Investigations of geomembrane integrity within a 25-year old landfill capping

Investigations have been undertaken at a 16 ha landfill capping to assess performance of the geomembrane component some 20–25 years after installation. The site has been the subject of quite extensive monitoring together with hydrological and other studies. Although environmental monitoring has shown no major concerns, there have been discrepancies in calculated water balance, leading to the recent investigations reported here. The capping, which is an interim solution, comprises about 1 m of cover soils over 0.375 mm LDPE geomembrane and surrounded by a perimeter drain. A robust final capping system will be constructed at a later date. Various remedial works were undertaken between 2010 and 2014 at the cap perimeter drains, also at a series of gas vent/probe holes through the geomembrane, to address the discrepancies in water balance, and the opportunity was taken to investigate the condition of the geomembrane which revealed a series of unanticipated gaps in the geomembrane. These investigations were subsequently extended over the whole cap to characterise the nature and extent of those defects and assess likely causes.The series of investigations reported here represents a significant case history, one of relatively few, and which describes: the approaches adopted to pursue the series of investigations; the findings of that work; options considered to address the issues; lessons learnt and the intervention strategies which are under consideration in response. It also has implications for other landfill caps and highlights the importance of construction processes including construction quality assurance to ensure the integrity of geomembranes following placement is not adversely affected, also the need for good records management to assess system performance in service and plan future interventions.