Luận án reducing the covertodiameter ratio for shallow tunnels in soft soils

Reducing the cover-to-diameter ratio for shallow tunnels in soft soils Reducing the cover-to-diameter ratio for shallow tunnels in soft soils Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. Luyben, voorzitter van het College voor Promoties, in het openbaar te verdedigen op maandag 12 september 2016 om 12:30 uur door Minh Ngan VU Civiel ingenieur Nationale Universiteit van Civiele Techniek, Hanoi, Vietnam, geboren te Hanoi, Vietnam. Dit proefschrift is goedgekeurd door de promotor: prof. Bosch copromotor: dr.

Broere Samenstelling promotiecommissie: Rector Magnificus, voorzitter Prof. Bosch, Technische Universiteit Delft Dr. Broere, Technische Universiteit Delft Onafhankelijke leden: Prof. van Tol, Technische Universiteit Delft Prof.

Vo, Hanoi University of Mining and Geology Prof. Thewes, Ruhr-Universität Bochum Prof. Bezuijen, Universiteit Gent Prof. Rots, Technische Universiteit Delft, reservelid Overige leden: Dr.

Bakker, Technische Universiteit Delft Keywords: tunnelling, stability, tunnel lining, ground movement, volume loss Printed by: Ipskamp Printing, Enschede Copyright © 2016 by M. VU ISBN 978-94-028-0028-9 An electronic version of this dissertation is available at http://repository. All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, includ- ing photocopying, recording or by any information storage and retrieval system, without written consent from the author.

To Mai Lan, Minh Hang and Chinh Duong A BSTRACT Despite the fact that shallow tunnels have the benefits of low short-term construction costs and long-term operational costs primarily due to the shallow depth of the station boxes, the limited understanding of shallow tunnelling in soft soils is an obstacle to the development of shallow tunnels in urban areas. This study carries out a theoretical in- vestigation of the effects of reducing the cover-to-diameter ratio C /D for shallow tunnels in soft soils. In stability analysis, the uplift, face stability and blow-out mechanisms are investigated. This study investigates interactions between the TBM and surrounding soil in tunnelling process, the stability of the TBM is not taken into account.

The relationship between the C /D ratio and the required thickness-to-diameter ratio d /D as well as the required support pressures will be derived in various soils. Ranges of support pressures are also estimated for the TBM. Structural analysis is carried out for the variation of deformations and internal forces of the tunnel lining when reducing the C /D ratio. Since the conventional design models are not suitable in the case of shallow tunnels a new structural analysis model, which includes the difference between loads at the top and at the bottom of the tunnel, is pro- posed.

Optimal C /D ratios with various d /D ratios for shallow tunnels in soft soils are also derived. With respect to ground movement analysis, this research investigates the areas affected by shallow tunnelling with a preliminary assessment of the risk of building damage by investigating surface and subsurface soil displacements. These areas are determined for different tunnel diameters in various soil types and are then compared to recent studies. The total volume loss is estimated at the tunnelling face, along the TBM, at the tail and includes long-term consolidation settlements.

By combining empirical models from the literature and the proposed new models, the volume loss components are estimated both for short-term construction and for the long-term consolidation effects. This shows that a no volume loss is feasible in shallow tunnelling with careful control of the support pressure. The boundaries of the influence zones in shallow tunnelling are identified and discussed on the basis of various case studies. The effects of the soil parameters on the influence areas are also investigated.

From these calculations, the limits and optimal C /D ratios for shallow tunnelling are deduced and recommendations and solutions for improving the shallow tunnelling pro- cess are proposed in this dissertation. vii A CKNOWLEDGEMENTS I consider it an honour to work with Prof.Bosch and Dr. Ir Wout Broere in this research. Johan, your speech at the first meeting about PhD studies has been lived in my mind “You are here not only for your PhD study, the more important achievement is the improvement of yourself”.

It has changed my attitude of the PhD study. I have a special thank for Wout, who has worked patiently with me-a recruit in tunnelling-not only for discussing and assessing to my sudden and strange ideas, but also with special guidance and even English correction. Without your help, I think it would be impossi- ble to write this acknowledgement. Johan and Wout, your guidance and suggestions in research process are really wonderful and I would like to express my profound gratitude and appreciation to you.

The research in this dissertation was supported by the Ministry of Education and Train- ing of Vietnam (Project 322), Hanoi University of Mining and Geology, Geo-Engineering Section and Valorisation Centre in Delft University of Technology. I am very grateful for their support and for the opportunity to carry out this research. For the period of my PhD study, I am grateful for the time spent with roommates and colleagues in the GeoEngineering Section. Patrick Arnold, thanks for your kind help not only on many things in a PhD study such as Latex and Matlab, but also many life prob- lems.

Nor Hazwani Md. Zain, Rafael Rodriguez Ochoa, Rui Rui and Hongfen Zhao who made me feel comfortable. I will remember the time with colleagues in GeoEngineering during BBQ, drinking events and especially, football matches between the United Nation team from Geo-Engineering section and Vietnamese team in TU Delft. For the Vietnamese community in Delft and in the Netherlands, I cannot find words to express my gratitude to you.

I cannot image how I could live in Delft without you. Thanks for the help from Chi and Phuong when I first came here. VDFC is a wonderful football club, I have had many amazing moments in some tournaments. This work would never been completed and perhaps begun without the support from my family.

I would like to thank my papa, mama and my younger sister, Dieu for your support. My wife, Mai Lan, thank you so much for your love, support, encouragement and patience. For my daughter, Minh Hang, it is really happy to see you growing up every morning. Thanks to my son, Chinh Duong who breathes new life into my research.

ix C ONTENTS Abstract vii Acknowledgements ix 1 Introduction 1 1.1 Aims of this research .2 Outline of this dissertation. 4 2 Stability analysis of shallow tunnels 7 2.3 Failure body models .1 Literature review concerning stability of tunnel face .2 Wedge stability model. 37 3 Structural analysis of shallow tunnels 41 3.2 Structural lining design .1 Influence of load and overburden on lining models .2 Influence of ground-lining interaction .3 A case study of Second Heinenoord Tunnel .3 Impacts of overburden on tunnel lining. 58 4 Ground movements and effects on buildings 71 4.2 Ground movement definitions and risk assessment.1 Ground movement definitions .2 Risk of building damage assessment .3 Effects of the C /D ratio on surface settlement.4 Effects of the C /D ratio on subsurface settlement.

89 5 Volume loss in shallow tunnelling 91 5.2 Volume loss at the tunnelling face.3 Volume loss along the shield. 97 xi xii C ONTENTS 5.4 Volume loss behind the shield .1 Volume loss at the tail .2 Volume loss due to consolidation .5 Total volume loss and case studies .1 Total volume loss. 119 6 Impact factors of influence zones 121 6.2 On the variation of influence zones with different categories of damage risk assessment .3 Effects of soil parameters on influence zones. 132 7 Conclusions and Recommendations 135 7.2 Recommendations for future research.

139 Bibliography 141 A Blow-out model 151 A.1 Uniform support pressure .2 Linear support pressure with gradient δp. 153 B Ground movement 155 List of Symbols 157 Summary 161 Samenvatting 163 Curriculum Vitæ 165 1 I NTRODUCTION You are here not only for your PhD study, the more important achievement is the improvement of yourself.Bosch Although tunnels are often designed well below foundation level in urban areas, shallow tunnels have many benefits with regards to the short-term construction costs and the long- term operational expenses. There are, however, limits to shallow tunnelling in urban areas with soft soil conditions, which should be investigated and solved. This chapter provides an overview of the general background to shallow tunnelling, the aims of this research and the outline of this dissertation.

I NTRODUCTION The demand for infrastructure in urban areas is increasing due to economic devel- opments and the growth of urban populations. Even though the construction costs are higher, underground infrastructure is a sustainable and safe construction choice for cities. Tunnels have become an important part of public underground infrastructure all over the world. Tunnel boring machines (TBM) are widely used in the construction of underground in- frastructure in urban areas due to the fact that disturbance at surface level can be re- duced significantly during the construction and their ability to limit settlements and damage to existing buildings.

In an environment with soft overburden, particularly in soft Holocene layers, buildings are generally built on pile foundations. The tunnel is of- ten designed well below the pile tip level. There are two reasons for doing this: to reduce interaction between the tunnelling process and the piles, and to avoid having to drive through old abandoned piles that are still present below the streets. This results in rela- tively deep track tunnels and also in deep station boxes.

1 When the tunnels are located at more shallow levels, above the pile tip level, this largely eliminates the impact on the pile bearing capacity due to the ground movement at the tip of the piles. This then reduces the required depth of the station boxes and therefore also the construction costs. Other benefits are the low operational cost in the long-term and the shorter travelling time from the surface to the platforms. Tunnelling in such conditions is only possible if there are no or hardly any obstacles in the subsurface of the streets.

A shallow tunnel, with a low cover-to-diameter ratio C /D may introduce un- foreseen or new limits, for example related to the face stability, the lining structure or ground movements and the subsequent impact on nearby structures. Also, the stability of the TBM and the tunnelling process may become an issue. For this reason, the focus of this study is on the impact of shallow tunnelling in soft soils. Firstly, the properties of the soil around the tunnel have important effects on the tun- nelling face stability.

With a shallow cover, if the support pressures at the tunnelling face are too small, the tunnelling face will collapse and the soil will move towards the TBM. When the support pressures are too large, this leads to problems of uplift, blow-out or fracturing. Furthermore, the relatively large difference between the support pressures at the top and the bottom of the tunnel and the relatively small bandwidth between the maximum and minimum support pressures, compared to moderate and deep tunnels, should be taken into account. Secondly, reducing the C /D value leads to a change in the overburden load on the tun- nel lining.

A common method used in structural tunnel design has been proposed by Duddeck and Erdmann (1985). Both his continuum model and the model without a re- duction in ground pressures at the crown are valid for moderate and deep tunnels with a depth C ≥ 2D. In shallow tunnels with a C /D ratio of less than 1, the overburden pres- sure on the crown and the invert is significantly different and the loads, which are used in Duddeck’s models, will not be realistic. Thirdly, underground construction in urban areas often leads to negative effects on exist- ing structures on the surface and on subsurface structures.