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 struts between them. Ontario's O. Reg. 213/91 permits timbering and shoring without engineering only where there is no hydrostatic pressure and the trench depth and width fall within the table to section 238, which specifies member sizes and spacing by depth and soil type. For example, for trenches 3.0 m deep or less in Type 3 soil, the table calls for 50 × 200 mm sheathing with no more than a 10 mm gap between boards, 200 mm square struts, wales of 250 mm square and a strut spacing of 1.2 m vertically and 2.4 m horizontally. Deeper trenches and Type 4 soil need heavier members. Alberta's OHS Code, Schedule 9, gives similar lumber tables for trenches from 1.5 m to 6 m deep.

Hydraulic and mechanical shoring

Hydraulic shores are aluminium or steel cylinders with pads or rails 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. Pneumatic and screw-jack shores work on the same principle. In Ontario, prefabricated and hydraulic support systems must be designed by a professional engineer, and the design drawings, showing the system's maximum depth and the soil types it may be used in, must be on the project. Some systems used to repair water main breaks are limited to 3.6 m of depth.

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 aluminium 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 4.5 m in stiff soil may only be good for half that in soft, wet soil. In Ontario, the design drawings must also state the maximum allowable clearance between the box walls and the excavation walls, and the support system must extend at least 0.3 m above the top of the excavation, or to the top where traffic is present.

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

Support systems must be installed from the top down, as the excavation proceeds, and removed from the bottom up as the trench is backfilled. Alberta's OHS Code says this expressly and requires a ladder to be used for the work. Workers must not enter the unsupported section of a trench to install shoring. 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. Backfill should follow removal closely so the walls are never left unsupported over a significant height.

Engineered systems

Deep excavations, wide excavations, excavations next to structures, and any system outside the regulation's tables or a manufacturer's data need a professional engineer's design. In Ontario this applies to trenches deeper than 6 m or wider than 3.6 m and to all prefabricated and hydraulic systems; Alberta requires protective structures deeper than 3 m to follow an engineer's specifications. Engineered systems include sheet piling, soldier piles and lagging, secant piles and braced or anchored walls. The engineer's drawings, and any inspection requirements, must be on site and followed exactly. 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 favour sloping or trench boxes; urban utility work beside buildings favours 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.