With the soft foundation treatment project of a highway in Jiangsu Province as engineering background, we explored the distribution and variation law of excess pore water pressure during the construction of bidirectional mixing columns, and analyzed the effective stress of the surrounding soil during hole expansion, and then determined whether hazards would occur during the soft foundation reinforcement process. We obtained the variation of pore water pressure and earth pressure during construction of group piles by arranging sensors at different locations and different depths. Results showed that during the construction process, the pore water pressure and earth pressure of soil around the pile changed drastically, and in particular, high excess pore water pressure was found in the adjacent of the pile position. Under the consolidation of surrounding soil, pore water pressure dissipated rapidly at first and then slowed down. The pore pressure accumulation away from the construction direction was smaller than that along the construction direction, hence impeding about 60% of the excess pore water pressure. In single pile construction, pore pressure had a significant linear relationship with the logarithm of radius ratio. Such distribution was identical to the solution of Vesic’s cavity expansion theory.The disturbance range was about 20 times of pile’s radius. In the meantime, we found that during the construction, pore water pressure was smaller than earth pressure, thus refraining from pile subsidence and soil liquefaction. On this basis, methods of modifying equipment or construction process can be adopted to avoid the subsidence of pile.