The Theory of Shoring

Shoring prevents cave-ins, Shoring, if designed and installed correctly, counteracts the force of a cave-in

For the shoring to do its job, the worker must stay within the protection of the shoring even when entering and exiting the trench.

Shoring Types
Shoring is the provision of a support system for trench faces used to prevent movement of soil, underground utilities, roadways and foundations. Shoring or shielding is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring systems consist of posts, wales, struts and sheeting. There are two basic types of shoring, timber and aluminum hydraulic. 
Hydraulic shoring. The trend today is toward the use of hydraulic shoring, a prefabricated strut and/or wale system manufactured of aluminum or steel. Hydraulic shoring provides a critical safety advantage over timber shoring because workers do not have to enter the trench to install or remove hydraulic shoring. Other advantages of most hydraulic systems are that they are light enough to be installed by one worker; are gauge-regulated to ensure even distribution of pressure along the trench line; Can have their trench faces “preloaded” to use the soil’s natural cohesion to prevent movement; and Can be adapted easily to various trench depths and widths. All shoring should be installed from the top down and removed from the bottom up. Hydraulic shoring should be checked at least once per shift for leaking hoses and/or cylinders, broken connections, cracked nipples, bent bases, and any other damaged or defective parts.

Pneumatic shoring works in a manner similar to hydraulic shoring. The primary difference is that pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneumatic shoring is that an air compressor must be on site. Air shoring involves using compressed air instead of hydraulic fluid to expand the trench jacks into position. Using the air type of system, pins are put in place to lock the jacks when a desired level of stability has been achieved. For the removal of this trenching system, air is again injected into the jacks to extend them, allowing the pin to be removed. These types of jacks are popular since they are cleaner than hydraulic jacks and there is no danger from the leakage of fluids or other lubrication.
1. Screw jacks. Screw jack systems differ from hydraulic and pneumatic systems in that the struts of a screw jack system must be adjusted manually. This creates a hazard because the worker is required to be in the trench in order to adjust the strut. In addition, uniform “preloading” cannot be achieved with screw jacks, and their weight creates handling difficulties.
2. Single-cylinder hydraulic shores. Shores of this type are generally used in a water system, as an assist to timber shoring systems, and in shallow trenches where face stability is required.
3. Underpinning. This process involves stabilizing adjacent structures, foundations and other intrusions that may have an impact on the excavation. As the term indicates, underpinning is a procedure in which the foundation is physically reinforced. Underpinning should be conducted only under the direction of and with the approval of a registered professional engineer.