Other than avoiding disturbance to residential areas, a wide variety of projects have benefited from the science and application of trenchless technologies. These include but are not limited to crossings under active taxiways, runways, and roadways/highways; driving intake structures from the middle of lakes; and establishing initial support for larger scale tunnels. Trenchless methods also make it possible to install utilities under rivers, canals and other obstacles with minimum disruption and harm to the environment.

Trenchless methods have been used for the last 50 years, and with advancements in technology for excavation and guidance systems, they are becoming even more common around the world when ground conditions and location permits.

Common Trenchless Technologies

Some of the most popular trenchless technologies methods include horizontal directional drilling, microtunnelling, horizontal auger boring, pipe ramming, and open face pipe jacking. Provided below is a short introduction to each method.

Horizontal Directional Drilling

Horizontal directional drilling (HDD) uses a steerable system for the installation of pipes, conduits, and cables in a shallow arc using drilling equipment from the surface. Traditionally HDD is applied to large scale crossings, such as rivers, in which a fluid-filled pilot bore is drilled initially and then enlarged by a back reamer to the size required for the product pipe. The reaming operation usually consists of using an appropriate tool to open the pilot hole to a slightly larger diameter than the carrier pipeline.

Typically, a “drilling mud”, such as fluid bentonite clay, is injected into the bore while cutting and reaming to stabilize the hole and remove soil cuttings.

Here are few extra tips on HDD method:

a) Select the crossing route to keep it to the shortest reasonable distance.

b) Find routes and sites where the pipeline can be constructed in one continuous length.

c) The HDD process takes very little working space versus other methods but on the pipe side of the crossing, sufficient temporary space should be rented to allow fusing and joining the carrier pipe if required.

d) Long crossings with large diameter pipe need bigger, more powerful equipment.

Microtunnelling

Microtunnelling is a method for installing pipelines by consecutively pushing a micro tunnel boring machine (MTBM) through the ground using a jacking system. This process is repeated until the microtunnelling machine reaches the reception shaft. As the machine advances, more tunnel liner or pipe is pushed from the starting shaft, through the entry portal.

The MTBM is operated from a control panel, normally located on the surface (distinction between microtunneling and general small diameter tunneling).

It simultaneously installs pipe as the soil is being excavated and removed, typically through slurry discharge lines.

If required, continuous pressure is provided to the face of the excavation to balance groundwater and earth pressure. Balancing these pressures is necessary in soft grounds to keep the tunnel structure intact during the excavation.

As the length of tunnel increases, the friction of the ground around the pipe increases as well. Generally, two methods are used to minimize this friction. Firstly, MTBM cutting head, cuts the ground a little bit larger than diameter of pipes, therefore the slight gap between the inside perimeter of the tunnel and the outer perimeter of the liner/pipe (overcut) reduces the friction considerably. Secondly, some type of lubricant, often bentonite slurry, is injected into this gap.

Auger Boring

Auger boring forms a bore from a launch pit to a reception pit, using a rotating cutting head. Spoil is removed back to the drive shaft by auger flights rotating in a steel casing. This method has limited steering capability (new technologies have added some capacity in this regard) but it is often more economical for short and straight drives compare to other methods. Auger boring machines come in different sizes and capacities. Ground conditions usually dictate how powerful of a machine is needed.

Typically, the anticipated application determines if auger boring is a suitable trenchless method. Bores smaller than six inches, usually fiber-optic or electric lines and small water pipes, might be more suitable for horizontal directional drill (HDD) which require higher budget for equipment and operation relative to the diameter and length of the bore. But for some water and gravity sewer lines that begin and end on grade, an auger bore rig could be used to potentially save cost.

Pipe Ramming

Pipe ramming is a trenchless technique for installation of pipes or casings (typically steel) in which a pneumatic tool is used to hammer the pipe or the casing into the ground. This technique is commonly used under highway and rail embankments and generally non-steerable.

The advantages of pipe ramming include:

a) Enables large diameter pipes to be installed at shallow depths.

b) No risk of ground settlement as steel pipe installed before excavation.

c) Fast set up time and can be used at steep angles.

d) Pipe ramming is one of the very few trenchless methods where a shutdown may not affect the resumption of the drive. The drive can normally be restarted after substantial down time

Pipe Jacking (Open Face)

Pipe jacking is a system of installation by pushing the pipes (typically reinforced concrete) hydraulically from a launch shaft such that they form a continuous string in the ground. This method is used for larger diameter excavations (>1.5 m), and personnel are usually required inside the pipe to perform the excavation or spoil removal process. The excavation can be performed manually (hand mining) or mechanically (mini excavators for example).

Conclusion

The capabilities and reliability of trenchless techniques have expanded over recent years. Many installations not feasible even a decade ago are now possible. Experienced engineers and owners know that selection of the most appropriate trenchless technique(s) is essential for successful, safe, timely, and cost-effective project completion.