Microtunneling vs HDD
HDD and Microtunneling: Trenchless Installation Techniques
Horizontal Directional Drilling (HDD) and microtunneling are two common trenchless methods of installing underground utilities with minimal surface excavation. Each of these techniques has very specific applications, advantages, and limitations; hence, the selection between the two becomes critical for the success of a project.
If you would like to know more about HDD or microtunelling check the further reading listed below. The Microtunneling and Horizontal Drilling - Recommendations published by the French Society for Trenchless Technology provides a good background on both technologies in Chapters 2 and 10.
Horizontal Directional Drilling (HDD)
HDD is a surface-launched technique of installation suitable for placing pipelines under obstacles such as rivers, highways, and environmentally sensitive areas. The concept of HDD mainly consists of a pilot hole drilled along a previously designed path, then the hole is enlarged to the desired diameter of the pipeline using reamers and pulled through the borehole with the pipe.
Advantages of HDD
- Cost-Effectiveness: HDD is generally less expensive than microtunneling, making it suitable for projects with budget constraints.
- Flexibility: HDD can handle various soil types, including clay, sand, and silt, and is effective for long-distance installations, sometimes exceeding 1,800 meters.
Limitations of HDD
- Accuracy: HDD is only accurate to +/- 100 mm, which may not be good enough for installations that have to have very precise.
- Reaming: Wider holes require a second passage using a reamer to enlargen the hole.
Microtunneling
Microtunneling is a remotely controlled, laser-guided continuous support to the excavation face, which is well-suited for high water table conditions and other soil types, including hard rock. It involves digging entry and exit pits. The MTBM is then launched from the entry pit and accurately guided to the exit pit.
Advantages of Microtunneling
- High Precision: Microtunneling provides accuracy within +/- 10 mm, making it suitable for pipelines that have to be laid at exact grades.
- Suitable for Difficult Conditions: Suitable for high water tables and can handle a wide range of soil types, from soft soils to hard rock. TBM’s tend to be very specialised depending on soil condition. Some TBM’s such as the Herrenknecht Multi-Mode TBM are suitable for many different soil conditions.
- Improved Safety: Microtunneling minimises the need for workers to enter the borehole, thus minimising many of the safety risks associated with underground construction. Large TBM’s require a crew of up to 8 people though, some of which are inside the tunnel or even the TBM as it moves, designing TBM’s with minimal crew size required would reduce operating costs.
Limitations of Microtunneling
- Higher Costs: Advanced technologies and equipment involve increasing the overall costs of microtunneling compared to HDD.
- Installation Lengths Limited: The technique has traditionally been applied to short-length installations, normally up to 300 meters compared to HDD.
Choosing Between HDD and Microtunneling
In selecting one among HDD and microtunneling, a project manager must at least consider:
- Project Budget: Projects that have slim budgets might prefer HDD, being a less costly method.
- Required Precision: Microtunneling is preferred in highly accurate installations, such as gravity-dependent pipelines.
- Soil and Environmental Conditions: In cases of a high water table or problematic soil conditions, microtunneling has better conditions, whereas HDD has flexibility on all sorts of terrain.
- Installation Length and Diameter: Longer and variable diameter installations can be done using HDD, while microtunneling is generally in shorter lengths with larger diameter installations.
In short, both HDD and microtunneling are very good trenchless technologies, each with its own strengths and weaknesses. A proper method should be selected based on thorough analysis of project-specific needs, environmental conditions, and budget for successful and effective installation.