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Subgrade Preparation for Wastewater Lagoon & Pond Installation

June 4, 2020

Industrial and Environmental Concepts (IEC) recommends proper subgrade preparation.  The pond subgrade is critical in draining liquid and venting gas for membrane liner systems.  The long-term performance of a pond liner system depends heavily on the condition of the prepared subgrade.

Most soils can be used for a subgrade as long as they don’t contain rock or materials that can puncture the liner.  Fine grained soils such as sand, loam and clay can be used for the pond subgrade.

The prepared surface should be uniform, well compacted, and free of sharp rock fragments, stones, tree roots etc. should be removed or covered with a protective material or geotextile. The surface should not have any natural or foreign object that protrudes above the surface of the subgrade.

Rock and aggregate in photo to be removed prior to liner installation.

The protective soil should be a minimum of (6″) thick and should be compacted. This bedding thickness may have to be increased depending on local site conditions. Another alternative for wastewater ponds is the use of a non-woven geotextile.

Smooth, rounded stones less than (2″) can remain within the prepared subgrade, however, these should compacted into the subgrade so they don’t protrude above the finished surface. The general rule of thumb is that all stones and rocks, regardless of shape and size, and clay lumps that lie above the subgrade surface should be removed.

The prepared subgrade should be compacted in accordance with design specifications and standard engineering practice. Generally, this means that the subgrade should be compacted to a minimum 95% of maximum dry density according to the standard Proctor test (ASTM D698). At a minimum, the subgrade should be firm and unyielding, and should be compacted to a level that permits the movement of construction equipment, liner deployment equipment, and other related traffic without causing rutting and/or deformation of the surface.  IEC recommends wastewater lined basin floors be slightly sloped to promote gas venting and drainage.

Compaction is especially important around pipe penetrations and concrete structures. In most projects, IEC can offer liner options for redundant sealing and recommendations around piping and structures.

Standing water, windrows and rock to be corrected prior to liner installation.

All ponds or lagoons to be lined should not include pockets or voids of any kind and should not be rutted or have high areas on the floor. In addition, the surface should be free of frost lumps and ice.

The prepared subgrade should also be shaped and graded to facilitate surface drainage both prior to, and during the installation of the lining system.

Care must be taken to maintain the prepared subgrade following completion. Vehicular traffic on the completed subgrade should be limited. Marks or ruts left in the subgrade by vehicular traffic should be smooth prior to pond liner installation. The subgrade should be protected from desiccation, flooding and freezing. Standing water should be removed so that the earthwork does not become saturated).

On projects that involve IEC, the subgrade will be inspected upon arrival at site. Our project supervisors will inspect the condition of the subgrade and will issue a “Certificate of Acceptance if requested . Corrective actions and activities to maintain the subgrade in a suitable condition for lining (including dewatering) are the responsibility of the owner or the general contractor.

For questions on lining or covering a basin, you can contact Dave Anderson at IEC’s office, 952-829-0731.