Tonstein als Wirtsgestein: Ein geomechanischer Beitrag über Opalinuston
Authors: Lempp Christof , Menezes Flora , Sachwitz Simon 
Published: January 2016

Der Opalinuston ist ein prominenter Vertreter der organikhaltigen Tonsteine oder Schiefertone, die in der Schichtenfolge aus Trias und Jura regional ein mehrere Zehnermeter mächtiges, relativ homogenes Schichtpaket bildet. In Abhängigkeit vom Verfestigungsgrad der Pelitgesteine variieren die geomechanischen Parameter Scherfestigkeit, Zugfestigkeit und Durchlässigkeit, die bei einer zu diskutierenden Nutzung als Wirtsgestein für die Lagerung radioaktiver Abfälle auch bautechnische Bedeutung erlangen. Ein Tunnelvortrieb im Opalinuston bot die eher seltene Gelegenheit, Proben aus unverwittert frischen Schiefertonen in größerem Umfang als bei üblichen Bohrungen zu gewinnen. Es werden geomechanische Laborversuche, insbesondere mehrstufige, triaxiale Kompressionsversuche und Hydraulic‐Fracturing‐Versuche an definiert räumlich eingespannten Quaderproben ausgewertet, um Scherfestigkeit, hydraulische Zugfestigkeit sowie Durchlässigkeits‐ bzw. Abdichtungseigenschaften des Opalinustons zu charakterisieren. Einführend wird in diesem Kontext die Bedeutung des Sättigungszustands und dessen Veränderlichkeit diskutiert. Die ermittelten geomechanischen Parameter werden verglichen mit vormals höher und mit ähnlich beanspruchten, äußerlich gleich erscheinenden Pelitgesteinen aus Japan und aus der Schweiz.

Clay shale as host rock: a geomechanical contribution about Opalinus clay. The Opalinuston is a prominent rock representing the type of organic clay shales or clay stones within the sequence of Triassic and Jurassic marine sediments in Southern Germany. The rock forms a homogenous unit some ten meters thick. The degree of consolidation of this type of pelitic rock depends mainly on the former load conditions, but is also dependent on the long‐term weathering and even on the present exposition. The geomechanical parameters such as shear strength, tensional strength and permeability vary with the state of consolidation and become important when the use is discussed of such rocks for radioactive waste disposal.

A tunneling project at the northern escarpment of the Swabian Alb (Southwest Germany) within the Opalinus clay offered the rare opportunity to obtain fresh unweathered rock samples in greater amounts compared to fresh drilling cores from which geomechanical investigations are usually undertaken. Consequently, the results of geomechanical laboratory testings are presented in order to compare here the results of multistep triaxial compression tests, of hydraulic fracturing laboratory tests and of some other tests for rock characterization with the corresponding results of Opalinus clay sites in Switzerland that were investigated by the Swiss Nagra Company for host rock characterization.

After a discussion of the relevant state of fresh Opalinus clay, especially of suction pressure conditions and saturation state, the results of triaxial shear tests are presented. Increasing shear deformation at increasing pressure and unchanged water saturation do not result in a significant strength reduction of the Opalinus clay. The rock shows increasing cohesion and stiffness, if multiple loading has repeatedly reached the failure point. Thus there is no increased permeability with continued shearing. Only at the beginning of the shearing process is a temporarily increased permeability to be expected due to dilatation processes. An increased permeability, however, is generated by hydraulically injected water when fracturing may occur. Such fracturing depends on the injection rate: Low injection rates lead to local structural disintegration, higher injection rates result in single crack propagation as is known from hydraulic fracturing tests. Such hydraulically formed fractures later tend to close with an increase of surrounding stress. This closure increase is comparable to a healing process because new hydraulic fractures can be generated consequently at higher stress levels. Such laboratory test results seem to be favorable when using fresh Opalinus clay as a host rock, but they are difficult to verify at standard in‐situ‐conditions with weathered or partly deconsolidated rocks.

Similar organic‐rich and differently overconsolidated pelitic rock types display a wide spectrum of geomechanical behaviors despite the fact that no visible or physical differences can be provided. A direct comparison of the Opalinus clay parameters from the Swabian Alb, as investigated here, and from Switzerland, as determined by the Nagra Company, show differences in the effective shear strength. Nevertheless, the scatter is much smaller than that occurring for undrained shear stress differences as a function of water content.

Consequently, for each project one must define exactly the relevant conditions within the different pelitic rocks in order to determine the useful geomechanical parameters. Additionally, it can be an advantage to use great depths because of the favorable geomechanical characteristics of Opalinus clay at high pressure. In this way some of the construction problems in Opalinus clay may be reduced if not avoided by using deep boreholes for waste deposition.