(1) The construction of the secondary lining should be carried out after the deformation of the surrounding rock and bolt support is basically stable. In tunnels with weak surrounding rock or at the portal sections, the secondary lining can be constructed in advance.

(2) Optimize the construction mix ratio of lining concrete to enhance its waterproofing ability

According to the waterproof design grade of the tunnel lining, when selecting the mix ratio, the coarse aggregate should adopt a continuous gradation, and an appropriate sand ratio should be used to improve the workability of the concrete and increase its compactness. At the same time, measures such as adding waterproof agents and high-efficiency anti-crack waterproof expanding agents should be taken to reduce the tiny pores inside the concrete and improve its waterproofing ability.

(3) Optimize the concrete pouring process to improve the compactness of the concrete

During concrete pouring, pour layer by layer symmetrically on both sides, from bottom to top in sequence, and control the height difference between adjacent areas. Combine the use of inserted vibrators and attached vibrators fixed on the formwork trolley to ensure dense compaction. Ensure continuous pouring, eliminate construction joints, and reduce weak waterproof links. After form removal, carry out spray curing in a timely manner to avoid shrinkage cracks on the concrete surface caused by premature form removal or untimely curing.

(4) Backfill grouting behind the lining

① Set PVC grouting ports at the arch crown. The grouting ports should protrude 3 - 5 cm from the inner edge of the lining for easy connection of the grouting pipes. The grouting ports extend to the outer edge of the cast-in-place lining and the inner side of the waterproof board. In special sections, after the waterproof board is laid, it is bonded to the inner side of the waterproof board with adhesive. Combined with the layout of construction joints, a circumferential maintainable grouting pipe is set at the position of each cast-in-place lining construction joint.

② Carry out arch back grouting after the lining construction is completed and reaches the design strength. This makes the primary support, waterproof board, and inner lining closely adhere, avoiding the possibility of water seepage due to accidental damage to the waterproof board, and also ensuring structural safety and preventing structural cracking and damage. The backfill grouting material is 1:1 cement grout, and the backfill grouting pressure is 0.1 MPa.

(5) Zoned waterproofing of the secondary lining

To prevent groundwater from seeping into the tunnel in case of failure of various waterproof measures, during construction, install external rubber water stops behind the lining at construction joints and deformation joints, and set circumferential maintainable grouting pipes in special sections to achieve waterproof zoning every 10 meters. In groundwater-sensitive areas, the zoning range can be appropriately reduced. Waterproof zoning can quickly identify water seepage points and reduce the influence range of groundwater.

(6) Precautions

① The vibration of the secondary lining concrete must be in place to prevent the occurrence of honeycombs and pitted surfaces. For the arch part, mainly rely on attached vibrators, and for the side walls, mainly use inserted vibrators. When using inserted vibrators, do not touch the waterproof board to prevent damage to it.

② All circumferential steel bar joints of the secondary lining adopt steel bar couplers. When welding longitudinal steel bars and other parts, use heat-insulating objects for isolation to prevent welding sparks from burning the waterproof board.

③ Pay attention to the setting of the steel bar protective layer of the secondary lining to prevent exposed concrete and exposed steel bars, and prevent the "rib-like" phenomenon on the lining surface. The protective layer should neither be too thick nor too thin.

④ During the construction of the secondary lining concrete, the formwork trolley needs to be adjusted properly to prevent large misalignments. When misalignments occur, it is not advisable to repair with concrete to prevent the concrete from falling and damaging the locomotive during operation. For misalignments, they can be cut, trimmed, and pasted with insulated plastic foam boards.

⑤ When the lining is constructed with waterproof concrete, when the tunnel passes through areas with corrosive groundwater, conduct a water quality analysis of the groundwater, and the lining should use corrosion-resistant concrete. Immediately carry out curing after form removal of the lining, and the curing time is generally 7 - 14 days. In cold regions, do a good job in the cold insulation of the lining.

⑥ Install the lining embedded parts correctly according to the design requirements, such as the embedded channels, lighting facilities, reserved embedded parts for operation ventilation, etc. in railway tunnels.

Only by precisely implementing the selection of the secondary lining timing, the optimization of the mix ratio and pouring process, the implementation of backfill grouting and waterproof measures, as well as various precautions during the construction process, can a strong, waterproof, and durable tunnel lining structure be created. In future tunnel engineering construction, with the continuous progress of technology, we should continue to explore more efficient and reliable construction techniques, and at the same time, strengthen construction management and quality supervision to firmly lay a solid foundation for the safe and stable operation of tunnel projects.