Andrea Franza

Discussion of “Observation of Ground Movement with Existing Pile Groups Due to Tunneling in Sand Using Centrifuge Modelling” by Ittichai Boonsiri and Jiro Takemura

This discussion aims to highlight the underlying cause of several aspects of the greenfield settlement data presented by Boonsiri and Takemura (Geotech Geol Eng 33(3):621–640, 2015). The discussion considers, for the geotechnical centrifuge tests that were reported, the effects of the boundary conditions imposed at the model tunnel on resulting settlements. Data obtained using the rigid boundary model tunnel in Boonsiri and Takemura (Geotech Geol Eng 33(3):621–640, 2015) are compared against other available data from tests using a fluid-filled flexible membrane model tunnel. It is demonstrated that the boundary conditions used to simulate tunnel ground loss have an important impact on the settlement mechanism; compared to a fluid-filled flexible membrane, a rigid boundary model tunnel results in wider settlement troughs, which do not vary in shape considerably with changes in relative tunnel depth, and can result in higher ratios between the area of the settlement troughs and the tunnel ground loss. The appropriateness of the different tunnel boundary conditions is also discussed.

Centrifuge Modelling of Tunnelling beneath Axially Loaded Displacement and Non-Displacement Piles in Sand

Tunnelling under piled structures is becoming more common in urban areas. However, there is limited guidance available for the prediction of settlements and the loss of bearing capacity of existing piles due to tunnel excavation. This paper aims to provide an improved understanding of the response to tunnelling of axially loaded displacement and non-displacement piles. Data are provided from a series of geotechnical centrifuge tests of tunnel excavation beneath single piles in dry silica sand. The tests evaluate induced settlements of the piles at varying levels of initial safety factor (i.e. the ratio between initial bearing capacity and applied load). Furthermore, a previously published analytical approach, based on cavity expansion theory, is used to investigate the variation of the residual safety factor at constant pile load with tunnel volume loss. The outcomes of both centrifuge and analytical investigations illustrate the importance of pile installation method and initial safety factor and improve understanding of tunnel-pile interaction mechanisms.

The Effects of Relative Tunnel Depth and Volume Loss on Vertical Settlements above Tunnels in Dense Sands

© ASCE. Past studies have provided data on the variation of settlement distribution above relatively shallow tunnels in sands. There is, however, a lack of research that considers the case of relatively deep tunnels. This paper compares the results of a set of plane-strain centrifuge tests in dry dense sands. The cover-to-diameter ratio, C/D, of the tunnels ranged between 1.3 and 6.3, thereby including relatively shallow and deep tunnels. Ground movements are measured using an image-based measurement technique. Gaussian curves are fitted to the settlement data in order to evaluate the characteristics of the settlement profiles. An assessment of greenfield settlement trough shape, both at the surface and subsurface, is carried out. The effect that relative tunnel depth and volume loss has on the settlement trough shape is demonstrated and discussed. The results indicate a non-linear trend of settlement trough shape with C/D, which suggest a transition between shallow and deep tunnels within the investigated C/D range. To account for highlighted results, new relationships are proposed to estimate settlement trough shape parameters in dense sands.

A Winkler-Based Method for the Assessment of Tunnelling-Induced Deformations on Piled Structures

© ASCE. In urban areas it is often required to assess the deformation distributions induced by tunnel construction on piled buildings. In this paper, a simple analysis method is presented for estimating these deformations. The proposed method is based on a two-stage procedure: (1) estimation of the greenfield ground movements caused by the tunnel excavation, and (2) analysis of the foundation/building on elastic springs subjected to a system of forces induced by the greenfield ground movements. Simple closed-form expressions are used for the evaluation of spring stiffnesses and tunnelling-induced forces. The proposed method is validated by comparing its predictions with those from 3D finite element analyses. It is shown that the simplified analysis method provides a good assessment of building deformations for several cases of tunnelling beneath piles and relative soil-structure stiffnesses. The proposed method represents a useful tool for preliminary parametric analyses due to the reduced computational cost and simple yet versatile implementation compared to 3D numerical analyses.