Prefabricated Drop-In Liners: The Case for Earlier Consideration in the Design Process
Prefabricated geomembrane liners are frequently excluded from specifications before their suitability has been assessed. Understanding what prefabrication changes, and what it does not, allows engineers and specifiers to keep all options open at the point where they matter most.
Prefabricated drop-in liners and tank liners account for roughly seven out of every ten liner jobs quoted in the field. Even so, the choice of liner has often already been made without considering prefabricated liners, usually at the point a material is named in the specification.
What Prefabrication Changes
A prefabricated liner is welded in a controlled factory environment, then carefully flaked and rolled for transport to site. Factory conditions eliminate the variables that compromise field welding quality: wind, airborne dust, temperature fluctuation and operator fatigue under time pressure. Every weld is testable before the liner leaves the facility, and non-conformances can be addressed without programme consequences. On site, the scope of work changes considerably. Installation involves unrolling, un-flaking and anchoring the liner to the prepared subgrade. A civil crew equipped with appropriate lifting plant can carry this out without the specialist welding trade labour that field fabrication requires.
How Many Panels the Job Requires
Flexible geomembranes can be factory-fabricated into single flat panels of up to approximately 60 m x 60 m, representing around 3,600 m² of contained area in one flat piece, depending on the unit weight of the chosen material. For many smaller containment structures such as pump sumps, tank liners, small ponds and hardstand bunds, a single prefabricated panel covers the entire installation area, removing the requirement for any field welding at all.
Larger containment structures continue to benefit from prefabrication. The liner can be supplied as several factory-fabricated panels, joined on site with a limited number of field welds at their junction points. Once panels require on-site welding to complete the installation, the work is correctly characterised as an installation rather than a drop-in liner event. However, the reduction in total on-site weld length is still substantial, and the skill level required of the installation crew reduces accordingly. Prefabricated panel construction also opens the material choice beyond HDPE, which cannot generally be prefabricated in the same manner.
Where the Material Decides
Prefabrication requires a material with specific characteristics. The process requires a membrane that can be flaked, rolled tightly for transport, and unrolled on site without cracking, delaminating or sustaining stress damage. This is the defining physical requirement, and it is the reason flexible membranes form the basis of most prefabricated liner systems. Flexible membranes also carry a secondary advantage in service: their greater elongation before yield allows the liner to conform to subgrade settlement rather than bridge voids beneath it. They are also materially less susceptible to environmental stress cracking under sustained load, a failure mode that has affected HDPE installations where subgrade preparation was less than specification required.
Membranes suitable for prefabrication include:
- PVC and PVC alloys
- Polypropylene
- Flexible polyethylenes with EVA additives
- Proprietary polyethylenes formulated for folding and prefabrication
- LLDPE grades formulated for fabrication. Standard LLDPE sheet typically requires additive packages before prefabrication is feasible, adding cost to the process. Grade confirmation with the manufacturer prior to specification is strongly recommended.
Highly crystalline membranes such as standard HDPE yield at approximately 12% elongation and carry a higher risk of environmental stress cracking under sustained stress. Where flexible membranes are more forgiving of subgrade imperfections, HDPE requires a well-compacted, smooth and flat subgrade free of any surface protrusions.
Factors That Guide the Decision
The questions below are most useful when answered before a material is named in the specification. Once a material is specified, the fabrication method follows from it, and options that may have been technically and commercially appropriate are effectively closed off.
- What is the liner containing, and what chemical exposure must it tolerate? This sets the shortlist of acceptable membrane materials, and therefore whether prefabrication is feasible.
- What UV exposure is anticipated, and what service life is required? This influences the stabiliser package, membrane thickness, and the need for a cover layer.
- What is the sub-strata condition and preparation standard achievable on this site? This determines whether a conformable flexible liner or a rigid membrane with high subgrade preparation requirements is more appropriate.
- What is the finished containment area and geometry, and how many panels does the installation require?
- What delivery access and laydown area is available on site? A rolled prefabricated liner requires vehicle access and a clear laydown area for unrolling prior to placement.
- What lifting plant is available, and what is its safe working load? A minimum safe working load of 3 tonnes covers most prefabricated liner sizes. Heavier panels require prior assessment.
- How much programme time is available for on-site welding and weld testing? Where programme time is constrained, prefabrication with its factory-tested welds can shorten the critical path for the liner installation activity.
Answered before the material is specified, these questions keep every option open, including a prefabricated liner, which for many containment structures by number, if not always by area, represents the most practical and cost-effective solution available.