Not all tunnels are created equally.

How tunnels perform during and after installation depends on several factors, including diameter, length, location, ground conditions, and if water is present. Outside of environmental variables, we also have to monitor project cost, maintenance investments, and the utility installed.

Environmental conditions and project costs can determine what installation method works best. Some cases may call for a traditional trench excavation and utility installation, but others may only need a boring machine.

When the conditions call for a tunnel to be bored, what method works best? Horizontal Directional Drilling (HDD) or Micro Tunneling? The answer, more often than not, lies in the application.

What is Micro Tunneling?

Micro tunneling uses a small laser-guided machine called a Micro Tunneling Boring Machine, or MTBM, to create precise paths underground from one open pit to another.

It can be expensive, but micro tunneling is designed for long runs that need pinpoint accuracy from Point A to Point B, often within ~10mm of the line. Installs are generally for pipes ranging from 4 to 12 feet in diameter, and tunnel lengths can extend for hundreds of feet.

Crews may combine micro tunneling with a process called pipe jacking to reduce the time they spend on site. Pipe jacking is an installation technique using a hydraulic jack to push pipe, conduit, or other utilities into the borehole.

Micro tunneling is safer than traditional excavation as well. Unlike digging a long trench, only two pits are needed for the machine. Workers also don’t need to be in the tunnel or near the boring device. This limits falling risks, unstable trenches, and other hazards.

How Does it Work?

Micro tunneling uses a remotely controlled boring machine from above ground, limiting interactions between the crew and the boring device.

As the MTBM travels along a pre-determined path from one pit to another, it relies on lasers, gyroscopes, and a CCTV feed to ensure it stays on the drill path. Workers can make adjustments as the machine digs through the earth.

Micro tunneling is incredibly accurate, thanks to all the technology used to guide the device. The machine boasts accuracy within ~10mm of the center, resulting in a precise dig without surprises.

Though digging a couple of pits may close off sections of a roadway or create a temporary nuisance for neighborhoods, it’s less intrusive than a traditional trench. There is minimal disruption by machinery and debris because the work is underground. This makes it a good option in busy areas, cities, and residential neighborhoods where the quality of life could be impacted.

The Micro Tunneling Process Explained

The project starts with two pits big enough to fit the machines, pipe, and other materials. One serves as the entry point for the MTBM, and the other is the exit.

Once the pits are prepared, the MTBM is fitted with a drill head and positioned at the entrance. As the machine bores through the soil, a locating system using lasers, gyroscopes, and other equipment keeps the drill bit on the guided path from one pit to the other. Workers on the surface reorient the drill head remotely if it drifts off target.

If workers are pipe jacking, the new pipe is pushed through the drilled hole, then connected to any existing utilities once it’s in place. After everything is tested, the pits are filled with gravel and repaved.

What is Horizontal Directional Drilling?

Less costly than micro tunneling and trenching, horizontal directional drilling (HDD) gets the job done with less impact on the bottom line.

HDD projects only need a launching shaft and exit shaft along with two shallow pits, requiring fewer workers, less prep, and less cleanup. A guided drill bores from the entrance shaft to the exit shaft are controlled from the surface.

Although HDD boring is less accurate than micro tunneling, it usually stays within ~100mm of the path. Despite being less accurate, horizontal drilling allows crews to control drill bits along pre-determined paths stretching thousands of feet.

Though the cost of HDD jobs is less than micro tunneling, it’s slower and requires more maintenance. The drill bit also enters at an angle before flattening its path underground, so pipes and utilities should be able to flex.

Less Prep, Longer Bores

Horizontal directional drilling is less invasive than micro tunneling because the pits can be shallower, though they still need to be marked for safety.

With less disturbance to the ground and surrounding areas, HDD jobs are minimally invasive, making them better for fragile environments and near waterways.

It’s also possible for HDD boreholes to dig much deeper into the earth compared to micro tunneling. This allows workers to avoid previously installed utilities and other potential problems.

How Horizontal Directional Drilling Works

Horizontal directional drilling starts with digging two shallow pits for the drill to enter and exit from. Once the pits are dug, a pilot borehole is drilled using a cutting tool and drilling fluid. The drilling fluid is formulated for the soil type to prevent overheating, loosen soil, and remove cuttings from the borehole.

When the ground is rocky or hard, a drill motor powered by hydraulic pressure helps the cutting tool continue along its path.

Once the pilot hole is made, a reaming tool is pulled back through the borehole to make it bigger. The process is repeated until the tunnel is big enough to fit the pipe through. On the reamer’s last trip through the borehole, HDPE, PVC, or metal piping is pulled through with it.

When Should You Use Which Construction Method?

Depending on the conditions and costs, one type of drilling may be more advantageous than the other.

Micro tunneling is preferred when:

  • The installation requires high accuracy and low maintenance
  • The tunnel is for a gravity sewer, though it’s applicable for other utilities like gas, water, and telecom lines
  • A large pipe is getting installed

HDD is preferred when:

  • Project cost is a driving factor
  • Maintenance and accuracy are not critical to the installation
  • The ground is rocky or hard

HDD has limitations, though. If there is a risk of soil disruption, losing control could increase the risk of a tunnel collapse. It also might not be useful for projects requiring high accuracy during the dig, such as gravity sewers.

Otherwise, horizontal directional drilling works for most other utilities, including water, cable, gas, and conduit.

Less Intrusive, Safer Installs

Both options have pros and cons, but the conditions and budget may still be deciding factors.

Micro tunneling makes sense when accuracy and reliability are key or in specialized cases. Otherwise, HDD is a low-cost option to install many underground utilities safely without causing problems and damage.

No matter which method you choose, always make sure to install a tracer wire system as well to ensure you can locate utilities in the future and prevent accidents.

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