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Tunneling inevitably induces changes in soil stress and, hence, may adversely affect the surrounding ground and nearby existing tunnels. A model test and a series of numerical analyses were carried out to investigate the response of the ground and existing tunnels in sandy soil to nearby tunneling according to the relative spacing. The test results showed that increasing the distance-to-diameter ratio (d/D) changed the settlement trough from W-shaped to U-shaped along the transverse section. In addition, the major settlement area induced by each advancing step along the longitudinal section was within 0.75D in front of the tunnel face. The numerical results showed that the overall downward movement of the existing tunnel was due to the combined action of soil pressure and excavation unloading. In this article, the mechanism of the W-shaped surface settlement is discussed; a modified Peck formula is used to describe the W-shaped settlement trough, and the relation of typical parameters to d/D is analyzed. [ABSTRACT FROM AUTHOR]

Many uncertain factors in the excavation process may lead to excessive lateral displacement or overlimited internal force of the piles, as well as inordinate settlement of soil surrounding the existing bridge foundation. Safety control is pivotal to ensuring the safety of adjacent structures. In this paper, an innovative method is proposed that combines an analytic hierarchy process (AHP) with a finite element method (FEM) to reveal the potential impact risk of uncertain factors on the surrounding environment. The AHP was adopted to determine key influencing factors based on the weight of each influencing factor. The FEM was used to quantify the impact of the key influencing factors on the surrounding environment. In terms of the AHP, the index system of uncertain factors was established based on an engineering investigation. A matrix comparing the lower index layer to the upper index layer, and the weight of each influencing factor, were calculated. It was found that the excavation depth and the distance between the foundation pit and the bridge foundation were fundamental factors. For the FEM, the FE baseline model was calibrated based on the case of no bridge surrounding the foundation pit. The consistency between the monitoring data and the numerical simulation data for a ground settlement was analyzed. FE simulations were then conducted to quantitatively analyze the degree of influence of the key influencing factors on the bridge foundation. Furthermore, the lateral displacement of the bridge pile foundation, the internal force of the piles, and the settlement of the soil surrounding the pile foundation were emphatically analyzed. The most hazardous construction condition was also determined. Finally, two safety control measures for increasing the numbers of support levels and the rooted depths of the enclosure structure were suggested. A novel method for combining AHP with FEM can be used to determine the key influencing aspects among many uncertain factors during a construction, which can provide some beneficial references for engineering design and construction. [ABSTRACT FROM AUTHOR]