Combined with the design and construction practice of a large-scale deep-buried soft rock tunnel in a hydropower project in the west, the invention proposes to use foundation anchor piles 7 and prestressed bolts 6 to pair the upper and lower half sections of the cavern before the deep-buried soft rock large-scale tunnel is excavated to the bottom. Targeted pre-reinforcement is carried out respectively, and then the left and right sides of the tunnel are excavated at the bottom, and the steel arch standing legs of the upper step cavern are led in time, and the excavation and support design and construction of the closed full-section support arch are systematic. method to achieve the purpose of ensuring the safety and stability of the tunnel structure. The concrete construction steps of this embodiment are as follows:
 a. According to the geological conditions revealed by the excavation, implement the necessary system support for the upper half-section cavern 1 that has been excavated on the upper steps of the tunnel, including the upper half-section mortar bolt 3, the upper half-section steel arch 4 and the concrete spray layer 5, etc., formed as figure 1 The upper half-section cavern system is supported to ensure the safety of the upper half-section cavern 1; during construction, the upper half-section mortar bolt 3 and the upper half-section steel arch 4 are arranged in sequence, and finally the concrete is sprayed to form a concrete spray layer 5.
 b. If figure 2 As shown, prestressed anchors 6 are arranged on the spandrels, waists, and arches of the upper half-section cavern 1 respectively, and targeted reinforcement is implemented. Fixed connection, through the tension and compression action of the prestressed anchor 6 and the force transmission effect of the mortar leveling layer 12 between the through-length channel 10 and the concrete spray layer 5 to form the upper half-section steel of the cave wall. Integral pre-stressed support strips for the 4 vertical legs of the arch frame (that is, the pre-stressed bolts apply force along the radial direction of the tunnel through the full-length channel steel and the mortar leveling layer to the upper half-section steel arch vertical legs as support) , to avoid that the foundation of the upper half-section steel arch 4 completely loses its bearing and supporting capacity to the surrounding rock of the top arch after being suspended during the excavation process, and can also limit the deformation of the surrounding rock in the corresponding parts during the excavation process. , to avoid instability; such as Figure 5 As shown, in this example, the through-length channel steel 10 is horizontally arranged on the wall of the upper half-section cavity 1, and the through-length channel steel 10 is provided with a through hole corresponding to the embedded position of the prestressed anchor rod 6, and the prestressed anchor After the rod 6 passes through the through hole, the bolt nut 11 is used to tighten and apply prestress. At this time, the long channel steel 10 is also used as the bolt backing plate, and the mortar leveling layer 12 is arranged between the long channel steel 10 and the hole wall for leveling. Increase the stability and force transmission of the prestressed anchor.
 c. Implement advanced system foundation anchor piles at the arches on both sides of the upper half-section cavern 1, so as to further strengthen the rock mass at the arches of the upper half-section cavern 1, improve its bearing capacity, and provide the upper half-section steel Strong support for the foundation of the arch 4. At the same time, the system foundation anchor piles 7 on both sides of the arch foot (viewed in the direction of the tunnel axis) implemented in advance before the bottom half-section excavation act as the "foundation pit" for the side walls on both sides of the lower half-section cavern 2. The load-bearing and stability-maintaining effect of the "enclosure pile" effect limits the unfavorable deformation of the side walls of the lower half-section cavern 2 and the tunnel bottom slump during the bottom-drop excavation process before the lower half-section support construction is completed in time. like Image 6 As shown, the end of the anchor pile 7 is made of a steel plate 13, and the other side of the steel plate is welded with a full-length channel steel 10 arranged at the arch foot of the upper half-section cavern 1, which further increases the overall reinforcement effect;
 d. Carry out bottom excavation of the lower half-section cavern 2. like image 3 , Figure 4 As shown in the figure, the bottom-fall excavation adopts the left and right subsection method, and the subsection ratio is reasonably allocated according to the principle of meeting the construction requirements and the minimum adverse impact on the stability of the surrounding rock. The side foundations are suspended at the same time to maximize the bearing and support capacity of the upper half-section steel arch 4. The cyclic footage of drop-to-bottom excavation is controlled at 1-2m, that is, after the optional half-width excavation is 1-2m footage, the upper half-section steel arch frame 4 is connected in time, the lower half-section steel arch frame 9 is installed, and the lower half-section mortar is applied The bolt 8 is then closed by spraying concrete, and then the other half of the excavation and support is carried out in the same way to form an annular supporting whole; The lower half-section shotcrete support forms the overall surface support structure of the cavern; the upper and lower half-section steel arches 4 and 9 are connected by mechanical connection with backing plate bolts.
 e. Repeat steps a, b, c, and d in sequence until the entire tunnel is penetrated.