The Hyperloop is an extremely promising mode of transportation. It will run through a system of tubes that have got an internal pressure of 0.1 % atmospheric pressure. The design of these tubes is essential for the economic potential of Hyperloop. Also it sets the framework for some of the technical aspects and the other way around. One might ask themselves whether Hyperloop tubes need to be constructed above ground or underground. There are three main reasons why the Hyperloop tube would be constructed underground:
When constructed above ground, the Hyperloop tube will be exposed to sunlight and will therefore become quite hot. When the temperature of the tube will rise, the compressive stresses in the tube will increase. When these compressive stresses will become too large, there is a chance of buckling of the entire structure. There are several ways to solve this. These are: high neutral temperature, dilation segments and impediments for buckling. The first one is hard to create with a tube system, as it is difficult to produce such large segments of tube at a high neutral temperature and transport them as well. The implementation of dilation segments is a possibility as well, but will create issues with the air-tightness of the tube, and will also lead to weak spots in the structure. The final way is an impediment for buckling, these will however be expensive and therefore also not an option.
One solution to all these problems is to place the tube underground. The underground will have a fairly constant temperature environment. Therefore, the problems mentioned above will not arise. The lack of temperature difference also minimizes the temperature strains, and therefore the chance of fatigue. Maintenance will therefore hardly be needed.
Moreover, most of the urban areas in the world are situated in coastal areas. This means that the structure has to be protected against the eroding effect of this saline environment. This way, the structure needs to be covered in a protective layer. However, a saline environment may break this protective cover and may cause corrosion of the structure itself. For a steel structure, this would mean the structure itself will lose strength and also will become porous, therefore the tube will lose its air-tightness.
The underground will also be a saline environment, when the pore water around the tube will be brackish. On the other hand, the tunnel will be buried deep in the soil. The pores will be completely saturated, so that corrosion cannot take place. This protective water layer is therefore likely to withstand any influences from outside
Urban planning and landscape pollution
In densely populated areas, it is extremely difficult to construct the Hyperloop tubes above ground. The immense radii of the various bends make it impossible to have the infrastructure fit within the current framework of urban planning. Moreover, the construction of the tubes (which will range around 3 m in diameter) will lead to landscape and noise pollution. Current trends in infrastructure construction show that this landscape and noise pollution are no longer accepted by a large portion of the inhabitants. Moreover, this landscape pollution would lead to an immense drop in property value. This should at all costs be prevented, as this will impose a negative effect on the social acceptance of the Hyperloop concept.
Structure-wise, it might be better to situate the tubes underground. Especially in earthquake prone areas, earthquakes might lead to a significant design load, which will become economically unfavourable. In the underground the earthquake loads on the structure may be lower, especially if the tube is situated at the correct position and in strong soil layers. Moreover, the segment division of the tunnel makes it more flexible and therefore more earthquake resistant.
There are however also some significant downsides to building the Hyperloop system underground. Some of them are:
The costs for underground construction are higher than that of above ground construction. The specific factor for this is highly dependent on the external circumstances, such as soil strength.
Since the costs are higher, additional ways of obtaining a turnover or reducing the costs need to be defined. One way of increasing the turnover of the system is to use the tubes being built also for the use of utilities. Such a combined hyperloop and integrated utility tunnel can be seen in Figure 1.
The additional utility space can be used for carrying cables for electricity, telecom, and/or be used for the transport of waste water or heavy showers. Since these utilities are now also placed underground, but hard to access, putting them in a hyperloop tunnel will increase the maintainability of these systems. Moreover, the utility companies will pay for the use of these tunnels, increasing the profitability of the tunnel system.
Longer construction time
Bored tunnels are constructed using Tunnel Boring Machines (TBM). These machines are able to bore at a rate of 15 m/day. When Hyperloop will be constructed underground, this rate of drilling is simply too low. It will take forever to realise a full-scale track, even when using multiple machines.
Various initiatives have been started on the speeding up of these tunnel boring machines. One of them is the Boring Company, which is one of the companies that is owned by Elon Musk. The Boring company aims on speeding up the process tenfold and therefore drilling 150 m/day. This will be achieved by a more autonomous boring process and continuous drilling procedures. At this moment, lining installation and drilling cannot happen in a parallel manner in soft soil conditions.
The issues of costs and construction time will need to be tackled, in order to make an underground Hyperloop viable. There are however many factors that contribute to the economic potential. Moreover, in some areas, there is no other option then positioning it underground. The rate of technologies that make fast boring and installation of tunnels possible will ultimately determine whether there is enough potential for an underground Hyperloop.