Settlements above tunnels in the United Kingdom--their magnitude and prediction

• Published in: Tunnels & Tunnelling International p. 56
• Main Authors: O'Reilly, Myles, New, Barry
• Format: Journal Article Trade Publication Article
• Language: English
• Published: Oxford NS Media Group Limited 01-05-2015; Compelo
• Subjects: Clay; Design; Environmental impact; Laboratories; Projects; Standard deviation; Studies; Tunnels; Urban areas; Water mains; United Kingdom; United Kingdom > UK
• ISSN: 1369-3999

SETTLEMENTS CAN be a problem with soft ground tunnelling in urban areas where buildings, both modem and ancient, can be put at risk, services, too, can be endangered and at times it has been deemed necessary to divert services before tunnelling is begun. These environmental considerations have led to a considerable research effort being devoted to the study of settlements caused by tunnelling through soft ground; much of the research work has been undertaken either directly by or under contract for the Transport and Road Research Laboratory. Measurements of settlement and ground movement made on tunnelling projects located, in the main, in built-up areas are reviewed. The ground conditions studied included stiff-fissured clays, glacial deposits and recently deposited silty clays, as well as cohesionless soils of low density, weak rocks and made ground. Many of the tunnels were driven in free air by use of shields, but compressed air was used in the weaker soils to maintain stability; the bentonite shield and chemical treatment of the ground were also used in loose sands. The data from these case studies are used to provide simple analytical tools that enable better prediction of the magnitude of settlements and ground movements caused by tunnelling through soft ground to be made. The analysis given above for cohesive soils is unlikely to be applicable to granular soils as the assumption that particle displacements away from the tunnel are directed toward the tunnel axis is not supported by laboratory studies. Further, the assumption of deformation at constant volume is untenable as some dilation or compaction of granular soils is almost inevitable during deformation. Again, data from the field are limited and inconclusive. Independent model studies reported by Potts |22! and Cording et al[231 indicate a rapid narrowing with large inward displacements of the settlement trough near the ground surface with the sand soils funneling down into the void created by the excavation (see Figure 3). This settlement mode was discussed by Atkinson and coworkers 1241 in terms of a dilating wedge over the tunnel crown, which develops until collapse occurs on surfaces that propagate vertically upwards from the tunnel haunches. The transverse distance from the tunnel centre line to the point of inflexion (y = i) is used to describe the width of the settlement trough and has been considered to be related both to the depth from ground surface to axis, Z, and to a lesser extent the diameter of the tunnel. Multiple linear regression analyses performed on the field data presented here, however, revealed no significant correlation between the trough width parameter, i, and tunnel diameter, although the expected strong correlation of i with tunnel depth, Z, was found. This was true for both cohesive and granular data groupings This finding is to some extent explained by [Glossop], who carried out an analysis based on stochastic/ numerical modelling techniques l2lL The analysis showed that at distances of more than about one tunnel diameter from the periphery of the tunnel the shape of the settlement trough is not significantly dependent on the tunnel diameter and the loss of ground may be considered to occur at a point 'sink' located at the tunnel axis Strictly, therefore, they are only applicable within these limits, but the indications are that the values would not be appreciably different for reasonable extrapolation beyond the limits of'B', 'C and TT above, but the limitation on cover must not be contravened. Further, the analysis given is two-dimensional and, although this may be satisfactory in the prediction of conditions subsequent to the tunnel construction, other significant ground deformations of a three-dimensional character may occur during the passing of the tunnel face He] [20] [3s]_ Considerable monitoring of ground and building settlement is now routinely carried out on most tunnelling projects in urban areas. Where the extent and/or magnitude of the predicted settlement are important, consideration should be given to arranging the construction programme so that the settlement profile is determined in a 'safe' location, e.g. under parkland, as early as possible in the project.