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Success on the Downs


Colin Warren and Paul Watson report on the monitoring success of the North Downs Tunnel

With a cross-sectional area of more than 165m 2, the 3.2km long North Downs Tunnel under Bluebell Hill between Maidstone and the Medway Towns is one of the largest built in the UK. The £80M, single-track twin bore tunnel on Section 1 of the CTRL route was driven from both the London and Kent ends at depths of up to 100m through a varying sequence of Upper, Middle and Lower Chalk.

The tunnel was completed five months ahead of schedule and came in £5M under budget. Much of this success was put down to the dedicated on-site design team as well as the use of observational techniques and the New Austrian Tunnelling Method (NATM).

The advantage of the NATM system compared to single-pass tunnel boring machine lining methods - particularly where crown excavation proceeds well in advance of bench and invert excavation - is that it allows validation of the ground model and geotechnical parameters used for design.

The was especially important on this project, given the weak nature of the chalk, which can make obtaining representative borehole samples difficult. This typically results in an overestimate of the degree of fracturing caused by drilling disturbance.

The true mass character of the chalk - including information on jointing, rock quality etc - was determined by geological mapping of the excavated tunnel face and by monitoring tunnel lining displacements and ground movements around and above the tunnel. Results were then fed back into the design process, to further refine it.

Monitoring instrumentation included:

lA total of 179 surface settlement points measured using precise levelling techniques. Settlement was measured on eight transverse arrays, mainly in the shallow cover areas and above the tunnel centreline;

lSeven surface extensometers installed above the tunnel centreline, three in deep boreholes and the others in holes close to both portals where ground cover above the tunnel crown was thin;

lFive arrays of rod extensometers installed from within the tunnel. Each array comprised three arms - one extending vertically up from the crown and the others at the base of the top heading - radial to the tunnel. Each arm contained 3m, 6m and 9m long extensometers and, in the three arrays in thick cover sections, an additional 15m long extensometer; and lRegular arrays of bireflex targets placed along the tunnel at between 5m and 40m intervals. These allowed precise 3D survey monitoring of the primary shotcrete lining. Accuracy of the readings, which was governed by the quality of the targets and the atmospheric conditions in the tunnel, was - 0.5mm.

The results were post-processed using the Dedalos tunnel deformation software.

This allowed presentation of the vertical, transverse and longitudinal tunnel lining movements at each array, plotted against time. The results were discussed at a daily meeting, allowing adjustments to be made to the tunnel advance length and face support details.

Design discussions between contractor Eurolink and RLE before tunnel excavation had identified the need for three trigger, action and evacuation levels. It was agreed that each of these thresholds would be tied in to an action in Eurolink's tunnel monitoring implementation plan and tunnel emergency plan.

Dedalos was modified to plot strain development in the primary lining with time. This was based on elastic theory and the early age and long-term deformation characteristics of the shotcrete, determined by testing carried out at Innsbruck University.

Determination of stress within the lining was based on the monitored compressive radial strain in the shotcrete lining.

The results also confirmed parameters adopted for secondary lining design with back-analysis using FLAC carried out at the each surface extensometer. This enabled validation of the insitu stress regime obtained from hydrofracture tests performed in boreholes, the geotechnical parameters adopted for design, and the amount of relaxation assumed ahead of the tunnel face. Where possible, modification of the tunnel profile and refinement of the design was carried out.

Successful completion of the tunnel was due to the integrated site team and the in-place monitoring regime, which allowed design to be optimised throughout construction.

Colin Warren is RLE lead geologist and Paul Watson is RLE senior field engineer on Contract 320 Thames Tunnel.

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