Large cross-section weak surrounding rock tunnel three-step and six-part short-distance construction method

The invention discloses a large cross-section weak surrounding rock tunnel three-step and six-part short-distance construction method, which comprises the steps as follows: (1) annularly excavating an upper step of an upper part arc pilot tunnel, transferring excavated earthwork of the upper step to a lower step; (2) excavating a left part of a middle step about 4.5-5 meters behind the upper step, and transferring excavated earthwork to the lower step; (3) excavating a right part of the middle step about 3-4 meters behind the left part of the middle step, and transferring excavated earthwork to the lower step;(4) excavating a left part of the lower step about 6-8 meters behind the left part of the middle step or about 3-4 meters behind the right part of the middle step, and conveying excavated earthwork of the left part of the lower step out of the tunnel; (5) excavating a right part of the lower step about 3-4 meters behind the left part of the lower step, and conveying excavated earthwork of the lower step out of the tunnel; and (6) excavating the rest part of the tunnel bottom about 20-25 meters behind the right part of the lower step, excavating continuously according to the steps in a circulating and interlacing manner, and finally realizing tunnel excavating construction. The method can be used for building a large cross-section tunnel in a safe, rapid and economic manner.