Undercrossing shield construction of a subway, the tunnel vault is about 3.0m away from the foundation pile tip of the existing building. In order to analyze the influence of shield undercrossing construction on the existing building and the influence of the existing building load on the tunnel structure, the calculation and analysis are carried out for the existing foundation forms, foundation reinforcement into raft, addition of equal length piles and increase of unequal length piles. Using the method of foundation reinforcement into raft foundation and long pile tip exceeding 0.5m below the tunnel floor can transmit the load of the upper building structure to the soil layer below the tunnel floor and reduce the load of the tunnel roof, thereby achieving the underpinning effect. According to the allowable deformation value of existing building, allowable internal force of segment and allowable deformation value of segment, using this underpinning method can meet the requirements of the various of allowable values of existing building, tunnels, etc.

The bearing capacity of the post-grouting super-long bored pile, calculated by the standard enhancement coefficient, is 66% larger than that of the measured. The FEM analysis shows that when the grouting radius at the pile end is 1.5 d and the permeating height of the cement slurry is 4.05 m from the pile end, the end resistance enhancement coefficient is 1.17, and the pile side average friction resistance enhancement coefficient is 1.31 within 12 m above the pile end. The calculated bearing capacity is close to the measured bearing capacity of the post-grouting pile by those enhancement coefficients. The calculation shows that: a) in the case of no grouting, the pile end reaction force is about 5.78% of the pile top load; b) in the case of grouting, when the grouting diffusion radius at the pile end is 1.5 d and the permeating height from the pile end is 4.05 m, the post-grouting pile end reaction force is about 6.78% of the pile top load; and c) after grouting, the load transmitted to the pile end cannot be significantly increased; d) using the reinforcement coefficient suggested by the Standard, the calculated bearing capacity is larger than the measured.

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