Design and construction method of additional cellar under existing building

The invention provides a design and construction method of an additional cellar under an existing building, applied to the development and the application of an underground space under an existing building on a soft soil foundation. The method comprises the following steps of: (1) temporarily processing and reinforcing an existing building; (2) arranging an underpinning base carrier for wholly underpinning the foundation of the existing building; (3) designing and constructing an underpinning pile foundation under the existing building; (4) carrying out the engineering design and the construction of a foundation pit combined with the cellar structure; and (5) carrying out the structural design and the construction of a sequential part of the cellar structure to achieve the aim of adding a cellar under the building. The design and construction method provided by the invention can provide a complete and reliable design and construction technology for developing and utilizing the underground space under the exis