Earth excavation method for secondary island building of silt stratum of oversized deep foundation pit

The invention discloses an earth excavation method for secondary island construction of a silt stratum of an oversized deep foundation pit, which comprises the following steps: (1) primary island construction: the primary island construction elevation is the original ground elevation; (2) excavating earthwork above the bottom elevation of the top beam supporting system, and constructing the top beam supporting system; (3) secondary island building is conducted, the elevation of the secondary island building is the same as the elevation of the top of the supporting beam, and a slope ramp is led from a soil outlet to a secondary built center island; (4) island-type segmentation and layering are conducted, and an excavator transfers the muddy soil layer of the foundation pit below the supporting beam in a stepped mode; (5) a cushion layer is constructed, a pile head is processed, pile foundation detection work is completed, and a waterproof layer is constructed; and (6) sectioning and timely closing the bottom p