Discussion: Greenfield tunnelling in sands: the effects of soil density and relative depth

Abstract

Contribution by N. Shirlaw and S. Boone The discussion contributors thank the authors for their paper summarising the results of 15 centrifuge tests on model tunnels in sand. Their paper represents a helpful contribution toward better quantifying the response of granular soils to tunnelling. It is the discussers’ opinion that much more use could be made of centrifuge testing on model tunnels in sand to solve practical tunnelling problems. While there are numerous published centrifuge model tests in sand, including the authors’, the data are typically from studies that are very limited both in their nature and in terms of possible practical application. For useful application, more tests are needed that represent real heading geometry and in saturated, rather than dry, sand. A tunnel driven in sand typically requires a support pressure, as provided by a pressurised tunnel-boring machine (PTBM). An important aspect of geotechnical analysis for use of PTBMs is to define target values for the pressure at the face and for the tail void grouting consistent with subsurface conditions (Shirlaw & Boone, 2009) and compliance with maximum specified values for surface settlement or surface volume loss, Vl,s. Maximum values for settlement or surface volume loss are typically set by the designer of the tunnelling project, while the pressures at the PTBM are set by the contractor or their engineer. In their paper, the authors categorise volume loss at the tunnel level, Vl,t, of 1% as ‘low’ and 5% as ‘high’. In practice, designers commonly require that a maximum surface volume loss of 1% is consistently achieved over PTBM tunnels in sand. The discussers have seen recent examples where a maximum value for surface volume loss of 0·5% has been specified for tunnels in granular soil. Reviewing the authors’ Figure 9 in the range from 0 to 1% tunnel volume loss, the loose and medium-dense sands (Id 0·3 and 0·5) are contractive, such that the surface volume loss is significantly greater than the tunnel volume loss at all values of C/D. In dense sand (Id = 0·9) the ground deforms at close to constant volume, with the surface and tunnel volume loss being similar. A contractor’s engineer with a target of 0·5% maximum surface volume loss will therefore have to target values for volume loss at tunnel level that are significantly less than the desired maximum value for the surface volume loss in loose and medium-dense sand. Even in dense sand, the target value for volume loss at tunnel level cannot exceed the maximum value for surface volume loss. In the discussers’ opinion, the very low values of maximum surface volume loss that are now commonly specified on tunnelling projects reflect a desire to fully use the capabilities of PTBMs to control ground movement. Controlling the ground movement at source becomes the primary, and often only, means of protecting buildings, utilities and other structures from damage during PTBM tunnelling. This approach transfers much of the settlement control problem from the project designer to the contractor’s engineer. The contractor’s engineer has the task of defining the necessary operating pressures for tunnelling in order to consistently perform below the specified maximum values for surface volume loss. Potentially, data from testing of model tunnels in geotechnical centrifuges could provide a useful basis for making this assessment. However, data currently available are not sufficient for this task. Frequently, when assessing potential near-surface implications of tunnelling through soft ground in urban environments, engineers apply well-known empirical methods that relate the unit volume of the ground surface settlement trough to the unit volume of the tunnel, expressed as ‘volume loss’, and characterise the shape of the settlement trough as an inverted standard normal probability distribution curve (e.g. Peck, 1969; New & O’Reilly, 1981; and others). While useful for elementary studies, these methods do not adequately address the roots of surface volume loss that occur at the tunnel level, as related to closure of the ground around tunnelling machines and linings, the effects of the attitude of tunnelling machines in very soft soils or when negotiating curves and face pressure control. The pressures applied at the face, along the shield and at the tail void are critical in the control of tunnel and surface volume loss. Until fundamental research and field data quantitatively examine the contributions of each ground loss source, field control will be dominated by subjective opinion, ‘rules of thumb’, different methodologies and trial-and-error field adjustments. Although graphs relating internal support pressure to tunnel volume loss are provided in the paper, these data cannot be readily applied in the assessment of support pressures for PTBM driven tunnels, as the true field problem is three-dimensional (3D) and progressive rather than the stationary plane-strain conditions of the authors’ tests. The authors show how a two-dimensional (2D) arch develops over the model tunnel, at least for the higher values of soil relative density, similar to work by Iglesia et al. (2013) and others. In practice, Wan et al. (2019) illustrate the 3D propagation and sequence of soil arches and displacement modes that develop over tunnels in London Clay at various stages of tunnel advancement at the face, around the shield and around the lining, with longitudinal as well as transverse arches. While the response of sand will be different to that of clay, the obvious 3D influences of the tunnelling processes are similar. In practice, the 3D relationships between support pressures, forward tunnelling rate and the resultant surface ETSI Caminos, Universidad Politécnica de Madrid, Madrid, Spain (Orcid:0000-0002-8510-0355). † Department of Civil Engineering, University of Nottingham, Nottingham, UK (Orcid:0000-0003-1583-1619). ‡ Formerly, Faculty of Engineering, University of Nottingham; now Shanghai Civil Engineering Co., Ltd of China Railway Group Ltd, Shanghai, P. R. China. § Golder Associates (HK) Limited, Wan Chai, Hong Kong, P. R. China. ∥ Golder Associates Ltd., London, Ontario, Canada. Franza, A. et al. Géotechnique [https://doi.org/10.1680/jgeot.19.D.002]