Summary

Trenching work exposes workers to many risks. Cave-in is the most serious and frequent risk during such work, but unfortunately, it is very often underestimated, as even a minor or partial cave-in of less than 1 m3 of soil can fatally injure a worker. An analysis of 59 reports of serious and fatal accidents in excavation and trenching work from the Commission des normes, de l’équité, de la santé et de la sécurité du travail (CNESST), between June 1973 and May 2015, shows that there were 51 fatalities and 25 serious accidents in Quebec. Wall collapses frequently cause occupational accidents on construction sites and must be avoided at all costs. In order to do this, slopes must be excavated at a safe angle, depending on the type of soil, or temporary retaining walls must be erected to support the slopes.

Cave-in accidents are a reminder that the shear strength of natural soil deposits is not uniform and that trench stability varies from one point to another within a deposit. Although Occupational Safety and Health Administration (OSHA) regulations, Health and Safety at Work regulations and various Canadian provincial regulations prescribe maximum allowable slopes for safe excavations, there is always a risk of landslide in an embankment.

Cave-ins occur frequently on small construction projects and cause fatalities and/or serious accidents. Many of these fatalities and serious injuries could have been avoided if the workers had identified the warning signs of a cave-in and had therefore been able to evacuate the excavation in time. Case studies reveal that workers do not always have time to evacuate the excavation because 1) the time between the completion of the excavation and the onset of the cave-in causes the workers to misinterpret the stability of the soil mass, 2) creep phenomena occur before the cave-in and 3) ground movements are initially too small to be detected by simple observation.

Indeed, wall failure is very difficult to predict by visual observation alone. Monitoring sensors can be used to identify small movements in a slope or wall, indicating an increased risk of cave-in, and warn workers of the imminent risk. They can then evacuate the excavation in time to avoid serious accidents or fatalities. Given the environment in which these monitoring sensors are used temporarily on construction sites, they must be quick to install and easy to use.

The Mini Pipe Strain Meter (MPSM) was previously developed at the National Institute of Occupational Safety and Health, Japan (JNIOSH) and was tested in their laboratory with Japanese soils to measure the increase in shear strain in the shallow subsoil of embankments. An increase in strain indicates an impending cave-in and the MPSM emits sound and light signals to warn workers in time to evacuate the trench.

Full-scale model tests of slope and wall failure performed in the laboratory at the JNIOSH showed that:

• The MPSM detected the risk of slope failure for those tests;
• Small shear deformations at shallow depths were clearly mobilized, which corresponded to the development of sliding surfaces in deeper parts;
• The identification of the second or third creep could give workers a few minutes to evacuate;
• When the risk of slope failure or wall cave-in increases, it is not perceptible by observation alone;
• No visible cracks were observed during the tests and no ground movements were visible before the failure;
• The length of time before failure depended on the soil and on the excavation conditions;
• The cave-in of a slope or wall can be predicted by measurement; therefore, the risk can be detectable by prediction.

By warning that a collapse is imminent, the MPSM helps reducing the risk of injury from cave-in. In short, the MPSM is not a system for preventing slope or wall to cave-in, but rather a method for monitoring the risks. Other safety measures should be used in conjunction with the MPSM.

As the MPSM was developed and successfully tested to monitor trench stability and wall collapse in typical soils of Japan, its performance with other types of soil remained unknown. Therefore, the focus of the present expertise was to determine whether the MPSM would perform effectively in situ in sensitive clay, a typical soil of the Champlain Sea, which makes up the subsurface of more than 80% of the inhabited territory of the province of Quebec. This expertise was possible as a larger study entitled Soil classification and selection of shoring systems for the excavation of trenches (IRSST Project #0099-5290) was concomitantly realized.

The tests results showed that:

• The MPSM is easy to install manually on site with a wrench key;
• The MPSM worked well during site tests in typical Champlain Sea clay;
• During these tests, the first warning signal (D1: yellow light) and the second warning signal (D2: red light) were triggered, indicating an imminent cave-in. D1 lasted 22 minutes, whereas D2 lasted 50 seconds;
• The MPSM provided a means of measuring any increase in the risk of a cave-in during trench excavation;
• The MPSM has potential for use on Quebec sites of Champlain clay if it is available at a reasonable price. It currently sells for about US$7,000 in Japan.

Further testing of the MPSM is nonetheless required with other types of Quebec soil and to assess its reliability and sensitivity regarding the influence of its placement with respect to the trench or slope.