The Measurement Of The Ground Response To Tunnelling During The Experimental Drive For The Channel Tunnel

Maconochie, D.J. (CSIRO, Division of Applied Geomechanics) | Potts, E.L.J. (Milburn Professor of Mining Engineering, University of Newcastle Upon Tyne) | Reid, A.G. (Department of Civil Engineering, University of Strathclyde)

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SUMMARY: An account is given of the installation and performance of remote reading multiple point borehole extensometers and Talbott strain cells which were installed in advance of the experimental drive for the Channel Tunnel in 1975. The analysis of the large amount of recorded data considered the influence of construction activities, geologic structure, face position and machine movements on the observed changes. A two-dimensional finite element model was considered based on deduced ground parameter and the results were compared with the observed displacement profiles. The limitations of the analysis are reviewed and comments made on the selection of the design parameters. RÉSUMÉ: Un compte rendu de l'installation et du fonctionnement en forage d'extensomètres telelecteurs à ancrage multiple et de cellules Talbott à extensomètres, qui furent installes à l'avance en 1975 dans la galerie de reconnaissance pour le tunnel sous la Manche, est presente. L'analyse de la grande quantite d'information enregistree prend en consideration l'influence des travaux, de la structure geologique, de la position du front de taille et des mouvements des machines, sur les changements observes. Un modèle bidimentionnel à element fini, base sur les paramètres deduits du sol, est considere et les resultats sont compares avec les profils de deplacement observes. Les limitations de l'analyse sont examinees et des observations sont faites sur le choix des paramètres du project. ZUSAMMENFASSUNG: Es wird uber die Einrichtung und die Arbeitsweise von Fernablesungs-Bohrlochestensometern mit mehrfacher Verankerung sowie Talbott-Zellen mit Spannungsmessern berichtet. Diese Gerate wurden 1975 im voraus in dem Kontrollstollen der Versuchsstreke fuer den Kanaltunnel installiert. Die Analyse der zahlreichen davon registrierten Daten beruecksichtigte den Einfluβ, den die Bautatigkeiten, die geologische Struktur, die Streblage sowie die Maschinenbewegungen ausuebten. Ein zweidimensionales, auf abgeleiteten Erdbodenparametern begruendetes endlich element-Modell wurde in Betracht gezogen und die Ergebnisse warden mit den beobachteten Verschiebungsprofilen verglichen. Die Begrenzungen der Analyse werden besprochen und Bemerkungen werden ueber die Wahl der Bauparameter gemacht. 1. INTRODUCTION In 1975 work on the Channel Tunnel project had progressed from the planning stage to the point where trial tunnel drives were about to commence from the British and French coasts. However, Government support for the project was withdrawn only days before a British tunnelling machine was to commence excavation of a 2 kilometre pilot bore of the central service tunnel. Included in that phase of the works was a programme of in situ geotechnical and lining studies which had progressed to a point whereby useful information could be obtained on the proposed tunnelling procedures provided a short length of tunnel was completed. Soon after the project was cancelled permission was given by the Channel Tunnel unit of the Department of the environment for a 250 m length of tunnel to be completed. The paper describes the development, installation and results obtained from ground instrumentation which formed part of a wider study by the consultants into a rational basis for the design of the tunnel as a whole. In this study, only part of which was completed, ground and lining instrumentation was installed in and around a length of tunnel which was made accessible by the presence of the Beaumont tunnel intersecting the proposed line of the service tunnel beyond the Shakespeare Cliffs at Dover. 2. DESCRIPTION OF THE TUNNEL The tunnel system would have comprised two running tunnels of 7.1 m inside diameter flanking a 4.5 m diameter service tunnel at 15 m centres and linked by cross passages at 250 m intervals. The tunnel alignment had been determined by a combination of railway and geological constraints but eventually the profile took the form of a very flat W, the steepest gradients being near the portals (Gould, Tough, 1975). The tunnel was to have commenced at the terminal area north of Folkestone and after heading north east taken a gentle curve to the south east and continued in almost a straight line to Pas de Calais. The undersea length of approximately 41 km would have been up to 110 m below mean sea level and between 40 and 60 m under the sea bed. The general geologic structure had been well defined over the years. The simplified succession of strata in which the tunnel would lie comprised an upper layer of Middle and Upper Lower Chalk containing over 90 per cent calcium carbonate but badly fissured and water bearing. Beneath this layer lies the softer Chalk Marl comprising 40 to 80 per cent calcium carbonate, the remainder being clay. It has low permeability, is relatively free from fissures and would be encountered over nine tenths of the route. The final layer to be found beneath the chalk but above the water bearing lower greensands is the Gault Clay, a stiff overconsolidated clay containing a calcium carbonate content ranging from 40 per cent in the upper part to 15 per cent at the base.

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