## 7 Conclusions  
In this study, we conducted model track experiments to assess the slip sinkage induced during an off-road tracked vehicle executing its mission. The model track system was installed on silty sand of different relative densities (40%, 60%, and 80%), while three different vertical stresses (7.0 kPa, 10.4 kPa, and 14.0 kPa) and longitudinal loads were applied, thereby comprehensively assessing the effects of various ground and load conditions on slip sinkage. We derived the following conclusions. 1. In the model track experiments, the soil block was formed and failure occurred along the bottom and side ground of the block regardless of the ground density and vertical stress conditions. That is, slip sinkage of the off-road tracked vehicle was induced by vertical displacement caused by shearing action in the soil block formed on the soil-track interface. This is similar to the phenomenon in which vertical displacement of the sample occurs in the soil direct shear test. However, as the soil block is rigidly constrained on only the side with grousers, it did not show the exact same tendencies. 2. Downward slip sinkage occurred in the soil-track interface under all experimental conditions. The extent of slip sinkage gradually increased with slip displacement, though the rate of increase gradually decreased. In addition, slip sinkage increased as surcharge mass increased and relative density of the soil decreased. We expect that it will be difficult to ensure tractive performance for heavy-weight off-road tracked vehicles driving on loose ground. 3. Normalized slip sinkage, defined as the ratio of slip sinkage to static sinkage, which is caused by vehicle weight, increases with slip displacement. However, it is not greatly affected by surcharge mass or relative density of the soil. From the experiment results, we proposed an equation for assessing the slip sinkage of off-road tracked vehicles. We used the proposed equation to predict the results of the additional model experiments for verification and confirmed that the slip sinkage of off-road tracked vehicles can be predicted relatively accurately using the proposed formula. 4. We proposed a method for assessing the compaction resistance of off-road tracked vehicles considering the effect of slip sinkage using the proposed slip sinkage equation. A parametric study was conducted taking into account the driving conditions of the off-road tracked vehicle. Overall compaction resistance was estimated to be about 7.35-54.01 times higher than when static sinkage alone was considered. These results indicate that the effect of slip sinkage should be considered when assessing the tractive performance of off-road tracked vehicles, and that tractive performance can be overestimated when compaction resistance is calculated considering only static sinkage. We expect that the results of this study can be used to predict the slip sinkage of off-road tracked vehicles driving on silty sand, which can then be used as a basis to assess motion resistance. By evaluating the static sinkage of off-road tracked vehicles using existing research, the extent of slip sinkage can be predicted quantitatively through the proposed formula. However, as the above conclusions are based on the results of experiments conducted only on silty sand, the results of this study should be verified and supplemented by additional studies under various soil conditions. Especially for clayey soils, slip sinkage is manifested in different ways because the shape of the failure surface on the soil-track interface (i.e., shear failure mode) is completely different from that of silty sand [39]. Also, cautious should be exercised when applying the results of this study to fastmoving tracked vehicles that exert dynamic loads to the ground surface; slip sinkage may differently appear. Further experiments conducted under various conditions are required to verify the outcomes of this study, through a full-scale test. Conceptualization, S.-H.B. and C.-K.C. ; methodology, S.-H.B. ; validation, S.-H.B. and S.-H.L.