Abstract: In the soft foundation treatment projects, the phenomenon that the spacing of vertical drains reduces when the consolidation efficiency of the foundation decreases often appears. Given this, the distribution function of horizontal permeability coefficient of soil in overlapping smear zones is established, and then the analytical solutions are obtained for the excess pore pressure and average degree of consolidation of vertical drains foundation with overlapping smear zones. Based on the analysis of the variation of the average degree of consolidation of the foundation with the spacing of the vertical drains for different cases, the reasons are explored when the spacing of the vertical drains reduces and the consolidation efficiency of the foundation decreases. Finally, we discussed the influence of the smear effect and well resistance effect on the minimum critical spacing of vertical drains. The results show that the overlapping of smear zones of adjacent vertical drains is the underlying reason for the minimum critical spacing of vertical drains in the foundation. The greater the smear effect is, the greater the minimum critical spacing of vertical drains is. The specific performance is that the minimum critical spacing of vertical drains increases with the increase of the disturbance degree of the foundation or the radius of the smear zones. The larger the well resistance is, the smaller the minimum critical spacing of vertical drains is. When the permeability coefficient of the vertical drain decreases, the length of the vertical drain increases or the radius of the vertical drain decreases, and the minimum critical spacing of vertical drains decreases accordingly.

Key words: vertical drains, consolidation, overlapping smear zones, disturbance degree of foundation, the minimum critical spacing