Shoring and shielding

Where there is no room to slope, or the soil is too weak for a practical slope, the walls must be supported or the workers must be shielded. Shoring holds the walls in place so they cannot move. Shielding accepts that the walls might move and puts a strong box around the workers so that soil cannot reach them. Both work only when the right system is chosen for the soil and depth, installed exactly as designed, and kept in use for the whole time anyone is in the trench.

Shoring versus shielding

The difference matters because it decides what the system protects and how it must be used:

• Shoring is an active support. Struts, wales, sheathing or hydraulic cylinders press against the walls and resist the earth pressure. Properly installed shoring prevents the walls from moving, which also protects nearby pavement, utilities and foundations.
• Shielding is a passive protective structure. A trench box or shield is placed in the trench, and workers stay inside it. The walls may still slough or collapse against the outside of the box, but the box resists the load and keeps the workers' space open. Shields do not protect adjacent structures from settlement.

Workers inside a shield are protected only while they stay inside it. The most common way shields fail to protect people is not structural failure but workers stepping out of the end of the box to guide a pipe, check a grade or retrieve a tool.

Timber shoring

Timber shoring uses vertical sheathing or uprights against the walls, horizontal wales and cross braces between them. OSHA Appendix C to Subpart P gives timber shoring tables for trenches up to 20 ft deep in Type A, B and C soils, specifying the size and spacing of cross braces, wales and uprights by depth and trench width. The tables assume specific wood species and grades, and the minimum sizes and maximum spacings must not be relaxed. Timber shoring is labor-intensive and now less common than hydraulic and box systems, but it remains useful for irregular excavations and where prefabricated systems cannot fit.

Hydraulic and mechanical shoring

Hydraulic shores are aluminum or steel cylinders with rails or sheeting that are lowered into the trench and pumped against the walls from above, so workers never have to enter an unprotected trench to install them. OSHA Appendix D gives tables for aluminum hydraulic shoring in Type A, B and C soils; otherwise, manufacturer's tabulated data apply. The tabulated data must be in writing, approved by a registered professional engineer, and a copy kept at the jobsite while the system is in use.

Hydraulic shoring is quick to install and remove and pre-loads the soil, which reduces wall movement. Its weak points are human: cylinders spaced too far apart, sheeting omitted in soils that need it, shores installed at an angle, and hydraulic pressure that bleeds off without anyone checking. Shores should be installed and removed from the top of the trench using the manufacturer's tools, with the pressure checked at each inspection.

Trench boxes and shields

A trench box is a pair of steel or aluminum side panels held apart by spreaders. It is lowered into the trench by the excavator, and workers enter only once it is in place. Boxes come in many sizes and can be stacked for deeper trenches. Slide rail systems, where panels slide down vertical posts as the excavation proceeds, are a hybrid that provides some shoring action in large or deep excavations.

Every box is designed for specific conditions. The manufacturer's tabulated data state the maximum depth for each soil type, how boxes may be stacked, the spreader lengths allowed, and how the box must be used. A box rated for 15 ft in Type B soil may only be good for 10 ft in Type C. OSHA 1926.652(g) requires shields to be installed so as to restrict lateral or other hazardous movement, forbids workers from being in the shield while it is installed, removed or moved vertically, and allows excavation up to 2 ft below the bottom of a shield only if the shield is designed for the full depth and there is no indication of soil loss from behind or below it.

These pressure figures are illustrative; real ratings come only from the manufacturer's tabulated data for the specific box, and some manufacturers state depth ratings directly rather than pressures.

Using a box correctly

• Check the box before use: welds, pins, collars, spreaders and the data plate showing the manufacturer and model. A bent panel or a missing pin can halve its strength.
• Use only spreaders, pins and stacking arrangements allowed by the tabulated data. Homemade spreaders and mismatched boxes are a common cause of failure.
• Dig the trench and set the box in stages, pushing it down as the excavation deepens, so that nobody is ever in an unprotected section.
• Keep the gap between the box and the trench wall small, and backfill it if the data require, so that a collapsing wall cannot gain momentum or tip the box.
• Keep workers inside the box. Laying pipe at the end of the box is safe only if the pipe end is still within the box or the end is protected by an end shield or sloped.
• Provide a ladder inside the box, and make sure workers can get out without leaving its protection.
• Move the box only when nobody is inside it, and never ride a box as it is dragged or lifted.

Installation and removal

OSHA 1926.652(e) requires support members to be securely connected, installed in a way that protects workers from cave-ins and structural collapse, and never subjected to loads beyond their design. Installation proceeds from the top down as the excavation deepens, and removal from the bottom up, with backfill following removal closely. Removal is often more dangerous than installation, because the trench has been open for a long time, the soil may have relaxed, and crews are in a hurry to finish. Workers must never enter an unprotected section of trench to install or remove a system.

Engineered systems

Excavations 20 ft or deeper, excavations next to structures, and any system outside Appendices C and D or a manufacturer's tabulated data need a registered professional engineer's design. Engineered systems include sheet piling, soldier piles and lagging, secant piles and braced or anchored walls. The design must be in writing, identify the engineer, and be kept at the jobsite. Any change in conditions, such as a different soil, more depth or a nearby surcharge, sends the design back to the engineer.

Choosing between systems

The choice between sloping, shoring and shielding is usually made by weighing soil, depth, space, duration, nearby structures and cost. Long pipe runs in open country often favor sloping or trench boxes; urban utility work beside buildings favors shoring that prevents ground movement; very deep or wet excavations need engineered systems from the start. Whatever is chosen, it should be decided before digging, with the equipment on site and inspected, rather than improvised once the trench is already open.