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Purpose: This study establishes a method for predicting ground vibrations caused by railway tunnels in unsaturated soils with spatial variability. Design/methodology/approach: First, an improved 2.5D finite-element-method-perfect-matching-layer (FEM-PML) model is proposed. The Galerkin method is used to derive the finite element expression in the ub-pl-pg format for unsaturated soil. Unlike the ub-v-w format, which has nine degrees of freedom per node, the ub-pl-pg format has only five degrees of freedom per node; this significantly enhances the calculation efficiency. The stretching function of the PML is adopted to handle the unlimited boundary domain. Additionally, the 2.5D FEM-PML model couples the tunnel, vehicle and track structures. Next, the spatial variability of the soil parameters is simulated by random fields using the Monte Carlo method. By incorporating random fields of soil parameters into the 2.5D FEM-PML model, the effect of soil spatial variability on ground vibrations is demonstrated using a case study. Findings: The spatial variability of the soil parameters primarily affected the vibration acceleration amplitude but had a minor effect on its spatial distribution and attenuation over time. In addition, ground vibration acceleration was more affected by the spatial variability of the soil bulk modulus of compressibility than by that of saturation. Originality/value: Using the 2.5D FEM-PML model in the ub-pl-pg format of unsaturated soil enhances the computational efficiency. On this basis, with the random fields established by Monte Carlo simulation, the model can calculate the reliability of soil dynamics, which was rarely considered by previous models. [ABSTRACT FROM AUTHOR]

In tropical and subtropical granite regions, quantities of boulders were encountered by the tunnel boring machine (TBM) threatening construction safety. The detection of potential boulders in the driving path of a TBM before construction begins in this region has always been a critical issue. However, boulder detection seems to be aimless owing to a lack of recognition of the boulder distribution law. In this study, the process by which boulders form and evolve is analyzed on the basis of the weathering mechanism of the strata. From this, a general law is hypothesized: on the vertical axis, boulder frequency decreases exponentially with increasing of vertical distance from the bedrock. Data from the literature prove the reasonability of this hypothesis. Subsequently, on the basis of this law, a simple approach for forecasting high-probability zones of boulder occurrence in TBM tunnel engineering was proposed, which was then applied to two newly built tunnels. The field data showed that all boulders were spotted in the forecasted high-probability zones, which verifies the effectiveness of this approach. Significant cost-savings will occur when more boreholes or geophysical prospecting are used in the forecasted high-probability zones rather than all over the tunnel line aimlessly. [ABSTRACT FROM AUTHOR]

Ground vibration and noise induced by underground railways have become a major environmental issue. Significant research has been carried out to investigate vibrations generated by a single tunnel, with less attention been paid to the dynamic interaction of twin tunnels. This paper presents a three-dimensional analytical model for the dynamic interaction of twin tunnels in a homogeneous half-space. The two tunnels are modeled using the cylindrical thin-shell theory. The soil, a homogenous half-space with two cylindrical cavities, is simulated using the elastic continuum theory. The translation between two cylindrical waves and the transformation between plane waves and cylindrical waves are introduced to satisfy the boundary conditions on the ground surface and the twin tunnel-soil interfaces. The accuracy of the proposed model is verified by comparison with numerical models. A numerical example is presented to investigate the influence of the dynamic interaction between twin tunnels on the ground vibrations. [ABSTRACT FROM AUTHOR]