Spatial arrangements and clustering of opening-mode fractures in experimental models of layered rocks

Arrays of opening-mode fractures were obtained in three-layer experimental models. They consist of a stiff, brittle competent Granular Rock Analog Material (GRAM) layer sandwiched between two more compliant incompetent elastomer layers. All layers are homogeneous with uniform properties and initially under hydrostatic stresses. Fracturing occurs during horizontal unloading (extension) under constant vertical compressive stress ${\sigma }_v$. The process begins with fractures having spacing ($S$) to layer thickness ($T$) ratio of $\Omega =S/T\ge 1$. At nominal extension strain ${\epsilon }_{xx}>{10}^{-3}$, new fractures form closely to some of the initial ones. They initiate at the layer top and bottom and propagate both vertically and laterally, forming a linear fracture cluster (or corridor) in plan view. The $\Omega$ value within the cluster is 0.01–0.05, consistent with the natural prototypes. Then, other initial or newly formed fractures become centers of cluster growth until the entire layer is filled with clusters at ${\epsilon }_{xx}>{10}^{-2}$, marking the transition from strongly clustered to a more uniform distribution of now closely spaced fractures. All these spatial fracture arrangements also exist in nature, and according to our results depend on ${\epsilon }_{xx}$, controlled by the process driving the fracturing of competent layers. This process involves the lateral spreading of the incompetent materials under the vertical (lithostatic) compression ${\sigma }_v$, and depends on the ${\sigma }_v$ value and the contrast in the elastic moduli between competent and incompetent layers. These findings provide new insights into fracture clustering phenomenon and can serve as a basis for improving its numerical modeling that should be able to reproduce the obtained results.