The tests consisted of a sequential twin-tunnel construction with varied centre-to-centre spacing in overconsolidated clay. Relatively complex apparatus facilitated a predefined volume loss whilst monitoring surface settlement, tunnel support pressures and pore-water pressures. The surface data were assessed against published estimation methods with the results highlighting some inconsistencies

Divall, S.

Goodey, R.J.

City Research Online

Divall, S. & Goodey, R.J. (2012). Novel apparatus for generating ground movements around sequential twin tunnels in over-consolidated clay. Online publication by Proceedings of the ICE - Geotechnical Engineering, pp. 10-11. City Research OnlineOriginal citation: Divall, S. & Goodey, R.J. (2012). Novel apparatus for generating ground movements around sequential twin tunnels in over-consolidated clay. Online publication by Proceedings of the ICE - Geotechnical Engineering, pp. 10-11. Permanent City Research Online URL: http://openaccess.city.ac.uk/4359/ Copyright & reuseCity University London has developed City Research Online so that its users may access the research outputs of City University London's staff. Copyright © and Moral Rights for this paper are retained by the individual author(s) and/ or other copyright holders.  All material in City Research Online is checked for eligibility for copyright before being made available in the live archive. URLs from City Research Online may be freely distributed and linked to from other web pages. Versions of researchThe version in City Research Online may differ from the final published version. Users are advised to check the Permanent City Research Online URL above for the status of the paper.EnquiriesIf you have any enquiries about any aspect of City Research Online, or if you wish to make contact with the author(s) of this paper, please email the team at publications@city.ac.uk.Novel apparatus for generating ground movements around sequential twin-tunnels in over-consolidated clay  S Divall and RJ Goodey Geotechnical Engineering Research Group, City University London, UK ABSTRACT: The tests consisted of a sequential twin-tunnel construction with varied centre-to-centre spacing in overconsoli-dated clay.  Relatively complex apparatus facilitated a predefined volume loss whilst monitoring surface settlement, tunnel support pressures and pore-water pressures.  The surface data were assessed against published estimation methods with the re-sults highlighting some inconsistencies.         1 BACKGROUND In construction using a Tunnel Boring Machine (TBM), the ground deformations towards a newly created cavity are of-ten known as volume loss.  Potential sources of tunnelling-induced ground deformation are described extensively by Mair & Taylor (1997).  The product of these ground defor-mations is apparent at the surface as a transverse settlement trough which is usually assumed to fit a Gaussian distribu-tion (Peck, 1969).   Tunnelling construction guidelines have been developed based, largely, on research from single tunnel arrangements (e.g. Peck, 1969; Mair, 1979; Taylor, 1984 and Attwell & Yeates, 1984).  Twin-tunnelling surface settlement predic-tions are often the superposition of two single tunnel predic-tions (O’Reilly & New, 1982).  The assumption is that the construction of a second tunnel is unaffected by the presence of the first tunnel.  Previous research, particularly numerical studies, has indicated that superposition may not necessarily be sufficient (e.g. Addenbrooke & Potts, 2001). This research programme aims to explore the ground movements in over-consolidated clay when constructing parallel tunnels with a small separation distance.  A number of plane strain centrifuge tests, using relatively complex ap-paratus to accurately simulate volume loss were carried out.  This enabled the simulations of a single tunnel construction, followed by a pause representing a construction delay, and then a second separate tunnel.   2 EXPERIMENTAL TEST SERIES 2.1 Model Geometries The use of a geotechnical centrifuge as a tool for examining geotechnical problems is well documented (Taylor, 1995).  Three largely identical tests, only varying in the tunnel cen-tre-to-centre spacing, have been conducted (Table 1).    Test Name Spacing (D) Fluid volume extracted from each tunnel (%) Test 1 Test 2 Test 3  1.5 3.0 4.5 3.0 3.0 3.0 Table 1 Tests performed  The tests were performed in a strong box at 100 g.  Mod-els consisted of preformed circular cavities in overconsoli-dated clay.  The twin-tunnels were bored equally spaced from the model centre-line.  All models had a cover to diam-eter ratio (C/D) equal to 2 and the tunnel axis level was ap-proximately 80 mm above the base of the strongbox.  The typical layout for the models is shown in Figure 1.   Figure 1 Schematic of a typical model  The newly developed apparatus provided support to the tunnel cavities using a fluid that could be removed in order to simulate volume losses.  The apparatus utilised a motor-ised Bishop ram as a syringe for removing the supporting fluid from within the tunnels.  The support pressure in the tunnels is controlled by a standpipe and, as such, the pres-sure automatically increases with g.  A full discussion of the apparatus details are given by Divall & Goodey (2011).   The instrumentation of the models included Druck pore pressure transducers (PPTs), pressure transducers and Linear Variable Differential Transformers (LVDTs).  A rack con-taining twelve LVDTs was bolted onto the top of the strong box to measure vertical surface settlement.  The movements within the soil mass were also recorded via a digital image-processing system.  The system monitored subsurface pat-terns of movement by tracking marker beads pressed into the front surface of the clay.     2.2 Test Procedure After the acceleration had reached 100 g the tunnels were isolated from the standpipe using a plug valve controlled by a rotary solenoid.  The centrifuge was left running overnight until pore pressure equilibrium had been reached in the mod-el.  Sequential tunnel constructions were simulated by oper-ating the equipment to drain 3 % of the total volume of the support fluid from each of the tunnels.  A time period repre-senting a construction delay of three minutes was allowed between these events.        -0.6-0.5-0.4-0.3-0.2-0.10-300 -200 -100 0 100 200 300Vertical Settlement (mm)Horizontal distance from the centre of strong box (mm)Twin-Tunnel LVDT Settlement DataSuperpositionAddenbrooke & Potts (2001)Hunt (2005)℄B℄A-0.3-0.25-0.2-0.15-0.1-0.050-300 -200 -100 0 100 200 300Vertical Settlement (mm)Horizontal distance from the centre of strong box (mm)Tunnel A LVDT DataTunnel B LVDT DataGaussian fit to Tunnel A LVDT Data℄B℄AFigure 2 Tunnel A and Tunnel B settlements Figure 3 Twin-tunnel settlements 3 SURFACE SETTLEMENT DATA Figures 2 and 3 show surface settlement data obtained from Test 2.  In Figure 2 the individual settlement troughs are ob-tained by taking the surface readings before and after tunnel construction events.  The surface settlement data associated with the first and second simulated tunnel constructions will be known as Tunnel A and B respectively.    As Tunnel A is excavated in what is effectively a green-field site these are shown to have good agreement with the Gaussian fit.  The settlements generated by Tunnel B show an increased magnitude as well a degree of asymmetry.  This resulted in higher observed volume losses than in the case of Tunnel A.  To examine this asymmetry, Gaussian curves can be fit separately to the left and right-hand sides of the set-tlement trough data.  The parameters i and K could then be calculated for Tunnel B settlements based on these and a measure of asymmetry generated.   Figure 3 shows the total surface settlement after both tun-nels have been excavated as well as some comparisons to ex-isting predictive techniques.   4 CONCLUSIONS The centrifuge model tests described have provided the be-ginnings to some very interesting data examining the small strain movements around twin-tunnels.  The accepted prac-tice of superposition has been shown to have some short-comings although two recent numerical studies have shown a better fit with the experimental data.   Tunnel A surface settlements were as expected for green-field construction, but Tunnel B surface settlements were not.  The test series shows the closer the centres of the tun-nels, the greater the added volume loss observed in the sec-ond bored tunnel.  As the volume extracted from each tunnel is precisely controlled, the reasons for this are unclear at this time and are a topic for further investigation. REFERENCES Addenbrooke, T.I. and Potts, D.M., (2001). Twin tunnel in-teraction - surface and subsurface effects. International Journal of Geomechanics, Vol.1, pp. 249-271.  Attewell, P.B. and Yeates, J., (1984). Ground movements and their effects on structures, Blackie and Son Ltd, At-tewell, P.B. and Taylor, R.K.   Divall, S. & Goodey, R.J., (2011). Apparatus for centrifuge modelling of sequential twin-tunnel construction. Inter-national Journal of Physical Modelling in Geotechnics – in press. Mair, R.J. and Taylor, R.N., (1997). Bored tunnelling in the urban environment, Proceedings of the Fourteenth Inter-national Conference on Soil Mechanics and Foundation Engineering, Vol.4, pp. 2353-2385.  Mair, R.J., (1979). Centrifugal Modelling of Tunnel Con-struction in Soft Clay, Ph.D. Thesis, Cambridge Univer-sity.  O’Reilly, M.P. and New, B.M., (1982). Settlements above tunnels in the United Kingdom - their magnitude and prediction, Tunnelling'82, Papers Presented at the 3rd International Symposium, Inst of Mining and Metallurgy, London, England, pp. 173-181.  Peck, R.B., (1969). Deep excavation and tunnelling in soft ground. Proceedings of the 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico City, Vol.3, pp. 225-290.  Balkema. Taylor, R.N., (1984).  Ground movements associated with tunnels and trenches.  Ph.D. thesis, University of Cam-bridge.   Taylor, R.N., (1995). Geotechnical Centrifuge Technology. Blackie Academic & Professional, Taylor, R.N.  

Novel apparatus for generating ground movements around sequential twin tunnels in over-consolidated clay

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