## 3 Results and Discussion
### Simulations vs. Field Experiments for the Grouped Shovels  
As is shown in Figure 14, the soil failure evolution under the influence of potato digging shovels (PDS) is a continuous cycle and can be summarized as follows: (1) Firstly, after the P-a of PDS sticks into and cuts soil, elastic deformation occurs and then evolves into plastic deformation [47], which spurs the emergence of horizontal cracks (Figure 14a). (2) Next, as PDS keeps moving forward, the horizontal crack extends, the soil mass is compressed and lifted upward and backward, and then an oblique crack emerges (Figure 14b). Here, the compaction to soil accounts mainly for the increase in the draught force [48]. (3) Finally, as a critical intensity of the combination of the normal and shear stresses is reached, the oblique crack extends to the soil surface and a failure plane is formed [48; 49], which leads to the rupture of soil mass and its separation with undisturbed soil (Figure 14c). Subsequently, the draught force decreases and the above process repeats, and thus the signal fluctuates.  
Figure 15 demonstrates the simulated and field measured draught force as well as the soil failure properties of grouped shovels. It is very clear that the draught forces for both DZ and YS begin to stabilize at a displacement of about 140 mm but decrease to 0 gradually at a displacement of about 560 mm. Meanwhile, the signals of the draught force are fluctuating constantly, which corroborate the reports by [PERSON] and [PERSON] [48], [PERSON] et al. [47] and [PERSON] et al. [50].  
Figure 12: Soil failure at different mounting angles. Displacement contour (**center**) and ploughing tracks in soil bin experiments (**right**).  
In Figure 15, there is a clear strain concentration band near the tip of shovels, and the strain near the shovel surface and the soil surface is higher than that in the inner soil. In addition, the great strain area of DZ is significantly larger than that of YS, and the thickness of the furrow slice produced by DZ (about 143 mm) is also smaller than that of YS (about 169 mm). The results mentioned here indicate that the grouped YS also reduces the compaction to soil and thus reduces the draught force. In addition, the biomimetic spatial surface of YS is able to change the stress state and motion state of the lifted soil mass continually, and this kind of change is conducive to reduce the friction and adhesion between soil-shovel interfaces [13], which improves soil mobility and makes the soil mass easier to be broken; this is also an important resistance reduction reason for biomimetic spatial surfaces. Moreover, compared with single YS, the grouped YS shows much more significant draught force reduction performance without the shank's interaction with the soil.  
As shown in Figure 16, the simulated and field measured draught forces of the grouped YS are 6.40 kN and 9.00 kN, respectively, while those of the grouped DZ are 7.99 kN and 10.39 kN, respectively. It is noticeable that the draught force of YS in simulations and field experiments are 19.85% and 13.33% lower than those of DZ, respectively. In addition, the fuel consumption of YS is 9.18% lower than that of DZ, which further verifies the simulated and field measured draught forces for the grouped shovels. Therefore, YS exhibits significant performance in energy consumption reduction.  
Figure 14: Soil failure evolution under the influence of PDS.  
Figure 13: Soil failure of run No. 12. Displacement contour plot (**left**) and equivalent plastic strain contour plot (**right**).  
Figure 16: Comparisons of the simulated and field measured results in terms of the draught force and the fuel consumption.  
Figure 15: The simulated and field measured draught force and the simulated equivalent plastic strain of DZ (**top**) and YS (**bottom**). The solid lines represent the simulated values and dash lines represent the field measured values.