What are the key points of tunnel construction?

Tunnel construction is a difficult point in road and bridge construction. It has a complicated construction process and is different from other projects. It has the characteristics of concealment. If the survey and design estimates are insufficient or some precautions are not done during the construction, the safety of the tunnel may be affected and the project cannot be carried out normally. Therefore, when carrying out tunnel construction, it is necessary to understand the key points of the construction, strictly control the quality of each key point, strengthen management, and ensure safety.

Jet machine

1. The spraying equipment should have good sealing performance, and the conveying material should be continuous and uniform to meet the requirements of spraying operations.

2. Determine the reasonable water-binder ratio, water consumption, cementing material consumption and sand rate to ensure workability and early strength.

3. Sequential spraying from bottom to top in segments, the length of the segment is not more than 6m; the thickness of the arch should not exceed 6cm at a time, and the side wall should not exceed 10 cm. When spraying in layers, the next layer should be sprayed on the previous layer. The concrete should be set after the final setting. If spraying is carried out after 1h of final setting, the surface of the spray layer should be cleaned with high-pressure air and water first. The nozzle should be perpendicular to the rock surface and the distance from the sprayed surface should be 0.8~1.2m.

4. The spray pressure should be controlled at 0.15~0.2Mpa, and the concrete should be wet cured after spraying for 2h after final setting, and the curing time should not be less than 14d.

5. The spraying workers should wear protective equipment such as dust masks, protective caps, protective glasses, dust masks, etc. The workers should avoid direct contact with alkaline liquid quick-setting agents. After accidental contact, they should immediately rinse with water and spray concrete. After completion, the equipment should be cleaned in time.

6. The surface of the shotcrete should be flat, without hollows, cracks, or looseness. The base surface should be leveled with shotcrete. The flatness should be checked with a 2 m ruler. The allowable deviation of the surface flatness is 5 cm for side walls and 7 cm for arches.

Common problems on site

1. There are cavities and not dense behind the vault and side walls of the tunnel.

2. The thickness of the sprayed concrete is insufficient, uneven, and the steel frame is not completely covered.

3. Use dry spray. All tunnels that do not use wet spraying are required to stop construction.

4. The spraying distance is too far, and the sprayed surface is above 1.2 meters, which cannot guarantee the compactness of the concrete.

5. The spraying sequence is wrong, not bottom-up, but rather random.

6. The thickness of one shot is too thick, causing the concrete to hollow, not compact and collapse.

Anchor rod

Anchor rod is a rod body made of solid or hollow steel in the rock mass in order to avoid the loose collapse of the rock mass during the excavation of the surrounding rock, which plays the role of reinforcing the soil and connecting it into a whole. The rock space is consolidated with mortar or other materials and fixed with steel pads.

Construction points

1. The anchor rod construction should be carried out after the initial spraying of concrete. The anchor rod shall be drilled in by special machinery. When using a general pneumatic rock drill, a special impactor shall be equipped. The main structural plane is vertical.

2. Arrange the position according to the design requirements, the allowable deviation of the hole position is not more than 10cm, and the insertion length of the rod shall not be shorter than 95% of the design length.

3. When the rod body is inserted into the anchor rod hole, the position should be kept centered and the insertion depth should meet the design requirements.

4. The mortar pouring into the mortar bolt hole should be full and dense, and an appropriate amount of micro-expansion agent and accelerator can be added into the mortar or cement slurry.

5. After the anchor rod is installed and the filled mortar is finally set, the supporting plate should be installed immediately and the nut should be tightened.

Common problems on site

1. The number and length of bolts are lower than the design value, and the distance between bolts is large. The lock foot anchor rod and the steel frame are not U-shaped welding. The on-site inspection should separate the leading small pipe, leading anchor rod, hollow anchor rod, and locking foot anchor tube (rod) in detail.

2. The raw material of the anchor rod is not up to the standard, and the wall thickness of the anchor rod and weighing inspection shall be checked on site.

3. The hollow bolt designed for grouting is not grouted or replaced by mortar bolts.

4. The penetration angle of the anchor rod does not meet the design requirements.

Steel frame

The steel frame is a support skeleton structure made of steel gratings, section steel, steel rails, etc., which are laid out according to the tunnel excavation contour line in order to keep the surrounding rock stable during the initial support period of the tunnel excavation in the tunnel escape pipeline project. After installation, it can achieve the purpose of supporting the stability of the surrounding rock and limiting the deformation of the surrounding rock. It is usually combined with steel mesh and shotcrete to bear the force.

Construction points

1. The steel frame shall be erected in time after the initial spraying of concrete.

2. The steel frame should not be sectioned on the vault with greater force and other places with greater force. The diameter of the main reinforcement of the grid steel frame should not be less than 18mm, and the welding between the steel bars should be firm.

3. Each section of the steel frame should be connected by bolts, and the connecting plates should be closely attached, and the gap should not exceed 2mm.

4. The outer edge of the steel frame should be tightly wedged with steel wedges or precast concrete blocks. The steel frame should be covered by sprayed concrete. The thickness of the protective layer should not be less than 40mm. The gap between the steel frame and the surrounding rock must be filled with sprayed concrete.

5. After the steel frame is processed, a trial assembly check must be carried out. The allowable error of the peripheral assembly is plus or minus 3 cm, and the plane warpage should be less than 2 cm; the erection position is accurate, and the deviation of the steel frame spacing, lateral position and elevation from the design position does not exceed Plus or minus 5 cm, the verticality error is plus or minus 2°; in the case of the partial excavation method, the steel arch frame is positioned by locking foot anchor pipes (rods) after installation, and the number is two. After the lower half of the excavation, the steel frame should be extended in time to form a closed loop; the two rows of steel frames shall be firmly connected longitudinally with a 22-diameter steel rod with a circumferential spacing of 1m to form an overall stressed structure.

6. When the steel frame is installed, the inner contour size should be strictly controlled, and the amount of settlement and deformation should be reserved to prevent intrusion into the lining clearance.

The rescue process of tunnel collapse is very complicated, and the rescue time is relatively long. The rescue situation usually takes 3 to 6 days. Therefore, it is very necessary to quickly open the “life channel” to transport air and food to the trapped persons, and to carry out information transmission and psychological intervention. ; The tunnel escape pipeline can reconfirm the length of the collapsed body and guide the construction of the escape channel.