the subgrade soil
• maintain a stable configuration when other geosynthetic components such as geotextiles or geonets are placed on top of the GM-GCL composite liner.
Designing a stable slope with a GCL consists of the following steps (Gilbert and Wright 2010):
1. Define the geometry, loading conditions and consequences of a failure for the slope during construction, operation and after closure.
2. Select appropriate material properties for the GCL and all other materials in the slope. Consider rate of loading, deformations, normal stresses and fluid pressures in this selection.
3. Analyse and evaluate the stability.
4. Take measures to mitigate any concerns about stability of the slope.
It is important to stress the fact that published values of interface friction and internal shear strength should not be used in detailed design. Performance tests using site- specific material and mimicking field conditions should always be conducted.
In this respect, the interface shear test (ASTM 6243) is useful in evaluating the interface friction of GCLs with soils and/or geosynthetic components as well as their internal shear strength. However, one needs to be aware of the factors and conditions that could affect the results obtained from this test. These include hydration of bentonite, hydration liquid, consolidation procedure, normal stress, specimen size, shearing device, gripping/clamping systems, magnitude of shear
displacement, shear displacement rate, properties of soil and geosynthetic materials forming interfaces on either side of the GCL and preparation conditions, and product type(s).
Improperly performed tests can give highly inaccurate results, so it is important to carefully consider testing procedures and to examine test data for inconsistencies.
E2.2.5 Equivalence of liner systems
The performance design trend imposes the quantitative evaluation of the equivalence of alternative liners and traditional liners. Nowadays, there is an increasing interest in the use of GCLs as a replacement for conventional compacted clay liners (CCLs). Because, in many jurisdictions, regulations prescribe acceptable barrier system configurations in terms of CCLs, this often raises the question whether a liner involving a GCL is equivalent to one involving a CCL (Rowe 2005, Bouazza 2002).
Rowe (1998) indicated that, when comparison between different products must be carried out, it is important to keep in mind that it is not possible to generalise about ‘equivalency’ of liner systems, since what is ‘equivalent’ depends on what is being compared and how it is being compared. Apart from their own features, the
performances of liner systems are related to the contaminant amount, concentration and decay
parameters, the aquifer characteristics and its distance from the bottom of the landfill, and the efficiency of capping and drainage systems. In this respect, to assess equivalence from an environmental perspective it is necessary to assess the equivalence in terms of contaminant impact on a receptor aquifer beneath a landfill by conducting a contaminant transport analysis. Rowe (1998), Manassero et al. (2000) and Rowe et al. (2004) provided a framework to model the contaminant transport through geomembrane/
compacted clay liner (GM/CCL) and geomembrane/ geosynthetic clay liner (GM/GCL) composite liners. Furthermore, Rowe (2005) stressed the fact that, when selecting parameters for use in conjunction with a contaminant transport analysis, consideration should be given to:
• the potential for clay–leachate/GCL–leachate interaction and its effect on hydraulic conductivity • the interaction with the adjacent GM and the effect on
leakage
• diffusion and sorption
• the leachate head and corresponding gradient • the provision of appropriate protection to the GM and
GCL to minimise potential squeezing and local thinning of the GCL
Siting, design, operation and rehabilitation of landfills
102
E.3
Requirements for GCLS for
basal and sideslope liners
The following parameters are considered minimum requirements for geosynthetic clay liners (GCLs) in landfill liners to maximise their service life.
It should be stressed that these requirements represent a minimum expectation for ‘good practice’. A higher standard might be required in certain applications and the onus is on the engineer of record to establish if a higher standard or requirement is needed.
Note that GCLs must maintain a hydraulic conductivity less than or equal to the design value for the
contaminating lifespan of the landfill (in other words, the period of time during which the escape of contaminant due to a failure of the engineered system would have an adverse impact on the environment (Rowe et al. 2004). This aspect must be taken into account in the design of the GCL liner and the liner system itself.
1. The geosynthetic clay liner shall be a reinforced, multi-layered system comprising two layers of geotextiles encapsulating a layer of dry bentonite. To minimise the potential problems, for applications where there is a risk of internal erosion (such as when the GCL rests on a permeable layer such as a gravel or geonet layer) or may be subjected to wetting– drying, a GCL with a scrim-reinforced carrier and thermal treatment with properties similar to or better than those for which there is test data in the
literature is recommended, unless it can be clearly demonstrated by test results that an alternative GCL is suitable.
2. It is important to select or specify a bentonite that has been specially formulated to meet the specific, unique demands encountered by geosynthetic clay liners in landfills. As a minimum, the bentonite shall meet the specifications indicated below:
Property Range or value
Montmorillonite content
> 70 wt%
Carbonate content* < 1–2 wt%
Bentonite form Natural Na-bentonite or >80 wt% Sodium as activated bentonite Particle size Powdered (e.g. 80%
passing 75 micron sieve) or Granulated (e.g. < 1% passing 75 micron) Cation exchange capacity ≥ 70 meq/100 g (or cmol/kg)
Free swell index ≥ 24 cm3/2g
* Carbonate here implies calcite, calcium carbonate or other soluble or partially soluble carbonate minerals.
3. Other design requirements and technical specifications for the geosynthetic clay liner (for example, Atterberg limits, organic carbon content, mass area of bentonite, mineralogy, shear strength and hydraulic conductivity under expected field stresses to water and permeant with chemical composition similar to expected leachate). 4. Provide a statement (with justification) on the
chemical compatibility of the GCL liner and the leachate. In particular, unless relevant testing has previously been conducted for very similar conditions (such as proposed GCL, stress level, leachate), the hydraulic conductivity tests supporting the design hydraulic conductivity should be conducted on samples hydrated to simulate expected field hydration and stresses and permeated with a simulated leachate that approximates that expected in the landfill until the ratio of the chemical
composition in permeant influent and effluent is ≥ 0.9 (see Petrov and Rowe 1997; Rowe et al. 2004). Similar compatibility studies should be conducted for compacted clay liners.
5. Provide a statement (with justification) on diffusion coefficients, partitioning coefficients and any other parameter used in the design or analysis (for example, see Rowe et al. 2004).
6. Equivalency comparisons between CCL and GCL base liner systems should incorporate a contaminant transport impact assessment (for example, see Rowe and Brachman 2004);