When trenching and using shoring, soil type only matters to a point. “One common mistake contractors make is placing too much emphasis on that soil typing. If you look strictly at the OCCUPATIONAL HEALTH AND SAFETY definition of what types of soil and what characteristics they have, almost anytime you dig, you’re digging in type C soil,” offers P bar Y Safety . “If it’s been previously disturbed, if it’s been dug up before, if it consists of fill, if there’s groundwater present—it’s almost always a type C soil.” One of the biggest hazards of trenching and excavation work is the risk of cave-ins. Trenching fatalities are primarily caused by cave-ins, with death occurring by suffocation or crushing when a worker is buried by falling soil.
Trench stability is affected by a number of factors, such as soil type, moisture content, depth of the trench, length of time during which the trench is left open, previous excavations or soil disturbances and vibration and excessive weight from heavy equipment nearby. Trench-related fatalities can also be caused by falls, exposure to hazardous gases, drowning, falling equipment or materials and electrocution.
“People think they’re invincible. They don’t realize how extremely dangerous trenches and confined spaces can be,”. OCCUPATIONAL HEALTH AND SAFETY defines the three soil types as follows:
- High Stability—Type A: Clay, silty clay, and hard-pan. No soil is Type A if it is fissured, is subject to vibration, has previously been disturbed, or has seeping water.
- Medium Stability—Type B: Silt, sandy loam, medium clay, and unstable dry rock. Previously disturbed soils, except those that would be classified as Type C. Soil that meets the requirement of Type A soil, but is fissured or subject to vibration.
- Low Stability—Type C: Gravel, sand, loamy sand, soft clay, submerged soil, dense unstable rock, or soil from which water is freely seeping.
If the type of soil wasn’t enough to worry about at a potential trench site, there are also other potential geological problems waiting for contractors. Shale limestone, sandstone, and other soft to medium hard rocks can slow down a project if a contractor doesn’t have the right tools to handle the job. Funny I was always taught there were four types!
falls into trenches or excavations
tripping over equipment, debris and spoil
excavated material or other objects falling on workers
exposure to underground services or overhead electrical cables
unstable adjacent structures
mishandled or poorly placed materials
hazardous atmosphere (noxious gases/lack of oxygen)
toxic, irritating or flammable and explosive gases
incidents involving vehicles and other mobile equipment
A Notice of Trench or excavation must be given in person or by phone
Mark and locate utilities: Employers must ensure all gas, electrical and other services are located or marked in or near the area to be excavated. If a service poses a hazard, it must be shut off and disconnected before the excavation activity begins. If a potentially hazardous service cannot be disconnected, the service owner must be asked to supervise the service’s uncovering during the excavation.
Think Adjacent structures: Constructors must prevent damage to adjacent structures by engaging a professional engineer who must specify in writing the precautions to be taken.
Soil strength: Determine the soil type to protect excavation walls from collapsing. This can be determined by doing things such as inspecting trenches and excavations following a rainfall, melting snow, thawing earth and overflows from nearby streams, storm drains and sewers. The soil type determines the strength and stability of the excavation walls.
Wall stability: Strip the wall of a trench or excavation of any loose rock or other material that may slide, roll or fall on a worker.
Equipment: Keep heavy equipment, excavated soil or rock and construction material at least one metre away from the upper edges of the trench or excavation. Do not position or operate a vehicle or machinery in a manner that could affect the wall’s stability.
Work space: Maintain a clear work space of at least 450 millimetres (18 inches) between the wall of an excavation and any formwork, masonry or similar wall.
Fall protection: Provide a barrier at least 1.1 metres (42 inches) high at the top if an excavation does not meet regulatory slope requirements and is more than 2.4 metres (eight feet) deep.
Protect yourself: Never enter a trench deeper than 1.2 metres (47 inches) unless the walls are sound, made of solid rock, properly sloped, shored or protected by a trench box. Never work alone in a trench.
Protective systems: Workers must be protected against trench or excavation cave-ins and other hazards using three basic methods:
- Sloping which involves cutting back trench walls at an angle, inclined away from the excavation.
- Shoring which helps support trench and excavation walls to prevent movement of soil, underground utilities, roadways and foundations. Timber and hydraulic systems are the most commonly used supports to shore up walls. Both types must be designed by a professional engineer.
- Prefabricated support systems (for example, trench boxes and shields) which can prevent soil cave-ins.
“Competent person”: Trenches and excavations must be inspected daily for hazards, and when conditions change, before workers enter them. This must be done by a “competent person”, as defined by the Occupational Health and Safety.
A “competent person”:
- has the knowledge, training and experience to organize the work
- is familiar with the OCCUPATIONAL HEALTH AND SAFETY and Construction Regulations and
- can identify existing and potential health and safety hazards and/or unsafe working conditions
Entry and exit: Provide safe access and egress for workers at excavations by means of ladders, steps, ramps, or other safe methods of entering or exiting. Trenches must have ladders placed in the area protected by the support system and be accessible in the event of a collapse.
Understanding the type of soil, which determines the strength and stability of trench walls, can help supervisors and workers plan their protection accordingly. As soil types and conditions can change over very short distances,:
Type 1: Often described as “hard ground to dig”, this type of soil, such as “hardpan”, consolidated clay and some glacial tills, is so hard that it is close to rock.
Type 2: This type of soil can easily be excavated by a backhoe or it can be hand-excavated with some difficulty. The sides of a trench will remain vertical for a short period of time. Examples include silty clay and less dense tills.
Type 3: This type of soil, when dry, flows through fingers and will not stand vertically. Sand, granular materials, silty or wet clays and all backfilled or previously disturbed material fall under this category.
Type 4: This type of soil must be supported and contained to be excavated to any significant depth. With its high moisture content, soil of this type is very sensitive to vibration and other disturbances. Examples include muskeg, quicksand, clay and silty clay.
Soil is a mixture of sand, gravel, silts, clay, water, and air. The amounts of these ingredients which determine its “cohesiveness”, or how well a soil will hold together. Cohesive soil does not crumble. It can be molded easily when wet, and is hard to break up when dry. Clay is a very fine grained soil, and is very cohesive. Sand and gravel are course grained soils, having little cohesiveness and often called granular. Generally speaking, the more clay that is in the soil being excavated, the better the trench walls will hold up.
Another factor in soil cohesiveness is water. Soil that is filled with water is termed saturated. Saturated soil does not hold together well, and is particularly dangerous in excavation work. However, the opposite can also be true. Soil that has little or no water in it, or oven-dry, can crumble easily, and will not hold together when excavated.