| Tunnel secondary lining layer, as the outermost layer of tunnel engineering concrete structure, not only plays an important supporting role in the tunnel surrounding rock stability, but also has serves as decoration and waterproof functions. Tunnel as underground building structure, its engineering geological condition is very complex, coupled with poor tunnel construction condition and unreasonable design, the influence of such factors tend to make different degrees of damage to the tunnel second lining structure, The tunnel second lining cracking phenomenon is the most common type of the second lining structure destruction, which adversely impacts the project quality and durability as well as tunnel engineering safety. Therefore, the research on tunnel second lining structure cracking mechanism and the influence of cracks on the structural safety is of great significance.Through the study, it is found that the causes of cracks in the road tunnel second lining is numerous, and has very complex mechanism. In this dissertation, the field geological investigation, theoretical research, on-site monitoring measurement, indoor and outdoor tests and numerical analysis are used to research on the several key technical problems such as the regional engineering geological conditions, lithologic characteristics, second lining cracks distribution characteristics, the deformation of surrounding rock and supporting load-bearing characteristics, second lining cracking mechanism, second lining crack propagation law and second lining crack safety evaluation system etc based on the tunnels along the expressway from Gucheng to Zuxi in Wudang Group schist zone. This article research results not only has realistic engineering significance but also has a strong theoretical value. In this study, the following results and conclusions are drawn:1. Research on engineering geological conditions and characteristics of surrounding rock in schist area with large fault.(1) In combination with tectonic geological and engineering geological conditions in the studied area, movement characteristics and geometrical motion regularity of nappe structure of Wudang Mountain is obtained. And regional geological settings, stratum lithology, geographic and geomorphic, geological structure conditions and hydrographic characteristics of this schist zone are investigated.(2) Through the engineering geological evaluation shows that under the effect of complex engineering geological conditions, the rock mass is affected by nappe, extrusion and uplift, corrugation, fold and fault develop well, the surrounding rocks are always broken, fragmentized or intensively disintegrated rock mass with residual tectonic stress, groundwater development, high mica content and weathering degree and poor mechanical performance, under the influence of various factors, the mechanic property and integrity of surrounding rock is significantly weaken, it is easy to cause engineering geological hazard problems in the tunnel construction process, such as rock mass large deformation, collapse, water gushing and mud gushing, the supporting structure occurs to crack due to great pressure, seriously impacting the safety of tunnel construction and supporting structure.(3) Based on laboratory test (X-ray diffraction, physical and mechanical experiment, the rock acoustic wave test), The physical and mechanical experiment results demonstrate that the metamorphic soft rock is characterized by plasticity, swelling, Theological property, easy to disturbance, damage degradation and anisotropy, etc.(4) Combined with the research results of schist engineering:the characteristic of tunnel surrounding rock is characterized with the water swelling, softening, disintegrating and Theological, it is easy to induce engineering geological hazards, such as large deformation of surrounding rock, landslide, floor heave, invasion and so on. The damage of surrounding rock is produced during construction due to poor ability to resist the external environment disturbance, the problem such as bearing capacity to reduce. And it is prone to local large deformation and stress concentration causing by poor structural plane bonding performance, low bonding shear strength and obvious anisotropic.2. The research of surrounding rock deformation and supporting load-bearing characteristics.(1) The characteristics of schist rock tunnel are summarized as large deformation, high deformation speed, great divergent trend and long deformation duration. The results of later monitoring reflect that the velocity of rock surrounding displacement continues to increase, which shows rheological properties in a long time.(2) There are apparent differences not only among surrounding rock deformation in different parts but also among surrounding rock deformation of various rock occurrence various and the deformation law of surrounding rock.(3) Schist tunnel deformation curve has three stages:deformation slow growth stage; continuous deformation stage, accelerating deformation stage.(4) The supporting structure bearing value has great discrepancy at different positions of the tunnel, which shows great deal of asymmetry and obvious anisotropy. High surrounding rock pressure of surrounding rock appears on the contact region between supporting structure and rock formations, the supporting structure bearing value changes with rock occurrence.This feature is roughly identical to the deformation law of surrounding rock.(5) The load sharing ratio of the secondary lining is relatively high, reaches about 80 to 90 percent, the bearing load proportion of initial liner is small, accounting for about 10%～20%. The secondary lining is easily lead to extensively crack caused by excessively high stress, which is a major cause that results in fracture in the secondary lining of schist tunnel.3. Second lining cracking characteristics analysis and research.(1) Secondary lining cracking characteristics of schist tunnel area are analyzed, combining the danger of cracking, secondary lining cracking investigation and detection methods, secondary lining crack types and so on.(2) Researching the second lining cracking characteristics, regularities of distribution and causes in the studied, and laying a foundation for the research of second lining cracking mechanism.4. Tunnel lining cracking mechanism research.From regional geological conditions, the characteristics of surrounding rock, landform, hydrology geological condition, in-situ stress, design factors, construction factors and other aspects to make comprehensive analysis on the tunnel lining cracking mechanism.(1) The influence of regional geological conditions on secondary lining cracking.Under the action of regional geology and tectonics, the rock is characterized with loose and broken structure, big water content, low strength and intactness, residual tectonic stress, high mica mineral content and so on. It is susceptible to encounter with geological disasters, such as water gushing, large deformation, or collapse during tunnel construction, the loose and broken surrounding rock would produce too large pressure on the supporting structure, resulting in cracking of tunnel lining structure.(2) The influence of schist engineering characteristics on secondary lining cracking.Based on the engineering properties of surrounding rock and laboratory test (X-ray diffraction, physical and mechanical experiment, the rock acoustic wave test), the engineering properties of rock are studied from the macro and micro perspective, the evolution of rock mass deformation and failure characteristics and engineering properties is obtained. The physical and mechanical experiment results demonstrate that the metamorphic soft rock is characterized by plasticity, swelling, rheological property, easy to disturbance, damage degradation and anisotropy, etc. Schist tunnel engineering properties make schist easy to produce plastic flow, larger plastic area and lateral extrusion, as well as the formation of bias and local stress concentration problems. Engineering characteristics lead to excessive deformation of surrounding rock and surrounding rock pressure on the supporting structure, resulting in second lining structure cracking.(3) The influence of groundwater on secondary lining cracking.The influence of underground water in rock mass is mainly manifested in the following two aspects:on the one hand, The coupling mechanical and physicochemical action between groundwater and rock change macroscopic mechanical properties of surrounding rock such as strength and stiffness, at the same time, the effects of the physical and chemical structure on fissure structure reduces the physical and mechanical properties of rock mass; On the other hand, through mechanics effect, the groundwater changes the distribution of the rock mass stress field, and leads to expanding of fissures in rock mass and seepage deformation, then making rock mass structural plane split extension and shear deformation, and increasing the porosity and connectivity. By changing and weakening the physical and mechanical properties of surrounding rock, the groundwater accelerates the destruction of the rock, so as to induce and intensify the cracking of lining structure.(4) The influence of in-situ stress on secondary lining crackingBased on the in-situ stress test and surrounding rock pressure monitoring result which shows that the study area is high ground stress area, effect of residual tectonic stress is obvious. Under the action of tectonic stress aggravated deformation of soft rock tunnel. The influence of ground stress is the important influencing factors to result in secondary lining cracking.(5) The influence of construction factors on secondary lining cracking.Theoretical research and field test result analysis show that different excavation methods have obvious differences in disturbance of tunnel surrounding rock, the construction disturbance increases the surrounding rock damage and degradation. The blasting vibration in construction o cause bigger damage n the surrounding rock and supporting structure, even leads to the large surrounding rock deformation, flaking and the linings craze. Reasonable construction scheme is of great significance to prevent second lining cracking. In addition, studies have shown that construction organization and construction control technology is an important effect on the second lining cracking.(6) The influence of design factors on secondary lining cracking.Some problems in the classification of surrounding rock and supporting structure parameters design are studied. Research shows that unreasonable classification of surrounding rock and support parameter design is one of the leading cause in tunnel lining structure cracking.5. Second lining cracking numerical simulation analysis.(1) Through the theoretical research, the theoretical calculation about the internal forces of secondary lining structure has been carried out, secondary lining structure numerical model is identified and established. Through the analysis of engineering problems under the influencing factors of schist tunnel second lining crack, several kind of common engineering problems in the study area such as the bias, hollow, uneven settlement, the uneven thickness of the lining structure and concrete strength are summarized, which would be used as the research content in numerical calculation.(2) To solve the problem of bias setting up four kinds of situations:without bias, bias, respectively in the vault, hance and sidewall. Based on the calculation analysis (second lining structural horizontal displacement, vertical displacement and crack calculation) of all kinds of bias case, the displacement cracks are less in on bias conditions; there are more displacement and cracks producing under the action of bias, especially in parts where the bias effect. It can be concluded that the bias make great difference on the cracking of second lining structure.(3) When there exist holes behind the tunnel second lining structure, the calculation analysis under different working conditions are carried out, the second lining structure displacement and crack calculation results show that the horizontal displacement of sidewall is the maximum in the condition that holes appears at the side wall, the horizontal displacement of hance is second, the vault is the last; as for vertical displacement, the results are opposite with that of horizontal displacement. For the second lining cracking, lining crack distribution is obvious influenced by the size of the hole, the bigger the void, the greater the number of cracks, the larger of crack propagation range resulting in more serious damage produced in the concrete.(4) Through several conditions of tunnel second lining structure thickness calculating results it can be seen that the vault vertical displacement is significantly greater than other parts when the lining thickness of vault is insufficient, the maximum horizontal displacement is produced on both sides of the arch, besides, the cracks are produced around the lining, and the number of which in vault are the most. As the insufficient thickness of second lining appears at the hance, the horizontal displacement of hance is the maximum, the maximum vertical displacement is produced in the vault, at the same the number of cracks on the surface of the hance is the most. As the insufficient thickness of second lining appears on the sidewall, the maximum horizontal displacement and the most cracks are both produced on the sidewall, the maximum vertical displacement is also produced in the vault. In general, the smaller is second lining thickness, the more cracks, and the wider is the extended range of cracks.(5) Through the calculation analysis of uneven settlement in different working conditions, tunnel lining structure displacement and cracks are less in the case of no uneven settlement, and shows obvious symmetry distribution; when uneven settlement is produced, the maximum horizontal displacement occurred at vault, deformation direction is to the left. When the whole lining structure stiffness is bigger, the overall performance of second lining structure is in the trend of inverse clockwise based on the bottom lines as the pivot; The maximum vertical displacement is produced on the left side of the base plate, vertical displacement value decreases along the inverted arch to vault in the subsidence area, the whole vertical displacement value on the right side is smaller. Cracks are first produced on the right side on the left side of the base plate and the right side of the inverted arch, and gradually spreading in all directions, but no cracks is find in the vault. In general, the greater the uneven settlement, the greater the size of crack coverage area.(6) Through analyzing the displacement and the cracks regularity about different intensity of second lining concrete structure can been founded:displacement value of C20 second lining concrete structure is the largest, the number of cracks is the most, the extended range of cracks is widest. C25 is less than C20. C30 is the least of all. Controlling the quality of concrete construction and enhancing the strength of concrete to the control structure cracking can get good effect.6. The research of second lining crack propagation law and safety evaluation.(1)Based on intensity factor of fracture mechanics theory, the numerical calculation results which are obtained under different bias condition, different parts, different regulations and characteristics of the expansion of crack show that intensity factor (K1) at crack tip changes smaller with crack width, the crack width had little impact on the propagation of the crack. However, crack tip intensity factor (KI) increases with crack depth. The value of intensity factor (KI) in bias parts is significantly greater than that of other parts, the effect of bias is advantageous to crack expansion, crack depth in the bias part is within the scope of 3.0 to 12.0 cm, intensity factor (KI) is less than the fracture toughness (KIC), crack propagation is in the safe and stable stage; When the crack depth is up to 15.0 cm, intensity factor (KI) is greater than the fracture toughness (KIC), crack propagation is in the instability of stage; Crack depth reaches 12 cm, intensity factor (KI) is close to the fracture toughness (KIC). After considering a safe reserve, to take 12cm as the threshold of unstable failure of second lining crack.(2) For different bias condition, dynamic expansion trend of cracks in different parts and the four stages of crack extension from the beginning to broken are studied, verifying the rationality that crack depth 12 cm as the critical depth value of instability and failure of lining structure.(3) Based on the existing qualitative and quantitative evaluations of second lining crack and above crack propagation numerical studies, increasing the crack depth as an evaluation index to revise the original specification, and proposing of tunnel second lining crack safety evaluation standard used in the schist area.(4) By refer to research achievements and experience of second lining crack treatment at home and abroad and the results of this article, the suggestions on the second lining of the tunnel cracking prevention and treatment in the schist area is put forward. |
PDF Full Text Request