HARDING PRIZE COMPETITION 2023 | BTS


A


Longitudinal joint


Longitudinal flange


Crown


B


Springline


Left, figure 2: Segmental grey cast iron ring


(a) Terminology (b) Finite element mesh Invert


First, a new set of bending stiffness reduction


factors were derived based on the results of an extensive numerical parametric study; these can be readily applied in routine engineering calculations. Subsequently, the rotational behaviour of a prototype tunnel joint geometry was fully characterised for the first time, enabling the development of a new model for simulating the mechanical response of GCI tunnel joints. Additionally, the analyses provided new insights into


the effects of removing the bolts and tightening the bolts to different extents on the joint response.


INTRODUCTION There are hundreds of kilometres of deep tunnels belonging to the London Underground (LU) metro network that were constructed with GCI linings. With an increasing number of underground structures being constructed in proximity to these tunnels and the absolute necessity to protect their serviceability, adequate methods of predicting their structural response are needed more than ever. This paper summarises some of the findings and


methods established by the Author for the structural assessment of GCI tunnel linings during his recently completed PhD studies (Ruiz López, 2022). GCI linings were constructed as a succession of


rings to form a tunnel, each ring comprising several segments. Rings and segments were bolted together at circumferential and longitudinal joints, respectively. The segments have a U-shaped cross-section formed


by the skin and the circumferential flanges with the longitudinal flanges closing the segments at both ends. The use of bolts in the longitudinal flanges was possibly


A Extrados


aimed at preventing excessive opening of joints (i.e., separation of segments) which can nevertheless occur (Thomas, 1977) and is thought to reduce the stiffness of the structure. Due to uncertainties in the mechanical behaviour of the joints, the stiffness of the tunnel lining is largely unknown which constitutes a fundamental barrier to achieving meaningful structural analyses. An experimental investigation into the behaviour of a


half-scale GCI ring was conducted at Imperial College London (Yu et al., 2017; Afshan et al., 2017) was aimed at overcoming these limitations – see the laboratory set-up shown in Figure 1. The investigation provided valuable insights into the impact of the joints on the structural response. Moreover, the availability of the experimental results motivated the development of a competent 3D finite element (FE) model replicating the laboratory set- up (Ruiz López et al., 2022) and which constituted the starting point of the research described here. The FE model, developed in the program ICFEP (Potts


& Zdravkovic, 1999), was employed subsequently to characterise the behaviour of joints and to evaluate systematically the influence of the joints on the structural response. The investigations are described below. In this paper, a new methodology is introduced


– based on bending stiffness reduction factors, to incorporate the influence of joints in preliminary assessments of GCI tunnel linings. The comprehensive characterisation of the rotational behaviour of a prototype joint is described subsequently. Lastly, the effectiveness of removing the bolts and tightening the bolts as mitigation measures for existing tunnels is evaluated.


B Extrados Left, figure 3:


Figure 3 Adopted sign convention for positive and negative bending (a) Positive bending (b) Negative bending


Intrados Intrados July 2023 | 31


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