Design of Sand-Based, 3-D-Printed Analog Faults With Controlled Frictional Properties

Laboratory experiments with surrogate materials play an important role in fault mechanics. They can improve the current state of knowledge by testing various scientific hypotheses in a repeatable and controlled way. Central in these experiments is the selection of appropriate analog, rock-like materials. Here, we investigated the frictional properties of sand-based, 3-D-printed materials. We performed uniaxial compression tests, direct shear, and inclined plane tests in order to determine a) the main bulk mechanical parameters of this new analog material, b) its viscous behavior, c) its frictional properties, and d) the influence of some printing parameters. Complete stress-strain/apparent friction-displacement curves were presented including the post-peak, softening behavior, which is a key factor in earthquake instability. Going a step further, we printed rock-like interfaces of custom frictional properties. Based on a simple analytical model, we designed the a) maximum, minimum, and residual apparent frictional properties, b) characteristic slip distance, c) evolution of the friction coefficient with slip, and d) dilatancy of the printed interfaces. This model was experimentally validated using interfaces following a sinusoidal pattern, which led to an oscillating evolution of the apparent friction coefficient with slip. Our approach could be used for simulating earthquake-like instabilities in the laboratory and mimic the periodical rupture and healing of fault sections. Additionally, our tests showed the creation of a gouge-like layer due to granular debonding during sliding, whose properties were quantified. The experimental results and the presented methodology make it possible to design new surrogate laboratory experiments for fault mechanics and geomechanics.