The engineering properties of soils such as compressibility, permeability and shear strength especially when considering temperature changes must be taken into account for performance of energy geo-structures. For that reason, the number of researches increase on the temperature effects on soil behavior in last decades.
Saturated clays made up of two phases and all components expand when heated. As a result of this expansion, there is a loss of strength in the parts where water is absorbed. The effect of repulsive and attractive forces also present here. As a result of phenomenon, contraction behavior occurs in normally-consolidated (NC) clays. Even if the soil cooled, the resulting deformation is largely irreversible [1, 2]. Delage et al. [3, 4], as a result of oedometer tests on OC Boom clay, observed that the change in cv and mv was negligible at temperatures exceeding 60 °C. Cekerevac and Laloui [5] reported that compression index (Cc) is almost the same and the compression index is independent of temperature, as a result of the tests at two different temperatures (22°C and 90°C). On the other hand, Marques et al. [6] emphasized that compression curves affected by temperature in the experiments completed at 10°C and 50°C, unlike the others. Shariatmadari and Saeidijam [7] revealed that the slopes of the consolidation curves of bentonite-sand mixtures at high temperature increase and it was related to the decrease in the void ratio at high temperature.
Studies have emphasized that many different factors affect the hydraulic conductivity of soils, such as viscosity of the fluid, properties of the solid matrix, physico-chemical interactions, and chemical content of the pore fluid [8, 9]. Because the viscosity of free pore water changes with temperature, the flow rate of the water changes and accordingly hydraulic conductivity changes [10]. Bouazza et al. [8] revealed that with the increase in temperature, there may be changes in the size of the voids in the clay structure and therefore the hydraulic conductivity may change. In addition, the redistribution of the pores also affects this behavior. It was reported that the porous structure and texture of the soils change with temperature and the permeability can change with the repositioning of the grains by changing their arrangement [11]. Ye et al. [12] observed that there is a linear relationship between hydraulic conductivity and temperature, and this is associated with a decrease in viscosity. In addition, Ye et al. [12] emphasized that the temperature affects the hydraulic conductivity, but its increase or decrease does not have a significant effect on the hydraulic conductivity.
Bentonite or sand-bentonite mixtures are preferred as a barrier material around energy structures. Bentonite is a material with a very low permeability, densely compacted with very low water content [13]. Considering that barriers should preserve their properties up to 106 years, bentonite is being a geological material that can preserve its structure for a long time. The sealing effect created by swelling pressure have highlighted its use as a buffer material [14]. In addition, properties such as the expansion of bentonite in contact with water and its large surface area support the use of bentonite as a buffer. In addition to these, there is a possibility that some of the bentonite, which can expand with high temperatures during the thermal period, will turn into non-expandable illite [15]. However, Wersin et al. [16] emphasized that although it was assumed that bentonite performance decreased above 120°C, leakage probably occurred above 150°C, but that bentonite still retains with its sealing ability. Sand-bentonite mixtures are also used as buffer material in order to prevent the possible negative effects of swelling behavior due to the high water absorption capacity of bentonite. The anti-shrinkage effect of sand contributes to the buffer material [14].
The performance of buffer materials around energy geo-structures may be improved by mixing of some additives. If thermally durable materials are added to the buffer materials the intact engineering properties may be preserved under high temperatures. In this study, the fiberglass material was choosen because of its superior properties under high temperatures. Fiberglasses are obtained by melting silica-based glass into thin strips. Commercial fiberglasses are made from pure silica rods and from silicate melts containing 50–70% SiO2 and 10–25% Al2O3 [17]. Although there are many different glass classes such as A, E-CR, C, D, R, S-glass, the most widely used is E-glass. Pure silica requires very high temperatures to be converted into fiber glass. Fiberglass is used in many fields such as insulators, transportation, construction due to its anti-corrosion behavior, good mechanical properties, heat resistance [18]. The thermal stability of fiberglass is very high, hence it can remain intact and durable for years at temperatures lower than the softening temperature. Fiberglasses are composed of metal oxides or alkalis (Figure 1) dispersed in a silica system, which is an example of a three-dimensional system of oxygen tetrahedra [19].
It should be noted that Djeghader and Redjel [20] revealed that in contact of fiberglass with water, water can enter between the fiber and whatever other material forming the mixture, due to capillarity, and this may weaken the structure of the fiberglass and lead to crack formation [22-24]. It also causes an increase in weight, expansion of non-reinforced sections, and deterioration of the soil matrix [25].
In this study, consolidation and hydraulic conductivity tests of fiberglass added sand-bentonite mixtures were carried out under room temperature and high temperatures. The effect of fiberglass on the compressibility and hydraulic conductivity behavior of sand-bentonite mixtures were investigated under high temperatures and temperature cycles.