Horizontal directional drilling (HDD) and microtunneling have become terms synonymous with trenchless technology. Both these methods have proved to be versatile and cost effective compared to traditional trenching methods and are important tools in a contractor’s toolbox.
While both these methods serve the purpose of laying pipelines, there are inherent differences that a contractor needs to know before deciding which method to use for a particular project. An important aspect before a method is decided is the geotechnical investigation of the project site to determine the geological and soil conditions prevalent in the region.
Horizontal Directional Drilling (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 excavation of an entry/launch shaft and an exit/reception shaft. However, sufficient work space 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.
|Free Download: An In-Depth Look At the Horizontal Directional Drilling Process|
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 1,800 meters and depths up to 15 meters. Pipeline diameters varying from 50 to 1,200 mm can be installed using this method. Accuracy achieved with HDD is +/- 100 mm. When used in shallow ground cover, HDD can cause ground movement such as heaving or collapse and loss of drilling fluid. 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.
The Drilling Process
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.
The bore diameter is approximately 50 percent larger than the 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 throughout 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. (To learn more about drill bits, check out A Complete Guide to the Usage and History of Drill Bits and Tooling.)
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.
MTBMs 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 3,000 mm can be installed using this method. Accuracy achieved is as good as +/- 10 mm, which may be more when large diameter pipes are involved.
This method is excellent for installing pipelines requiring precise grade, such as gravity sewers. It is also used for installing other pipelines such as for 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, can make it a better choice.
The Microtunneling Process
The microtunneling process is similar to pipe jacking and requires adequate working space at both ends. MTBMs work best in soft formations and high water tables. A launch shaft is excavated to the depth of pipe installation and the jacking frame is set up. The thrust wall at the back of the launch shaft helps the product pipe to be pushed into the borehole created by the MTBM. This method requires stable ground conditions and hence unanticipated soil conditions such as ground water or 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. A closed loop slurry system transports cuttings and drilling fluid back to the surface for settlement and recirculation. 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. (To learn more about the process, see Planning a Microtunneling Project: What You Need to Know Before You Begin.)
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 site. Another important factor that decides the method is the kind of pipeline to be installed. For pressure pipelines such as gas, water mains, pump sewers and for 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 long run. Knowledge of different trenchless installation processes can enable contractors to make the correct choice to complete a project successfully.