The last category of common applications for geomembranes is underlayment. Similar to liners, an underlayment doesn’t necessarily contain every drop of liquid in a reservoir, but still plays an important role in drainage and soil stability. For example, most geomembrane uses in road construction and repair qualify as an underlayment. Even when they’re used to stop fuel and oil leaks from soaking into nearby soil or groundwater, they’re also adding durability to the road surface by improving its resistance to lateral spreading forces.
Geomembranes make an ideal underlayment for concrete pads and foundations installed over loose, fast-draining, or unstable soil. Some products are combined with geotextiles that actively hold soil to create hill and slope stabilization systems that are incomparable to previous methods. However, other underlayment installations are far simpler. A layer of impermeable geomembrane like BTL Liner’s ArmorPro is great for ensuring a green roof doesn’t leak. An underlayment doesn’t necessarily come in direct contact with liquids, or at least not as constantly as with storage ponds and tanks; so, they have a different set of requirements for reliable performance.
Full Buried Installation
Geomembranes chosen for underlayment purposes generally need to be designed for full buried installation. Most reinforced and unreinforced materials qualify for this kind of installation, but reinforced geomembranes definitely hold their shape better when installed against steep slopes. Even green roofs generally bury geomembranes layers under at least a few inches of ballast to prevent UV damage. Underlayment is typically replaced on a regular schedule, even when it’s buried, so the materials don’t necessarily need to last quite as long as permanents liners do. If the application doesn’t call for completely burying the underlayment material, it will at least need a ballast system to keep it anchored to the ground for temporary uses.
Traditional full cover ballasts, like gravel and river stone, are only generally used to protect green roof installations and some rural road designs. Most geomembrane underlayment that require ballast rely on sandbags, gravel cages, anchoring spikes or strips, or even large landscaping stones to keep the material in place. Green roof designs call for large diameter gravel to allow water to quickly drain down to the surface and towards the drains. Mulch layers serve as the ballast over the planted surfaces, but there’s always a need for another fill material around protrusions like vent pipes and skylights so there’s no chance of ponding water.
No Need for Secondary Underlayment
Some types of pond and tank liners require a secondary underlayment material like geomembrane to protect them from rough soil and abrasion damage. When you choose a heavy-duty liner material like BTL Liner’s ArmorPro, there’s no need for another underlayment below it to prevent damage. The same is true when ArmorPro is chosen as an underlayment material on its own. Eliminating a whole layer of material can dramatically reduce the cost of a project, allowing your company to expand the size or scope of the installation.
Dealing with Difficult Soil Types
Geomembrane underlayment can keep gravel from sinking right into soft, malleable soils that have a steady source of water rising from below the surface. Expansive clay, a type of soil found widely throughout the South, Southeast, and Midwest, is particularly challenging for forming stable roadbeds. Major highway projects like Interstate 35 from Texas to Minnesota have dealt with these kinds of conditions by installing geomembranes as an underlayment. Since the clay absorbs a lot of water, swells, and then cracks as it rapidly shrinks in dry weather, roads installed above the soil often crack severely. Undulations can lead to roads that are highly uneven and very damaging to tires. Geomembranes are ideal for keeping difficult soil from damaging the road above by adding a solid layer that flexes and shifts without telegraphing damage to the asphalt or concrete above.
Moisture Control in Areas with High Water Tables
Roads and foundations installed in areas with high water tables also benefit from an impermeable geomembrane that doesn’t let water seep up into the roadway or wash out the soil below the surface. In many mountainous parts of the Southeast, Northeast, Northwest, and Midwest, underground streams and springs cause recurrent potholes on rural roads and highways. Geomembranes that don’t allow moisture permeation work best for stabilizing these areas. For best results, the material should be contained within multiple layers of gravel so there’s plenty of support to distribute the weight as vehicles travel over the surface.
Which Materials Work Best for Underlayment?
For high stress installations like roadways, HDPE materials are generally chosen based upon their superior durability and adaptability to buried installation. However, HDPE geomembranes do run the risk of cracking when installed in environments that experience very cold winter temperatures. The material also becomes brittle and hard to handle during winter installations, and regular exposure to freeze and thaw cycles shortens the lifespan of the membrane. This is easily overcome by choosing an HDPE liner that features protective LDPE layers. The higher cold resistance and flexibility of the LDPE creates an underlayment that can work in all weather conditions. You’ll find this combination of materials in BTL Liner’s ArmorPro product line.
Replacement Schedules for Green Roof Materials
The appropriate schedule for replacing the waterproof geomembrane on a green roof depends on the exact material chosen and the installation details. You can generally expect to replace the membrane every 10 to 30 years, depending on the thickness of the material and the aggressiveness of the plants on the roof. If weeds and grasses move in that have strong and penetrating roots, you may find a green roof needs a new liner in just five years. Proper maintenance and weeding is essential since you only want shallow and gentle rooted plants growing on the roof. As long as you keep the material covered with mulch, soilless growing medium, and gravel where needed, you won’t need to worry about UV degradation shortening the lifespan of the geomembrane.
Each year brings the discovery of new ways to use geomembranes as an underlayment or liner. Within the next few decades, continuous improvements in polymer manufacturing and additive options will expand use even further. By understanding the basic characteristics and benefits of a geomembrane product like BTL Liner’s ArmorPro, engineers can easily find plenty of new opportunities for putting underlayment to good use.