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Trench Shoring Methodology
Delving the Depths of Shoring Installation
By Matt Wilkinson
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| For slide rail installation, the operator digs some of the trench out, and then pushes the panel down to depth. The dig-and-push installation method used for slide rail systems can be benefi cial when working near existing structures where vibration is a concern. |
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When it comes to trench shoring, there are several different systems contractors can use, each with its own unique installation process. It’s important to have an understanding of these differences, because the equipment required for installation and the equipment you have available may dictate which shoring method you choose. Site conditions — such as soil type, groundwater, proximity to existing structures and utilities — will also have an impact on which installation methods are feasible.
Timber!
In the past, most trench shoring consisted of timber.
As the trench was dug, timber sheets would be placed and supported with timber wallers and struts. Installing all-timber shoring can be costly, labor intensive and dangerous because workers have to construct the shoring from inside the hole. For these reasons, and others, many contractors today choose to support timber with some form of hydraulic shoring. Hydraulic shoring is usually made of aluminum or steel and comes in two forms — vertical and horizontal.
When applicable, vertical shoring is the easier of the two forms to use because it can be installed by one worker. It consists of two vertical rails, which are pushed out against the trench walls by interconnected hydraulic cylinders. As the trench is dug, plywood sheets are placed along the walls. A worker then sets the vertical shore into the trench and pumps it out against the walls, repeating the process down the length of the trench. Depending on soil conditions, the plywood sheets may not even be necessary.
Horizontal hydraulic shoring is always used with sheets. It consists of two horizontal wallers, which are pushed out against the trench walls by interconnected hydraulic cylinders. Once the trench is dug and the sheets are in place, the horizontal shores are lowered into place and supported by hanging chains. Workers then pump the wallers out against the trench walls.
Strong as Steel
Sometimes steel piling is preferred over timber, due to its higher strength characteristics and economic cost. Steel sheet pile is installed by driving the piles into the ground, outlining the trench before it is dug. The trench is then excavated to depth. The sheets can be driven in a number of different ways, from dig-and-push to crane-mounted hammers. One option is an excavator-mounted vibrator, which connects directly to an excavator’s hydraulic system. It uses a jaw that clamps onto the sheet pile, holding it steady, while its vibration loosens the soil around the sheet, allowing the operator to push it to depth.
Sometimes steel piling is strong enough to shore the trench walls on its own, using only a toe-in for support. If soil conditions won’t allow this, the piling can be supported a number of different ways, including the same horizontal hydraulic shoring discussed above. In situations where even more support is needed, heavy-duty hydraulic waller frames and bracing struts may be used.
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| In the past, most trench shoring
consisted of timber. As the trench was dug, timber sheets would be placed
and supported with timber wallers and
struts. Many contractors today choose
to support timber with some form of
hydraulic shoring. |
|
Shores that Slide
Slide rail systems consist of four basic components — corner posts, spreader posts, panels and spreader beams. Slide rail systems are installed using the dig-and-push method and should only be used when there is no groundwater present. First, the posts are set and then panels are placed in tracks that run down each post. The operator digs some of the trench out, and then pushes the panel down to depth. Rows of panels can be added to achieve the needed depth. If an object is going in the trench that makes using spreader beams impractical, a sacrificial beam or external wallers can be used. The dig-and-push installation method used for slide rail systems can be beneficial when working near existing structures, where vibration is a concern.
Trench Boxin’
Trench boxes come in a few different forms, but installation is fairly straightforward for all of them. Heavy-duty boxes are typically made of double-walled steel panels of varying heights and lengths. Heavy-duty boxes with articulating struts are installed using the dig-and-push method as the trench is excavated. Boxes with static struts are simply set into place after the trench is dug out. Panels are stacked on each other to achieve the desired depth. As you progress down the length of the trench, boxes can either be lined up end-to-end or picked up and moved as you backfill.
Another option is to use a drag box, which has modified struts that are manufactured to allow an excavator to pull it down the trench without picking it up. Other variations include four-sided boxes, manhole boxes and high-arch spreaders that make working around manholes and existing utilities easier. One of the more recent trends is the use of smaller, aluminum trench boxes that can be man-handled and lifted with a backhoe. Whichever box you choose, it’s important to remember that they are meant to be used as shields to protect workers from cave-ins, not as a means of shoring trench walls.
At the end of the day, always follow the manufacturer’s installation guidelines and any engineering plans that exist for your project. They exist to help you get the job done and keep your workers safe.
Matt Wilkinson is a marketing representative for Mabey Bridge & Shore Inc., Elkridge, Md.
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