric testing is required wherever a hazardous atmosphere exists or could reasonably be expected, such as near landfills, sewers, gas mains, fuel storage, contaminated sites, or where fuel-powered equipment runs in or near the excavation. Testing uses a calibrated multi-gas detector, typically measuring oxygen, flammable gases as a percentage of the lower explosive limit (LEL), carbon monoxide and hydrogen sulphide. The detector should be bump tested each day before use, and readings taken from outside the excavation by lowering a sampling probe or the detector itself before anyone enters. Test at the top, middle and bottom, because gases stratify. Where the excavation meets the definition of a confined space, the full confined space requirements apply, including an assessment, a plan, an entry permit and continuous monitoring.

Ventilation

Where a hazardous atmosphere is possible but can be controlled, forced ventilation with an air mover or blower can keep the air in an excavation fresh. The blower intake must be in clean air, upwind and away from engine exhausts, and the air should be delivered to the bottom of the excavation where heavier gases collect. Ventilation does not replace testing; it is checked by continuous monitoring. It is not a fix for an active gas leak, which must be stopped by the owner.

When an excavation becomes a confined space

Most open trenches are not confined spaces, but some excavations meet the definition. In Ontario, the confined space provisions for construction projects in O. Reg. 213/91 define a confined space as a fully or partially enclosed space that is not both designed and constructed for continuous human occupancy and in which atmospheric hazards may occur because of its construction, location or contents or because of work done in it. A deep, narrow excavation near a sewer, a pit with a vault or chamber, a shaft or a tunnel can qualify. Alberta's OHS Code, Part 5, regulates confined spaces and restricted spaces, and other provinces have similar rules. When a space qualifies, the employer needs a written program, a hazard assessment, a plan, entry permits, testing, ventilation, attendants and a rescue plan. HSE-211 covers confined spaces in detail.

Even when an excavation does not meet the definition, the same thinking applies: anticipate the atmospheric hazard, test, ventilate, keep someone at the top, and have a plan for getting a worker out without sending someone else into the same air.

Contaminated sites deserve particular planning. Where historical use suggests fuel, solvents or other chemicals in the soil, the employer should obtain environmental site information before digging, plan for monitoring and personal protective equipment, and arrange disposal of contaminated spoil and water.