What is the reverse method in construction engineering?

Top-down method is a construction method of multi-storey basement in high-rise building, which is completely opposite to the traditional construction method. The traditional construction method of multi-storey basement in high-rise building is to excavate the slope by large excavation or excavate vertically after supporting with supporting structure, dig to the design elevation and pour the reinforced concrete floor, then construct the basement structure of each floor from bottom to top, and then construct the above-ground structure after the underground structure is completed. The top-down construction method is as follows: firstly, the underground continuous wall is built around the building, and a small number of middle supporting columns (called middle column piles) are built inside the building according to the axis of the column network, and then the underground beam-slab structure is constructed. When completed, the underground and above-ground structures will be constructed simultaneously. After the basement floor is completed, composite columns and walls will be built. However, before pouring the reinforced concrete floor in the basement, the allowable number of floors of the above-ground superstructure should be determined by calculation.