A trench box is designed as a temporary earth retaining structure allowing the sides of the excavation to be cut vertical or near vertical. This quick and easy method ensures ground stability within a trench and helps to minimize the excavation area to avoid damage to neighboring structures and utilities in the ground. The primary function of any trench support method is to protect people from caving ground. The secondary function is to provide support to nearby structures and allow equipment access to the work. For deep trenches the most feasible and cost effective support method should be devised by weighing different alternatives for trench method of excavation, pipe laying, back-fill, schedule and obstructions. In any given project several trench support methods may be used to accommodate different conditions. There is no “one-size-fits-all” solution to the process of selecting and designing a trenching support method. The first steps are to read the plans, specifications and geo-technical reports to understand the constraints and conditions that will be encountered.
Where the soil will not remain open without caving, a form of trench support can be utilized. Temporary support methods such as trench boxes or hydraulic shoring have been utilized. Trench boxes are generally used in open areas, it is a structure that supports the sides of an excavation and is designed to prevent cave-ins. Trench boxes are different from shoring because, instead of shoring up or otherwise supporting the trench face, they are intended primarily to protect workers from cave-ins and similar incidents. The excavated area between the outside of the trench box and the face of the trench should be as small as possible. The space between the trench boxes and the excavation side are back-filled to prevent lateral movement of the box. The permeability of the geologic material must be low enough to avoid the necessity of de-watering for these methods to be successful. For most shallow trenches bracing system should be used. For the utility trench excavations, cross-trench bracing is used, but it somewhat restricts the work area.
Prefabricated trench boxes are used to maintain trench integrity during excavation and back-filling operations. These can be quickly setup and placed in a section of trench. The trench box is slid along the trench with excavation occurring just ahead of the trench box and back-filling occurring in the back half of the trench box. Hydraulic shores can also be used to provide additional stability to the trench until it is back-filled. The temporary shores are placed in the excavation immediately after the trench is excavated to provide a temporarily supported trench between the excavation and back-filling operations.