Research On The Evaluation Technique Of Cable Tunnel Load Bearing Characteristics Under Crack Defects

As a common form of tunnel,cable tunnels are concealed underground works,and external works such as foundation excavation and adjacent tunnel construction pose a threat to the structural safety of cable tunnels,which are susceptible to cracking hazards and affect their load-bearing performance.This thesis takes a single-layer lined cable tunnel in Shijiazhuang city as the research object,and uses numerical simulation to study the cracking characteristics and load-bearing performance of the cable tunnel under the influence of foundation excavation and shield construction disturbance,and proposes a tunnel load-bearing performance evaluation scheme based on the numerical simulation results.The main research contents and research results are as follows:(1)The Cohesive Zone Model(CZM)based on damage fracture mechanics is used to simulate tunnel lining cracking,taking into account the advantages and disadvantages of the common crack simulation methods.The principal structure and mechanism of the CZM model are explained,and the method and principle of global embedded cohesive unit based on the cohesive force model are summarised.(2)Based on the relevant research,a simulation replication of the three-point bending test for plain concrete beams and four-point bending test for reinforced concrete beams was carried out using cohesive units to achieve the calibration of tunnel lining CZM cracking parameters and validation of the model.(3)A structural model of the tunnel load was established and the process of mapping the displacement field of the lining units in the differential software to the Abaqus tunnel crack model was implemented and verified for correctness with the help of a Python secondary development based on the idea of displacement interpolation.(4)Three types of external works are studied: new foundation excavation disturbance,new shield vertical underpass construction and horizontal underpass construction.The foundation excavation disturbance mainly includes different excavation depth,different burial depth of tunnel,different spacing and different section size working conditions.The new shield construction includes tunnel-tunnel different spacing,cable tunnel different burial depth,cable tunnel different size and other study conditions.The cracking and load carrying characteristics of the cable tunnel are analysed for each condition.The process of lining crack emergence,expansion to instability is systematically described in terms of initiating load,crack distribution form,crack characteristics,expansion law and load carrying limit state.The concept and quantification formulae of the residual load carrying capacity of the tunnel are proposed and the residual load carrying capacity of the tunnel under different working conditions is calculated.(5)A regression prediction model between the commonly used engineering evaluation indicators and the residual load carrying capacity is constructed,and three classes of tunnel load carrying performance are classified using the residual load carrying capacity as the criterion.The indicators for judging the tunnel bearing performance levels are determined based on the importance of the influencing factors in the prediction model,and an evaluation scheme for the tunnel bearing performance is proposed.