The material through which the tunnel was dug was primarily chalk, not the relatively soft and yielding earth that lay under the sand and clay below the Thames, and while this actually eased certain concerns in the construction and operation of the tunnel it presented new challenges, as well (Margron, 1996). The relative continuity of the geological floor that lay under the waters of the English Channel, especially in the line of chalk that was followed for the Channel Tunnel, was a major reason for the selection of the tunnel construction as it provided rigidity and support without the same type of reliance on tunnel shielding as was used in the earlier construction of the Thames Tunnel. The chalk types and lack of homogeneity did make for more difficult tunneling on the French side of the tunnel, but this has been expected due to extensive exploratory drilling on the French side and while it made progress more difficult geological conditions were still considered favorable for the tunnel and work progressed as expected (Margron, 1996). Different methods were employed by the English and the French in shoring up the tunnel as work progressed, with sealed iron and reinforced concrete linings being used by the French and iron-cast linings being used by the English, with varying segment numbers and segment lengths for the lining (Harris, 1996). These different lining methods were more a result of varying budget and time concerns and varying expertise and engineering understandings than they were responses to the different chalk substrate through which the two teams were tunneling. The tunneling itself was as revolutionary as the tunneling under the Thames had been; though the Chunnel was not entirely without precedent in terms of undersea tunnels its length and...

The challenge here was in drilling through and removing the chalk itself rather than in maintaining the integrity of the tunnel as it was being constructed, though of course with the much greater weight of the sea flowing on top of the tunnel this was also a concern. The different material through which this tunnel was built changed the concerns from one of ongoing leakages and sudden break-ins of sand and water to the potential for more catastrophic collapses, and the iron and concrete lining methods that had been developed by the time of the Channel Tunnel's creation proved invaluable in maintaining integrity during construction and use (Harris, 1996).
Conclusion

Building tunnels underwater still presents major engineering challenges every time it is proposed and undertaken. The practice is no longer unheard of, and there are several significant success stories of major tunnels being completed beneath large waterways. There are also stories, of course, in which project delays and cost overruns stand as embarrassing testaments to engineering difficulties, and stories of outright failures. Neither the Thames nor the Channel Tunnel can be counted as such failures, but rather they represent the ever-growing knowledge and capabilities of modern engineers.