While the creation of subterranean piping systems has traditionally involved the highly disruptive and labour-intensive process of digging up the earth to gouge out installation channels, modern engineering has developed sophisticated subsurface construction methods that remove the need for open trenching completely.
These trenchless piping methods are currently enjoying a rapid rise in popularity throughout the construction and civil engineering sectors, because of their reduced cost as well as ability to minimize disruption to existing built environments during the process subsurface construction.
Two of the most popular trenchless methods for pipe installation today are horizontal directional drilling (HDD) and microtunnelling.
While both of these methods similarly permit the installation of subsurface pipes without the cutting of open trenches, according to Andy Robinson, director of Boregis Ltd. and a 30-year veteran of the underground utility installation sector, they each evolved along completely independent lines of development.
“HDD’s roots are in the oil industry and date back to the early part of last century,” said Robinson, who is a facilitator for the Design and Construction of Microtunneling Projects course run by the American Society of Civil Engineers (ASCE) on behalf of Engineering Education Australia (EEA).
“HDD employs many of the techniques and processes common to vertical well drilling, but today HDD equipment is tailor-made for horizontal, or near horizontal, installations.”
While the HDD process was originally developed by the oil industry, modern engineers frequently employ it for telecommunications or power projects because of its suitability for long-distance installation lines.
“HDD typically involves a surface launched and surface retrieved drilling process that is ideal for the installation of long continuous lengths,” said Robinson. “While it is best suited to the installation of smaller diameter pressure pipelines, typically less than 1.2 metres, for oil, gas and water transmission, it is also used for the direct installation of telecommunication and electrical cables.
“With HDD, the product pipe is installed after the hole for it has been created by the drilling process.”
In sharp contrast to HDD, which was created during the formative development of the modern oil industry, microtunnelling has been around for just several decades, and originally focused on sewerage pipe installation.
“Microtunnelling originated in Japan in the 1970s and was a development of conventional pipe jacking to enable smaller pipes to be installed mechanically,” said Robinson. “It was specifically designed for excellent installation accuracy, and is primarily used for the installation of gravity pipelines, in particular sewers.
“Unlike HDD, microtunnelling is undertaken from and to below ground shafts or pits, the same as other methods of tunnelling.”
A key difference between HDD and microtunnelling lies in the way they deal with the surrounding earth or soil in which the pipes are installed, and thus their suitability for different ground conditions.
“HDD supports the ground initially using a thixotropic mud in place of the excavated ground, and has problems supporting heavy granular soils such as gravel, cobbles or fragmented rock,” said Robinson.
“In contrast, microtunnelling installs the pipe lining in a single pass, and thus provides immediate support to the ground. However microtunnelling is generally not suitable for excavation of rock or where there is the risk of cobbles or boulders at the smaller diameters.”
While in certain situations both trenchless installation methods are applicable, the disparities between the approaches mean they do not often compete against each other directly.
“There is crossover between the two methods. Some small diameter gravity sewers are installed by HDD, especially when there is plenty of pipe gradient available. Similarly, some pressure pipelines are directly installed by the microtunnelling method,” Robinson said. “However, because of the different attributes of the two methodologies it is relatively rare that they compete for the same work.”
The older method developed by the oil industry remains more popular at present because of its widespread use for the installation of small-scale pressure pipelines.
“In some parts of the world, HDD has largely taken over from conventional open trench, especially for small diameter pressure pipelines and cable installations. It is therefore the more widely used method,” said Robinson. “Microtunnelling is generally limited to deeper gravity pipe installations in poorer ground where conventional trenching is not practical or desirable.”
The increasing popularity of trenchless technology has led America’s peak body for the civil engineering profession to create its own protocols and support mechanisms for the sector.
“ASCE Utility Engineering and Surveying Institute (UESI) have a committee focused upon trenchless installation of pipelines, responsible for leading ASCE’s engagement with industry and responding to industry needs within the trenchless sector,” said Robinson.
“ASCE also supports both microtunnelling and HDD methodologies through engineer training courses and the preparation of training videos, and has sponsored the development of a number of related publications and guidelines.”