Convergence monitoring is of great importance to evaluate the tunnel health condition. This paper introduces a real-time convergence monitoring system of shield tunnel with straight joints based on wireless tilt sensors. Wireless tilt sensors have the advantages of real-time monitoring, flexibility and ease of installation, no interruption to tunnel operation and no demand of inter-visibility. A model of calculating horizontal convergence from tunnel inclination for different diameter tunnels is proposed. Optimal sensor measurement positions are adopted to improve the accuracy of the proposed model. For a shield tunnel with six segments, one wireless sensor is installed at the optimal position of 123° with a maximum relative error of 6%. For a large diameter shield tunnel with ten segments, two wireless tilt sensors installed at the optimal positions of 107° and 143° are obtained. The absolute errors of tunnel horizontal convergence for the two sensors scheme are less than 0.33 mm, which shows the high precision of this scheme. The proposed model is verified for tunnels with asymmetric deformation induced by surcharge load. The proposed model has a maximum relative error of 4.5% in layered ground. A field monitoring case of Shanghai metro tunnel proves the applicability of the proposed method.

Wireless Underground Sensor Networks (WUSNs), an important part of Internet of things (IoT), have many promising applications in various scenarios. Signal transmission in natural soil undergoes path loss due to absorption, radiation, reflection and scattering. The variability and dynamic of soil conditions and complexity of signal attenuation behavior make the accurate estimation of signal path loss challenging. Two existing propagation models for predicting path loss are reviewed and compared. Friis model does not consider the reflection loss and is only applicable in the far field region. The Fresnel model, only applicable in the near field region, has not considered the radiating loss and wavelength change loss. A new two stage model is proposed based on the field characteristics of antenna and considers four sources of path loss. The two stage model has a different coefficient m in the near field and far field regions. The far field distance of small size antenna is determined by three criteria: 2 D²/λ, 5 D, 1.6 λ in the proposed model. The proposed two stage model has a better agreement with the field experiment data compared to Friis and Fresnel models. The coefficient m is dependent on the soil types for the proposed model in near field region. It is observed from experiment data that the m value is in the range of 0~0.20 for sandy soils and 0.433~0.837 for clayey silt.

Soil disturbance has always been the major concern in shield tunneling activity. This paper presents the investigation on the micro-scale responses of the soils during shield tunnel excavation in sandy-cobble stratum. The code paraEllip3d is employed in discrete element method (DEM) analysis in which the soils are mimicked as an assembly of ellipsoids. Triaxial tests on the micro-scale responses of cobbles are carried out using the materials sampled from the tunnel face during construction period, and corresponding DEM simulations are performed to calibrate the micro parameters for the ellipsoids. On this basis, the face instability process during the shield tunneling in cobbles is studied using 1 g model test as well as corresponding DEM simulation. The micro-scale responses of cobbles are investigated by triaxial test as well as corresponding DEM simulations. Multiple material responses are discussed in the DEM simulations, including the stress–strain relationship, the contact distribution, and the force chain evolution in the elementary and model test. Finally, the mechanism of tunnel face instability in cobbles are discussed on the basis of aforementioned investigations.