Since the excavation is one of the most hazardous work in the whole construction field, in every jobsite we must avoid, as much as it is possible, all threats to workers.
In our case, trench excavation, the worst possible accident is the collapse or fall of a trench.
What are the conditions for a possible soil fall? A good starting guide is provided us by OSHA:
- Tension Cracks.Tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench
- Sliding , may occur as a result of tension cracks
- In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s vertical face shears along the tension crack line and topples into the excavation.
- Subsidence and Bulging.An unsupported excavation can create an unbalanced stress in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench.
- Heaving or Squeezing.Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in the drawing above. Heaving and squeezing can occur even when shoring or shielding has been properly installed.
- Boilingis evidenced by an upward water flow into the bottom of the cut. A high-water table is one of the causes of boiling. Boiling produces a “quick” condition in the bottom of the cut, and can occur even when shoring or trench boxes are used.
Of course, we can add to this list:
- Existence of underground utilities, which can lower the resistance of the soil
- Unexpected underground water or natural gas source
- Change of weather, indeed rain or snow can rapidly change soil resistance.
- Vibration and weight. Every equipment must be placed in a safety distance from the open trench.
The most diffused ways to prevent or to limit these hazard in a trench excavation are the shoring and shielding systems.
Shoring is the provision of a support system for trench faces used to prevent movement of soil, underground utilities, roadways, and foundations. Shoring is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring systems consist of walls, struts, and sheeting.
Shoring must be used for trenches with a depth more than 5ft. With a depth of 4ft or less shoring is relatively useless for preventing accidents to worker, such as a cave-in. In this case it can be placed a shielding system just to strengthen the walls and proceed to the excavation and boring work. If I can recap quickly: Shoring is the process of bracing walls to prevent collapse, while trench shielding is done only to protect workers in the event of a collapse.
Generally we can find 2 general types of shoring, both of them will provide security to the worker and to the excavation itself.
1.Timber shoring. (Soldier-pile). It grants positive trench wall support, it can be used in almost any soil conditions, and it as a lot of advantages. It consists of a provisional structure, made of wood sheets placed vertically in the trench’s walls (the soldiers). They are reinforced with metallic cross braces and provide the support for tension cracks on the trench sides.
The main advantage of this kind of shoring is its high adaptability to every ground conditions. In case of the presence of lateral obstacles (such as preexistent pipes) it can be possible to place the soldier on the side of them, surrounding the obstacles, building the structure with the exigence of the jobsite itself.
2.Hydraulic or pneumatic shoring: is the use of hydraulic pistons that can be pumped outward until they press up against a prebuilt structure, pushing outwards the trench walls. 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.
Pneumatic Shoring works in a manner similar to hydraulic shoring. The primary difference is that pneumatic shoring uses air pressured metallic rods in place of hydraulic pressured ones. A disadvantage to the use of pneumatic shoring is that an air compressor must be on site.
A third category are the “trench boxes”. They are pre-made structure for shielding a trench. Their primary function is to prevent hazards for workers in case of cave-ins and general collapses to the trench. Trench boxes are generally used in open areas, but they also may be used in combination with sloping and benching. The box should extend at least 18 in (0.45 m) above the surrounding area if there is sloping toward excavation. This can be accomplished by providing a benched area adjacent to the box. The shield does not support the trench walls; it is only there to protect the people in the event of a cave in. It is designed to be dragged along the trench using the backhoe or a crane.
If the planned trench is located near slopes, special attention should be paid to the presence of any pressurized water mains as any damage to the pipes could lead to slope failures. For excavation depth of less than 1.2 m, shoring support may not be required for ground that is found to be self-supporting. However, if external loads are likely to be present, or if there is doubt as to the stability of the trench sides due to the presence of weak ground or high groundwater, especially in inclement weather, the trench sides should be supported even if the excavation depth is less than 1.2 m.
Ground movement due to trench excavation should be minimized as it can damage existing services running parallel to or across the trench. In addition, when existing services cross a trench line may be undermined by the excavation, they should be supported either by propping from underneath or by hangers suspended from ground surface.