Trenches are quite common in many engineering and construction sites. They are meant for laying pipes, phone lines as well as lots of other constructions. While some are quite deep, others may be extremely shallow. Depending on the quality of soil, trench walls won’t support themselves for a long time. An aluminum or steel trench box secures the trench walls to make it safe to work there without the risk of walls collapsing on people or equipment. Trench boxes are also known as trench shields, manhole boxes, tap boxes, or sewer boxes.
Before excavation starts, the site must undergo a thorough risk assessment to highlight any possible risks, the staffing required and the equipment required. The necessity of additional access is also assessed.
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Then the trench has to be considered. How deep should it be? How large should it be? Trenches of more than 5 feet require support either from shoring, sloping, or trench box. But if the trench is over 20 ft deep, its support must be designed by a registered engineer. How will people enter the trench? It is by steps, ladders or a ramp? The trench must always be safely accessed by employees within 25 feet, in times of emergency. 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 workers on the location must wear protective gear, high visibility clothing, hard hats, steel-toed boots, etc.
Ensure that all heavy tools as well as equipment are kept far from the trench’s edge.
It’s probably easier to install a manhole box than extract it because of the moving earth in the area around the trench. It’s recommended that a chain sling be used for extraction, using any of these 3 methods.
Straight pull–this involves simply attaching a sling to two extraction/lifting points and lifting it out.
Half pull–this is simply attaching a sling to one side of a manhole box, lifting it as much as possible, then switching the sling to the opposite side and repeating the action till the sewer box 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.
To sum up, trenches do save lives. It’s legally required that they be used and they have to be planned for. So long as they’re used and maintained properly, they make work so much easier and safer.