Shear strength behaviour of sand-bentonite mixtures with pumice additivite under high temperature

Depending on increase in the number and type of energy geostructures, studies on the change in soil behavior against heat increase becomes more important. The engineering properties such as permeability, volume deformation of surrounding soils around energy structures mustn’t alter in the presence of heat and thermal cycles. Pumice is a material used in many fields especially for thermal insulation. For that reason, pumice can be used for increasing the resistance of soils in the presence of heat. In this study, the shear strength behavior of sand-bentonite mixtures was investigated with pumice additive under high temperature. In the experiments, 10% and 20% pumice were added to 10% and 20% sand-bentonite mixtures and compaction, direct shear tests were conducted. The direct shear tests were performed both room temperature and under 80°C. The results have shown that the pumice additive increased the shear strength of sand-bentonite mixtures under high temperature when compared the test results under room temperature.


Introduction
The increase in the number of energy geostructures increased the importance of interaction between the soil and energy structure and soil behavior under high temperatures. Today use of many structures or elements such as energy piles, buried power cables, solid waste and nuclear waste storage areas, etc., which cause soils to be exposed to high temperatures. The behavior of soils in the presence of temperature varies in many respects. For example, the shear strength of the normally consolidated clay increases with increasing soil temperature. Also the studies have shown that the slope of the shear strength envelope is independent of the temperature [1]. In previous literature studies, it was found that the effect of temperature on shear strength is significantly dependent on volume changes caused by heating called thermal consolidation [2]. In another study, the triaxial compression tests on Pontida silty clay showed that in the temperature range of 18°C to 115°C, the shear strength and hardness of this silty clay decreased with increasing temperature [3].
Pumice is a type of volcanic rock used in many different sectors such as filtration, ceramic, construction, textile, agriculture, chemistry, especially heat and sound insulation. Pumice is a highly porous material and usually there is no connection between these pores. Lack of connection between the pores will not allow water to flow. Therefore, pumice was characterized as a low permeability material. In addition, it has very low very thermal conductivity. Pumice is available in two forms in nature, as acidic and basic. Acidic pumice has white and dirty white color and basic pumice has brown or black color. The density of acidic pumice is lower than the basic pumice.
Pumice is an amorphous aluminum silicate formed as a result of volcanic activities. Chemical structure of pumice includes 60-70% SiO2, 13-15% Al2O3, 1-4% Fe2O3, 1-2% CaO, 1-2% MgO, 2-5% Na2O, 3-4% K2O and it also contains TiO2, SO3 [4,5]. Aluminum oxide in the chemical structure of pumice is known to provide high heat resistance [6]. The ton price of the pumice used in the experiments is about $4.4 (US Dollars). This additional information shows that the use of pumice in such applications is also very cost-effective.
Especially in solid waste and nuclear waste storage areas, only bentonite or bentonite-sand mixtures are used as a buffer because of its low permeability and high swelling properties [7]. Generally, bentonite is mixed with sand and used as a reinforced filling material with high thermal conductivity and stiffness [8][9][10]. The use of bentonite by mixing with sand instead of using it alone is because sand content will compensate occurrence of shrinkage cracks.
In this study, shear strength behavior of sandbentonite mixtures was investigated under room and high temperature in the presence of 10% and 20% pumice. The effect of pumice addition on the shear strength behavior of sand-bentonite mixtures were examined.

Material Characterization
In this study, the tests were carried out on the sandbentonite mixtures. Sodium bentonite was supplied from Eczacıbaşı Esan Mining Company. Pumice was provided from Pomza Export Mining Company. 20.7% of the sand used is fine grained and it was used as sieved through No.6 sieve. Also the bentonite and pumice was sieved through No.40 sieve. The sand and bentonite samples were dried for 24 hours in the oven at 105 o C. Pumice has a natural water content of 2% to 5% and it was used without drying. The 10% bentonite-sand and 20% bentonite-sand mixtures were used in the tests and these mixtures will be called as 10B-90S and 20B-80S, respectively. The dry unit weight of the pumice is 12.9 kN/m 3 . The physico-chemical properties of the used materials are given in Table 1.

Method
Direct shear tests were performed according to American Society of Testing Materials Standards ASTM D3080 (2011). The samples were prepared by compressing in three layers in a metal mold at maximum dry unit weight and remaining on the 2% wet side of the optimum water content. After compaction of the samples, they were left submerged condition for 24 hours and weights were placed on them in order to prevent swelling. The samples were placed into mold having dimensions of 6x6 cm in the test instrument and left for consolidation, then the test was carried out. Direct shear experiments were performed both at room temperature and under 80ºC.
The high temperature in the experiments is provided by the heat rod specially designed for shear box. The K type thermocouples were used to control temperature inside both soil and water. The temperature of the water and soil were measured and recorded by using digital thermometers.

Test Results
The samples were prepared according to the maximum dry unit weight and 2% wet side of the optimum water content values obtained from Standard Proctor tests. The maximum dry unit weight and optimum water content values are given in Table 2. The maximum shear stress values of 10% and 20% bentonite-sand mixtures in the presence of 10% and 20% pumice were given in the Figure 1 and 2. Direct shear tests were carried out under two different temperature conditions, under room temperature and high temperature (80ºC) and the results were compared. The internal friction angle of the additive free mixture with 10% bentonite was 33.1º at room temperature. This value decreased with 10% pumice additive but reached 36.7º with 20% pumice additive. At 80ºC, the internal friction angle increased from 32.2º to 33.8º with 10% pumice and 35.1º with 20% pumice by showing a significant trend as pumice was added to the 10B-90S mixtures. These findings indicate that pumice additive has an effect on increasing the internal friction angle of 10% bentonite-sand mixtures under high temperature. However, this increase was not seen in maximum shear stress values. Pumice additive decreased the maximum shear strength value from 140.2 kPa to 138.3 kPa under 98 kPa normal stress at 80ºC however at other stress levels this value increased. Under the 98 kPa normal stress, shear stress values reduced to 141.2 kPa with 20% pumice additive, while it was 152 kPa for additive free mixture. The cohesion value decreased for all 10% bentonite mixtures when shifting from room temperature to 80ºC (Table 3).

Conclusions
In this study, direct shear tests were conducted on the pumice added 10% and 20% bentonite-sand mixtures under room temperature and high temperature. It has been observed that the internal friction angle increases as the pumice amount increases for 10% and 20% bentonitesand mixtures at room temperature and 80ºC. The maximum shear stress value increased under high temperature at 98 kPa normal stress level for 20B-90S mixtures. As a result of the direct shear tests, it has been observed that the pumice additive did not increase the shear strength under room temperature. On the other hand, it was determined that in the 20% bentonite-sand mixtures pumice additive increases the shear strength under high temperature. The experimental results showed that pumice additive can be used as an additive to increase shear strength of sand-bentonite mixtures where high temperature is present.