Effects of existing surface structures on settlements induced by tunneling

Reliable prediction of potential structural damage caused by tunneling-induced ground movements is a very important factor in assessing the environmental impacts and costs of urban soft ground tunnel projects. Conventional practice relies on prior empirical data to estimate ground movements for "greenfield" conditions (no structures present) and then evaluates the effects of these displacements on overlying structures. This approach can be highly conservative (i.e., grossly overestimates the potential structural damage), imposing unrealistic or unnecessary constraints on the tunnel design or leading to unrealistic estimates for remedial costs. The alternative approach is based on predictions from non-linear finite element analyses that are highly sensitive to modeling of stress changes caused by tunnel construction and soil behavior. A recent design procedure presented by Potts and Addenbrooke (1997) has been used to evaluate effects of soilstructure interaction in the construction of the Jubilee Line Extension project in London. Their procedure uses simplified 2D FE models and simulates an overlying structure as a deep elastic beam (with equivalent axial and bending stiffness) with soil properties typical of conditions in London clay. This thesis provides an independent assessment of the results presented by Potts and Addenbrooke (P&A). Underlying assumptions regarding the ground loss at the tunnel cavity, the initial Ko stress state, the modeling of soil behavior and drainage conditions are all examined using the non-linear 2-D FE program Plaxis. Although these analyses confirm some key findings, such as the linearity between ground loss and centerline surface settlement, they also show that the initial Ko value, drainage conditions and modeling of non-linear soil stiffness are all important factors in predicting the ground movements. Analyses using the Hardening soil model are successful in replicating the modification factors proposed by P&A for modeling effects of soil-structure interaction for a shallow tunnel in London clay. Although it is not possible to develop generic design methods for different ground conditions, the P&A design procedure offers a logical methodology that can be applied with other site-specific soil profiles. Further research is needed to address the underlying uncertainties in modeling 3-D stress changes around advancing tunnel headings for different methods of construction. Thesis Supervisor: Andrew J. Whittle Title: Professor of Civil and Environmental Engineering Effects of existingsurface structures on tunneling induced settlements MEng 2001 2002 TABLE OF CONTENTS p. CHAPTER 1 INTRODUCTION AND GENERAL PRINCIPLES 1.1 GENERAL INTRODUCTION 1 1.2 SUGGESTED DEFINITIONS AND SYMBOLS FOR THE DEFORMATION OF FOUNDATIONS 2 1.3 GROUND MOVEMENT DUE TO TUNNELING 5 1.4 CLASSIFICATIONS OF BUILDING DAMAGE 9 1.5 THE CONCEPT OF LIMITING TENSILE STRAIN 11 1.6 STRAINS IN SIMPLE RECTANGULAR BEAMS DESIGN CURVES FOR DAMAGE 13 1.7 EVALUATION OF THE POTENTIAL OF DAMAGE TO BUILDINGS DUE TO SUBSIDENCE 19 1.8. PROTECTIVE MEASURES 21 CHAPTER 2 THE APPROACH OF POTTS AND ADDENBROOKE ON THE INFLUENCE OF A STRUCTURE ON TUNNELING INDUCED GROUND MOVEMENTS 2.