Ground movements associated with tunnels and underground excavations are very difficult to predict accurately. Prediction is often based on an empirical approach based on a volume loss empirically established from previous case histories in similar grounds.
Higgins et al (1996) employed a more rigorous analysis, based on assumed volume loss, for the prediction of settlements resulting from the construction of the station box together with the running and the station tunnels in Westminster. They estimated a resultant movement of 45mm on the northern edge of the Big Ben clock tower base and 24.4mm on the southern edge. The tunnels were estimated to produce 17.1mm and 8.4mm on the northern and the southern edges respectively. Using a base width of 15m and Gedometer level of 55m above base gives a tilt at the Gedometer of 75.5mm due to box and tunnels and 32mm due to tunnels alone.
The above prediction was carried out based on a volume loss of 2-2.5% (which was judged to be conservative based on similar case histories) and without making allowance for deep struts provided to reduce ground movements due to station box excavation. Assessment of the volume loss associated with the tunnels in Westminster has indicated a volume loss of 3% to 3.5% in the short term. Using this volume loss and allowing for the consolidation settlement measured six months after tunnel drive (equivalent to 4% total) would have resulted in a predicted tilt in the order of 134mm due to tunnels and box excavation, and 57mm due to tunnels alone. The presence of the deep struts and difference between the construction sequence model used in the prediction and that actually used make any comparison between the predicted settlements and tilt and that actually took place invalid.
The above predicted settlements and associated tilt or those that occurred and have been reversed by compensation grouting would have most certainly led to severe to very severe damage particularly at the junction between the tower and the Palace of Westminster. The compensation grouting has demonstrated its ability to deal with this type of problem and kept the settlement and tilt to a very small value.
The technique is very versatile and can be adapted to deal with the developing settlement whatever its magnitude or profile. It does not rely on complicated prediction to succeed, only accurate monitoring which can respond to the speed of the developing settlements and the heave resulting from grouting. Obviously, there is a practical limit to the speed of grout injection and the number of repeats, therefore there is a limit on how much settlement can be compensated in a given time. For a tunnel induced settlement, for example, the tunnel may have to be slowed down if theresulting volume loss is larger than that which can be compensated practically.
The tunnel routes on Contract 102 of the Jubilee Line Extension pass beneath and influence many prestigious structures in central London and the settlements during excavation of these tunnels would have lead to severe damage to these structures. The adoption of the relatively new techniques of concurrent and observational compensation grouting has resulted in a huge reduction in the magnitude of settlement and the control of the damage that would otherwise have resulted.
Compensation grouting under the clock tower of Big Ben, although not dissimilar to any other structure on the project, has presented a unique case owing to its sensitive nature, historic value and the highly stressed soils under the tower compared with the soil surrounding it. Nonetheless, both the observational and the concurrent grouting methods have provided a highly refined control (to less than 1mm) of the tilt of this tall and sensitive structure.
Given the disparity between the predicted and actual tunnel losses, and the resulting settlements, the provision of a compensation system is viewed by the writers as a prudent insurance against the serious and expensive damage that may occur by reliance upon prediction alone.
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The authors wish to express their appreciation to the Jubilee Line Extension project team for allowing the publication of this paper, to BBA for providing the monitoring information, and to WH Baker of Geobase, and P Sola of Geocisa y Technicos, for their contribution in the development of the AG compensation strategies.