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More than a decade ago, a third-party case study described a geocomposite drainage layer installed as part of a landfill cap system. The project was typical and progressing exactly as planned with all appropriate construction and installation procedures being followed according to standard industry practice. Yet, as the filler sand was being placed on the composite, onsite construction quality assurance personnel began noting a tearing sound. Somehow, this geocomposite that had passed all standard testing of the time was failing as the geotextile was actively delaminating from the geonet. How could this happen?
In late 2018, Richard Thiel published a follow-up article discussing the shortcomings of the test methods used in ASTM D7005. To summarize, he shows that there is significant variability of peel strength across a roll width. For this reason, the five samples required for ASTM D7005 is likely to overlook potential defects in the geocomposite as it did more than a decade ago.
In this article, we will review some of the suggestions presented by Thiel and offer additional suggestions on best practices for effective quality control.
The exact lamination strength, or peel strength, of a geocomposite is important to know when the product will be responsible for maintaining long-term interface shear strength. For this reason, establishing specific design specifications is critical. Thiel writes that the product’s lamination integrity depends on a number of factors including the unbonded edge distance, which should be restricted to less than 15 cm, and the uniformity of the bonding between layers. Laminated areas should not display weak spots, blisters, or holidays.
While it has been suggested that a lamination peel strength target value of 1.0 pound/inch (175 N/m) is appropriate for most projects, the drawbacks of the current test method has prompted Thiel to suggest alternative specifications. Both alternatives were discussed in detail at the Geosynthetics 2019 conference with feedback was forwarded to the ASTM Committee D35 on Geosynthetics. Until then, designers should consider doubling the total area tested by either increasing the width of each specimen or the doubling the number of the specimens.
For the installation phase, it is recommended to specify the size and weight of the construction equipment allowed on the slope in addition to the minimum allowable soil thickness spread over the geocomposite (12 inches, or 30 cm). Increasing the soil thickness could help offset the weight of larger equipment if the project requires heavier machinery. Otherwise, the smaller the equipment, the better.
Design engineers will typically develop specifications and continue using those same figures for years until updates to standards are published. However, changes to standards take time and it can take years before recommended changes go into effect. By anticipating changes, engineers can maximize the performance of their designs.
Manufacturing and field quality control
Ultimately, geocomposite performance falls to the manufacturers who produce the product and the field inspectors who ensure that the product arrives and is installed properly. Geocomposite manufacturers are constantly balancing transmissivity and lamination strength through temperature and pressure management. Improving transmissivity with lower temperatures, for instance, can weaken lamination strength. Onsite quality control procedures such as regular cleaning of the equipment and regular inspections of the product being extruded can help avoid the shipment of inconsistent product not meeting the manufacturer’s technical specifications.
Once the product has arrived at the construction site, field inspectors have the last opportunity to identify potential weaknesses in the product. Visual inspections of the geocomposite should be made with a focus on looking for nonlaminated areas. Panels should also be physically tested at regular intervals by pulling the geotextile away from the geonet. Finally, extra care should be given to ensure that the cover soil is being hauled, dumped, and spread over the liner according to specifications to avoid damage by poor operational technique or excessive loading.
The feedback given for ASTM D7005 should prompt everyone in the industry to rethink the quality control processes implemented for geocomposites. Designers can start by retooling their specifications. Manufacturers can modify their processes and apply new procedures to improve output. And installers can emphasize on-site inspections.
Thiel, R. and Narejo, D. “Lamination strength requirements for geonet drainage geocomposites.” Proc., 18th Annual GRI Conf./ASCE GeoFrontiers Conf., ASCE, Reston, Va. (2005). Accessed March 11, 2019. https://www.rthiel.com/uploads/ngrey/037%202005%20Final%20GeocompositeLaminationStrength%20by%20Rick.pdf.
Thiel, R. “Geocomposite lamination strength ASTM test method: Time for an update.” Geosynthetics Mag. (2018). Accessed March 11, 2019. https://geosyntheticsmagazine.com/2018/08/01/geocomposite-lamination-strength-astm-test-method-time-for-an-update/.