Editors’ message: The housing crisis from underground—damage to a historic town by geothermal drillings through anhydrite, Staufen, Germany

Abstract

Geothermal energy can be used for heating, electric power generation and other applications (e.g. Seyboth et al. 2008). The confidence of the public in this technology is currently compromised by several bad examples where inappropriate implementation of geothermal installations has caused avoidable damage. A dramatic but instructive case is presented here. This article is a plea both for the reasonable use of geothermal energy and for more geologic and hydrogeologic competence and education in geothermics. Connoisseurs of classical German literature know the historic town of Staufen (7,800 inhabitants; Fig. 1) as the town of Doctor Faust, immortalised by the eponymous drama written by Johann Wolfgang von Goethe (1749– 1832). The town is currently suffering severe damage that began immediately following geothermal drillings in the city centre. Staufen is located in the Upper Rhine Graben, within the tectonic transition zone between the central graben and the Black Forest. The graben is filled with several kilometres of sediments; the Black Forest mainly consists of crystalline rocks. In the transition or foothill zone (German: Vorbergzone), Mesozoic sedimentary rocks are present below the surface or crop out as ridges and hills, reflecting the fault pattern and tilting of the strata. In this region, the Upper Triassic (Keuper), which is also present below Staufen, includes gypsum and anhydrite layers and lenses of substantial thickness, which is common knowledge and documented in publications and geologic maps. Even a standard German-language textbook entitled Geologie von Mitteleuropa (Geology of Central Europe) mentions noteworthy anhydrite occurrences in this region (Walter 1992). Anhydrite transforms into gypsum when it comes in contact with water, resulting in a volume increase of up to 61% that can generate pressures of 5–10 MN/m. Geotechnical engineering books, including traditional German textbooks, discuss this problem and recommend avoiding any contact between anhydrite and groundwater in all types of construction activities (Prinz 1991). There are numerous well-studied examples, internationally and in southwest Germany, where swelling of anhydrite caused severe geotechnical problems during construction of roads, tunnels or buildings. In the USA alone, an oftcited study by Jones and Holtz (1973) calculated that shrinking or swelling materials inflicted at least $2.3 billion USD annually in damage to houses, buildings and roads. This geologic and geotechnical knowledge appears to have been ignored during the Staufen project, and a predictable disaster has run its course. In late 2007, seven boreholes, up to 140 m deep, were drilled into the gypsumand anhydrite-bearingKeuper formations, creating hydraulic contact between anhydrite and confined groundwater. The boreholes were cased in the uppermost part, while the deeper parts remained uncased. A few weeks after drilling, cracks started to form in nearby buildings. Geodetic measurements soon revealed that the ground was rising. Since then, the impacted area has increased steadily; 131 houses are currently affected, with some houses seriously damaged (Fig. 2). The highest uplift exceeds 1 cm/month, with no signs yet of slowing. The uplift rate and the degree of damage generally decrease with increasing distance from the drillings (statement to the press, released in December 2008 on the website of Staufen, www.stadt-staufen.de). One type of crack is particularly prominent and frightening: vertical, open cracks that seem to be oriented towards the area of the drillings and tend to cut the impacted buildings from the ceiling to the roof (Fig. 3). It is conceptualised that these types of radially divergent, vertical cracks also exists underground, caused by serious swelling of a cylindrical zone around the drillings. At the N. Goldscheider ()) Centre of Hydrogeology (CHYN), University of Neuchâtel, Rue Emile-Argand 11, 2009, Neuchâtel, Switzerland e-mail: nico.goldscheider@unine.ch Tel.: +41-32-7182645