To prepare the subgrade before installing a GEOMEMBRANE LINER, you must achieve a smooth, stable, and uniform surface free of sharp objects, voids, and moisture. This involves a meticulous sequence of steps: clearing and grubbing, excavation and grading, compaction and proof rolling, and final surface preparation to meet strict engineering specifications. The goal is to create a foundation that prevents puncture, minimizes stress, and ensures the long-term integrity of the liner system. A poorly prepared subgrade is the primary cause of liner failure, making this phase arguably the most critical in the entire installation process.
Phase 1: Initial Site Assessment and Clearing
Before a single piece of equipment rolls in, a thorough site assessment is non-negotiable. This isn’t just a walk-around; it involves geotechnical investigations to understand soil composition, groundwater levels, and overall stability. You’re looking for potential problems like organic topsoil, which decomposes and settles over time, or expansive clays that swell with moisture. The rule of thumb is to remove all unsuitable materials, including vegetation, roots, rocks larger than 1.5 inches (38 mm), and any man-made debris. This initial clearing, often called “clearing and grubbing,” sets the stage for everything that follows. If the native soil is weak, you might need to excavate to a specified depth and replace it with a select fill material, like a well-graded sand or gravel.
Phase 2: Excavation and Grading for Positive Drainage
This is where the shape of the subgrade takes form. The key principle here is positive drainage. The surface must be graded to direct any potential liquid (like rainwater or leakage) toward collection points, such as sumps or drains. The standard minimum slope is 2% (a 2-foot drop over a 100-foot run). For larger containment areas, slopes between 3% and 5% are common to ensure efficient flow. Grading is done with heavy machinery like motor graders and bulldozers, guided by surveyor stakes to hit precise elevation targets. It’s a process of cut and fill—moving earth from high spots to low spots—to create the designed contours. The table below outlines common slope specifications for different applications.
| Application | Recommended Minimum Slope | Rationale |
|---|---|---|
| Landfill Base | 2% | Balances drainage with stability of waste layers. |
| Pond or Reservoir Floor | 3% to 5% | Ensures complete drainage for inspection and cleaning. |
| Landfill Cap | 5% to 10% | Promotes rapid runoff, minimizing infiltration into the closed landfill. |
Phase 3: The Science of Compaction
Once graded, the soil must be compacted to a specific density to prevent future settlement that could tear the geomembrane. This isn’t about just running a roller over the dirt a few times; it’s a controlled process. The target is typically to achieve 95% of the Maximum Dry Density as determined by the Standard Proctor Test (ASTM D698). For critical applications, the Modified Proctor Test (ASTM D1557) might be specified, requiring 95% of a higher density standard. Compaction is achieved in lifts (layers) of 6 to 8 inches (150 to 200 mm) thick. Each lift is moisturized to near the soil’s “optimum moisture content”—the water level that allows soil particles to slide into the densest arrangement—and then compacted with a sheepsfoot roller, vibratory roller, or smooth drum roller, depending on the soil type. Proof rolling, which involves dragging a heavy, smooth-wheeled roller across the surface, is the final test. Any deflection or soft spots under the roller indicates inadequate compaction, and those areas must be reworked.
Phase 4: Final Surface Finishing and Verification
After compaction, you have a strong base, but it’s likely too rough for direct contact with the thin geomembrane. The final surface finishing is what separates a good installation from a great one. The goal is a surface that is “smooth and uniform to the eye and touch.” This means:
Removing All Protrusions: Every single rock, root, or clod of hard clay larger than 3/4 inch (19 mm) must be removed. A common method is to use a fine-toothed landscape rake or a motorized rock picker.
Filling All Voids: Any small depressions or holes must be filled with fine, sandy material and tamped down by hand. These voids can create air pockets under the liner, which can expand in heat and stress the material.
Creating a Protective Layer: In many cases, especially over rocky or coarse soils, a geotextile cushioning layer is installed directly on the subgrade. This non-woven geotextile acts as a puncture-resistant blanket, protecting the geomembrane from sharp particles. Alternatively, or in addition, a layer of fine sand or screened soil (often called a “bedding layer”) at least 6 inches (150 mm) thick is placed and meticulously leveled. The surface must be tested with a 10-foot (3-meter) straightedge; the tolerance is typically no gap greater than 1.5 inches (38 mm) beneath the straightedge.
Critical Checks and Tolerances
Before you even think about unrolling the liner, a final verification is done. This is a quality control checkpoint. Moisture content is critical; the subgrade must be dry. A simple field test is to walk on the surface in clean boots—if you leave no imprint, it’s dry enough. If moisture is present, it can vaporize after the liner is installed, creating bubbles that stress the seams and material. The surface hardness is also tested, often with a device like a Dynamic Cone Penetrometer to ensure uniform strength. The approved subgrade should be ready for immediate liner placement, and protected from traffic and weather to maintain its condition. Any rainfall means you have to wait for the surface to dry completely and re-check for erosion or soft spots.