Factors affecting the hydraulic performance of a geosynthetic clay liner overlap

Highlights

• Addresses inadequacy of published GCL overlap experimental data.

• Evaluating effect of four contributing factors on GCL overlap hydraulic performance.

• The most critical factors are overlap width and minimum overburden confining stress.

• Design guidelines developed assist practitioners to optimise the GCL overlap criterion.

• Findings support reduction in liner failure and potential groundwater contamination.

Abstract

Geosynthetic clay liners (GCL) are increasingly being used as a major component of barrier systems replacing compacted clay liners due to their very low permeability and speed of installation. Researchers and practitioners have identified the critical role of the GCL overlap that presents constant challenges encountered in maintaining the designed hydraulic conductivity. This study presents a series of flow box tests conducted to evaluate and improve understanding of the combined effect of each of the four contributing factors, namely, overlap width, supplemental bentonite applied at the overlap, the overburden confining stress, and the hydraulic head acting on the GCL overlap. The findings reveal that the overlap width is the most significant parameter affecting the design hydraulic conductivity. The application of a minimum overburden stress to maintain the designed hydraulic performance is also recognised as important. The effect of confinement due to higher hydraulic heads is of interest to practitioners. The supplemental bentonite has the least effect on the GCL overlap hydraulic performance even though it enhances the function of the overlap seam. This knowledge addresses the inadequacy of published GCL overlap experimental data comparing the effect of different factors affecting its hydraulic performance. It also assists industry practitioners to design and specify the overlap criterion for a specific application depending on the product specifications and site conditions. The results of this research will help minimise failures of liner systems in barrier applications such as landfills, mines, tailing dams substantially to reduce the potential risk of groundwater contamination.

Introduction

The demand for geosynthetics is increasing rapidly due to their vast number of uses in geo-environmental engineering applications (Cheah et al., 2016; 2017; Jayalath et al., 2018; Weerasinghe et al., 2019a). Geosynthetic clay liners are one such geosynthetic material that is widely accepted as a critical element in composite hydraulic barrier systems replacing compacted clay liners due to the low permeability characteristic of the bentonite encapsulated within the geotextiles (Booker et al., 2004; Brachman and Gudina, 2008; Gassner, 2009; Rowe, 2005). GCLs are commonly used underneath a geomembrane and in some cases above a layer of compacted clay liner in a composite barrier design. The GCL product is used in a number of applications such as, in landfills and mines to reduce contamination, resisting hydrocarbon leakage in secondary containment facilities and in vertical cut-off barriers such as irrigation canals (Benson et al., 2005; Daniel, 2012; Part, 2001; Rowe, 1998). However, the performance of the GCL relies on the fact that the liner system is continuous and has no leakage between GCL panels, which has the potential for impacting hydraulic performance.

Much research has been conducted to improve the GCL performance and the product has improved immensely over the past two decades. However, the variability among products by various manufacturers, nonconformity in installation quality and site-specific characteristics in different regions impose a constant challenge in maintaining the performance at the GCL overlaps. Cooley and Daniel pioneered in observing large flow rates collected through the GCL overlaps compared to the non-overlapped portions in a model-scale laboratory setup. (Cooley and Daniel, 1995; Daniel et al., 1997). With the identification of this additional flow through the overlap, numerous research were conducted to investigate various factors affecting its hydraulic performance (Batali, 2006; Batali and Didier, 1997; Egloffstein et al., 2012; Fox et al., 1998; Rowe et al., 2016; Weerasinghe et al., 2019b; Xiong et al., 2009).

The overlap width is one of the critical factors considered to affect the liquid permeation through a liner. Early research identified that a 300 mm overlap width is conservatively sufficient to maintain the liquid flow at the seam to be less than or equal to the flow passing through the GCL itself (Egloffstein, 2001). A GCL overlap with bentonite supplementarily added to the overlap in comparison to a GCL overlap with no bentonite showed a significant reduction of a power of two in permeation (Benson et al., 2004; Cooley and Daniel, 1995; Rowe, 2012). This allowed the manufacturers to reduce the overlap width to an optimal length by restraining the liquid flow using the permeable characteristics and self-seaming ability of its own supplemental bentonite (Seiphoori et al., 2016; Yang et al., 2015). Limited research has been conducted to evaluate the effect of the form of bentonite applied at the GCL overlap (granular, powder, or paste), and therefore, mostly relies on the manufacturer or the specific GCL product (Abuel-Naga et al., 2013; Benson et al., 2004; Brachman et al., 2010; Kendall and Buckley, 2014; Rowe et al., 2016). However, it was later identified that reduced overlap widths may result in leakages and even separation of panels due to environmental conditions such as shrinkage (Brachman et al., 2018; Rowe et al., 2017b; Thiel and Richardson, 2005).

Research has identified that the hydraulic conductivity at the GCL overlap reduces significantly with the increase in overburden confinement stress due to loading such as waste or soil cover. This is attributed to lower void ratios in bentonite resulting from higher confining stresses (Bouazza et al., 2002; Giroud et al., 2004; Rowe et al., 1997). The effect of hydraulic head acting on a liner system is another important aspect discussed in literature. The hydraulic head in a municipal solid waste is generally very low (i.e. less than 300 mm), but in cases such as ponds and other water retaining structures it is significantly high (CETCO, 2010). Shackelford et al. (2000) reported that hydraulic conductivities of laboratory permeated GCL specimens were not affected by the hydraulic head for values up to 6 m. In contrast, Rowe et al. (2017a) observed that the hydraulic head imposed an overburden pressure on the GCLs exhumed from the field. However, experimental or field data published on the effect the overburden confinement and hydraulic head acting on the GCL overlap to its hydraulic performance are limited.

Many studies have suggested recommendations for improvement of these individual factors (Batali and Didier, 1997; Bouazza et al., 2006; Brachman et al., 2018; Egloffstein, 2001; Kowalewski et al., 2016; Yang et al., 2015). Identification of the best combination of overlap width and supplemental bentonite, however, also depends on other factors such as the quality of the manufactured product, swell index of bentonite, mass per unit area, and temperature (Maubeuge and Ehrenberg, 2014; Rowe, 2012). Relying on these research results, different manufacturers have developed installation guidelines for the overlaps of their specific GCL products in various regions of the world. Common practice varies between a wide range of 150–1200 mm of longitudinal and end overlap widths with granular and powder supplemental bentonite ranging from 0.4 to 0.6 kg/m or paste bentonite with a water to bentonite ratio ranging from 1:4 to 1:10 by mass (CETCO, 2010; Geofabrics Australia, 2015; NAUE, 2019). The current industry practice to select the most suited GCL for a specific project relies mostly on the mass per unit area of a GCL. The users tend to follow the manufacturer's installation guideline for the product overlap, disregarding the condition of the specific application. For example, a municipal landfill application having a low hydraulic head and a high overburden confining stress acting from the waste might differ from a scenario of a pond with a higher hydraulic head but less confinement acting on a bottom liner GCL overlap. The GCL product specification might fulfil the requirements of both applications, but the overlap specifications might have to differ depending on the two scenarios.

This research study specifically focuses on evaluating four factors identified to be affecting the hydraulic performance of the GCL overlap: the overlap width, supplemental bentonite applied on the overlap, the overburden confining stress and hydraulic head acting on the overlap. The research study presents a flow box test series conducted to investigate the effect of these factors on the GCL overlap hydraulic performance. The experimental results are analysed to identify the optimal combination of the most influential factors. The research findings address the inadequacy of published overlap test data available and provides evidence to improve insight on the contribution of the effect of each of these factors on the GCL overlap hydraulic performance when in conjunction with each other. The study aims to increase understanding of how the overlap criterion could be used as a measure in improving the overall liner hydraulic performance based on the application addressed. The importance of recognising how the overlap specifications could be reformed based on different site conditions is highlighted as a significant outcome of this study. The scope of this research work was limited to a bottom liner GCL application and tap water was selected as the permeant liquid for all experiments. Further, this study does not investigate the effect of other permeants and environmental conditions such as variation in temperature and settlement of subgrade layers of soil (Petrov and Rowe, 1997; Rowe et al., 1997; Viswanadham et al., 2012).