Mechanized tunneling induced ground movement and its dependency on the tunnel volume loss and soil properties

peer reviewedThis study investigates the ground movements due to mechanized tunnel excavation by applying two-dimensional finite element analyses. To assess the contribution of the compressibility and plasticity of the soil on the ground movements, different constitutive models are employed to describe the soil behavior. The influence of volume loss around the tunnel on the surface volume loss is investigated, and a quadratic correlation between them is proposed. Consequently, the empirical Gaussian distribution curve, which is generally used to determine the tunneling induced settlement trough, is improved by applying the proposed quadratic correlation between surface volume loss and tunnel volume loss. Furthermore, the settlement trough width parameter has been derived by a linear function of tunnel volume loss as well. The proposed equations are validated via a case study of centrifuge tests from the literature. The results show that the proposed modification enhances the empirical solution by having better knowledge on the model parameters. Additionally, tunnel overburden and coefficient of lateral earth pressure at rest (K 0 ) are taken into account to study their influence on the tunneling induced surface settlements. Finally, global sensitivity analysis is applied to evaluate the relative importance of corresponding model parameters in terms of their influence on the ground movements

Numerical investigation of tunneling in saturated soil: the role of construction and operation periods

peer reviewedThis paper numerically investigates the slurry shield tunneling in fully saturated soils with different hydraulic conductivities in short- and long-term scales. A fully coupled hydromechanical three-dimensional model that accounts for the main aspects of tunnel construction and the hydromechanical interactions due to tunneling process is developed. An elasto-plastic constitutive model obeying a double hardening rule, namely hardening soil model, is employed in the numerical simulations. The research mainly focuses on assessing the influence of soil hydraulic conductivity and the method to simulate backfill grouting in the tail void on the evolution of ground subsidence, excess pore water pressure and lining forces. Two different consolidation schemes have been taken into account to computationally address the tunnel construction in soil with low and high hydraulic conductivities. In addition, different methods are employed to simulate the tail void grouting as a hydromechanical boundary condition and to study its effects on the model responses. Finally, the influences of infiltration of the fluidized particles of grouting suspension into the surrounding soil and its corresponding time–space hydraulic conductivity evolution on the displacements and lining forces are studied

Model validation and calibration via back analysis for mechanized tunnel simulations - The Western Scheldt tunnel case

peer reviewedIn this research, Finite Element (FE) method is applied to simulate the shield supported mechanized excavation of Western Scheldt tunnel in the Netherlands. Both 2D and 3D numerical models are created to predict the system behavior. Sensitivity analysis and parameter identification techniques are utilized to calibrate and validate the model based on field measurement. The mechanical behavior of the soil is modeled by an advanced elasto-plastic model, namely Hardening Soil model correlating small strain stiffness (HSS). Global sensitivity analysis is carried out in this paper to evaluate the relative sensitivity of model response to each input parameter. Thereafter, a parameter identification technique (back analysis) is employed to find the optimized values of the selected parameters. To accomplish this, the computationally expensive FE-model is replaced by a meta-model in order to reduce the calculation time and effort. Moreover, a soft soil constitutive model based on the modified Cam-clay model deals with primary compression of fine grained soils, is assigned to the clay layer to further improve the numerical prediction of system behavior. Due to the importance of model subsystems, such as face pressure and volume loss, the sensitivity of model response to subsystems has been evaluated. The results show that optimized parameters obtained via back analysis make the numerical simulation capable to well predict the ground settlement

Prediction of tunnel lining forces and deformations using analytical and numerical solutions

peer reviewedStructural design of linings requires a reliable prognosis of lining forces and deformations. In engineering practice, both analytical and numerical solutions are popular to be employed to predict the system behavior. This paper employs the commonly accepted analytical solutions to calculate the lining forces and deformations for both shallow and deep tunnels, the results are compared to the numerical results for corresponding equivalent boundary conditions, initial conditions and identical material properties. Afterward, more sophisticated constitutive models for soil/structure elements in conjunction with more realistic construction aspects are taken into account. The comparison of the results of analytical and numerical solutions highlights the differences between these two well accepted methods as well as the effect of considering realistic features in numerical simulations. Moreover, the lining forces and deformations obtained from plain strain condition are compared to the 3D numerical results. The results show that the analytical bedding model is able to reasonably predict the lining behavior for both shallow and deep tunnels even if the soil is assumed to be an elastic material. In numerical solutions, lining forces and deformations depend to a large extent on the applied soil constitutive model and construction method. The face support pressure, backfill grouting and arching effect cannot be captured appropriately in plain strain condition, which leads to the discrepancy between the model responses obtained from 2D numerical/analytical solutions and realistic 3D simulations

Sequence of footfalls in elephant walk after [5].

<p>The static loading conditions (loading steps 1 to 4) simulated by FEA are marked and quantified within the sequence. The leftmost loading step is loading step 1, with the elephant at a standstill. Black bars indicate ground contact of the respective foot. fl  =  left forefoot, fr =  right forefoot, hl  =  left hindfoot, hr  =  right hindfoot. See text for a detailed description of the loading steps.</p

Hardening soil model parameters.

<p>Hardening soil model parameters.</p

Capture of elephant footprints geometry using 3D laser scanner.

<p>A total of six footprints were scanned, i.e., three pairs, each of them consisting of one forefoot imprint (right) and one hindfoot imprint (left). Each pair is pictured by a photograph (top), 3D surface plot (center), and a 2D longitudinal section plot (bottom).</p

2D-plot of relative density versus settlements for back analysis of applied stress [kN/m²] by FEA for a circular plate (d = 0.32 m).

<p>The diagram applies to subsoil conditions of Rhine sand. According to the deformation characteristics illustrated at the top right corner of the diagram, blue curves apply to the flexible loading characteristics of the elephant's foot, and the green curve ( = 350 kN/m² loading step 5) applies to rigid loading characteristics used in the FEA model. The relationship is detailed in the text. The range of stresses that can be back-calculated from in situ conditions of relative density of subsoil (0.3 and 0.47) and measured values of <i>s</i> (20.28 mm, 21.16 mm, and 26.32 mm) is marked by a box.</p

Triaxial test results for the determination of stiffness E₅₀ [kN/m²] of Rhine sand with an initial density of e = 0.6.

<p>Blue, green and grey line: Deviatoric stress is plotted against axial strain for experiments conducted at 50 kN/m², 100 kN/m², and 150 kN/m² confining pressure, respectively. The stiffness E₅₀ is the secant stiffness over the first 50% of the deviatoric stress.</p

3D-plot of relative density versus settlements for back analysis of applied stress [kN/m²] by FEA for a circular plate (d = 0.32 m).

<p>The diagram applies to subsoil conditions of Rhine sand. This diagram can be used to estimate the load having produced a fossil footprint if the original subsoil parameters were the same as our experimental subsoil, Rhine sand.</p