Channel Tunnel and the Thames

¶ … Channel Tunnel and the Thames Tunnel: Soil Composition and Building Methodology

Subaqueous tunnels present unique engineering challenges, for some obvious and some not-so-obvious reasons. Complications regarding materials, work crews, machinery, soil, and of course water all make tunnel-building beneath rivers, seas, and other bodies of water a major undertaking, even when tunnel length and soil composition might otherwise make an easy go of things. By comparing to subaqueous tunnels that are similar in their revolutionary qualities but hugely different in terms of their execution, some of the common problems that crop up in subaqueous tunnel construction can be better appreciated. The following pages present a brief comparison of the Thames Tunnel, the first known subaqueous tunnel ever constructed, and the Channel Tunnel, a more modern marvel that connects England with continental Europe. Similarities in these two tunnels are highlighted, and the differences in the requirements and approaches to these two very different tunnels is fully discussed.

Thames Tunnel

Initial work began on a tunnel running underneath the Thames in 1825, though it would be almost two years before a satisfactory set of test holes could be drilled and it took nearly two decades for the tunnel to be completed (Skempton & Chrimes, 1994). Given that a project like this had never been attempted and that the worksite lay entirely dormant for seven years while funding was worked out, this timeline is actually more impressive than it first seems, and the fact that the tunnel was successfully completed at all is a testament to the ingenuity and the perseverance of the engineers and work crew that brought this tunnel into being. The soil through which the tunnel was dug was itself not problematic, however the layers of soil above the tunnel were clay and sand, the latter of which posed significant problems to the tunnel's engineering and construction at the outset. In areas where the clay layer between the sand and the tunnel was thick, progress was relatively rapid and easy for the tunnel-makers, but where the clay was thin the sand presented a constant problem of sand runs breaking into the tunnel. On five occasions, this led to the river's intrusion into the tunnel, significantly delaying progress and raising safety concerns (Skempton & Chrimes, 1994).

Fortunately, the solution was found in engineer Sir Marc Isambard Brunei's advent of the tunneling shield, a now-commonplace (and far more complex) tool that allowed the insertion of a tunnel casing, what would now be deemed a lining, while work progressed. (Skempton & Chimes, 1994). This tunnel shield propped up the tunnel along its length as it was being constructed, but was especially useful at the tunnel ends where the sand breaks were ore common due to the thinner clay layers. Here, they propped up the clay that was prone to grow brittle and crack when thinned out, while elsewhere they simply provided an easing of working conditions and greater safety against the less-likely sand breaks and inrushes of water. No significant engineering problems appear to have been encountered in the actual digging of the tunnel; though progress was very slow as the labor consisted entirely of hand-held tools, it was the tunnel support that provided the most significant soil-based engineering challenge and that required innovation in methodology to overcome.

The Chunnel

The Channel Tunnel, affectionately known as the Chunnel, is a far cry from an early-nineteenth century manually dug tunnel. Its subaqueous nature presented related challenges of support and safety during construction, but in most other regards it was a different undertaking altogether. 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).