Both methods have gained popularity because of their non-disruptive nature and their lower carbon footprint compared to open-cut pipeline installation methods. However, these methods have a few important differences, knowledge of which can help contractors choose the right method for a project.
Launch and Retrieval
HDD – HDD is a surface launched trenchless installation technique used to install pipelines where conventional tunneling can prove to be expensive. It is a surface-to-surface method and does not require the excavation of an entry/ launch shaft and an exit/ reception shaft. However, sufficient workspace has to be provided at the drill rig side and the exit side for storage of pipes and auxiliary equipment.
HDD has been used successfully to install pipelines through environmentally sensitive areas, long crossings, and delineated lands. It is very popular for installing pipelines under palustrine wetlands, streams, creeks, airports, etc.
Microtunneling – Microtunneling is a pit launched trenchless installation technique and uses laser-guided steerable remote control. This method provides continuous support to the excavation face. An entry or launch pit is excavated and provided with a jacking system and thrust wall. An exit pit is excavated at the spot where the microtunnel-boring machine (MTBM) and the product pipe will be retrieved. A slurry separation plant is provided at the surface in order to manage the slurry.
Microtunneling is used to install pipelines that cross infrastructure such as highways, railway lines, major rivers, etc. MTBMs use high end guided systems with live monitoring for real-time correction capability, giving a high degree of accuracy compared to other trenchless methods. (Read also: Pits and Excavations in a Trenchless Project, a Primer.)
Soil Type, Size and Accuracy
HDD – HDD is best suited for clayey soils, non-cohesive sand, and silt because of its ability to stay suspended in the drilling fluid (such as bentonite) for removal. It is used for long installation lengths, in some cases greater than 1800 meters, and depths up to 15 meters. Pipeline diameters varying from 50 to 1200 mm can be installed using this method. The accuracy achieved with HDD is +/- 100 mm. When used in shallow ground cover, HDD can cause ground movements such as heaving or collapse and loss of drilling fluid.
Microtunneling – MTBMs work best in soft formations and high water tables but they can be used to bore through all types of soil, sand, clay, and even through hard rock. Installation lengths can exceed 225 meters with varying depths. Pipeline diameters varying from 250 to 3000 mm can be installed using this method. The accuracy achieved is as good as +/- 10 mm, although may be more when large diameter pipes are involved. (Read also: Soil Types and How They Affect Trenchless Construction.)
Best Suited For
HDD – HDD rigs require an assortment of systems to enable accurate installment. Since it is launched from the surface and arcs down into the ground before it comes out the other end, this method cannot be used for installing gravity pipelines. HDD is best suited for installing pressure lines, water lines, cables, gas pipelines, pump sewer systems, and conduits. Installation of pipelines using HDD is cheaper than microtunneling.
Microtunneling – This method is excellent for installing pipelines requiring precise grades such as gravity sewers. It is also used for installing other pipelines such as gas, water, and cable lines. Even though microtunneling is a bit more expensive than HDD, the accuracy, reliability, and lower maintenance cost of pipelines post-installation, makes it a better choice in most cases.
The Boring Process
HDD – The HDD method is a two-stage process. The first stage involves drilling a pilot hole using a steerable guided drill along a planned path. The pilot hole is approximately 1 to 5 inches in diameter along the proposed centerline of pipe alignment. When the drill string reaches the exit point, the second stage is initiated.
The second stage involves attaching a reamer of approximate size as the product pipe to the end of the drill string after removing the drill bit and pulling it back to the entry point. Sometimes several passes may be required before the desired bore diameter is achieved. (Read also: Why Reamers Are Important to Trenchless Boring, Plus Available Types of Reamers.)
The bore diameter is approx. 50% larger than pipe diameter to enable easy pull-through of the pipe string. The product pipe is attached to the reamer and pulled through the borehole. All along the process, electronic monitoring of the drill path is carried out to obtain the maximum degree of accuracy. A slurry system helps circulate the drilling fluid and helps transport the drill cuttings back to the surface.
Microtunneling – A launch shaft is excavated to the depth of pipe installation and the jacking frame is set up. The rotating cutter head of the MTBM excavates the ground as it progresses. The laser guidance system installed in the MTBM relays grade and position to the operator at all times. The thrust wall at the back of the launch shaft helps the product pipe to be pushed into the borehole created by the MTBM.
A closed-loop slurry system transports cuttings and drilling fluid back to the surface for settlement and recirculation. This method requires stable ground conditions and hence unanticipated soil conditions such as groundwater or the high water table may require corrective measures such as freezing or dewatering. A wide range of cutter heads is available to cut through different soil types or through rocks if needed.
Make an Informed Choice
- The choice between HDD and microtunneling depends on factors such as pipe size and material, allowable tolerances, available area, and ground and subsurface soil conditions at the site.
- Bends in HDD pipelines can become weak points that translate into increased maintenance costs and risk of pipe failure.
- Microtunneling has greater pipeline integrity due to its straight or curved pit to pit design reducing maintenance costs and risk of failure.
- For pressure pipelines such as gas, water mains, pump sewers, and cable pipes, HDD is a better option and also cost-effective.
- For installations that require precise grade and accuracy such as in gravity sewers, microtunneling is the preferred method.
- Microtunneling is costlier than HDD but it has more accuracy and reliability in the end.
- For smaller budget projects where accuracy and long-term costs are not a major consideration, HDD is preferable.
- Where accuracy and long-term costs are priorities, microtunneling though expensive, is the right way to go.