Concrete Hyperloop

Abstract

In August of 2013 Elon Musk published his paper ’Hyperloop Alpha’, in which he describes an alternative transportation system for the to be built railway connection between Los Angeles and San Francisco. The Hyperloop system that Musk describes relies on partial vacuum tubes through which vehicles can travel at speeds up to 1000 km/h with great efficiency. An innovative breakthrough in the transportation sector if it can be made into a reality. Therefore his company SpaceX organised a contest to which many teams applied to built a concept vehicle to be used in the tubes. In 2017 a team of students from TU Delft won the overall prize for the best design and afterwards the team leaders founded Hardt to develop their ideas and make Hyperloop into a reality. BAM Infraconsult supports Hardt and together they develop the Hyperloop and its tube infrastructure. Currently the preliminary design for the tubes is made in steel and no alternatives have been considered. This research focusses on an alternative tube design in concrete. The tube has to not only satisfy the strict requirements that apply to the Hyperloop technology in general, but it also has to adhere to the specific design philosophy of the Hardt vehicle concept. A concept in which the vehicle is suspended from tracks on the ceiling of the tube by a series of magnets. The report is based on the following research question: Is a concrete design a feasible alternative for the design of airtight Hyperloop tubes compared to the considered steel tubes? To answer this question two philosophies are considered and researched into depth. Firstly a design in prefabricated normal strength concrete is analysed, making use of prestressing and conventional reinforcement, following the requirements set in the Eurocode. Secondly an innovative ultra high performance steel fibre reinforced concrete (UHPFRC) design is considered. Again prestressing is applied, however conventional reinforcement is not present in this design as it relies on steel fibres and prestressing alone. This design is modelled according to the French UHPFRC guidelines. A requirement for the design is that the inner diameter of the tubes is equal to 3.6 m and that the tubes span 30 m between pylons. The design methodology is based on a variable thickness of the tubes in order to find the optimal concrete structure that satisfies all requirements. Checks are made for multiple failure modes after which the governing minimum thickness is derived. Other than proving to be structurally sound according to the Eurocode, the tube also has to abide to a strict deflection limitation under dynamic loading conditions. In order to research this criterion the tube design is validated using finite element modelling. Here a combination of different parameters related to the concrete material characteristics and the dimensions of the tube is researched by means of a sensitivity analysis. Using the dynamic amplification factor specified by Hardt, the maximum deflections of the tube are checked and the parameters that have the greatest influence on the deflection are discussed. The obtained results are compared to the steel tube. Furthermore the possibilities for the construction of a concrete tube are considered, including a recommendation for the connections between tube sections. Moreover the transportation of the tubes to the building site and final assembly are taken into account. After all factors are considered a cost estimation is made for the design of the concrete tubes and compared to the cost estimation of the initial steel design of the tubes. This cost estimation does not only consider the material and production costs of the tubes, but also takes into account transportation, assembly, maintenance and site costs plus an allowance for risks and profit. It is concluded that a design in normal strength concrete is possible, but not practical. Its dimensions make the production and assembly very difficult. However a design in UHPFRC is feasible, as its required dimensions are considerably better. Furthermore it satisfies all structural requirements without the use of conventional reinforcement and even the strict deflection criterion at dynamic loading conditions can be achieved. Moreover the design in UHPFRC is a competitive alternative to the steel design, as it has a higher score when all criteria are considered.