Construction technology of secondary lining of tunnel

The secondary lining is carried out after the surrounding rock and the initial support deformation are basically stabilized according to the measurement situation, and it closely follows the excavation surface to meet the safety distance requirements for excavation and lining. The secondary lining uses a 12m formwork trolley, and the concrete is transported to the construction site by a concrete transport truck, and pumped into the mold. The concrete adopts the vibrating method with the attached vibrator as the main insertion vibrator and the auxiliary. Reinforced concrete lining section, the steel bar is cut and formed outside the hole, and the hole is installed in the hole using a multi-functional operation platform.

Construction process of tunnel secondary lining

1. Steel bar production and installation

The main lining and stirrup of the lining are processed by the steel bar processing plant. The processing and manufacturing process of the steel bar should meet the design and specification requirements.

In order to ensure the accurate positioning of the second lining steel bar, the thickness of the steel bar protective layer meets the requirements. specific methods:

(1) First, the surveyors use the coordinates to stake out the center points of the two front and rear rebars within the range of the self-made trolley on the leveling layer and the tarpaulin, determine the normal direction, and ensure the verticality and elevation of the rebar The accuracy of the rebar connection in the arch. The verticality of the steel bar binding is determined by the three-point hanging ball method.

(2) Measure the elevation of the center point of the reinforcing steel bar on the filling surface of the invert arch with a level gauge, calculate the difference in height between the center of the circle and the upper center of the invert arch, and use a self-made tripod to determine the center of the circle.

(3) After the center of the circle is determined, use the ruler method to check whether the size of the positioning rebar meets the design requirements, readjust the position that does not meet the requirements, and fix the rebar after all the requirements are met. The fixing of the steel bar is controlled by an adjustable supporting rod welded by steel pipes on a self-made trolley.

(4) After the positioning reinforcement is fixed, mark the location of the circumferential main reinforcement with chalk on the support bar according to the design reinforcement spacing, and mark the installation position of the longitudinal distribution reinforcement on the positioning reinforcement, and then start binding the reinforcement within this section. All steel bar intersections should be tied.

2. The second-line trolley is in place

The heavy-duty steel rail is used for the lining trolley track, and the bottom surface is directly placed on the concrete surface filled with the construction invert arch to ensure the stability of the trolley. Track layout control standards: first, the track center distance is 6m, and the allowable error is ±25px; second, the track surface elevation is 375px above the center of the tunnel filling surface, and the allowable error is ±25px. The center line of the formwork should be adjusted to coincide with the center of the trolley beam as much as possible, so that the trolley is in a good state of stress during the concrete pouring process.

The positioning of the trolley in the curved section should consider the change of the length of the left and right overlap caused by the difference between the length of the inner and outer arcs, so that the arc is smooth, and the joints are reduced.

The lining trolley travels to the position of the vertical mold, adjusts to the accurate position with the lateral jack, and performs the positioning and re-testing until it is adjusted to the accurate position. After the trolley is stretched into place, check whether the connection of each node of the trolley is firm and there is no misalignment and displacement. The five-point positioning method is used to check whether the template is warped or twisted, whether the position is accurate, to ensure the lining clearance, and at the same time it is easy to overcome the stagger at the joint of the lining ring. In order to prevent the trolley from floating when pouring the side wall concrete, it is necessary to add wooden braces or jacks on the top of the trolley. Also check whether the working window is in good condition. When measuring the pay-off, the design reserves should be considered.

3. Treatment of construction joints and deformation joints

The tunnel circumferential construction joint is provided with a composite waterproof structure of a mid-buried rubber water stop belt and a back-attached water stop belt, and the longitudinal construction joint is made of a composite waterproof structure of a mid-buried steel edge rubber water stop belt and a water-swelling rubber water stop bar . During construction, the waterstop is installed in the center of the plug template of the lining trolley, and is fixed by a U-shaped card to ensure that the location of the waterstop is accurately installed, and the ring and longitudinal construction joints are waterproof.

Construction joint construction should be smooth, straight, clean and free of water seepage on both sides. Construction joints shall be cut with cement mortar and loose layer on the concrete surface. Chiselling makes exposed fresh concrete area no less than 75%.
The deformation joints of the tunnel are set at the place where the stratum changes significantly, the light and dark boundary and the section where the section changes obviously. The deformation joint adopts the compound waterproof structure of middle buried rubber waterstop, externally attached waterstop and caulking material.
The position, width and structural type of the deformed joint should meet the design requirements; both sides of the joint should be flat, clean and free of water seepage; the back of the joint should be set with a backing material that has no adhesion to the joint material; the joint is dense.

4. Construction of second lining concrete

(1) Concrete pouring

The concrete is poured in layers and alternately symmetrically. The thickness of each layer should be less than 0.5m. The height difference on both sides should be controlled within 1.5m. The vertical distance from the hose nozzle to the pouring surface should be controlled within 2.0m to prevent the concrete from segregating. The pouring process should be continuous to avoid “cold joints” caused by the stop. If the intermittent time exceeds 1h, it will be treated as construction joints.

When the concrete is poured to 50cm under the working window, the dirt near the window should be scraped off, and the mold release agent should be applied. Putty should be applied at the joint between the window and the panel to ensure a close combination and no leakage.

(2) Concrete vibration

Vibration and tamping should be fixed by inserting the vibrator to ensure the compactness of the concrete; the arching line is supplemented by external hammering of the wooden hammer mold and tamping of the inserting vibrator to suppress the generation of air bubbles on the concrete surface. During the pouring process, it is strictly prohibited to drag the concrete with a vibrating rod.

(3) Lining concrete capping

When the tunnel lining is capped, the steel pipe injection method is used to select the appropriate concrete slump, and the injection is capped from the injection port of the arch. In order to ensure that the top concrete is tightly attached to the surrounding rock, the concrete should be pumped back one by one from the top sealing port. The concrete pump should be continuously operated, the conveying pipe should be straight, the turning should be slow, the joints are tight, and the pipeline is lubricated before pumping. In the top lining, the plastic grouting pipe is pre-buried every 20 meters in the longitudinal direction, and the grouting treatment is carried out after the lining.

(4) Demoulding

According to the construction specification, the strength of the on-site pressure test of the last capped concrete test piece shall be used to control. In general, the concrete strength shall reach more than 8.0MPa. The initial support is not stable, and the concrete strength should be more than 70% of the design strength when the secondary lining is applied in advance. Under special circumstances, the demolding time should be determined according to the test and monitoring measurement results.

(5) Concrete curing

Wash the outer surface of the template with water before removing the mold, and spray the concrete surface with high-pressure water after removing the mold to reduce the heat of hydration. The curing period is not less than 14 days.