# Dynamic Shear Degradation of Geosynthetic-Soil Interface in Waste Landfill Sites
## 4 Laboratory Test
### Evaluation of Disturbance Function and Parameters  
The increase of the disturbance, \(D\), represents an increase in the degree of damage in the geosynthetic-soil interface. In the experiment results, the shape of the disturbance function varied in accordance with the chemical conditions. Figure 7 demonstrates the comparison results of the disturbance function curves according to the chemical conditions for different submersion periods and normal stresses. In all of the cases, a more substantial disturbance was observed in the behavior of the sample after long-term submersion (of 850 days). The disturbance at the same deviatoric plastic strain trajectory, \(\xi_{D}\), showed larger values under acid conditions in case of short-term submersion, which means that the shear resistance of the geosynthetic-soil interface is more vulnerable to acid conditions in the short term. For behavior after long-term submersion, the disturbance showed larger values under basic conditions, which means that the shear resistance of the geosynthetic-soil interface is more vulnerable to basic conditions in long-term exposure. The characteristics of the interface-shear behavior can be expressed by the disturbance function curves. Conversely, the disturbance function curve can be reproduced by a mathematical combination of the functional form of the curve and the disturbance parameters \(A\) and \(Z\). The parameters represent the intrinsic material characteristics and are determined by a linear regression of the curves from the laboratory test results. The specific procedure and technique are described in a previous study [25]. Table 3 shows the average values of the \(A\) and \(Z\) parameters based on the test results. The \(A\) parameter increased with the increase in the submersion period under the same chemical and normal stress conditions, and is denoted by the shape of the disturbance curve that tends to move left [4]. The geosynthetic-soil interface therefore underwent more damage at low plastic strain levels. Specifically, the interface approached failure despite low plastic strain according to the increase in the  
Figure 7: Typical disturbance function curves: (**a**) 30 days submersion, 0.3 MPa of normal stress; (**b**) 30 days submersion, 0.6 MPa of normal stress; (**c**) 850 days submersion, 0.3 MPa of normal stress; (**d**) 850 days submersion, 0.6 MPa of normal stress. submersion period. On the contrary, the \(A\) parameter decreased with the increase in the normal stress under the same chemical and submersion conditions, which is demonstrated by the tendency of the disturbance curve to move right. Therefore, the geosynthetic-soil interface manifested more resistance against damage at low plastic strain levels. This trend is identical to the characteristics of the shear stress degradation that were analyzed previously. The parameter \(Z\) increased with the increase in the submersion period and the normal stress under the same chemical conditions. This is denoted by the tendency of the curve slope to increase, which also means a rapid increase of the material damage; in particular, the damage significantly grew with small incremental increases in the plastic strain. The chemical stability of the HDPE geonet was verified by a few previous papers [36; 37], therefore it was deduced that the shear stress degradation of the geosynthetic-soil interface mainly originated due to the different patterns of soil particles. Though the partial decay and breakage of nonwoven fabric filament were found in focused ion beam (FIB) observations, however, the frequency and severity of the observed damage indicate that they might be a minor factor of damage on geosynthetic itself [4]. This hypothesis was successfully verified by the FIB observation.