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In this article the face stability of slurry shield-driven tunnel in an aquifer is investigated. The influences of slurry infiltration at the face and soil layering are taken into account. The results show that due to the infiltration an additional margin for the support pressure is required. At larger slurry infiltration distance or larger cover depth to tunnel diameter ratio (C/D), a higher margin is required. It also appears that soil layering affects the hydraulic gradient that stabilise the face of cohesionless soil. TBM excavation in a semi-confined aquifer (compared to an unconfined aquifer) leads to higher excess pore water pressures in the soil around the TBM and a reduced infiltration of slurry into the soil in front of the TBM. The excess pore water pressures in a semi-confined aquifer are present over a larger area than in an unconfined aquifer, increasing the risk of a blow-out during excavation.

This article presents a simple numerical investigation of ground responses to tunnel-boring machine (TBM) tunnelling with active face support. Unlike other numerical modelling in which a flow inward the excavation chamber was assumed, in this modelling a flow inward the ground surrounding the face was assumed. The later flow is caused by the penetration of slurry (for slurry shield) or foam (for EPB). The effect of soil layering (semi-confined and unconfined aquifers) was also taken into account. The numerical results show that the ground response to TBM tunnelling with active face support can be significantly influenced by soil layering. In a semi-confined aquifer, at a distance larger than approximately three times of tunnel diameter from the face, the excess piezometric heads are the same at any depths. This finding is helpful to improve the solution for the excess pore pressure caused by TBM tunnelling in a semi-confined aquifer proposed by Xu et al. (2019). The existing analytical solution of excess pore water pressure distribution for the case of semi-confined aquifer matches well with the numerical result. It appears that for an unconfined aquifer, contour of the excess pore water pressures is like a radial distribution. The steady state model of piezometric head for the homogeneous soil results in excellent matches with the values derived from the numerical modelling. For a semi-confined aquifer, the ground surface heave was observed. Though the face collapse potential was found for the unconfined aquifer, the displacement around the face was very small.

Mechanized tunnelling with EPB-TBMs bases its success on the ability to guarantee a uniform and constant front-face pressure during the excavation, using the excavated soil properly treated with chemical additives. This front-face pressure, measured with pressure sensors inside the working chamber, is kept constant during the excavation phase by balancing the amount of incoming soil, the foam injected, and the amount of soil extracted through the screw conveyor in the working chamber. During the standstill phase, required to install the tunnel lining elements, particularly in the coarse-grained soils, the pressure inside the chamber tends to decrease until the next resumption of the excavation. This effect, mainly linked to the permeability of the soil outside the TBM, tends to increase settlements induced on the surface and, therefore, should be minimized. In this paper, a very simple calculation model proposed by Bezuijen and Gerheim Souza Dias in 2017 for tunnelling in saturated sandy soils, is applied to an Italian tunnelling project. A comparison of model predictions and data recorded real time on the site during the excavation is illustrated and discussed. The predictive ability of the calculation model proposed for saturated sand seems to be useful also for TBM applications in gravelly soil under the phreatic surface, highlighting the weight of the main factors affecting the specific boundary value problem; moreover, it can be a useful tool to address the problem and to individuate technical countermeasures finalized to minimize the phenomenon and its negative effects.

© 2017 Elsevier Ltd This work investigates the effect of the rotational component of input motion induced by the kinematic interaction between a pile group and the surrounding soil on the seismic behaviour of a structure. To this end, a simple analytical model is developed by deriving the pile group behaviour from the seismic response of a single pile, taking into account equilibrium and compatibility of displacements at piles’ heads. Closed-form solutions in the frequency domain are provided for both the translational and the rotational motion of a group of unevenly distributed identical piles, rigidly connected at the top and displaced by the surrounding soil, which is subjected to purely translational oscillations. The proposed solutions, applicable to any subsoil conditions, highlight that pile group layout is the crucial parameter governing the magnitude of the foundation rotation. Further, new transfer functions from the soil surface in free field conditions to the top of a SDOF system are introduced, which take into account the translational and/or rotational kinematic effects. An application of the above concepts to a case study is presented, highlighting that the rotational component of input motion may be important for tall structures on small pile groups.