Deep Soil Mixing is a widely used ground improvement method in the Nordic Countries, particularly for improving the shear strength and stiffness of soft clays. However, simulating this process presents significant challenges due to extreme shear deformations. Traditional numerical methods like the Finite Element Method are unsuitable, as severe mesh distortion occurs during large deformation. In contrast, the Material Point Method provides a more reliable approach by allowing material points to move across a constant computational grid, avoiding mesh distortion. In this paper, the Generalized Interpolation Material Point Method, an advanced Material Point Method, is employed to simulate the mixing of Finnish clay. The mixing performance is assessed using the Lacey mixing index. The results reveal a highly heterogeneous mixing pattern and suggest that mixing efficiency does not improve indefinitely.