4.1 Scope of application and characteristics of construction method
The mechanical configuration of this report is determined according to the on-site hydrogeological conditions. Meanwhile, the palm surface excavation method can be selected according to the stability of the surrounding rock. Generally, the surrounding rock has reliable stability, the compressive strength of the rock mass is less than or equal to 60MPa, and the layer thickness is less than or equal to 30 cent meters, and in the absence of water, by using cantilever roadheader for excavation and slag discharge operations. The construction process of the steel arch frame adopts the unmanned arch erection and bolting-grouting integrated operation trolley for operation. Relatively, spraying concrete by large concrete wet spraying trolley.
If the integration of the surrounding rock is appropriate and the surrounding rock is not available for excavation with a cantilever roadheader, the drilling and blasting method should be used for excavation, the loader and dump truck should be applied in the slag discharge operations, before carrying out unmanned arch erection.
To summarize the above two points, this construction method is available for tunnels with a stable surrounding rock and excellent integration, and the excavation method is a two-step tunnel [11]. At present, the construction of weaker broken surrounding rock requires preliminary shotcrete, and it is applicable in the case of tunnel advanced support construction.
4.2 The characteristics of construction method
Improving the efficiency of arch installation. By adopted steel arch installation trolley to replace the work platform. Then, the whole folding steel arch can be lifted from the ground with the telescopic arm of the trolley, and the steel arch is accurately positioned to the design position. Therefore, the construction efficiency of the steel arch lifting and positioning has been significantly improved.
Achieving the unmanned arching operation. To implement the unmanned arch erection of bolting-grouting integrated operation, changing the connection mode between the segments of the steel arch, optimizing replacement scheme of the longitudinal connection bar and the steel mesh. Sequentially, decreasing the labor intensity and the number of workers erecting the steel arch quickly and safely.
Ensuring the safety of support construction. After tunnel excavation, the initial lining construction stage is a period that is a frequent occurrence of safety accidents. This method effectively reduces the number of operation and construction personnel and enhances the safety of support construction.
4.3 Installation procedure and control points of unmanned arch erection
The construction process is shown in Fig. 11.
Steel arch installation Firstly, before the steel arch is installed, check the clearance of the excavation section, and start installation after it is qualified. Secondly, after determining the centerline of the route, elevation could be ensured. Then to measure the lateral position to ensure that the steel arch is not skewed, not twisted, and is located on the same vertical plane. Thirdly, according to the measuring position, the whole steel arch is lifted at once by the arch installation machine. Furthermore, according to the measuring point and using a grab arm of the arch installation machine, the middle A segment of the arch is fixed, then open the unilateral segments B and C through another arm. The locking and anti-detachment function of the articulated slab is used to ensure that the unilateral arch is firmly supported on a stable foundation. Finally, the third grab arm is used to open the B and C segments of the other side in turn and firmly fixed on the foundation. The on-site installation of the steel arch is shown in Figs. 12 and 13.
To ensure that the steel arch is placed on a solid foundation before the whole ring is closed, the slag and debris under the bottom feet of both sides of the steel arch are removed before installation. When the arch foot (arch wall) is overcutted, soil cannot be backfilled, and the concrete cushion block should be used for adjustment.
Key points of quality control for unmanned arch construction In terms of raw materials, the mechanical properties and process performance tests (yield strength, tensile strength, and elongation) must accord to the requirements of the current national standard ‘Carbon structural steel (GB/T700)’, which can ensure the types and specifications of steel used to make steel frames to satisfy the requirements. During processing, materials such as steel bars and section steel should be flat and harmless, hence cracks, oil pollution, granular or flake corrosion should not occur on the surface. The sectional length is determined according to the design size and the excavation method adopted, and the drilling position on the web should meet the design requirements. The plane warping should be less than 2 cm, and the joints should be strictly prohibited at the vault of the steel frame. Welding of steel arch processing should not have false welding, weld surface should not have cracks, welding slags. The joint adopts double-sided arc welding and steel plate for auxiliary welding simultaneously. The size of the steel plate is determined according to the mechanical calculation, and using the four-sided girth welding to machine it which has 12 mm thickness of the weld, and each side is lapped by 50 mm.
When the steel arch is constructed, its footing should be placed on a solid foundation. Timely erection of arches after removal of uncompacted waste residue and miscellany at bottom feet and initial spraying of concrete. If the arch footing (or arch wall) is overcutted, concrete cushion blocks should be used to adjust the tighter so that the allowable longitudinal deviation of the arch foot is controlled at ± 10 cm, the lateral position deviation is ± 2 cm, and the verticality is ± 1°, and not backfilled with soil. Each section of the steel frame should be close to the surrounding rock, along the outer edge of the steel frame every 2 m with concrete precast block wedge.
Safety Measures Before the tunnel support operation, the working surface should be checked to remove loose rocks and concrete blocks. Working face electricity should meet the requirements of temporary electricity. In addition, the illuminance should meet the needs of safe operation, not less than 50lx.
When the surrounding rock is less-fragmentized, concrete should be sprayed immediately after blasting to close the surrounding rock, and when necessary to close the palm surface.
The unmanned archer should be regularly overhauled and maintained following the provisions. Meanwhile, completing the protection facilities.