1, preface
With the increase of a large number of tunnel projects, typical tunnels, such as extra-long tunnels, large-section tunnels, multi-arch tunnels, deep-buried tunnels and tunnels with weak surrounding rocks, appear constantly, which promotes the continuous improvement and perfection of tunnel construction technology. However, due to the particularity of tunnel engineering, influenced by geology, design, construction, operation, operation management and maintenance, there are more and more tunnel diseases, some of which have affected the normal operation of vehicles and even endangered driving safety. In order to reduce the number of diseased tunnels, it is necessary to study the causes of tunnel diseases and corresponding measures. In underground engineering cavern construction, the understanding of surrounding rock is one of the most important tasks. Only by clearly and correctly judging the nature of surrounding rock can we take effective measures in construction and successfully realize the construction goal.
Most tunnel surrounding rock diseases occur in soft and expansive surrounding rock. Weakly expanded surrounding rock refers to the surrounding rock with saturated compressive ultimate strength lower than 30MPa and integrity coefficient lower than 0.4. Its engineering properties are mainly manifested in poor stability of surrounding rock itself, developed joints and severe weathering of rock. The lithology is mostly mudstone, argillaceous siltstone and loose gravel soil, which is easy to soften when it meets water. Before construction, the site of soft and expansive surrounding rock should be carefully investigated and the design documents should be carefully read. The construction scheme should strictly follow the principles of "paying attention to geology, leading the conduit, strictly grouting, short footage, strong support, quick closure, diligent measurement and quick feedback", and strictly implement the relevant norms and standards of tunnel construction.
2. Classification of tunnel diseases
According to the manifestations of diseases, tunnel diseases can be divided into lining cracking, lining leakage, tunnel base boiling and mud leakage. It is necessary to investigate and analyze the causes of tunnel diseases before they are rectified. Only by investigating the causes clearly can we take targeted engineering measures according to the causes of the diseases.
For example, lining cracks are caused by bending, shearing and compression. As for why cracks occur, it may be the result of construction quality problems, design defects or a combination of factors. It is difficult to generalize only by analyzing specific problems. Similarly, according to the different causes, the corresponding treatment measures are also different. According to the causes, tunnel diseases can generally be divided into diseases caused by improper route selection, diseases that may be reserved due to unreasonable design, diseases reserved due to improper construction and diseases caused by improper maintenance.
3, tunnel surrounding rock disease and its influence
(1) deformation and failure of initial support. The structure sinks and the sprayed concrete cracks and falls off. Most deformation occurs in the arch ring above the arching line; The section with severe deformation is more than tens of centimeters, which encroaches on the structural scope of the secondary lining, resulting in insufficient section size of the secondary lining structure, and the initial support has to be removed and redone.
(2) The secondary lining is damaged. The main manifestations are: longitudinal splitting of vault concrete, obvious extrusion shear failure, peeling of surface concrete and exposure of steel bars; The deformation at the joint of arch foot wall is staggered.
(3) Frequent landslides, such as bare cave landslides before the initial support is completed, and the most serious and costly landslides after the initial support is completed.
In addition to the influence of engineering geology and hydrogeological conditions, there are other reasons for tunnel surrounding rock diseases, such as improper and untimely construction treatment; Weak supporting structure and other reasons. Due to the defects of construction methods and unreasonable construction procedures, this disease manifests itself as sinking, deformation and invasion on the final concrete surface. The geological structure of the tunnel is broken and weak. After excavation, a high-strength closed structural ring should be formed as soon as possible to restrain the excessive deformation and relaxation of surrounding rock, which is the effect and ultimate goal of the initial support.
4. How to effectively treat tunnel surrounding rock diseases?
(1) Optimized mining method
It is not appropriate to divide the excavation area too much in the excavation method, which will not be conducive to the formation of initial support as soon as possible, and it is difficult to ensure the integrity of the structure. After the upper excavation is completed, the initial support of the top arch is carried out by using the lower height. In case of extremely broken and weak surrounding rock, the upper core soil can be reserved, and a pneumatic pick can be used to gouge along the excavation contour line to form the upper initial support, and the middle core soil can be reserved, which not only avoids the excavation face from being exposed too much, but also has the dual functions of resisting the main body and the construction platform. Measures taken for the lower excavation: a. To prevent the upper initial support from sinking and deformation, such as increasing the length of deep-hole locking anchor rod (L ≥ 5m) at the upper steel leg; B. Set temporary longitudinal joists and temporary supports, etc. C, try to avoid disturbing the upper surrounding rock, especially near the excavation face, through "reserved protective layer" and manual excavation.
(2) Adopt composite lining structure.
Inverted arch composite lining structure design should be adopted for weak expansive surrounding rock tunnel. This is because, when the tunnel has reasonable supporting conditions, the rock strata will gradually release energy, adjust the internal force of the supporting lining of each layer, give full play to the compressive strength of the external support under the action of three-dimensional stress, and the steel arch provides resistance in time to limit the deformation of surrounding rock and reduce the contact stress borne by the lining. The lining is mainly subjected to the pressure of uneven expansion surrounding rock which continuously increases in the later period, which makes the lining structure stable for a long time. According to the relevant experimental research and the investigation of cracked lining of railway operating tunnels in China, it is proved that in tunnels with loose and broken strata and weak expansive surrounding rocks, the bottom pressure is often close to or greater than the vertical ground pressure, which often leads to cracking of the bottom plate, bulging of the tunnel bottom and inward movement of the lower part of the side wall, which makes the lining unstable and damaged. Therefore, the inverted arch is the key part of the structure, and the design and construction should be paid enough attention.
(3) the influence of the inner contour type of the lining section
The research on soft expansive surrounding rock tunnel at home and abroad shows that the shape of the inner contour of tunnel section has great influence on the expansibility of rock. According to German scholars' research on the expansion law and finite element calculation, the results show that the bottom drum with horseshoe cross-section profile is almost four times that of a circle. According to 268 groups of in-situ stress data measured at home and abroad, it is concluded that the horizontal stress at a certain depth in the earth's crust is often greater than the vertical stress, and sometimes it is even 3~5 times greater, so the circular profile and the nearly circular oval profile are suitable for tunnels with weak expansive surrounding rocks.
According to the survey data, in order to meet the clearance requirements, most tunnels at home and abroad adopt horseshoe-shaped cross section, and a few adopt circular cross section. In recent years, Chinese scholars' experimental research on the mechanical characteristics of tunnel supporting lining and inverted arch proves that the failure mode of horseshoe-shaped section under different principal stresses is that inverted arch and its connection with the side wall are the weak links of the structure. Therefore, in the design and construction of expansive surrounding rock, on the basis of the existing single-line and double-line horseshoe-shaped sections, the annular connection between the side wall and the inverted arch should be adopted to reduce stress concentration and give full play to the role of the inverted arch. The straight walls designed by Guanjiao Tunnel and Cuijiagou Tunnel in China have no inverted arch lining, and the diseases are serious. During the construction of Xianglushan hydraulic tunnel in expansive surrounding rock stratum, the straight wall section was changed into horseshoe inverted arch section, which improved the stress conditions.
(4) Secondary lining concrete construction
As far as tunnel surrounding rock construction is concerned, the concrete construction sequence of "arch first and then wall" from top to bottom is extremely undesirable. Disadvantages are as follows:
1) Hanging the upper part will cause deformation and settlement of the structure; 2) When a complete section is not fully supported at the beginning, concrete construction will interfere with each other, which is not in line with the efficiency and timeliness of NATM construction principle; 3) It is difficult to deal with structural reinforcement and construction joints.
The bottom inverted arch can be poured in advance, and the bottom constraint conditions can be established to form a closed loop. The advancement of bottom concrete can also achieve several effects: a. Seal the foundation to prevent it from being softened by water immersion. B, establishing a bottom constraint condition for the completed initial support of the wall arch to form a closed loop with the initial support structure of the wall arch. C. The bottom inverted arch moves forward, so that the load acting on the second lining concrete is transferred and distributed on the foundation surface according to the stiffness ratio, and the bearing capacity of the foundation is improved.
5. Conclusion
Because of the complex geology of weak expansive surrounding rock and frequent changes, the prerequisite for successfully crossing weak expansive surrounding rock is geological work. Only by doing geological work well and mastering accurate surrounding rock conditions can we correctly choose a good construction scheme and an effective treatment scheme.
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