Trenchless methods of construction and rehabilitation have satisfied many of the demands of busy cities, such as less interference, faster installation and environmental sustainability. But it’s easier said than done when it comes to selecting the right pipe material.

Many factors will influence the choice of pipe such as its usage, whether for storm water drains, sewer drains, water pipes or gas pipes; whether the pipe will function under gravity or pressure; the method of installation; project fund availability; type of soil and how it will affect the pipe material.

The most commonly used pipe materials in trenchless technology are concrete, cast iron, ductile iron, vitrified clay, polyvinyl chloride (PVC), polyethylene (PE), high-density polyethylene (HDPE), glass reinforced plastic (GRP) and steel.

The Favored Pipe for HDD and Pipe Rehabilitation

The choice of pipe for horizontal directional drilling (HDD) applications and pipe rehabilitation is PE and HDPE. HDD is a widely used trenchless method for installing pipes under cities, river crossings, embankments and through environmentally sensitive areas such as wetlands and lagoons, without disturbing the surface.

It can be used for water pipelines, sewer pipelines, storm water drains, gas pipelines and electrical and communication ducts. PE pipes can also be used for trenchless rehabilitation methods such as sliplining and pipe bursting that replace old and worn out pipes under city streets and front yards without having to dig them up.

To understand the benefits of PE and its usefulness in trenchless technology, we will take a look at PE, its properties and the installation process in HDD using butt welding to create a seamless, long lasting pipeline. (For more on different pipe materials, see The Lifespan and Dangers of Old Piping Materials Vs. Plastics and Liners.)

What Is PE?

PE is a thermoplastic polymer that can be shaped by flow and can be re-melted on heating. The starting material to produce PE is ethylene, a colorless gas, hence the name polyethylene. A PE pipe compound consists of PE resin combined with ingredients such as stabilizers, colorants and antioxidants depending on the application for which it is manufactured.

When melted, the thermoplastic can be molded into any desired shape and size by the extrusion process. When physical forces act on the polymer, they immobilize the polymer chains, preventing them from slipping, and then form the solid state upon cooling.

Characteristics of PE Pipes

PE pipes are virtually impervious to attack by moisture, water and soil, and are non-conductors of electricity. Other properties include:

  • Corrosion resistance – PE is not subject to electrochemical-based corrosion processes induced by electrolytes present in soil.
  • Abrasion resistance – The smooth internal surface of the pipe assists flow and prevents solids from accumulating or scratching the surface and also prevents buildup of biological matter and bacteria.
  • Flexibility – PE allows change in direction with minimal fittings; diameters up to 6” can be transported in long lengths as coils.
  • Strain-ability – PE can undergo high deformation without fracturing, allowing a buried PE pipe to deflect and take support from surrounding soil, and is able to resist seismic forces.
  • Join-ability – Two PE pipes can be joined to each other using the thermal fusion process known as butt fusion to make a leak-proof joint.
  • Toughness – PE resists crack propagation and retains toughness even at low temperatures.
  • Fatigue resistance – PE is resistant to fatigue even when there are continuous and repetitive pressure variations and pressure surges.
  • Long service life – PE lasts up to 100 years.

Butt Fusion of PE Pipes

Butt fusion involves the simultaneous heating and joining of two pipe ends. It is a thermofusion process, where heating of the two pipe ends is carried out using an electric heater plate until the molten state known as the fusion temperature is reached. At this stage controlled pressure is used to bring the pipe ends together and the pressure is maintained until a homogeneous joint is formed and the cooling period has elapsed.

The joints formed by butt fusion are fully resistant to end loads and have been found to have comparable performance under pressure to the pipe itself. Butt welding machines are used for the process and are capable of controlling parameters such as temperature of the heater, alignment and pressure.

Trained welders, good butt welding machines and correct parameters are important to ensure that joints do not fail due to contamination or incorrect technique.

PE Pipes in HDD

When it comes to installation of pipelines using trenchless methods such as directional drilling, the most preferred PE types are high density polyethylene (HDPE) and PE100, which has a minimum required strength of 10 MPa. PE100 has higher strength, higher resistance to crack propagation and greater toughness. PE100 has three types: standard, high melt strength and high stress crack resistant (HSCR).

PE100 HSCR is used where trenchless installation has to run through rocky geology, requiring additional protection. PE has been used in the trenchless industry for many years now and has proved to be very advantageous. (For more on HDD, check out A Step-by-Step Guide to HDD.)

HDD is a trenchless method in which the drill is launched at a pre-determined angle from a drill rig on the ground to a pre-determined exit point on the other side. The drill initially creates a pilot bore through which a reamer of different sizes is pulled back from the exit side of the drill path. Once the reamer of the same size or slightly larger than the pipe diameter is pulled back to the entry point, the pipe string is pulled with it.

Often brittle pipe materials break or get damaged during the pullback process, especially at the joints. Since PE pipes are ductile and flexible, straight long lengths can be manufactured, minimizing the need for fittings and joints. Pipe sections can be easily butt fused to create a pipe string of the desired length that can be pulled into the drilled borehole with ease and no fear of breaks or damage.

While other pipe materials are also frequently used, PE has become the material of choice because of its inherent characteristics. The rate of leakage in butt fused joints, if done properly, is zero and gain a tensile capacity as good as the pipe itself. PE has made it convenient to install pipes with minimum chance of failure post-installation and longer, and a problem-free service life.

River crossings, railway embankments, highway crossings and environmentally sensitive areas are best benefited by HDD. PE has a low carbon footprint, making it a choice material for crossings under lagoons, wetlands and levees.