Faults and fractures form the largest contrast of fluid flow in the subsurface, while their permeability is highly affected by effective pressure changes. In this experimental study, fractured low-permeability Flechtingen (Rotliegend) sandstones were cyclically loaded in a MTS tri-axial compression cell.
Two different loading scenarios were considered: “continuous cyclic loading” (CCL) and “progressive cyclic loading” (PCL). During continuous cyclic loading, a displaced tensile fracture was loaded hydrostatically from 2 to 60 MPa in several repeated cycles. During progressive cyclic loading, the load was increased with a step-wise function (15, 30, 45 and 60 MPa) and unloaded after every loading step. For full elasticity of rock matrix deformation each rock sample has been preconditioned up to 65 MPa. After that, an artificial tensile fracture was introduced into the sample using the Brazilian Disk test. The fractured sample was installed into the MTS triaxial cell at a given offset of 0.5 mm and hydrostatic loading was applied accordingly. The fracture permeability was measured continuously using the cubic law calculated from the hydraulic aperture. Fracture closure was measured using LVDT extensometers during the entire experiment and the resulting fracture closure and stiffness was calculated accordingly. The total deformation of the sample was corrected by the amount of elastic deformation of the rock matrix to obtain the fracture closure only. Potential changes to the fracture surface topography before and after the experiments were analysed from high-resolution surface scans obtained by a 3D profilometer using the fringe pattern projection.
The scale-independent roughness exponent was calculated using power spectral density method assuming self-affinity. The fracture aperture distribution and contact-area ratio was calculated by matching the best fitting principal planes of the bottom and top surface and applying a grid search algorithm. The results showed a “stress-memory” effect of fracture stiffness during progressive loading that can be used to identify previous stress states in fractures. This effect is characterized by a transition from a non-linear to a linear (reversible to non-reversible) behaviour of specific fracture stiffness when a previous stress-maximum is exceeded. Furthermore, the evolution of fracture permeability shows less reduction during progressive cyclic loading compared to continuous cyclic loading.
The data measured during the flow-through experiment under varying effective pressure are provided in the file “MTS_data.zip”. The data are provided as separate text-files as well as in Excel format with different spreadsheets, such that each figure in the paper can be recalculated and that the underlying data is comprehensive. The name of all three rock samples is given in the file name including the type of the experiment (CCL or PCL). The fracture surfaces and the fracture aperture distributions are found within the file “Surface_data.zip”. This file contains the fracture data of each of the three rock samples as point cloud data (text-files), as well the data calculated from the surfaces.
Seismic wave propagation in granular soils can induce large strain amplitudes in case of strong earthquakes. Seismic motions are irregular in frequency content and in amplitude, and have three different components in orthogonal directions. In this context, the main objective of this PhD research deals with nonlinear effects observed in granular soils under such complex loadings. A dynamic triaxial press was developed for dry and undrained saturated sand samples. Axial and lateral stresses can be applied independently with large amplitudes for various loading shapes. An innovative laser-based non-contact measurement technique was developed to continuously monitor the sample radius Dry and undrained cyclic tests performed on Leman Sand at various frequencies from 0.1 to 6.5 Hz show that the behaviour of this granular material is frequency-dependent at medium to large strains. Sand stiffness, which depends on stress conditions, seems to influence the extent of frequency effects on soil behaviour: for tests with lower stiffness, the soil response to low frequency is significantly amplified compared to the high frequency range. The overall rate-sensitivity may be enhanced by the angularity of the grains. Other cyclic undrained saturated tests on Leman Sand demonstrate that the superposition of two different loadings, one axial and one lateral (bidirectional tests), induce coupling effects in the nonlinear soil response. Experimental results are finally modelled with the linear equivalent method and with a multi-mechanism elastoplastic model (ECP Hujeux). Nonlinear effects observed in laboratory experiments, and particularly the increase of strain amplitude leading to cyclic liquefaction of dense sand, are well captured by the elastoplastic model. Assessing the behaviour of granular soils under earthquake loadings clearly requires to take into account the nonlinear features of sand behaviour in terms of pore pressure generation and strain amplitude.