State-Of-The-Art Review of Geosynthetic Clay Liners

3.1. Hydraulic Conductivity and Chemical Compatibility

Working as a hydraulic barrier, hydraulic conductivity is the most critical parameter to evaluate the effectiveness of the GCLs. Generally, the barrier effect of GCL is achieved by the bentonite within it, which has extremely low hydraulic conductivity similar to that used in the lubricant in pipe jacking [ 46 ]. The only exception is GM-GCL in which geomembrane acts as the water barrier.

2(OH)6, Mg3(OH)6), sandwiched between two layers of silicon-oxygen tetrahedral ((Si4O10)4−). Each clay sheet is composed of several or dozens of these structure cells, and layers of structure cells are connected to each other via Van der Waals force. These connections are comparatively weak so that the water molecules can intervene easily, causing swelling of the smectite on the macro level.

Bentonite is basically composed of smectite, whose crystal structure consists of a layer of aluminium-oxygen (or magnesium-oxygen) octahedron, (Al(OH), Mg(OH)), sandwiched between two layers of silicon-oxygen tetrahedral ((Si). Each clay sheet is composed of several or dozens of these structure cells, and layers of structure cells are connected to each other via Van der Waals force. These connections are comparatively weak so that the water molecules can intervene easily, causing swelling of the smectite on the macro level. Figure 4 gives an illustration of the crystal structure of smectite and the crystalline-swelling mechanism.

2 for 1 g of smectite, and a vast number of negative charges on it. To achieve charge balance, the surface will adsorb the cations in the pore water, forming a “diffuse double layer” (DDL) [50,

H

) can be calculated by the equation proposed by Bolt (1956) [

H = D k T 8 π n ε 2 ν 2

(1)

D

= dielectric constant of solution;

k

= Boltzmann constant (

k

= 1.38 × 10−23 J/K);

T

= absolute temperature;

n

= molar concentration;

ε

= elementary charge (

ε

= 4.8 × 10−20 esu); and

ν

= ionic valence. Moreover, electrostatic forces can attract numerous ions into the space between the clay particles, which causes a large difference of ionic concentration between the clay mineral surface and the pore solution. Driven by concentration gradient forces, water with lower electrolyte concentration is taken up by osmosis between the clay particles thus pushing them apart, which makes the smectite further swell.

The smectite has high specific surface area, e.g., about 800 mfor 1 g of smectite, and a vast number of negative charges on it. To achieve charge balance, the surface will adsorb the cations in the pore water, forming a “diffuse double layer” (DDL) [ 49 51 ]. Figure 5 gives an illustration of the typical diffuse double layer. The thickness of DDL () can be calculated by the equation proposed by Bolt (1956) [ 52 ]:where,= dielectric constant of solution;= Boltzmann constant (= 1.38 × 10J/K);= absolute temperature;= molar concentration;= elementary charge (= 4.8 × 10esu); and= ionic valence. Moreover, electrostatic forces can attract numerous ions into the space between the clay particles, which causes a large difference of ionic concentration between the clay mineral surface and the pore solution. Driven by concentration gradient forces, water with lower electrolyte concentration is taken up by osmosis between the clay particles thus pushing them apart, which makes the smectite further swell.

43,45,−12 m/s to 2 × 10−10 m/s, which decreases with the increase in confining pressure [

After water-swelling, an immobile water phase will be formed by the bound water molecules and an impermeable material will be formed by hydrated bentonite, leading to the decrease of hydraulically active pores, and manifested as anti-seepage of bentonite [ 30 53 ]. Generally, the hydraulic conductivity of bentonite to water is within the range of 2 × 10m/s to 2 × 10m/s, which decreases with the increase in confining pressure [ 15 54 ].

2+, Mg2+, Al3+), ion exchange will occur and reach equilibrium, and the thickness of DDL will significantly decrease. This will cause an increase of the hydraulic conductivity of GCL, which can be one order of magnitude or more for some permeating conditions [56,57,

In some conditions, if the permeant solution contains high concentrations of cations, especially bivalent cations and trivalent cations (e.g., Ca, Mg, Al), ion exchange will occur and reach equilibrium, and the thickness of DDL will significantly decrease. This will cause an increase of the hydraulic conductivity of GCL, which can be one order of magnitude or more for some permeating conditions [ 55 58 ]. The triggering mechanism of ion exchange on increase of the hydraulic conductivity was presented by Egloffstein (2001) [ 59 ]. Figure 6 gives an example of ion exchange between monovalent sodium ions and bivalent calcium ions. The ion exchange increases the inner-crystalline attraction, thus reducing the space between the silicate layers (layers of structure cells) and the mass of bound water. With this change, the micro structure of bentonite changes from smaller, finely distributed clay mineral flakes to larger clay mineral crystals, leading to an increase in hydraulic conductivity.

53,55,59,60,61,62,63,64,61,66,67,

GCLs are exposed to leachates other than water under most circumstances. Assessment of the effect of chemical solutions on the hydraulic behaviour of GCLs is of great importance. The hydraulic conductivity to the permeant liquid of GCL needs to be assessed on a case-by-case basis. The common method adopted is the “compatibility test”, in which the sample is permeated with a leachate similar to that anticipated. Factors influencing the hydraulic conductivity of GCLs have been studied through extensive compatibility tests [ 25 65 ]. Generally, the hydraulic conductivity can be affected by both internal and external causes. The internal factors involve the smectite content, aggregate size, exchangeable metals and void ratio of bentonite in GCL, and the external factors include the concentration of cations (monovalent and divalent) in the permeant liquid, pre-hydration of the GCL, etc. In the last 10 years, increasing concern has been expressed about the compatibility of GCLs in extreme environments and conditions, such as hyper-salinity, strongly acidic, strongly alkaline, excessive cold and excessive heat environments, which can greatly deteriorate their anti-seepage performance [ 14 68 ].