4PL SOLUTIONS

By removing the shoring installer from the unshored trench and making shoring equipment more available and easier to install, trench jacks have undoubtedly significantly impacted excavation safety. Utilizing trench jacks for shoring still has safety hazards that users should understand to help protect their workers.

 

These things rarely happen; however, workers must be informed of the risks they are taking before starting the project. The following are hazards and safety procedures associated with using vertical aluminum shores.

 

Injury to back and muscles from lifting heavy objects - An eight-foot long 52-88 extension trench jack weighs approximately 120 lbs. A two-person crew can safely lift, set, and remove it from the trench. Anything longer or heavier should be lifted and set with equipment such as a backhoe or boom truck.

 

Overhead lifting hazard - When jacks are hoisted by a sling from a tractor bucket or boom truck, the swinging jack presents a danger to workers guiding it. Loose plywood and rocks can also fall off onto workers. Workers should stand clear and navigate with a lead rope.

 

Finger and hand protection - Trench jacks have moving parts connecting the cylinder and the rail. When the jack swings open, fingers can be crushed under the cylinder block, and when swung closed, fingers can be sheared off if they are between the block and the rail leg.

 

When the hydraulic hose is connected to the block fitting, and when the jack is being lifted by hand, shearing and crushing are most likely to happen. Awareness through safety instruction and hand placement at a distance of 12 inches from the blocks is a safe practice.

 

Trench jacks may have optional finger guards; however, getting fingers under the block is still possible, and wrists are cut and banged when the jack folds or unfolds.

 

Bank collapse with a worker standing on it - When the jack is being set, it is still possible for the trench wall to collapse from the additional weight and activity around it. Trench jack installation should closely follow the excavation activity. During jack removal, the arch column is literally removed with the load still on it.

 

Pipe bedding and initial backfill cut the trench depth, adding stability before removing the jack. If backfill operations closely follow jack removal, the length of the unshored collapsible trench wall becomes short. Soil arching back to the backfilled area is likely, and trench wall failure becomes less likely.

 

Remote backfill operations such as excavator wheels, vibraplates, or remote-operated compactors must always be used for compaction outside the shored area. When trench jacks are being removed to allow pipe installation and reset, there is a greater likelihood of trench wall collapse. Equipment and personnel nearby risk losing the ground under their feet. Keep equipment and personnel except those needed to remove the jack a safe distance away.

 

This type of operation is not uncommon and most often works safely; however, if there is any evidence of trench wall collapse, the procedure should be discontinued, and a different method of getting production materials into the trench or another shoring system should be used. Several bad accidents have occurred in conjunction with this type of operation.

 

Get the surcharge loads right - Equipment over 20,000 lbs and large spoil piles over two feet high quickly add additional surcharges, especially in the top 10 feet, that can easily overload the trench jack. If one cylinder fails, a progressive failure to the bottom of the trench and then down the length of the trench is possible. A boom truck or backhoe outrigger placed next to a trench jack can trigger this.

 

To adjust for additional surcharge load, move the load away from the trench, spread the load with timber pad or steel plate, or decrease the trench jack spacing. Centering the load on the jack places most of the burden on that jack.

 

The alternative, centering the load between the jacks, distributes the load evenly between the jacks. However, it increases the possibility of the arch void falling out or arch shear failure at the jack. One alternative may not be any better than the other.

 

Trench Jack fold-up failure - If all jacks were unfolded into the trench from one side of the track, it is possible to get a bank failure that can lift the rotating jack leg. This type of failure is not common; however, it has been documented enough to be a concern. One story details that 40 feet of trench folded up the jacks and collapsed.

 

The solution is to rotate the jack, so the rotation leg is on the other side of the trench. The problem is that the installers must move to the trench's other side to set and pressurize the jack. Two soil conditions that this would most likely happen are medium-dense to loose non-cohesive soils and soft clays with high surcharge loads.

 

Loose trench jacks in the trench - Jacks not pressurized are not setting up arching and preventing trench collapse. In this condition, the jacks can also fall on workers below them. Jacks should not leak at all. Pressure can change slightly up or down due to temperature changes or increase due to loading; however, it should never loosen up in the ditch.

 

If Jacks are left overnight, they should be checked before entering the trench in the morning. Tap them with a hammer or bar of metal, and they will sound loose if they are. Remove and replace jacks that bleed off. If the trench wall has voids where the cylinder hits the wall, use wood blocking to extend the connection to the soil.

 

Non-vertical trench walls - For trench walls that are not vertical, an inverted A shape indicates that the trench jack is unstable. Assuming a coefficient of friction of O.1 between the soil and the aluminum rail and applying a factor of safety of 1.5, calculations indicate that the slope of the trench wall should not exceed 3 degrees, or the jack will lift and fail to provide an arching point.

 

In sloped trenches, extending the jack 18 inches above the hinge point does not provide roll-off protection for workers below because the jack is spaced. Place fabric or boards behind the jack rail to stop objects at the surface and bank ravel from falling on workers.