Laboratorial Prototype for Detection of Defects on Geomembranes - The Geophysical Approach

Abstract

Landfills are engineering facilities designed and constructed with a barrier system (lining system) intending to assure the protection of the environment. This system includes active and passive barriers. The passive barrier comprises a compacted clay liner (CCL) and/or a geosynthetic clay liner (GCL), while the active barrier includes a geomembrane (GM), protected by a geotextile (GTX), and a drainage layer known as primary leachate collection system (PLCS). The effectiveness of lining systems in service conditions depends, above all, of the performance of GM. A critical issue on their performance is the defects, which, unfortunately, seems to be unavoidable (Nosko and Touze-Foltz 2000; Peggs 1996; Peggs and Wallance 2008; Rollin et al. 2004; Rollin et al. 2002). Most of these appear during the placement of the PLCS (Barroso et al. 2007; Colucci and Lavagnolo 1995). Indeed, data reported by Nosko and Touze-Foltz (2000), collected at more than 300 sites, from 16 countries, showed that 71% of the damages were caused by stones during PLCS installation. Also according to these authors, the number of defects per hectare is about 12.9, whereas Rollin et al. (2002) refers a value of 17.4 defects/ha. It should be noted that these values concern to GMs installed under a strict Construction Quality Assurance (CQA) programmes. Higher values can be expected in landfills without CQA

Although there are some test methods do detect and locate defects in GM liners after the placement of the PLCS, as stated by Beck et al. (2008), namely the soil-covered GM method (mobile probe) and the grid method (permanent), the existing methods present some disadvantages. They are labor and time consuming and, so, very expensive. These conditions lead us to the development of a quick and low-cost, but also accurate, test prototype to check the GMs integrity after the placement of the granular layer.

The methodology consists in the development of a prototype combining the mobile probe method (ASTM D7007) with the multicables resistivity equipments presently used for geophysical surveys. This prototype is endowed with ways that allow to the semi-automatic data acquisition (detection location of the defects) and its processing in real time. The functionality of the prototype is presently being verified in a pilot plant, at one of ISEL’s (Instituto Superior de Engenharia de Lisboa) laboratory. The experimental work under way includes different types of lining systems and defects. A bigger scale pilot plant is being constructed at LNEC’s (National Laboratory of Civil Engineering) campus, to verify, at real scale, its functionality. Afterwards, the prototype will be checked in situ, at a true landfill.

In this paper the prototype development is presented under the geophysical scope of view. Some limitations, drawbacks and results are presented. Test methodology and its results are presented by Lopes et al. (2011).