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Computational modelling of the effect of underground excavations on adjacent structures has shown great potential to aid the assessment of tunnelling-induced damage to structures. However, the complexity of the mechanisms involved and the uncertainties connected to the use of sophisticated constitutive laws still limit the application of numerical modelling in civil engineering practice. This paper evaluates the effectiveness of soil models with different levels of complexity when predicting tunnelling-induced displacements of the soil surface, and consequently the assessment of building deformations. The performance of a non-linear elastic, a linear elastic–perfectly plastic and a critical-state-based kinematic hardening soil model were compared with the results of centrifuge testing of a tunnel excavation in sand. Results demonstrated that both the non-linear elastic and the kinematic hardening models are suitable to reproduce the effect of soil–structure interaction on the soil surface displacements and the building deformations, while also demonstrating the limitations of these methods in predicting local soil strains around the tunnel itself. [ABSTRACT FROM AUTHOR]

Building monitoring and protection are important components of underground projects in urban areas. Typically the procedures applied for the assessment of settlement-induced damage to buildings are based on simplified assumptions that do not take into account soil–structure interaction. Assessment methods based on the relative stiffness between the structure and the soil exist, but they are rarely applied in practice due to concerns about the accuracy and reliability. The primary aim of this work is to use the large amount of monitoring data provided by the Crossrail project in London to improve understanding of building performance and existing damage assessment methods. The paper gives an initial overview of the available monitoring data by presenting four representative case studies for load-bearing masonry buildings on shallow foundations. Structural data are then used to evaluate the consistency of predictions produced by different relative stiffness formulations. The results show the effect of building stiffness on the soil surface settlements and clarify the effects of various assumptions made during prediction. The conclusions highlight opportunities to improve prediction procedures and the need for more detailed monitoring data for future tunnelling projects. [ABSTRACT FROM AUTHOR]

At present, there are few well-documented case studies of circular shaft construction, making it difficult for designers to estimate reliable ground movements arising from such construction. This paper describes field observations of ground surface settlement assembled during the construction of 27 circular shafts built for three major tunnelling projects in London: Crossrail, National Grid's London Power Tunnels project and Transport for London's Northern line extension. Two categories of shaft construction were identified: support before excavation (SBE) and excavation before support (EBS). For the SBE category, the shaft was first supported by pre-installed walls followed by excavation of the soil between the pre-installed walls. For the EBS category, the ground was progressively excavated in sections followed by construction of the shaft lining. Interpretation of the field observations showed the importance of the shaft construction method on ground movements. Settlements were much more significant for EBS shaft construction than for SBE shaft excavation, although settlement arising from the installation of pre-installed walls or dewatering operations should not be overlooked. Normalised charts are presented to help the industry make estimates of settlements due to circular shaft construction in London, with due consideration for different shaft geometries and construction methods. [ABSTRACT FROM AUTHOR]

Understanding the magnitude and distribution of ground movements associated with deep shaft construction is a key factor in designing efficient damage prevention/mitigation measures. Therefore, a large-scale monitoring scheme was implemented at Thames Water's 68 m deep Abbey Mills Shaft F in East London, UK, constructed as part of the Lee Tunnel Project. The scheme comprised inclinometers and extensometers which were installed in the diaphragm walls and in boreholes around the shaft to measure deflections and ground movements. However, interpreting the measurements from inclinometers can be a challenging task, as it is often not feasible to extend the boreholes into ground unaffected by movements. The paper describes in detail how the data are corrected. The corrected data showed very small wall deflections of less than 4 mm at the final shaft excavation depth. Similarly, very small ground movements were measured around the shaft. Empirical ground settlement prediction methods derived from different shaft construction methods significantly overestimate settlements for a diaphragm wall shaft. The results of this study will help to inform future projects, such as the forthcoming 25 km long Thames Tideway Tunnel with its 18 deep shafts being constructed adjacent to existing infrastructure. [ABSTRACT FROM AUTHOR]

This paper describes part of the monitoring undertaken at Abbey Mills shaft F, one of the main shafts of Thames Water's Lee tunnel project in London, UK. This shaft, with an external diameter of 30 m and 73 m deep, is one of the largest ever constructed in the UK and consequently penetrates layered and challenging ground conditions (Terrace Gravel, London Clay, Lambeth Group, Thanet Sand Formation, Chalk Formation). Three out of the twenty 1·2 m thick and 84 m deep diaphragm wall panels were equipped with fibre optic instrumentation. Bending and circumferential hoop strains were measured using Brillouin optical time-domain reflectometry and analysis technologies. These measurements showed that the overall radial movement of the wall was very small. Prior to excavation during a dewatering trial, the shaft may have experienced three-dimensional deformation due to differential water pressures. During excavation, the measured hoop and bending strains of the wall in the chalk exceeded the predictions. This appears to be related to the verticality tolerances of the diaphragm wall and lower circumferential hoop stiffness of the diaphragm walls at deep depths. The findings from this case study provide valuable information for future deep shafts in London. [ABSTRACT FROM AUTHOR]

Recent advances in strain measurement using optical fibres provide new opportunities for monitoring the performance of piles. Brillouin optical time-domain reflectometry (BOTDR) is an innovative technique that allows measurement of full strain profiles using standard optical fibres. Distributed optical fibre strain sensors were installed at two sites, and the details of the installation methods are presented. A theoretical analysis that compares pile behaviour evaluated from localised strain measurement at discrete locations and from distributed strain measurements is conducted for the case of a vertically loaded pile in layered soil. Two problems are considered: a load test aiming to establish the load transfer function, and changes in pile-soil interface stress due to nearby tunnelling. Advantages of such a distributed reading for monitoring the performance of pile foundations in layered soils are demonstrated from the results of both cases. Cost analysis results show that the BOTDR measurement is a cost-effective pile monitoring method compared with discrete strain measurement methods such as vibrating-wire strain gauges and fibre Bragg gratings. [ABSTRACT FROM AUTHOR]