Prevention of speleothem rupture during nearby construction Corinne Lacave Martin W. Koller Urs Eichenberger Pierre-Yves Jeannin Abstract The cave of Milandre, located in the Swiss Jura, is characterised by a rich speleothem popula- tion. Some of the most decorated galleries lie at low depth (40–50 m). A new highway will be built, with a tunnel, just over a decorated part of the cave. In order to prevent damage in the cave, a study was conducted in the framework of the impact study. One of the goals was to evaluate the risk of speleothem rupture caused by explosive shots that will be used to excavate the tunnel portal. To this end, the speleothems’ vulnera- bility to ground shaking was compared with the observed accelerations induced by experimental explosions. This procedure allowed to determine maximum acceptable explosive charges. Keywords Cave protection Explosive shots Speleothem Milandre Vulnerability Introduction The cave of Milandre, located in the Swiss Jura, is characterised by a rich speleothem population (Gigon and others 1986). This site was proposed to be of national value. Some of the most decorated galleries are semi-active galleries lying at low depths (40–50 m). A new highway (A16) will be built just over a decorated part of the cave. This part of the highway will necessitate the construction of a tunnel, which implies the use of explosive charges to excavate the portal. In order to prevent damage in the cave, a study was conducted to assess the environmental impact. One of the goals was to evaluate the risk of spe- leothem rupture due to explosive shots that will be used to excavate the tunnel portal. A few similar experiences have occurred in other caves where explosions had to be used in the cave tourist settings. Montovani (2001, personal communication) experienced the driving of two tunnels near a unique soda- straw (tube thin stalactites) population in Europe (3–4 m long), in the cave of Choranche (Vercors, France). No damage to the soda-straws occurred, even though shaking was observed due to explosions in a particular rock layer. This shows that the nature of rock and the associated wave-attenuation characteristics can produce very variable ground shaking. This is the reason why it is strongly recommended to proceed to in-situ test explosions before any future work. Another experience was made by the Tunnel Study Center (CETU, France) during driving an artificial well in the cave of Orgnac (Arde`che, France). Some in-situ measurements were conducted in the cave to assess vibration values. The CETU pointed out (2001, personal communication) that the shot plan is as important as the total explosive charge used. Explosions in other caves near rich speleothem popula- tions have been often conducted blindly, without any knowledge or estimation of the rupture risk to speleo- thems’. In the present study, the speleothems’ vulnerability to ground shaking was compared with the observed ac- celerations induced by experimental explosions. The first part of this paper presents the way that the speleothems’ vulnerability has been studied and the second part shows the results of the test explosions. Conclusions are derived from a combination of both approaches. Speleothem vulnerability Laboratory resistance tests In the last decade, some investigations have been made to check the influence of earthquakes on speleothems in caves (examples can be found in Forti 1998; Gilli and others 1999; Cadorin and others 2000). A more complete study was conducted in the cave of Milandre in order to determine whether earthquakes can break speleothems, and whether broken – or unbroken – speleothems can be used for palaeo-seismic investigations (Lacave and others Received: 9 March 2002 / Accepted: 21 September 2002 Published online: 14 November 2002 ª Springer-Verlag 2002 C. Lacave (&) M.W. Koller Re´sonance Inge´nieurs-Conseils SA, 21 rue Jacques Grosselin, Carouge, Switzerland E-mail: corinne.lacave@resonance.ch Tel.: +41-22-3010253 Fax: +41-22-3010270 U. Eichenberger P.-Y. Jeannin Swiss Institute for Speleology and Karst Studies (ISSKA), CP 818, La Chaux-de-Fonds, Switzerland Original article 892 Environmental Geology (2003) 43:892–900 DOI 10.1007/s00254-002-0719-4