Burying utility lines is an effective way to ensure there is no damage or loss caused by structures or weather. However, successfully burying these lines without damaging existing utilities has always been a problem. Using slot trenching, however, this installation method is becoming more desirable.

Slot Trenching Defined

When installing underground utilities such as cables, conduits and pipes, workers often use narrow trenches first. These corridors are known as slot trenching. While workers can create slot trenches using conventional excavation methods, using techniques such as hydro excavation is preferential as it is nondestructive and has a greater operational efficiency. (For more on hydrovac excavation, see An Overview of Hydrovac Excavation.)

To create a useful slot trench, workers begin by excavating a pot hole. The scope of work determines the depth of the initial hole and grade of the following channel. Slot trenches are often used not only to install new lines but also to determine the clearances between, above and below buried utilities. Workers also utilize this trenching technique if they need to find directional drill heads which have hit subsurface utilities or otherwise become lost during tunneling.

Necessary Slot Trenching Tools

Completing a slot trench with traditional excavation tools is possible. However, doing so is dangerous to both workers and the environment. For a safer, more environmentally friendly excavation, it is best to use a method known as hydro excavation. With hydro excavation, workers remove layers of soil with pressurized water in combination with an air vacuum. The water cuts the ground away from the area while the vacuum lifts the debris from the area.

To correctly complete a hydro excavated slot trench, workers need a high-pressure water system to break up the soil. They also need a vacuum source. The vacuum can be a positive displacement blower, which moves air relatively great distances and is best used for deeper trenches. The other vacuum option is the fan system, which easier to operate and works best for shallower trenches.

Areas where freezing temperatures make it challenging to access subterranean utilities can still use hydro excavation for slot trenching purposes. Heating the water helps to soften the ground and make trenching easier than traditional methods. In these areas, a heat source for the high-pressure water system is an additional tool necessary to complete the project.

Why You Should Use the Slot Trenching Method

Slot trenching is the preferred method for laying subterranean utility lines. It saves money and time with improved accuracy, improved worker safety and is more environmentally friendly than traditional methods.

1. Worker Safety

With traditional trenching methods, worker safety is always a concern. The most significant risk is that while digging this trench, no matter how narrow, there could be collapses which could cause severe injury or even death to workers in the hole. To prevent potential cave-ins, workers would have to slope the walls, shore them up and shield them with trench boxes to give them more significant support.

Workers are unable to create the narrow conduits necessary for utility installation using traditional trenching methods. This more massive hole means the excess area would need to be backfilled before laying the lines and finishing the job. Backfilling the holes requires the use of heavy machinery which leads to the potential for injury.

By methods such as hydro excavation for slot trenching, workers are no longer in the hole. The conduits created by this method are very thin, forming a narrow slot big enough for the utilities but not necessarily for the construction crew to be inside. This mitigates the risk of injury or death due to a collapsed trench. There is no heavy equipment used nor any need to backfill part of the work area before laying the utility lines. (To learn more about safety, check out Trenchless Operations Safety Do’s and Don’ts.)

2. Improved Accuracy and Reduced Time

As slot trenching only requires a small excavated corridor, there is better control when creating the pot hole and subsequent trench. There is less risk of accidentally hitting other underground utility lines, potentially causing damage to these pipes and cables. A single nick in a water, gas or power line could cause an interruption of service of the affected utility to the surrounding customers. The use of methods such as hydro excavation to dig the slot trench helps to cut around the existing lines without damaging them.

Traditional trenching is labor intensive as workers must manually remove soil from the area. They also must shore the trench sides to ensure a safe working environment. Add to that the time it takes to backfill part of the newly dug hole to accommodate the pipes or cables, and workers could spend weeks or more just laying new utility lines.

Nonmechanical digging reduces the time it takes by eliminating the need for shoring and backfilling. The newly dug slot trench is narrow and the perfect width and depth for the utilities. Instead of taking weeks, it takes days to complete the project.

3. Environmental Factors

As nonmechanical methods of slot trenching are more accurate, less of the surrounding environment is disturbed during construction. With traditional excavation methods, large areas of soil are moved to complete the trench. This potentially disrupts wildlife habitats. Less soil removal is necessary by using hydro excavation and other nonmechanical methods, leaving nearby wildlife habitats undisturbed. It also doesn’t stir up as much dust and debris in the air, lowering the surrounding air quality and possibly contaminating underground water supplies due to the use of heavy equipment.

Choosing slot trenching methods over traditional excavation methods is better for workers and the environment alike. It also reduces work time and provides better accuracy to help avoid costly damage to existing utilities.