Energy foundations and other energy geo-structures – geotechnical contributions to alleviate the global energy crisis

Abstract

The subsurface of Earth contains an enormous potential of geothermal energy that can be used for heating and cooling purposes. With advances in the development of the ground-coupled heat pump, it is now economical to use this geothermal energy available in the ground. Shallow and deep foundations of simple houses and modern high-rise buildings, concrete structures of cut-and-cover tunnels such as bored piles and diaphragm walls, and tunnel linings and anchors of NATM and TBM tunnels and even sewer systems can be used as absorber elements for geothermal heat exchange. The technology is based on absorber pipes laid in concrete structures and a heat transfer fluid circulating through the system. The heat is extracted from the ground or stored in the ground to heat or cool the buildings connected to it. This promising technology results in a significant reduction in fossil energy consumption with moderate installation and low operating costs. This paper describes the fundamentals and design principles of this highly innovative technology and some of its applications in Austria, ranging from two test installations at the 15 km long Lainzer Tunnel to the Vienna U2 subway extension and the new Vienna Central Station. In addition, the results of a large-scale field test with thermally loaded energy piles focusing on load displacement behavior are presented. Four of the stations on the U2 subway extension in Vienna, which opened in 2008, have been covering most of their heating and cooling needs from geothermal energy generated by energy diaphragm walls, energy piles, and energy slabs on the station buildings for nearly a decade and a half. As this was the world's first infrastructure building of this type and scale at the time of construction, one of the diaphragm walls of the Taborstraße station was equipped with numerous sensors to obtain data sets on the long-term thermo-mechanical behavior of these wall elements over a period of about 14 years of operation. The foundation of the new Vienna Central Station serves as a geothermal heat exchanger. The basic heating and cooling needs of the station building are covered entirely by geothermal energy. Recent large-scale pile load tests on two energy test piles under coupled thermal and mechanical loading show that the thermal influence is clearly visible, but neither the load displacement behavior nor the bearing capacity is deteriorated by the thermal loading. The presented projects – all from the last two decades – show promising and sustainable performance in terms of geothermal energy generation and energy storage potential as well as thermo-mechanical behavior of the energy foundations and energy geo-structures.