Causes and treatment of cracks in tunnel lining
Tunnel cracks are one of the common diseases of tunnel linings. Cracking of linings is manifested in both the primary lining and the secondary lining, of which the secondary lining is the most serious.
The distribution of lining cracks is the most on the left side of the lower line, followed by the left side of the upward line, and the right side of the downward line. The distribution of lining cracks is concentrated in sections. The lower left is mainly concentrated in the sections K10 + 300 to K10 + 585 and K21 + 765 to K21 + 885. The location of the cracks corresponds between the up and down lines, and between the left and right sides of the down line, reflecting the uniformity of the crack generation mechanism.
Most of the tunnel cracks appeared in the early stages of tunnel construction, and some even had a large number of cracks during the tunnel construction process, and they were less likely to appear 5 years after construction.
The formation and development of tunnel cracks is a time process. Tunnel cracks are usually small and scattered at first, and gradually deteriorate over time. This shows that the formation and development of tunnel cracks are related to the deformation, relaxation, and spalling of surrounding rock. The formation and development of cracks and their closure are constantly changing over time. Concrete cracks can be divided into oblique cracks, longitudinal cracks and transverse cracks according to the occurrence. The oblique cracks are bounded by the maximum buried depth of the tunnel (approximately at K10 + 750). The cracks in sections K21 + 620 to 750 tend to import, and the cracks in sections K23 + 750 to 855 tend to export. According to the survey results, oblique cracks accounted for 16%, longitudinal cracks accounted for 59%, and transverse cracks accounted for 25%. The cracks generally open from the inside of the lining and gradually decrease along the thickness of the lining, and generally do not penetrate the entire lining thickness.
Most of the cracks are located in places such as cave entrances where the rock layer is thin, the rock mass is loose, and the cracks are developed. The distribution of cracks is 49% of the partition wall, 33% of the arch top, and 18% of the side wall. The openings accounted for 40% and the cave body accounted for 60%.
Cracks can be divided into unidirectional misalignment, bidirectional misalignment and tridirectional misalignment according to the misalignment mode.
The number of unidirectionally displaced cracks is the largest, showing as opening. Two-way staggered cracks are followed by open and shear staggers. Three-way staggered cracks. The amount of crack displacement varies from 0.1mm to 1mm, followed by 1mm to 2mm, and the maximum displacement is 6mm to 8mm. Most cracks are wide and narrow down until they are sharp.
Analysis of crack causes
The types of these cracks are roughly divided into dry shrinkage cracks, temperature cracks, load deformation cracks and construction joints. In this paper, the tunnel lining is used as the force entity, focusing on the horizontal stress (mainly due to temperature shrinkage due to dry shrinkage and thermal expansion and contraction), uneven settlement of the lining, and uneven arch force (saddle-shaped stress Force) and other three aspects for analysis.
The dry shrinkage cracks are mostly surface, the texture is small, and the direction is irregular. The occurrence of cracks has a great relationship with the construction process, including the choice of the driving method, the concrete vibration, the determination of the water-cement ratio, the removal of the mold, the arch backfilling, and maintenance. The demoulding is too early, and the concrete begins to be stressed before it reaches the design strength, which causes cracks in the concrete, especially the backfill of the vault is not dense. The top pressure is dispersed on the sides of the arch waist, and the “saddle” load appears on the arch The form of ground pressure distribution is very different from the original design of the lining. When the arch waist is opened, the shrinkage and cold shrinkage of the concrete itself are also important reasons for cracks.
A large amount of heat is generated during the hydration of the cement, and a temperature gradient is formed between the interior and the surface of the concrete to generate stress. When the temperature stress exceeds the binding force inside and outside the concrete, temperature cracks will occur. The tunnel lining concrete has dry shrinkage, thermal expansion and cold shrinkage. Since the surrounding rock of the lining hinders the free expansion and contraction of the lining, temperature stress is generated inside the lining concrete. The magnitude of this temperature stress is related to the dielectric properties (elastic modulus) , Linear expansion coefficient), rising and falling temperature (temperature difference), the resistance of rock and soil media to the tunnel wall and the length of the tunnel. Concrete is a material with low tensile strength, so it can often resist the compressive stress generated during heating, but it is difficult to resist the tensile stress generated during cooling.
When the tensile stress inside the lining exceeds the tensile strength of the tunnel lining concrete, cracking occurs in the tunnel lining, and this cracking starts from the middle of the tunnel. The maximum horizontal stress of the tunnel is in the middle of the tunnel. If the tensile strength of the tunnel lining concrete is exceeded, the lining cracks. The maximum horizontal stress still occurs in the middle of each section after cracking. When it exceeds the tensile strength R of the concrete, a second crack will occur. This continues until the intermediate maximum tensile stress is less than the tensile strength of the concrete. The cracks are stable and no longer increase.
Rectification of concrete cracks
According to the crack conditions of the secondary lining, repairing the stable cracks can be divided into 3 cases:
1. If the crack width is less than 0.3mm, the surface is mainly closed and observed. The characteristics of cracks, surrounding rocks, etc., are covered with carbon fiber when necessary.
For fine cracks smaller than 0.3mm, first use a wire brush to roughen the concrete surface to remove surface attachments; then fill the concrete surface with epoxy resin slurry, use the capillary action of the crack to absorb the repair glue, and close the crack; then use the repair material to coat Cover the surface until it is flat with the original structure surface, and finally treat the concrete surface with scraper and coloring material to make it the same color as the surrounding lining concrete.
2. For cracks with crack widths ranging from 0.3m to 0.5m, perform low pressure injection repair. The epoxy resin slurry is used for the injection to repair the cracks in the lining. The following steps should be followed:
(1) Clean the crack surface and use dry, oil-free compressed air to remove dust and scum inside the crack.
(2) Use sealing glue to adhere and fix the base of an injection cylinder on the concrete surface on both sides of the crack at a certain distance, while sealing along the entire length of the crack.
(3) After the sealant is cured, it can be injected.
(4) Use a special syringe to inject the low-viscosity crack repair glue into the crack cavity and maintain the pressure.
(5) After the glue injected into the crack is solidified, remove the injection cylinder and remove its base, if necessary, level the concrete surface with a grinding wheel.
(6) The surface should be cured, preferably covered with a plastic film.
3. For cracks with a width greater than 0.5mm, first perform low pressure injection to repair the cracks. In addition, for severe cracks such as wide cracks, wide openings, dense distribution, unfavorable combination, or staggered longitudinal rings or linings, certain serious diseases (such as It is located in the section of reinforced concrete second lining area that is not in the surrounding rock. Carbon fiber reinforcement should be pasted on the surface.
All cracks with a width of ≥ 0.5mm are first repaired by injection, and a low-viscosity, high-strength repair glue is injected into the crack cavity with a certain pressure. After the low-pressure injection is completed, the carbon fiber is stuck after the repair glue reaches the strength.
The causes of tunnel cracks are complex and changeable, which mainly include geology, design, construction and other aspects. The engineering geological survey must be strengthened. In addition to the topography, climate, stratum, age, lithology, and hydrology of the tunnel site, efforts must be made to identify structures, bad geological bodies, and hazards to the project. , Mismeasured. Improving engineering design capabilities, eliminating hasty practices, and careful design and construction are essential ways to prevent disease.