Soil – Unsupported vertical trenches

Common problems

The terms soil and earth are commonly referred to in the excavation process to describe the naturally occurring materials uncovered on a project. Soil conditions vary from one site to the next. Soil may be loose or partially cemented, organic or inorganic. However, most soils can be referred to as a mixture or an accumulation of mineral grains that are not cemented together. An exception is hard rock, which remains firm after exposure to the elements.

Proper planning and supervision can avoid unsafe working conditions caused by soil sliding. Unless such safety precautions have been implemented, sliding soil failure can occur in all types of excavations (including sloped trenches and excavations with braced trench boxes).

Soil failure is defined as the collapse of part or all of an excavation wall. The most common soil failure is typically described as an unexpected settlement, or cave-in, of an excavation. Soil sliding is the most common factor leading to soil failure.

Soil

Soil Mechanics


A number of stresses and deformations can occur in an open cut or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation. The following diagrams show some of the more frequently identified causes of trench failure.

Tension Cracks


Tension cracks usually form at a horizontal distance of one-half to three-quarters times the depth of the trench, measured from the top of the vertical face of the trench.

Soil

Sliding or Sluffing


This may occur as a result of tension cracks.

Soil

Toppling


In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s vertical face shears along the tension crack line and topples into the excavation.

Soil

Subsidence and Bulging


An unsupported excavation can create an unbalanced stress in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench.

Soil

Heaving or Squeezing


Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated below. Heaving and squeezing can occur even when shoring or shielding has been properly installed.

Soil

Boiling


Boiling is evidenced by upward water flow into the bottom of the cut. A high water table is one of the causes of boiling. Boiling produces a “quick” condition in the bottom of the cut and can occur even when shoring or trench boxes are used.

Soil

Statistics and safety

Most engineering projects involving pipelines, foundations, landfills, mining, etc., are initiated with an excavation of trenches for infrastructure to be installed. Trenching is inherently dangerous since it presents the risk of cave-ins, which may result in severe injuries, death, or consequential damage to adjacent properties.

Therefore, trenches must be designed with extreme precaution. A number of work-related injuries and deaths in the construction industry have been attributed to trench cave-ins. In the U.S., trench cave-ins account for about 1% of work-related deaths. This amounts to approximately 1000 work-related injuries and between 60 and 100 deaths per year (Thompson and Tanenbaum 1977, Suruda et al. 1988, White 2008). The majority of these deaths occur in sewer line construction when the trenches are not shored.

There have been reports of shallow trenches collapsing and resulting in fatalities; 79% of reported fatalities occurred in trenches less than 4.5 m deep, and 38% occurred in trenches less than 3 m deep (Eivemark and Hall 2000). In Canada, each province enforces strict regulations with respect to safe excavations practices in an attempt to prevent fatalities and serious injuries resulting from trench collapses.

Regulations

The regulations specify the maximum allowable height of an unsupported vertical trench (i.e. safe height), maximum sloping and benching angles, minimum allowable distance from other structures, and minimum distance for stockpiling of excavated or backfill materials from the trench.

Table below summarizes the maximum allowable height of an unsupported vertical trench as regulated by each Canadian province before safety measures such as benching, sloping, bracing, or trench boxes must be implemented to access the work space. Regardless of soil type, Canadian provinces recommend safe heights in the range of 1.2 m (WorkSafeNB 2016) to 1.5 m (Ministry of Labour Alberta 2009).

Maximum allowable height of an unsupported
vertical trench suggested by provinces in Canada

Trenches are typically excavated into unsaturated soils; therefore, trench stability is governed by the matric suction distribution profile between the soil surface and the groundwater table (Pufahl et al. 1983, Whenham et al. 2007, De Vita et al. 2008, Vanapalli and Oh 2012). Importantly, this indicates that the critical height (i.e. maximum depth of a trench that can be excavated without failure) of an the unsupported vertical trench should be determined by considering the matric suction distribution profile in addition to the soil type. In other words, adhering to a safe height suggested by Canadian province regulations may not be a reasonable approach in geotechnical engineering practice, because in-situ soil conditions are not considered.

This is a dangerous example of what you should not do

Wall failures in unsupported vertical trenches

The Occupational Health and Safety Code (Alberta 2009) nicely summarized the details of trench failure. A trench wall collapse might involve multiple tons of soil, which is more than enough weight to suffocate a human. Rescue attempts may be more difficult when the wall failure involves previously disturbed soil (Figure 1). In this case, failure is typically initiated at the base of the trench wall (Zone 1). This localized failure (or movement) leads to the failure in Zone 2. Finally, the failure in Zone 3 occurs due to the self-weight of the soil. This failure mechanism is a plausible explanation for why rescuers are sometimes trapped along with the first victim(s).

trenches
Figure 1. Trench wall failure mechanism involving previously disturbed soil (Alberta 2009)

The guide for excavation work (Manitoba 2007) categorizes the mechanisms of trench collapse into four types.

Failure mechanism

Spoil pile slide – occurs when the excavated material is not placed far enough away from the edge of the excavation. A minimum distance of 0.6 m is recommended for every 1 m of excavation depth (Figure 2)

Figure 2. Spoil pile slide (Manitoba 2007)

Side wall shear – common to fissured or desiccated clay-type or alluvial soils that are exposed to drying (Figure 3)

Figure 3. Side wall shear (Manitoba 2007)

Slough-in (cave-in) – common to previously excavated material, fill, and granular soils where the water table is above the base of the excavation, or where soils are organic or peat (Figure 4)

Figure 4. Slough-in (cave-in) (Manitoba 2007)

Rotation – common in clay-type soils when excavation walls are too steep, or when the moisture content increases rapidly (Figure 5)

Figure 5. Rotation (Manitoba 2007)

As mentioned previously, most trenches are excavated into unsaturated soils. Since the shear strength of a soil is significantly affected by matric suction, the critical height of a trench should be estimated by considering the distribution of matric suction. Instability of an excavation or a trench is caused by stresses and deformations that are normally attributed to increases or decreases in the moisture content (or matric suction) of the soil (NIOSH 2013).

Adapted by OSHAcademy; and A.Richard,G.Brennan,W.T.Oh

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