Undercutting-covered excavation semi-reverse construction method of cross-transfer subway station

An undercutting-covered excavation semi-reverse construction method of a cross-transfer subway station, wherein a section of a node of the cross-transfer subway station covers three underground floors, wherein a first underground floor is a station hall floor, and a second underground floor is a platform floor of a subway line A and a third underground floor is a platform floor of a subway line B;

• directions of the subway line A and the subway line B are orthogonal;
• construction is carried out by an arch covering method at the station hall floor and the platform floor of the subway line A;
• and after a shield tunneling machine for interzone construction of the subway line A is pushed through the station, construction of the platform floor of the subway line B is carried out by covered excavation using a reverse construction method under the section constructed by the arch covering method;
• the undercutting-covered excavation semi-reverse construction method comprises the following steps:
• step 1:
• stepwise excavating a left pilot tunnel of an arch;
• after the completion of excavation, initially spraying 40 mm thick concrete to close surrounding rock, applying anchor bolts, arranging a grid arch frame, erecting a temporary steel arch frame for the left pilot tunnel of the arch, and spraying concrete again to a thickness of 350 mm;
• step 2:
• stepwise excavating a right pilot tunnel of the arch;
• after the completion of excavation, initially spraying 40 mm thick concrete immediately to close surrounding rock, applying anchor bolts, arranging a grid arch frame, erecting a temporary steel arch frame, and spraying concrete again to a thickness of 350 mm;
• step 3:
• excavating remaining surrounding rock of a middle pilot tunnel of the arch of the station and building in a temporary steel arch frame for the middle pilot tunnel of the arch;
• step 4:
• applying a bottom bedding course, and applying arch foot joists on two sides after the bottom bedding course reaches a strength;
• step 5:
• removing part of the temporary steel arch frame by means of a spatial effect, laying a waterproofing course, and applying a second lining structure of an arch covering and reserving a side wall construction seam;
• step 6:
• after the second lining structure of the arch covering reaches a design strength, carrying out step-slope excavation of middle-layer core soil;
• step 7:
• excavating surrounding rocks on two sides of the middle layer of the station layer by layer from top to bottom;
• excavating to current layer at each step and driving anchor bolts downwards by 0.5 m, and carrying out next step of excavation after the completion of applying the anchor bolts on two sides and spraying concrete;
• step 8:
• after excavation to a design elevation, backfilling grouted tunnel wastes to a bottom elevation of a medium slab of the second underground floor, and after a foundation bearing capacity required for pushing the shield tunneling machine is met, carrying out consequent operation of a medium slab structure and second lining structures for side walls of the first and second underground floors from bottom to top;
• step 9:
• after the second lining structure of the medium slab of the second underground floor reaches a design strength, backfilling a subgrade, laying a track, and pushing the shield tunneling machine for the construction of the subway line A through the station;
• step 10:
• using an interzone tunnel of a left line of the subway line B as a construction passageway, cyclically excavating the platform floor of the subway line B, and timely applying a temporary steel support;
• after the completion of each round of excavation, gradually removing the support, timely applying a second lining structure of the third underground floor, and after the completion of applying the second lining structure of the third underground floor, carrying out next round of excavation;
• and
• step 11:
• after the completion of the second lining structure of the third underground floor, excavating remaining interzone ports of the subway line B, and applying an internal structure after a shield tunneling machine for the construction of the subway line B is pushed through the station.