Microtunnelling is not a better answer in every project. Its advantage appears when the hardest part of the job is no longer the pipe itself, but everything above and around it: live roads, railways, rivers, dense utilities, built-up areas, groundwater, traffic management and the cost of reinstating the surface. In those conditions, comparing trenchless installation with open-cut excavation only by the metre rate can lead to the wrong decision.
Open-cut excavation is still hard to beat in simple ground. If the route runs through open land, at shallow depth, with good access, few services and no major groundwater issue, digging a trench will often be cheaper and easier to organise. Microtunnelling starts to make sense when opening the ground creates more problems than the pipe installation itself.
What is the real advantage of microtunnelling?
Microtunnelling is a trenchless method used to install pipelines between a launch shaft and a reception shaft. A microtunnel boring machine is pushed forward by hydraulic jacks, usually with purpose-made jacking pipes behind it. The system is remotely controlled and can keep tight control of line and level.
That accuracy is important for gravity sewers and other pipelines where fall, alignment and invert level matter. Open-cut excavation gives direct access along the whole route, but it also means opening the ground along the whole route. Microtunnelling concentrates the work at shafts and the working area around them.
This is the key difference. The pipe still has to be installed properly, but the surface is not opened continuously. That is why microtunnelling becomes attractive where the surface is valuable, busy, sensitive or difficult to reinstate.
Where open-cut excavation starts to lose out
The difference becomes clear under roads, rail lines, junctions, canals, rivers, airport areas and dense urban streets. In those locations, open cut is not just trenching. It can mean traffic management, permits, lane closures, diversions, surfacing removal, highway reinstatement, temporary works, service diversions and a higher risk of disruption to nearby businesses and residents.
This is where microtunnelling stops looking like an expensive specialist method and starts looking like a way to control the project. The more disruptive the surface works would be, the more valuable the trenchless option becomes.
For street works, reinstatement is not a minor afterthought. Roads have to be reinstated to the required standard, and poor reinstatement can create later defects, claims and repeat visits. Avoiding a continuous trench can reduce that whole package of risk.
Groundwater and difficult ground conditions
Groundwater is one of the main reasons open-cut excavation becomes harder than it first looked. A trench in wet ground may need dewatering, sheet piles or trench boxes, temporary works, pumping, monitoring and a tighter safety plan. The deeper the trench, the more serious this becomes.
Microtunnelling can offer better control because the excavation face is supported by the machine and the pipeline is installed from shafts rather than from a long open trench. It does not remove geotechnical risk. Ground investigation is still essential. But in water-bearing or unstable ground, it can give a more controlled way to install the pipe.
This is especially relevant where settlement, loss of ground or disruption to nearby utilities would be difficult to accept. The question is not only “can we dig it?”. It is “can we dig it safely, keep it dry, keep the surface working and reinstate it without creating another problem?”.
Why the metre rate can be misleading
Microtunnelling often has a higher direct installation cost per metre than open cut. That is not the whole comparison. Open cut carries costs that sit outside the pipe rate: excavation, shoring, dewatering, service protection, traffic management, access restrictions, temporary surfacing, reinstatement and the wider cost of disruption.
In an open field, those extra costs may be small. In a town centre or under a strategic road, they can become the main part of the decision. A trenchless method may still look more expensive in the construction line item, but cheaper or safer once the full project impact is counted.
| Project condition | Open-cut excavation | Microtunnelling | Practical meaning |
|---|---|---|---|
| Open land, shallow depth, few services | Usually simple and cost-effective | Often hard to justify | Open cut normally wins on cost |
| Busy road or junction | High traffic management and reinstatement burden | Surface disruption is concentrated around shafts | Microtunnelling becomes much more attractive |
| Rail, canal, river or major crossing | Open cut may be difficult, costly or unacceptable | Can pass beneath the obstacle with limited surface impact | Trenchless often becomes the practical option |
| Dense utility corridor | High risk of clashes and protection works | Route can be driven below or between constraints, if designed properly | Ground investigation and accurate records are critical |
| High groundwater or deep trench | Dewatering and temporary works can dominate | Face support and shaft-based working can improve control | Decision depends heavily on ground conditions |
| Short, simple connection | Often quick and cheap | Set-up cost may be disproportionate | Microtunnelling may be overkill |
The table shows the main pattern. Open cut wins when the surface is easy to open and easy to put back. Microtunnelling wins when opening the surface triggers costs and risks that are larger than the pipe installation itself.
Where microtunnelling works best
The strongest cases are crossings and constrained corridors. These include sewers under live roads, pipelines beneath rail infrastructure, utilities crossing rivers or canals, and routes through areas where a long trench would block access or disrupt critical operations.
It also suits projects where line and level have to be controlled closely. Gravity pipelines are a good example. A small error in fall can create operational problems, so the ability to drive a controlled alignment is not just a construction benefit. It affects the future performance of the asset.
Microtunnelling also becomes more attractive at depth. A deep open trench needs more temporary works, more excavation, more spoil handling, more safety control and more reinstatement. A shaft-and-drive method can reduce the amount of open excavation, although the shafts themselves still have to be designed and built properly.
The limits of microtunnelling
Microtunnelling is not a magic way to avoid civil engineering. It still needs launch and reception shafts, working space, jacking equipment, slurry handling or spoil management, specialist operators, jacking pipes and careful control of jacking forces.
The method also depends heavily on ground information. Unexpected boulders, obstructions, mixed ground, buried structures, old piles, unrecorded utilities or major changes in soil conditions can cause serious problems. A trenchless route gives less direct access to the face than an open trench, so investigation and risk planning matter even more.
Short drives can be poor candidates because the set-up cost is high compared with the length installed. Complex geometry can also be a problem. Microtunnelling can achieve controlled alignments, but it is not chosen casually for awkward routes without proper design checks.
When should a client take microtunnelling seriously?
It should be considered seriously when the route passes under something that cannot sensibly be opened up: a live road, railway, watercourse, airport pavement, major junction, dense utility corridor or heavily used industrial site.
It should also be considered where the trench would be deep, wet, unstable or difficult to keep safe. In those cases, the saving may not come from the pipe installation rate. It may come from avoiding complex temporary works, long traffic management periods, difficult reinstatement and repeated disruption.
The third trigger is accuracy. If the pipeline needs tight grade control, especially for gravity drainage, the ability to control line and level can be a major reason to choose microtunnelling over a more basic trenchless method or a difficult open-cut installation.
A simple decision example
Imagine the same 300 m sewer laid in two places. In the first case, it runs through open grassland with shallow cover, no major services and easy access. Open cut will probably be the sensible choice. The trench is manageable, reinstatement is simple and the cost of surface disruption is low.
Now put the same sewer under a busy urban road with side streets, utilities, traffic restrictions and expensive reinstatement. It is no longer the same project. The pipe may be the same size, but the construction environment has changed completely. In that second case, microtunnelling may be the better decision even if its direct installation rate is higher.
This is the practical point. Microtunnelling does not win because it is always cheaper. It wins when it avoids the costs and disruption that open cut creates around the excavation.
How to compare both methods properly
A fair comparison should include more than the pipe and excavation cost. It should include shafts, ground investigation, temporary works, traffic management, utility protection, permits, programme risk, reinstatement, spoil handling, dewatering, access restrictions and the cost of disruption to third parties.
It should also consider what happens if something goes wrong. An open trench in a simple field is easy to inspect and adjust. An open trench in a congested street can turn into a much larger problem. A microtunnel drive with poor ground information can also be costly to recover. Both methods have risks. They are just different risks.
The better question is not “which method is cheaper per metre?”. It is: which method gives the lowest realistic project risk for this route, this ground, this surface and this programme?
Summary
Microtunnelling outperforms open-cut excavation when the surface above the pipeline becomes the main constraint. In open, shallow and uncomplicated ground, traditional trenching often remains the logical and cheaper option. In urban streets, under traffic, beneath rail or water crossings, near dense utilities, in deeper excavations or in high groundwater, the balance can change quickly.
The real advantage of microtunnelling is not that it avoids all excavation. It does not. It still needs shafts, specialist equipment and careful planning. Its advantage is that it avoids opening the whole route from above. Where that reduces traffic disruption, reinstatement work, groundwater problems and construction risk, microtunnelling deserves to be taken seriously.
Sources:
https://www.ukstt.org.uk/pf/microtunnelling/
https://pipejacking.org/assets/pj/static/PJA_intro.pdf
https://www.gov.uk/government/publications/specification-for-the-reinstatement-of-openings-in-highways
https://www.wsp.com/en-gb/insights/2023-trenchless-tunnelling-technology-benefits
