Research Of Deformation Monitoring Of Mountain Tunnel During Construction Based On The 3D Laser Scanning

The highway tunnel construction in China has experienced rapid development nowadays,and the geological conditions confronted during tunneling are becoming more complicated,whereas the monitoring and measurement technology for mountain tunnel construction has lagged behind.In this study,the 3D laser scanning technology was applied to monitor the deformation of mountain tunnels during construction with focus in three aspects,tunnel monitoring method,point cloud data processing and deformation analysis,application in engineering practice.In contrary to traditional single-point monitoring methods,the 3D laser scanning technology with the efficient and non-contact measurement is more competent in guiding the dynamic design of the NATM tunnel.According to the deformation monitoring principle of 3D laser scanning technology,the performance of two tunnel point cloud data registration methods were compared through field tests.Although the proposed plate target can be easily used in the mountain tunnel,the registration accuracy is approximately 3.4mm.The times of error propagation of the rear view method is reduced,and consequently the registration accuracy reached 2mm.Compared with traditional monitoring technology,3D laser scanning technology has the advantages of full spatial deformation analysis,high efficiency and non-contact measurement,but has the limitations of more error sources and low precision.The tunnel deformation analysis method based on point cloud data is studied.The central axis of the tunnel is fitted by the plane projection method.The projection of the central axis on the XOY plane is determined by the perpendicularity of the axis at a certain point perpendicular to the point cloud boundary.The projection of the central axis on the XOZ plane is determined according to the dome curve.The cloud data of a certain thickness is intercepted and projected to the tunnel section,and then the convergence in any direction can be obtained by constructing the polar coordinate system of the section.Aiming at massive point cloud data,a method for calculating the full space deformation field based on moving average method is proposed.Through the artificial 20mm radial convergence test,it is verified that the method can maintain the correct recognition rate of over 78%with the increase of the data reduction ratio.Different from the random sampling method and the curvature sampling method,the proposed method can maintain the tunnel deformation characteristics.Finally,the proposed method was applied to the K38+385-K38+421 segment of the Whenzhou Nanshan Tunnel to monitor the full space deformation field of class IV surrounding rock.According to the spatial time transformation relationship of the deformation field,the final convergence value of the section is evaluated be 27 mm.Compared with the traditional monitoring methods,the 3D laser scanning technology has error factors such as registration error,scanner resolution,shotcreting engineering and measuring shadow,and this is not suitable for monitoring small deformation fields below 2mm.The back analysis of the surrounding rock parameters of the tunnel gives the elastic modulus E=400MPa,the cohesion force c=150kPa,and the internal friction angle ?=25° as the equivalent parameters,which can be used as the basis for optimization design.Given the obtained parameters,the thickness of the primary lining can be reduced to 20cm,steel arch spacing can be increased to 1.4m,which can reduce the engineering cost while meeting the requirements of the design code.Using the longitudinal deformation profile of the tunnel,it is preliminarily judged that the optimal support time for the second lining is 25 days after excavation.