Research On Crystallisation Formation Mechanism And Prevention Technology Of Tunnel Drainage System

With the development of China’s transportation construction,the problem of crystalline blockage of tunnel drainage system is becoming more and more prominent,which can cause damage to the tunnel drainage system and lining structure and even threaten tunnel safety if not treated in time.Based on the Mao Ya Mountain Tunnel of Kanglue Expressway,this paper investigates the crystallisation mechanism of the tunnel drainage system based on on-site research,mechanism analysis and experiments on the influencing factors;develops a new type of descaling agent and simulates the descaling process of the actual drainage pipes through a laboratory device;and proposes specific recommendations on the use of the descaling agent based on its characteristics and the results of the simulation experiments,and proposes improvement measures from the perspective of prevention and treatment.The main research results and conclusions are as follows:（1）Through the on-site water quality and crystalline material research in Mao Ya Mountain Tunnel of Kanglue Expressway,it was found that the Ca²⁺,Mg²⁺,CO₃^2-and SO₄^2-contents in the water samples were small,while the average concentration of HCO₃^-ions was found to be high,up to 208 mg/L,which was easy to cause the formation of crystalline precipitation,and the flow rate of water in the pipeline was not large and the pH showed to be weakly alkaline,which was also favourable to the formation of crystallization.The crystalline form was found to be mainly irregular and square by means of EDS,SEM,XRD and other instrumental analysis,with some fusiform,rhombic and spherical particles,and the main peak of the XRD diffraction pattern was consistent with calcite,the tunnel crystalline material was judged to be mainly calcite type calcium carbonate.（2）A theoretical analysis of the factors influencing the tunnel drainage system has been made.Generally speaking,excluding unchangeable pressure and biological effects,the higher the concentration of crystallisable ions in the solution,the higher the concentration of CO₂,the higher the pH,the higher the temperature,the smaller the slope and size of the drain,the easier it is to precipitate ions from the surrounding rock and concrete,the easier it is to form crystals in the tunnel drain and the faster the crystallisation rate.（3）The effect of crystallisation ion concentration,pH,temperature and CO₂ contact surface on the crystallisation process was investigated experimentally.The single factor experiment showed that:an increase or rise in the initial concentration of crystallising ions,pH,temperature and CO₂ contact surface would all promote crystallisation.The increase in crystallisation rate was more pronounced for the same pH change interval,while the increase in crystallisation rate was more uniform for the same temperature change interval,and the effect of CO₂ on crystallisation in the tunnel drainage system was not negligible.pH and temperature had a more pronounced effect on crystallisation than the other factors.The highest crystallisation rate of 18.80%at 30 min was achieved at pH=9.1,and 17.05%at 30°C.Changes in the concentration of crystallising ions have a more even and lower effect on the crystallisation rate.（4）Scale removal experiments were carried out based on calcium carbonate scale samples to investigate the scale removal rates of different descaling agents and their synergistic effect of compounding,while the response surface methodology was used to optimise the descaling agent formulations.Firstly,oxalic acid and 1-Hydroxy Ethylidene-1,1-Diphosphonic Acid（HEDP）were excluded from the experiments,and the descaling rates of three main descaling agents,namely citric acid,sulfamic acid and2-Acrylamido-2-Methylpropane Sulfonic Acid（AMPS）,were continued to be evaluated on the calcium carbonate scale samples,and it was concluded that sulfamic acid had the best combined effect.For the choice of descaling adjuvants,Diethylene Triamine Penta（Methylene Phosphonic Acid）（DTPMPA）and 2-Phosphonobutane-1,2,4-Tricarboxylic Acid（PBTCA）was the most effective.Continuing to optimise the descaling agent formulation by the response surface method,a reasonable choice of descaling rate formulation needs to be made,taking into account the actual engineering requirements（e.g.cost）and the descaling effect to be achieved.When the mass fraction of sulfamic acid is between 7.5%and 8.5%,DTPMPA between 0.2%and 0.8%,and PBTCA between 0.2%and 0.8%,the scale removal rate is 85%to 99.8%.The mass fractions of 8.382%sulphamic acid,0.385%DTPMPA and 0.269%PBTCA are referred to as the optimum descaling agent formulation（the lowest cost formulation to achieve 95%scale removal）with the advantages of suitability for use in tunnel temperatures,negligible corrosion of tunnel pipes and metals,low mass fractions required and fast and efficient scale removal performance.（5）The effect of flow rate and water sample/descaler mass ratio on the descaling speed was investigated by simulating the descaling process of a real drainage pipe with a home-made experimental device.After the descaling agent was added to the water stream,the reaction rate with calcium carbonate crystals was fast,and the descaling speed tended to increase as the flow rate and percentage of descaling agent mass increased.At a flow rate of 0.3 m/s and a mass ratio of 1:9,10 g of calcium carbonate crystals were treated in 5.03 s at a descaling rate of 1.99 g/s.At a flow rate of 0.1 m/s and a mass ratio of 3:7,the descaling rate was the lowest at 0.48 g/s.The effect of the change in flow rate on the descaling speed is greater than that of the water sample/descaling agent mass ratio,and the effect of the change in parameters on the descaling speed is more pronounced at low parameter levels,and there is a law of diminishing marginal utility.（6）Propose specific management solutions for crystalline blockage,so that the use of descaling agents can be more reasonable,convenient and safe in practical application.Based on the mechanism of crystalline blockage in tunnel drainage system,the proposed measures are proposed from the perspective of prevention and treatment respectively,and the advantages and disadvantages of various measures are analysed,and suitable methods to prevent and control crystallisation in tunnel drainage system are selected according to the actual situation.