Your crew is digging a trench to fix a leaking water line. The ground looks solid. It seems safe. One of your workers climbs down a ladder and begins working on the pipe. Suddenly, one side of the trench wall collapses, burying your worker. Luckily your crew quickly fishes him out and lifts him out to safety.
Unfortunately for you, your worksite is frozen and safety officers want to talk to you. You’re now subject to an investigation, fines by Alberta OH&S, and possibly termination.
It was a quick and easy job. You were under pressure to get it done.
How could this have been avoided?
How Shoring Boxes Save Lives
One of the biggest hazards during buried utility work happens when workers enter a trench, which can be dug in all types of ground conditions. The biggest danger in these jobs is the trench walls caving in, trapping the workers under dirt, clay, and sand.
The collapse of a trench wall can be caused by soft or loose soil, permeating groundwater, rain, and people or machinery working near the trench.
One cubic yard of soil can weigh as much as a car.
Enter the shoring box.
Technically there is a difference between shoring boxes and trench or shielding boxes. Shielding boxes aren’t meant to hold up the sides of a trench; they’re there to protect workers from cave-ins.
A shoring box is a system of panels supported by pistons and used to brace trench walls to prevent collapse. They are often used when workers have to enter an excavation, which can be when repairs are needed on an existing utility line, or a new one is being installed.
Our clients most often use them when working on leaking waterlines in plants and refineries, or when installing new utility lines to houses in residential neighbourhoods.
Shoring boxes are designed to prevent cave-ins in unstable soil conditions. Often the excavation we’re working in has been excavated before and backfilled with sand. Other times a leaking water line has flooded the area making the soil unstable. The shoring panels are usually made from aluminum, and are transported to the work site and assembled there. The panel lengths vary and are chosen based on excavation size and the needs of the workers who will be entering.
In June 2018 an Edmonton man was pinned and buried up to his neck when a trench collapsed during a routine maintenance job for an Epcor contractor. The crews were using shoring, but he was outside the shoring box when the collapse happened. The job was frozen and the man was eventually freed and taken to a hospital.
In another incident, the director of an excavation company was given jail time and his company fined half a million dollars after one of his workers was killed in a trench collapse in the Oliver area of Edmonton. The worker had been working with a crew that was connecting a sewer line to a new home. There was no shoring in the trench and it hadn’t been sloped when the sides collapsed.
This shows how dangerous unshored trenches can be.
How Shoring Is Installed
The underground utility line to be worked on is first located by hydrovac. Once visually confirmed, a decision is made on how much room will be needed for workers to enter the excavation or trench, which determines how long and wide the shoring box panels need to be, and where it will be positioned.
The shoring box is then constructed at ground level, and carried by a mechanical excavator using slings to where it will be sunk.
It is then sunk using a hydrovac truck. The hydrovac operator digs underneath each edge and removes only enough earth to sink the shoring box so that the surrounding soil and clay are right up against the outside edge of each shoring panel. If any soil collapses along the edges and slides underneath, or if the excavation was done before the shoring box is placed, the area between the edge of each panel and the surrounding trench might need to be backfilled.
As the shoring panel sinks, another layer of panels can be added on top to make the shoring box as tall as is needed to complete the job safely. An excavator can be used to apply downward pressure to each corner of the box.
This is assuming the surrounding soil is stable enough for workers to walk on. If not, the entire thing may need to be constructed and lowered all at once.
Common Mistakes When Using Shoring Boxes
The most common mistake is not having the shoring box extend deep enough for the complete depth of the trench. What happens is that earth starts falling in underneath the shoring box, which is not only a huge pain but also potentially dangerous for workers.
Other problems are sinking the box unevenly, and leaving space between the edges of the shoring box and the excavation walls. Both can result in the shoring box actually shifting or tipping, which is obviously dangerous for both the workers and the utility being worked on.
Other Things to Remember
It’s important to make sure there is adequate lighting, ventilation, and any water is pumped out before someone enters the trench. A professional engineer will inspect the soil and the shoring box. It’s also important to inspect trenches for cracks in the soil during the course of the job. Entry and exit point ladders need to be securely tied off. Any excavated soil or other materials need to be kept away from the edge of the trench. Certain situations might call for a gas monitor and a confined space permit.
You can read more about OH&S requirements for trenching and shoring here —> http://www.qp.alberta.ca/documents/OHS/OHS.pdf
Why Experience Is So Important
Workers that haven’t had much experience building and sinking shoring are likely in for a rough time. There’s a lot to it, and it’s not as easy as it looks. You can really waste a lot of time.
On the other hand, it can be done efficiently and competently.
Having a hydrovac operator on site that has seen it all and will skillfully and safely build and sink your shoring box can make your job a lot easier.
Take the pressure off yourself and hire someone with a wealth of experience and knowledge!