A tunnel, while having no universally agreed upon definition is at its most basic, a hollowed out tube of some distance through water or the earth. This tube is completely enclosed on all sides, having two entrances at each end, known as portals. Throughout the 19th and 20th century, it has been commonplace to see roads, and highways, and powerlines being constructed above ground where space has been available. However as society moves further through time, and life becomes increasingly congested, tunnels provide some of the last available space for transport mediums, water and sewage transportation, and even power and telecommunication lines.
Each tunnel constructed serves as an answer to a particular engineering challenge, such as an obstacle that needs to be passed, whether that object be a mountain, a body of water, or an entire city due to lack of space for other alternatives. A tunnel may be chosen in favour of a bridge to cross a water body due to space constraints (i.e. in a large city), safety concerns (such as crossing the Tsugaru Strait in Japan, an area swept with typhoons), or feasibility.
To ensure safety, minimise expenditure and keep the project free from unnecessary delays, an in depth planning process must take place before construction can commence in any tunnel. Since the method of tunnel construction in Australia, and indeed worldwide depends almost solely on the material through which it passes, it is essential that any tunnel project starts with a comprehensive investigation of ground conditions. Once these conditions are established, engineers can then excavate the tunnel, support, and line the tunnel, using construction methods suitable for the medium they are passing through.
Australian Tunnel Planning
Australia is the 6th biggest country by land area on Earth, and therefore has many different geological formations. Australia enjoys a unique position where none of the land area lies close to a tectonic fault line, making its landscape extremely stable.
It is important to take a full survey of the proposed area to gain an understanding of such things as the soil type, rock structure and the level of the water table. If these factors are not taken into consideration the tunnel may; sink under its own weight due to inadequate foundations when constructing in soft soil; cave-in due to instability of the surrounding geological structure when tunnelling through rock; or flood due to insufficient waterproofing.
As mentioned above, Australia is home to many different types of rock strata. The city of Sydney, and in particular, Sydney harbour, lies on a bed of Hawkesbury sandstone. This particular type of rock tends to remain in large chunks that make it both hard to work through as well as hard to remove. This was one of the major deciding factors when choosing to cross Sydney Harbour with an immersed tunnel rather than a bored tunnel, as no excavation was required through this difficult medium.
Brisbane also has an underlying rock bed made up of Meta-sedimentary rock, called Neranleigh-Fernvale bed. This rock bed runs much of the Brisbane area, from being seen as part of Mt Gravatt in Brisbane’s south, to Samford in Brisbane’s north. This deposit is also wide spread throughout the Gold Coast. Brisbane City also lies on a bed of Brisbane Tuff a welded ignimbrite formed when Brisbane was once part of a volcano from ash flow and then compacted.
These rock formations are highly favoured by construction companies building tunnels below the water table, such as the Clem Jones Tunnel and the tunnels required for pipes leading to the Gold Coast Desalination plant, due to their low water permeability, thereby creating a natural waterproofing. This property does not come without costs, as these rock formations are some of the strongest in Australia, requiring a unique design of Tunnel Boring Machine, as can be seen in the below case study of the Clem Jones Tunnel....
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