Study On Crystallization Mechanism And Blockage Law Of Limestone Fracture Water In Highway Tunnel

The crystallization of limestone fissure water in highway tunnel is one of the important threats in the process of tunnel construction,whether in the construction period or in the operation period of the tunnel,and in every position of the drainage system.Based on the construction process of Anlan Expressway,this paper studies the cause of tunnel crystallization blockage by sample detection,model fitting and mechanism analysis.The factors affecting the crystallization of limestone fissure water in highway tunnel are discussed,and the crystallization mechanism of tunnel drainage system is clarified.The main findings are as follows:（1）The crystallization sample at the longitudinal drainage pipe of Hejiazhai tunnel is analyzed.It is found that crystallization is common,the moisture content of the sample is high,and the crystallization has nothing to do with the surrounding rock grade.Through SEM、XRD、XRF analysis,it is found that the crystal shape is different,such as square shape,spindle shape and lamellar shape,which is affected by salt ion,temperature,pH and kinetic effect.In addition,although the crystal color is different,the results show that the main component of the tunnel crystal blockage is calcium carbonate in the form of calcite.XPS results show that the sample contains Si、SiO₂、CaCO₃、Mg O,the main components are CaCO₃.According to the actual scaling situation,the scaling trend of tunnel drainage system is analyzed by saturation index method,stability index method and thermodynamic dissolution method.（2）By analyzing the water samples of Hejiazhai Tunnel,it is found that the water quality varies greatly from one location to another,which is related to geological structure,environmental climate,etc.The water quality of tunnel longitudinal drainage pipe solution can be divided into two types:the first kind of main ion is Ca²⁺、HCO₃^-、SO₄^2-,the pH is low,the second kind of solution is strong alkaline,the main ion is Ca²⁺、Na⁺、K⁺、CO₃^2-and so on.The Na⁺、K⁺concentration in the solution is generally positively correlated with the pH,and the Mg²⁺、SO₄^2-is negatively correlated with the pH.Some ions dissolved in shotcrete increase the SO₄^2-content from the face of the palm to the drain pipe.（3）The effects of Ca²⁺concentration,temperature,pH、CO₂contact surface and saltion(Na⁺、Mg²⁺、K⁺)concentration on the crystallization process were studied by setting the simulated water under two conditions:the increase of Ca²⁺concentration,temperature and pH promoted the crystallization.In addition,the increase of carbon dioxide contact surface also promotes crystallization,but under strong alkaline conditions,the effect of carbon dioxide contact on crystallization is more obvious.The addition of salt ions can slow down the precipitation of crystals,the effect of Na⁺、K⁺is limited,but the degree of crystallization mitigation of Mg²⁺in weak alkali conditions is obvious.（4）By comparing and exploring the crystals generated under different conditions in SEM,it is found that in precipitation formation,crystals with different morphologies are often not formed into a single crystal.addition of salt ions will change the microstructure of the CaCO₃,and the crystal stability is slightly different.Temperature and pH can also change the morphology,stability and particle size of the crystal.（5）The water quality of tunnel drainage system is simulated by indoor simulation,and the crystallization law of different working conditions is discussed.It is found that bellows produce more precipitates than smooth tubes.Compared with other working conditions,the material and velocity of pipeline have a significant effect on precipitation formation.By fitting the cumulative crystallization volume-time curve,the blockage time of simulated drainage system is predicted.（6）Combined with field investigation and previous experimental results,it is concluded that the crystallization of limestone fissure water in highway tunnel mainly includes the dissolution process caused by the action of groundwater and carbonate rock,the cement hydration process of shotcrete under the long-term scour of fissure water,and the crystallization blockage process of drainage pipe solution.Crystallization is mainly caused by high pH water quality easy to absorb carbon dioxide and react with calcium ion in drainage solution.In addition,from the geological,construction factors,climate and other aspects of in-depth analysis of the limestone tunnel drainage pipe blockage reasons.Finally,the countermeasures are put forward from two aspects:on-line monitoring,directional cleaning and anti-clogging methods of tunnel longitudinal drainage pipe blockage,and the development of new environmental friendly slow release agent.The anti-scaling countermeasures mainly include drainage system process optimization and standard construction conditions.