Very Low-Density Polyethylene in Tunnel Lining

Plastics are among the most versatile materials on the planet. They are used in everything from small consumer packaging to citywide projects. One of the reasons plastics are so versatile comes from their inherent chemical stability despite changing their overall structure. Changes to the molecular structure of polyethylene (PE) for instance, can alter the compound’s physical properties like density, rigidity, and tensile strength while retaining most of its thermal and chemical resistances. Some of the more common types of PE are high-density PE (HDPE) and linear low-density PE (LLDPE). The main difference between LLDPE and HDPE lies in their respective molecular structures. HDPE has fewer branches, lower molecular weight, and greater crystallinity than LLDPE. The result is a compound whose molecules can be packed closer together, producing a stronger, denser, more rigid material when compared with the highly branched LLDPE.

There are other, lesser-known variations to the molecular structure of PE that possess unique physical properties which makes them excel in tunneling applications. In this article, we will explore a variation known as very low-density PE (VLDPE), define some of its physical properties, go over its technical specifications when applied as a tunnel lining product, and explain why it can be a superior alternative to other tunnel lining products.

Physical properties of VLDPE

Polyethylene comprises a long chain of carbons and hydrogens (ethylene) bonded together. As mentioned earlier, the polymer’s physical properties can change depending on how these carbons and hydrogens are arranged on the chain, which can be altered through various manufacturing processes. The main structural distinction comes with the variety and density of the short and side chain branches that form off the primary chain. VLDPE is noted for its many branches, low density, and low crystallinity, which makes products of this material type lighter and much more flexible than other variations of the same thickness.

In many applications, strength and durability are the prime concern, but there are situations where a lighter, more flexible product is desired. Consider liners for concrete tunnels. These tunnels are already structurally sound and do not need added reinforcement from a liner. They do, however, often require protection from water infiltration that can damage the concrete in many ways. For instance, water often carries chloride ions that are capable of permeating through concrete and corroding the underlying steel structures that reinforce the concrete.

With a VLDPE layer, you can prevent water infiltration and protect the integrity of the concrete and its reinforcing steel structures. Of course, VLDPE wasn’t the first solution created by engineers, but it is certainly one of the latest and greatest.

Benefits of using VLDPE for tunnel lining

For decades, engineers have turned to PVC for their tunnel lining needs. With VLDPE, engineers have access to a lightweight alternative that is about 25–30% lighter than PVC, making it easy to install and less expensive to transport. The material’s enhanced flexibility further simplifies installation as it is capable of conforming to the irregular shapes and surfaces typically encountered in tunnels. Taken together, VLDPE’s light material weight and flexibility streamlines installation by reducing the burden and fatigue on installation crews and equipment.

It is also important to note that VLDPE’s light weight makes it practical to manufacture wider sheets, further reducing on-site labor through fewer welds. With fewer welds seams, engineers can also expect increased system reliability. Finally, as with all geosynthetics, VLDPE is non-toxic and seaming does not produce toxic fumes for a safe tunnel working environment. VLDPE liners can also be customized to further enhance crew safety and quality control through the use of a coextruded differing color signal layer.

VLDPE tunnel liners available in North America as AGRUFlex

In the past, engineers relied heavily on PVC liners. Today, they also have access to VLDPE, an alternative modern product that can equally protect concrete tunnel structures against the infiltration of water and gases. AGRUFlex is a VLDPE liner now available in North America from AGRU America for tunnel lining applications. In addition to offering efficient corrosion protection in tunnels, the product can be economically installed and can feature additional layers for improved safety and quality control. For instance, AGRUFlex can be extruded with a white surface to reflect light and enhance tunnel visibility for a safer working environment. A colored layer can be used to enhance quality control processes by improving the visible detection of liner damage. An additional conductive signal layer enables the use of Construction Quality Assurance Testing for possible holes, punctures, tears, cuts, cracks, and similar breaches over the partial or entire area of the installed.