Field Testing Analysis And Numerical Simulation Study Of Short Tunnel With Small Clear Spacing Under Complex Geological Conditions

Small clear spacing tunnel has become one of the main tunnel structures for road construction in the mountainous and complex geological conditions because of the advantages of preventing leakage of water well, relatively low cost, relative independence of the construction of the two tunnels and simple process, etc. in recent years. However, under complex geological and topographical conditions, the small clear spacing tunnel is a relatively new form. In this paper, Zeng Jiading Tunnel of A8 section and Lei Gongpu tunnel of A9 section on the Yongning highway in the city of Sanming had been taken as examples. For different geological strata and different terrain conditions, the construction process was monitored and measured, and the measuring information was analyzed and determined. A representative cross-section was selected and numerical simulation of finite element method was finished by Plaxis. The step by step construction process was simulated, and then the simulation results were analyzed.The main research work in this paper is shown as follows:(1)The two small clear spacing tunnels under complex geological and topographical conditions including Zeng Jiading tunnel and Lei Gongpu tunnel were monitored and measured. The main mointoring items include:observing the outside and inside rock of cave, surface subsidence, arch settlement, the surrounding convergence, movement of the middle pillar, axial force of anchor, pressure of surrounding rock, and stress of steel arch, etc.. And then the obtained data was settled and analyzed.(2)According to the measured data, the regularity for change of surrounding rock and the mutual impact of the excavation of the adjacent holes was analyzed, and provided reference for timely inverting of upper arch and second supporting construction; the stability of the middle pillar was determined, providing guidance for the reinforcement of mediate rock column; in order to determine the correctness of the excavation method of the small clear spacing tunnel and the rationality of the supporting structures and surrounding rock was analyzed, and them provide reference for the optimization of the supporting system.(3)The monitoring data of the two small clear spacing tunnels under different geological conditions during the excavation process was compared and analyzed, them the the key monitoring positions and time was determined, providing reference for the future monitoring under similar conditions.(4)The residual soil stratum of K68+850 cross-section of Zeng Jiading tunnel and the collapse deposits of semi-hard semi-soft rock strata of K72+520 cross-section of Lei Gongpu tunnel had been selected to simulate and calculate by finite element method. The finite element numerical simulation emphasized on the changes of all kinds of characteristics during the excavation process, including displacement field, stress field and the distribution of plastic point. Through comparing the changes of all kinds of the left and right holes after completion, the mutual impact of the first and second holes during excavation was analyzed, furthmore the forces of initial supporting structure was analyzed, them the safety and rationality of the construction on the field is determined.