Trenches are fairly common in many an engineering or construction site. They're designed for laying phone lines, pipes and several other constructions. While some are deep, others can be very shallow. Based on the soil's quality, trench walls support themselves for a short time. Steel or aluminum trench boxes support trench walls to make sure it's safe for work to be done without walls falling on equipment or people. Other names for trench boxes are manhole boxes, sewer boxes, tap boxes, or trench shields.
Before excavation starts, the site must undergo a thorough risk assessment to highlight any possible risks, the staffing required and the equipment required. The need for extra access is also evaluated.
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Then the trench will have to be looked at. How deep does it need to be? How big should it be? Trenches that are more than 5 ft require support from either shoring, trench box, or sloping. But if the trench is over 20 ft deep, its support must be designed by a registered engineer. How will people access the trench? It is by steps, ladders or a ramp? The trench needs to always be safe for access by workers within 25 feet, in emergency cases. The atmosphere of the trench may also require testing for low levels of oxygen or poisonous gases. Trench boxes are made to be simple to install but it's unsafe to stack boxes over each other.
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Looking after the trench
Check for any signs of movement or damage by inspecting the trench box/trench support daily.
All staff must put on protective gear, steel-toed boots, high visibility clothing, hard hats and so on.
Ensure that heavy tools and equipment is kept far away from the trench's edge.
It is probably more difficult to extract a trench box than install it because of the earth's movement around the trench. It's advisable to use a chain sling for extraction, using any of these three methods.
Straight pull--a sling is just attached to the two lifting or extraction points and lifted out.
Half pull--a sling is attached to one side of a trench box, lifted as high as possible, then the sling is switched to the opposite side and the action repeated till the trench support is removed.
Single pull--this involves attaching a single chain sling leg to an extraction/lifting point and raising the panel corners in turns; once the manhole box moves easily, it's taken out with the straight pull.
In summary, trenches help save lives. They must be planned for and it's a legal requirement to make use of them. Provided they're well maintained and used, they do make work so much safer and easier.