Microtunneling is a common trenchless construction technique. Communities use this method to help lay new utility and sewage lines. Oil and gas companies use microtunneling to drill new wells.

Planning a project where microtunneling is used to create the initial bore requires knowing what may lie beneath the surface. Different soil conditions, obstructions, rocks, and boulders can all make it difficult to use these machines to complete the project. (Read: Microtunneling: Key Planning Elements to Know.)

What Types of Obstructions Can Impede Microtunneling?

When it comes to microtunneling, many obstructions may get in the way. During the planning stages of any underground construction, a geological survey is necessary. The survey can help planners identify potential obstructions they may encounter during tunneling. Rocks and subterranean boulders are only a small part of what may lie beneath the surface and cause issues for drillers.

In urban areas, underground utilities are a problem for all subterranean projects. Having accurate underground maps helps to avoid these obstructions. Sadly, not all subterranean utility maps are as accurate as they need to be to avoid potential issues when microtunneling.

Other potential obstructions that may occur during microtunneling is preexisting construction materials. Concrete sheet piles and working slabs from previous works sites can cause issues for microtunneling projects. By reviewing building plans for the site, workers can determine if any shoring methods which may compromise the project had been employed,

Workers may encounter natural obstructions during the microtunneling process as well. Rocks, cobbled soil, and boulders may be present in the underlying soil. A geological survey, along with a ground-penetrating radar (GPR) review of the site, can help to map out any potential problem areas. (Read: Understanding the 4 Stages of Site Investigation.)

What Can You Microtunnel Through?

Rock is a vague term when determining if microtunneling is an option. The term “rock” covers an array of material types and characteristics. Some rocks are brittle, while others have a confining pressure of over 300 megapascals (MPa).

The ability to microtunnel through sedimentary rock dates to the 1990s using machines of all sizes. Sedimentary rock, when drilled, acts more like dense soil and is easy for the boring machines to tunnel through quickly.

Sandstone, on the other hand, may make it more difficult to complete the project. Sandstone wears the cutting tool and can limit the drive length. If the cutting tool becomes too worn, workers need face access to replace the tools before continuing.

Harder igneous and metamorphic rock can be tunneled through. However, this advancement has come about relatively recently, only after the turn of the century. Before the early 2000s, microtunnel boring machines (MTBM) did not have face access. (Read:Microtunneling vs Horizontal Directional Drilling: Understanding the Differences Between These Key Trenchless Methods.)

The lack of access made it virtually impossible to change the worn cutting tools during the drive. Even today, only boring machines with an outer diameter larger than 59.25” have the face access necessary to take on an abrasive rock formation.

To determine if microtunneling is an option, planners should perform a detailed geological study. In the study, they should learn what type of rock is in the soil. The rock should be classified by the drive and within a specific drive length. It should mark the location of any transition zones from rock to soil as well as the orientation and spacing of rock bedding.

Planners should also conduct point load and punch penetration tests.

Can Microtunnel Boring Machines Mine Through Boulders?

Microtunnel boring machines can break through a variety of dense structures. However, the components and boulder diameter are a factor in whether the MTBM can handle the job.

Boulders can be any rock structure that measures one to 30 feet clast. Most subterranean boulders have a clast of one to three feet. Very large rocks with a clast of over 30 feet are known as floaters and are often elongated and occur near the bedrock.

Planners must calculate and consider the strength of the boulders which occur in the microtunneling path. Boulders from local bedrock, which have not been transported by alluvial or glacial deposits are often weaker than those which have been transported “down-stream” to their current location. However, these boulders are rare.

According to the paper “Tunneling in Cobbles and Boulders” by Steven W. Hunt, most boulders have a maximum unconfined compressive strength of 22,000 to 45,000 psi (152-310 MPa) and a minimum unconfined compressive strength of 5,000 to 25,000 psi (34 to 172 MPa). If the compressive strength of the boulder is calculated to be less than 2,900 psi (20MPa), a disc cutter is unnecessary. For boulders with 10,000 to 15,000 psi (69 to 103 MPa), there is a need for a disk cutter to be added to the cutter head. (Read: How Does Rock Mechanics Help Engineers Better Understand How Rocks Will React to Tunneling Projects?)

Potential Hazards, Risks, and Consequences of Microtunneling through Boulders

While some MTBM can cut through boulders under a certain size, there are always some hazards associated with tunneling through these structures. Boulders that have over 20 to 30% diameter, have no face-chamber access, or disc cutters can cause the MTBM to get stuck or require a rescue of the shaft-tunnel and abandonment of the MTBM-tunnel. If it doesn’t get stuck, the cutter or rock crusher bar gets worn.

Other potential consequences include a severed pump and slurry light wear, which may cause pump failure or line rupture. The intake port can become severed, which results in jammed slurry lines and enlarged holes. Boulders can jam against the earth pressure balance (EPB) screw conveyor intake.

Damage to the MTBM is only one risk companies need to consider. For instance, if the construction team determines a boulder is too large to cut through, blasting or splitting the obstruction may be considered. While this is a viable solution, the blast may cause voids, ground loss, or sinkholes.

In Conclusion

With proper testing and planning, workers can determine what size and drive length of microtunnel boring machine is adequate for their needs. While some machines can handle abrasive rocks and small to mid-sized boulders, others cannot. A thorough geological survey can keep potential hazards from becoming a reality.