Analysis and design of partial ground improvement in deep excavations

Excessive wall deflection and ground settlement are common problems for deep excavations in soft clay and frequently damage adjacent buildings. To reduce wall deflection or ground settlement, the in situ soil is usually strengthened by jet grouting or mechanical deep mixing method. Finite element analysis can be conducted to determine improvement ratios, improved depths and improved zones of the soil to reach a balance between cost and safety. However, the conventional two dimensional finite element method does not take into account corner effects, which will affect the determination of improved zones. A three-dimensional finite element method is thus appropriate for analysis. In this study, an excavation 51.65 m long, 23.75 wide, and 9.31 m deep was used as an example to show how to design the improved zone using ideas of corner effects and the three-dimensional finite element method. It was found from a series of parametric studies that the lateral wall deflection was virtually not affected if the area within 8 m from the corners was or was not improved, which was subsequently used for the ground improvement in the actual excavation. The wall deflections were carefully observed during excavation. It was found that the measured wall deflections were close to those originally designed. The adjacent buildings near the excavation site were thus protected.