Bearing Behaviors Of Control Measures For Foot Settlement Of Tunnel Steel Arch

The phenomenon of tunnel collapse or substantial settlement at the crown are mainly caused by the failure of support at the foot of steel arch during tunnel construction in weak ground. In order to ensure the effective support of steel arch, several control measures for foot settlement have been developed. These measures and steel arch work together to prevent the tunnel settlement on the basis of playing the roles themselves. Whether these measures are safe and effective to support earth pressure are the main concerns in design. However, the current designs for some of these measures are immature, and there is a lack of corresponding design theories. Therefore, it's necessary to conduct thorough researches so as to provide valid theory support for the designs. The thesis focused on the work mechanisms and bearing behaviors of feet-lock pipe, longitudinal support beam (LSB) and temporary invert (transverse support), and gave the corresponding analytical methods through load-structure method. The main contents are as followed.(1) The mechanical model of feet-lock pipe combined with steel arch was first established by considering the load transmission and deformation compatibility between the two members. The results from this model were compared with those from the existing model and field measurements. Possible values of load acting on the steel arch were also discussed.(2) In view of the inconsistent understanding about reasonable installation angle of the feet-lock pipe, theoretical expression between the installation angle and crown settlement of the steel arch were derived and proved that the reasonable installation angle should be greater than or equal to 45° when rise-span ratio of the upper-section is less than 0.3, on condition that the steel arch can get enough support reaction along axial direction of the feet-lock pipe. This conclusion was consistent with those from many engineering practices. For safety consideration, the installation angle was suggested to be 30° when the rise-span ratio is 0.3, and the installation angle was suggested to be greater than or equal to 45° when the rise-span ratio is greater than or equal to 0.5.(3) Based on the work above, the mechanical model of feet-lock pipe combined with steel arch under unsymmetrical earth pressures (UEP) was first established. The force and deformation characteristics of feet-lock pipe under UEP were illustrated. Increasing the stiffness and installation angle of feet-lock pipe on the deep buried side were suggested to control the differential settlement between the two tunnel feet.(4) In view of the weakness of the Winkler beam model on elastic foundation, a two-parameter foundation model of Pasternak which can reflect the continuity of the foundation was first established to model the feet-lock pipe, and applied in the holistic analysis of the feet-lock pipe combined with steel arch. The influences of subgrade coefficient and shear stiffness of the foundation on the mechanical behaviors of feet-lock pipe were investigated through examples and comparison with the Winkler model.(5) The parameters of feet-lock pipe in current application are some unreasonable, which is unbeneficial for the support effect and economy. After considering the anti-shear factor of feet-lock pipe, foot settlement, subgrade reaction and steel consumption, the sizes of feet-lock pipe in the project example were improved to obtain better performance in support effect and economy. Finally, the suggested design parameters of feet-lock pipe were given for the different geological conditions.(6) From perspective of foundation bearing capacity at the tunnel foot, the mechanical model for stability evaluation of the ground at the foot was first proposed based on the bearing mechanism of steel frame and feet-lock pipe. The calculation formulas for foundation load and settlement at the foot under symmetrical earth pressures (SEP) and UEP were derived respectively. Then the influences of different design parameters on the load transfer of ground at the foot were studied in an example tunnel. Furthermore, the influence of subsequent excavation below the upper-section on the settlement at the foot were also studied. The conclusions gave theoretical guidance for foot stability of the tunnel.(7) For the ground composed of soil or heavy weathering rock, it is more reasonable to consider the changes of subgrade coefficient along the depth of feet-lock pipe. For this consideration, the mechanical analysis of feet-lock pipe with changing subgrade coefficients was conducted. The differential equation based on the assumption of m method was solved by the finite difference method and power series method respectively, considering the advantages of the different methods.(8) Based on the analysis of the work mechanisms of longitudinal support beam (LSB) and the auxiliary support system of steel arch, the mechanical models were established respectively, and the internal forces and deformation of LSB were derived in detail. On these basis, the performance of LSB with different support conditions were investigated. This work can provide theoretical support for design of LSB.(9) The mechanical model of temporary invert (transverse support) was first established as a circular beam on elastic foundation considering the specific construction technology, and then was applied in the holistic analysis of temporary invert (transverse support) combined with steel arch considering the load transmission and deformation compatibility between the both members. The calculation formulas for loads acting on the temporary invert (transverse support), foundation load and settlement at the foot under SEP and UEP were derived respectively. On these basis, the performances of temporary invert (transverse support) under the influence of different factors were studied respectively.