Excavation can cause the sides of a trench to crack, slide, topple or subside. A variety of methods are used to shore it up and prevent the trench destroying itself and injuring any workers inside. Shoring involves supporting the sides of the trench to prevent the soil from moving. Shallow trenches do not need shoring, but workers can instead slope the sides of the trench to prevent it collapsing. Shoring is only used in cases where the trench is too deep to slope the sides.
Hydraulic shoring uses an aluminium hydraulic pressure system to keep trench walls supported. The worker fits the trench with vertical posts or horizontal rails, known as "wales," in places where the trench walls need support; he then uses an aluminium hydraulic cylinder or cross-brace to put pressure on the supports to hold them in place. These vertical and horizontal supports can be made from aluminium or timber.
A worker can fit the system directly against a trench wall if the soil is stable. For trenches dug in less stable soil, the worker can place steel panels or plywood sheeting behind the supports. The trend in excavation, according to the Occupational Health and Safety Administration (OSHA), is to use hydraulic shoring instead of other methods.
Pneumatic shoring is a similar system to hydraulic shoring. Instead of using cylinders under hydraulic pressure, the air pressure in the pneumatic cylinders holds up the supports on the trench walls. A disadvantage to pneumatic shoring is that the system needs to have an air compressor on site for the cylinders to work.
Slide Rail System
The slide rail system is a preformed support system made up of vertical rails and steel panels. The trench is partially dug and an excavator pushes the rails and sheets into the ground. Then the process is repeated every few feet until the trench is complete. Steel spreaders lock into place in the rails across the width of the trench until the trench is finished. Then the spreaders are removed, leaving an open space. Slide rail systems can protect a trench of up to 40 feet in depth from collapse.
You can also use a screwjack system to support the sides of a trench. This is a manual system of extendable struts. The disadvantage of a screwjack is that you need to tighten the struts of a screwjack by hand while inside the trench.