Trenchless Pipeline Installation Methods and Their Pros and Cons
A geotechnical investigation of the project site is key to deciding the best method and equipment.
It’s an understatement that the benefits associated with trenchless pipeline installation methods far outweigh the deficiencies. The most appealing factor, however, is the fact that it does not require the disruption of traffic, blocking off entire road sections or digging up an entire stretch of road to get the work done. A launching and receiving pit is about the only excavation usually needed in most of these methods.
Trenchless technology offers a variety of methods for the installation of different pipe sizes and types, depending on location, soil type and depth of penetration. Some of the trenchless pipeline installation methods are horizontal directional drilling (HDD), horizontal auger boring (HAB), pipe jacking, pipe ramming and impact moling. All these methods have proved to be time and cost effective when compared to the trenching methods. However, these methods come with some advantages and disadvantages, knowing which will help gain a better understanding of which method will be most beneficial for a particular project.
Horizontal Directional Drilling (HDD)
HDD is a steerable tunneling method and is a two-stage process i.e. the pilot bore and the back reaming process, and can be used for small and large diameter pipelines. The optimum angle for the rig carriage incline to push the drill rod for drilling the pilot bore is 12° but the adjustment can be done anywhere between 5° and 30°. This method requires the use of a lubricant known as drilling mud, usually, bentonite that is circulated in the pilot bore as the drilling progresses. The drilling fluid facilitates the removal of the drill cuttings and also stabilizes the borehole.
Horizontal Directional Drilling Pros
There are several pros when installing pipelines using HDD. A major advantage is a minimum impact this method has to environmentally sensitive areas such as wetlands, estuaries, rivers and lakes. It is also a fast method of pipeline installation and enables the operators to accomplish creditable accuracy even when crossings are complicated. Continuous monitoring and control during the operation allow operators to navigate in constrained spaces between existing utility lines. Drilling can even be carried out at greater depths to bypass existing utilities altogether and requires only one launching shaft at the entry point and one reception shaft at the exit point.
Horizontal Directional Drilling Cons
The cons of HDD are not many but are significant. HDD operation can be carried out only by highly skilled operators and the cost of equipment and machinery requires the project to be of sufficient bore length to be profitable.
Horizontal Auger Boring (HAB)
HAB utilizes a casing system fitted with a cutting head and rotating auger that is jacked into the earth from the launching shaft. As the auger proceeds, it transmits torque to the cutting head. The soil cuttings generated from the cutting head are transmitted back to the launch pit via the rotating augers. Once the casing and auger are in the earth, the next set of casing and auger blades are attached to the previous casing and auger and the process is continued to the reception shaft.
Horizontal Auger Boring Pros
The major advantage of HAB is that the casing installation and excavation are carried out simultaneously. It only requires two pits, one for launching and one for excavation. The method is suitable for all types of soil except for mixed soils containing boulders.
Horizontal Auger Boring Cons
HAB is disadvantageous in that there is considerable investment in equipment because the method calls for availability of different sizes of cutting heads and augers for the varying casing sizes.
Pipe ramming is carried out by using a percussive hammer attached to the end of the pipe or casing that pushes it into the soil. The leading edge of the pipe or casing creates the borehole and installs the pipe. The spoil enters the pipe and is removed after the installation is complete.
Pipe Ramming Pros
The advantage of pipe ramming is that the method is very economical and does not require expensive equipment. The pipe can be installed either as one piece or in segments and can be installed in almost any type of soil. It is best suited to medium sized pipes and for shorter lengths.
Pipe Ramming Cons
The major disadvantage of pipe ramming is that there is no control over the line and grade of installation. There is also the chance of the pipe deflecting if met with hard obstructions like hard boulders that are larger than the casing. This method can only be used for steel pipes because of the percussive nature of the installation. The ranges of pipe size that can be installed using this method are limited from 2” to 55”.
Also known as the compression method, the installation is carried out by pushing or pulling the pipe through the borehole created by a mole. The mole is a streamlined tool in the shape of a bullet and is propelled using a power source. The energy is transferred to the mole by compressed air or hydraulic mechanism which allows the mole to propel through the ground. Once the borehole is complete the mole is removed and the pipe to be installed can be pushed or pulled through.
Impact Moling Pros
Impact moling is very advantageous for small diameter pipelines requiring short installation lengths. The method does not require any expensive equipment and can be used to install any kind of utility line, making it an economic method. The moling process helps in compressing the soil around the mole thus stabilizing the borehole.
Impact Moling Cons
Impact moling has some disadvantages such as inaccuracy in line and grade since the system is unguided. This can cause damage to existing utility lines that have not been detected before the start of the project. Pipe sizes that can be installed is limited to 6” or less though some techniques allow for larger sizes, and installation length is limited to 60 feet though some techniques allow up to 200 feet.
The variety of methods available in the trenchless installation toolbox is sufficient to meet the different types of soil and environmental conditions that exist at the installation site. In all these methods the most important thing is the geotechnical investigation of the project site to decide the best method and equipment suitable for the project. The overall cost and time that can be saved by using trenchless methods are considerable, which is why it is becoming an essential tool in the hands of many contractors whether for pipeline installation or rehabilitation.