A "Zuckerdolomit"-sample from the Piora Mulde (Switzerland), studied by an in situ applied load experiment using neutron time-of-flight diffraction Eine "Zuckerdolomit"-Probe aus der Piora-Mulde (Schweiz), getestet in einem in situ Belastungsexperiment mit Neutronen-Flugzeit Diffraktion

“Zuckerdolomit” is a rock, which - due to its geomechanical behaviour – has frequently been discussed by geologists, last but not least, because its importance from the geoengineering point of view. A sample collected in the Piora Mulde (Central Switzerland) was studied in a long term experiment applying uniaxial load to a cylindrical sample (30 mm O, 60 mm length). The experiment was carried out at the time-of-flight-diffractometer EPSILON-MDS in combination with the pressure device EXSTRESS at the pulsed neutron source IBR-2 in Dubna (Russia). During a previous experiment texture and residual strain were determined for the same specimen (Scheffzuk et al., 2008). Applied load was increased in steps up to 36 MPa, in each case separated by an exposition time of 24 h. This final level was followed by a period retaining the load by fixed pistons (17 days). Afterwards, the sample was completely unloaded and measured within this state before load was increased again up to the level which was reached already prior to unloading. The attempt to continue the experiment by a further increase of applied load failed: The sample immediately collapsed into a sandy (medium grain size) material. The entire experiment was carried out over a time span of about 591 hours. Dolomite- and anhydrite lattice spacing were determined for each stage of deformation, by calculation from the obtained neutron diffraction patterns. Till the 17 days interruption of the experiment, all dolomite spacing reacted more or less uniform to the increasing compression, - in contrast the anhydrite responded more irregular. Deviatoric loading was observed during the time of clamped pistons. As a result of unloading, the dolomite lattice spacing responded with medium strain release. In comparison, strain release of the anhydrite spacing was much stronger. From the experimental results one may conclude: The sample could be uniaxially loaded up to ~36 MPa, but collapsed if repeated loading up to about 38 MPa followed a state of complete unloading after a previous “longer” rest under a fixed final load of ~ 36 MPa. It needs to be assumed, that – besides the residual strain state - distinctly different elastic constants of the components increase the effects of texture relations within the rock. In addition, the chosen time scale may have determined the achieved experimental result. Clearly, the hydration of anhydrite to gypsum may be one, but not the only reason for the special behaviour of the material.