| Nowadays,a large number of long tunnels have been built in China.The construction quality and defects of the tunnels affect the stability and operational safety of the tunnel.The important factor that determines the stability of the tunnel is the quality of the invert structure.The tunnel invert is a concealed structure made by reinforced concrete,which is interspersed between the tunnel packed layer and the bed rock.Due to construction period,environmental restrictions and insufficient inspection and monitoring,construction defects of the invert can be caused.Therefore,it is necessary to inspect the construction quality of the invert in the tunnel acceptance process.A variety of detection methods have been used in the quality inspection of inverted structures.The drilling core and the ground penetrating radar are two popular methods,but difficulties and limitations in their application affect detection precision and inspection efficiency for the engineering needs.In this paper,the multi-channel analysis of surface wave method(MASW)is used to detect the tunnel invert structure.Surface wave propagates near the surface with strong amplitude,slow attenuation and dispersion.This method uses multi-channel array geophones to acquire Rayleigh surface wave data for inverting underground layered elastic parameters.Base on a physical model of invert structure,the dispersion of the Rayleigh wave are analyzed.Different from the conventional stratum model structure,the invert structure is a strong non-dispersive model.Therefore,a complex root is the solution of the dispersion equation,which corresponds to the leakage mode wave.For stable calculation,I only use the real part to obtain the corresponding dispersion curve.The elastic wave field propagation is simulated by the finite difference method.The obvious leakage wave influence and "mode jumping" phenomenon appear on the dispersion spectrum.So it is difficult to extract the fundamental and high mode dispersion curves from the observed data.In this paper,F-K spectrum is used as the objective function and Bayesian method is used to invert the invert structure parameters,such as height and velocity.In order to verify the effectiveness of the multi-channel analysis of surface wave method for detection of the tunnel invert structure.A synthetic model is used to test inversion sensitivity.The method is well sensitive to the thickness and velocity parameters of the invert structure,and the results are consistent with the model.Data acquisitions were carried out in tunnels under construction,and field data were processed.The results show that the MASW method can determine the thickness and mechanical modulus of the invert structure.The observation instrument is improved for convenient performance and high efficiency.The MASW method,a non-destructive testing(NDT),can be applied to the quantitative evaluation of the tunnel invert structure,and can also be applied to the detection of similar model structures,such as railway tunnels and road structures. |
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