| Recently,tunnels and underground engineering have developed rapidly with an unprecedented speed.The target of our country’s tunnels and underground engineering are towards complex mountainous areas,cross-river passageways,and deep underground.Meanwhile various large-scale and complex structural forms,tunnels and underground engineering with high-level construction difficulties are emerging constantly.To solve the problems of surrounding rock instability,excessive ground subsidence and roof fall during the construction of shallow buried tunnels under construction or weak and broken strata,tubal curtain support measures have been adopted in a large number of shallow buried tunnels and underground projects.However,the selection of design parameters based on engineering experience can no longer to meet the challenges by which the current tunnel and underground projects faced.Therefore,numerous factors that influence the selection of tubal curtain design parameters are studied in this paper through the combination of theoretical analysis,numerical simulation and engineering examples.The main work is as follows:(1)The deformation law of shallow buried tunnel excavation and the theory of surrounding rock stability of shallow buried tunnel supported by pipe curtain are summarized.By referring to relevant domestic and foreign references,the research results of domestic and international pipe curtain support are summarized.The factors that influence the deformation of the backfill soil layer in the shallow buried tunnel are analyzed.Settlement law and prediction formula for surface settlement of shallow buried tunnels are concluded.The deformation of the strata in front of the tunnel face under the lining support,the extrusion deformation of the palm face and the deformation of surrounding rock behind the palm face are studied.The theory of the deformation,the stability of the palm face,and the stability of the soil between the pipes in the shallow tunnels are systematically illustrated.The current research on the design parameters of the pipe curtain is analyzed and summarized.(2)The mechanism of pipe curtain support is studied,and the formula of tubal curtain selection is deduced.The stress and deformation process of tunnel surrounding rock and pipe curtain under the support of pipe curtain are analyzed.A mechanical model was established to analyze the stress of a single steel tube in a shallow tunnel in backfilled soil area,and a calculation formula was given.At the same time,the factors affecting the design of pipe curtain are analyzed.The results showed that the tunnel section size and surrounding rock level have a great influence on the design parameters of pipe curtain under a certain depth of burial,whereas the excavation cycle footage,excavation method,excavation step distance and the length of pipe curtain section make a little difference to the deformation of pipe curtain.The implicit formula between the tunnel depth and the tube diameter and the distance between the tubes is deduced by using the Pasternak double parameter elastic foundation beam theory,which involves various parameters.Based on genetic algorithm,Software is used to calculate the theoretical pipe diameter and pipe spacing under different burial depths,and software is used to fit the relationship between burial depth and pipe diameter and pipe spacing,which intuitively presents the relationship between burial depth and pipe diameter and pipe spacing.(3)The influence of depth and section size of backfill on pipe diameter and spacing of pipe curtain was studied.By using finite element analysis,a total of 102 kinds of conditions with different tube diameter and tube curtain pole spacing are simulated under different buried depth and tunnel section size.Based on the settlement of the vault,the tubal curtain deflection and road surface settlement,the influence of the factors such as buried depth and section size of tube screen that affect pipe diameter and tube spacing was further studied.The recommended values of the reasonable steel pipe diameter and steel pipe spacing under different buried depths and different span conditions under the backfill soil layer were given.The relationship between buried depth and pipe diameter,buried depth and pipe spacing,tunnel span and pipe diameter and pipe spacing was fitted by software.The relationship between pipe diameter and buried depth obtained by numerical simulation was compared with the relationship between pipe diameter and buried depth derived from Pasternak double parameter elastic foundation beam theory.It is found that the trend of the relationship between buried depth and pipe curtain design parameters obtained in the two cases was basically coincided.In addition,the influence of tunnel depth and tunnel span on tube curtain selection was significantly analyzed,which indicated that the influence of tunnel depth on tube curtain selection was much greater than that of tunnel span on tube curtain selection.The flat rate of tunnel has some influence on the selection of tube curtain.(4)Taking the Maoyakou tunnel project,the data obtained by numerical simulation and on-site monitoring were compared,and the design parameters of the original pipe curtain were optimized.Based on the unchanged design parameters of Maoyakou’s original pipe curtain,finite element software was used to simulate the settlement of the tunnel in Maoyakou,and the results were compared with the field monitoring and measurement results.The results showed that the finite element simulation results were consistent with the on-site monitoring results.At the same time,using the formula which is deduced from the theory of beam on elastic foundation Pasternak double parameters,the original pipe curtain support engineering parameters of Chongqing Maoyakou tunnel were optimized.The finite element method was used to simulate the original pipe curtain design parameters and the optimized design parameters.The results showed that after parameter optimization,the tube met the requirement of the strength,stiffness and stability,and costs were under control.The recommended values of the design parameters of the tube curtain obtained in this paper can be referred to the projects close to this project.The relationship derived from the theory of Pasternak’s double-parameter elastic foundation beam can be used for various engineering design and construction references. |
