The invert is a reverse arch structure set at the bottom of the tunnel to improve the stress conditions of the upper supporting structure. It is one of the important components of the tunnel structure, and it is the foundation of the tunnel structure. On the one hand, it needs to effectively transmit the ground pressure on the upper part of the tunnel to the underground through the side wall structure of the tunnel or the load on the road surface, and it also effectively resists the reaction force from the ground under the tunnel. In fact, it is a kind of foundation beam that can bear the permanent load of the ground and the temporary load of the road surface (dynamic load). The invert and the secondary lining form the entire tunnel, increasing structural stability.
The role of invert
1. Solve the problem of insufficient foundation bearing capacity, reduce ground subsidence, prevent the uplift and deformation of the bottom drum, and adjust the lining stress;
2. Seal the surrounding rock to prevent excessive deformation of the surrounding rock and improve the overall bearing capacity of the mechanism;
3. Increase the support resistance of the bottom and the wall to prevent shear damage caused by internal squeezing.
Construction requirements for invert
The invert arch should be constructed in a timely manner according to the surrounding rock stress condition and the initial support detection situation, and the traffic in the tunnel should be guaranteed to be unimpeded, without affecting the advancement construction in front, and the construction should be carried out in strict accordance with the excavation plan, and the construction of the arch wall should be carried out. Early arch support and invert arch molding concrete construction to realize early closure of the support structure; formwork must be used during invert arch construction to ensure the quality of concrete pouring; invert arch cushion, initial support, molded concrete, inverted arch Arch backfilling should be constructed separately to ensure the construction quality of each unit; the invert arch should be formed at one time, and should not be poured in stages, the joints should be smooth and waterproof, and the over-excavated parts should be backfilled with concrete with the same strength as the lining; The invert, filling layer and floor concrete should be constructed as soon as possible to ensure the construction of the arch wall concrete and the second lining and the normal working length of the lining trolley. The floor concrete should be strictly controlled to ensure the thickness and elevation of the concrete pavement.
During the construction of invert and slab, drainage facilities such as central drainage ditch, horizontal drainage pipe, and longitudinal drainage pipe should be implemented as required to ensure smooth drainage in the tunnel, and attention should be paid to the provision of deformation joints through the second lining; the invert of the opening section should be in Excavated into the hole within 150m and closed into a ring. Set up guardrails around the excavation of the inverted arch, and install a dense mesh safety net; set up warning signs at the exit. During invert construction, trestle bridge construction is adopted to ensure continuous invert construction and ensure that the excavation of the tunnel and the transportation of materials in the tunnel are not affected by the invert excavation, and the strength of the poured invert concrete will be satisfied. After the traffic intensity is required, the trestle bridge will be removed for the next stage of invert construction.
Invert construction process
1. Trestle construction
In the construction of the trestle bridge, attention should be paid to the flatness and width of the installation to ensure the safety of vehicles and the smooth passage of different construction vehicles. One side of the trestle bridge is erected on the completed invert backfill and base plate construction site, and the other side is erected on the original ground. The trestle bridge should be erected in consideration of the bearing capacity of the trestle bridge, and the foundation of the trestle bridge should be reinforced according to different geological conditions.
The excavation of the inverted arch soil layer should be assisted by mechanical cooperation and manual excavation; for soil and rock excavation, the excavation length must be controlled, and the excavation vibration must be avoided during blasting operations to affect the formed lining and other structures. The over-excavation and under-excavation should be controlled as much as possible. The invert section should be checked immediately after excavation, and the insufficient part should be re-excavated to ensure the thickness of the invert; the tunnel invert excavation should be smooth to avoid stress concentration in. The debris at the bottom of the side wall steel frame is clear and clean to ensure a good connection with the inverted steel frame, creating a reasonable working surface for the construction of the low side wall.
2. Initial support and second lining steel pre-embedded
(1) The excavation of the invert and the initial support of the invert should be closely connected. Before installing the steel arch frame, a 5-250px low-grade concrete cushion should be laid on the base, and then the invert arch steel frame should be installed, and then sprayed concrete or The use of moulded concrete facilitates the installation of inverted arch reinforcement and installation of templates;
(2) The inverted arch steel support installation must meet the design and specification requirements, and be welded smoothly and firmly with the side wall arch frame;
(3) The extension length of the pre-embedded steel bars at the side walls of the two side linings on both sides of the invert should meet the requirements for welding with the second lining steel bars in the circumferential direction, and the joints should be staggered so that the number of steel joints of the same section is not more than one percent of the total. Fifty. The spacing between the second and second lining steel bars of the inverted arch must be ensured, and the layer spacing must be ensured by welding the positioning steel bars.
3. Invert pouring and invert filling
(1) Before pouring the invert arch, the mixing ratio of concrete should be strictly demonstrated, and the top form with reserved vibrating holes should be used to ensure the compactness of the concrete. The invert concrete can be built with pumped concrete; the invert should be constructed before The position of the settling joint should be designed in advance. The settling joint should ensure that the second lining concrete, arch wall, and invert are disconnected on the same vertical plane, and the settlement joint should be treated as required to ensure the quality of construction, and the surrounding rock will not change.
In large lots, there should be as few or no expansion joints as possible to ensure that the lining does not have water-proof and drainage problems.
(2) Invert backfilling should start construction after the invert concrete reaches 70% of the design value. When invert backfilling, a layer of concrete should be poured to ensure that the rubble does not directly contact the invert, and the rubble is manually put in, and then the concrete is poured. The order of placing the rubble shall be carried out. The amount of stone backfill shall not exceed 25%, the distance between the stone shall be more than 250px from the edge of the formwork, and the distance between the auxiliary stones shall not be greater than the maximum diameter of the coarse aggregate.
(3) Treatment of construction joints and deformation joints. The surface of the joints should be cleaned of floating slurry and sundries, brushed with no less than the strength grade of the structural concrete or coated with concrete cross-section treatment agent, poured concrete in time, and applied formwork to the construction joint Or other measures to form a predetermined shape to ensure that subsequent projects are closely connected; the maximum allowable settlement difference of the settlement deformation joint should not be greater than 30mm, the width of the settlement deformation joint should be 20~30mm, and the thickness of the concrete structure at the joint It should not be less than 300mm, and the color seam should be set with a backing material that does not adhere to the caulking material or a water-swellable water-stop strip to meet the requirements of sealing and waterproofing, adapting to deformation, convenient construction, and easy maintenance.
The construction quality of the tunnel invert is an important indicator to ensure that the tunnel bears the permanent load of the ground and the temporary load of the road surface. The construction of the invert must be meticulous and constructed in accordance with the construction requirements to ensure the integrity of the tunnel structure, make the tunnel structure instability, pavement settlement, cracking, and tumbling Phenomena such as mud will not appear after the tunnel is delivered to use, so as to ensure the safety and normal operation of the tunnel.