# Dynamic Shear Degradation of Geosynthetic-Soil Interface in Waste Landfill Sites
## 4 Laboratory Test
### Test Apparatus and Conditions  
The Multi-Purpose Interface Apparatus (M-PIA), which was originally manufactured in 2012, and was modified to improve its performance and accuracy in 2015, was utilized to conduct the cyclic simple shear test. Figure 3 describes the outline and the shear-box details of the M-PIA. The structure and technical items are explained in the studies by [PERSON] et al. [27]. The materials in the test of the geosynthetic-soil interface were the geosynthetics and the granular soil. Jumunjin sand was used in the present study as a typical type of granular soil. The sand specimen was prepared by the air pluviation method at a relative density of 60%. A funnel with a metallic square outlet with a width of 7 mm was used to drop the Jumunjin sand into the shear-box. The sand was spread in the shear-box at a constant speed to a height of 60 cm. The physical characteristics are summarized in Table 1.  
Figure 3: M-PIA: (**a**) photo and (**b**) shear-box details. Figure 2: Shear stress of RI, FA, and observed states. The present study is based on the idea that the chemical aggressor in the leachate that is generated from the decomposition process of the landfill waste will affect the shear behavior of the geosynthetic-soil interface under the monotonic and dynamic-loading conditions. As an antioxidant such as carbon black is added to the manufacturing process for high-density polyethylene (HDPE), it is postulated that soil decay will be the main reason for the shear strength degradation of the geosynthetic-soil interface, rather than damage to the geocomposite. A few studies on the shear strength degradation of the geosynthetic-soil interface under dynamic-loading conditions have been performed [17; 29]. Others have studied only the chemical conditions decay of raw materials [30; 31]. However, for the present study, the simultaneous occurrence of the chemical effect on the geosynthetic-soil interface and the dynamic-loading conditions were considered. Previous studies have shown that pH values changes dramatically due to the complicated biological and chemical reactions in leachate [20; 32]. Therefore, in the present study, the chemical conditions of leachate were incorporated by using buffer solutions of different pH values. Both geosynthetic and soil specimens were submerged in the basic, neutral, and acidic solutions for 30 days and 850 days in order to consider the short- and long-term responses of the interface, respectively. Considering the unit weight of a waste landfill and the area of the specimen, 0.3 and 0.6 MPa of normal stresses represent heights of 20 and 40 m of landfill waste, respectively. As such, these are the forces that were applied to the specimen. The unit weight of a hazardous waste landfill site is 15.9 to 17.3 kN/m\({}^{3}\)[19]. The dynamic conditions were also considered in order to simulate an earthquake load. The predominant frequency range of the earthquake load is between 0.1 Hz and 15 Hz, based on previous studies [33; 34]. It is often assumed that soil behavior is independent of the seismic-loading frequencies between 0.1 Hz and 10 Hz. Additionally, ASTM D3999 [35] recommends the frequency of 0.5 Hz or 1.0 Hz in cases where uniform sinusoidal loading is applied. Based on the previous research and recommendation, 0.5 Hz of sinusoidal loading was chosen for the cyclic simple shear test. As such, this study is limited by 0.5 Hz of load conditions and the effect of frequency shall be investigated in a further study. Additionally, a few pilot tests were performed in order to decide the amplitude of the shear strain. If the amplitude was more than 5%, the specimen failed too early, indicating difficulties with regards to the attainment of a sufficient amount of data for an analysis of the shear stress-strain