Investigation of fault behaviour during shear process for weak and strong materials using 3D printing technologies

Morphology of rock joints (faults, fractures) has been recognized as the key factor controlling their mechanical behaviour, including the pre-peak and post peak phases as well as dilatancy. It also appears playing a significant role in two mechanical behaviours that have been observed on natural fracture during shearing: (i) joint dilation, or (ii) joint closure in association with asperities crushing, and rock matrix plastic deformation. We examine how (i & ii) occur in the joint, discussing their relationship with normal stress, joint morphology and intact matrix mechanical properties. To do this, two innovative methodologies based on 3DP technologies using a sand and phenolic binder on one side and a polymer (PA12) and binder jetting technology on the other one are applied to built fractures in a weak matrix, and in a strong matrix respectively. Joint surface roughness are built as fractal property with a self–affine replication, in accordance with natural observations. Results of direct shear tests under constant normal stress reveal that the mechanical behavior of the joints is first controlled by the mechanical parameters of the material (UCS/σ_n ratio), then by the joint geometry. In the case where the UCS/σ_n ratio is high (>40) corresponding to a strong material compares to the mechanical solicitation, no significant damage is notice on the joint and the maximal dilation angle is controlled by the steepest angles of the shorter wavelength asperities, which may only represent a small percentage of the surface roughness. In a the case of a weak material the joint behaviour is more complex, and is controlled by a specific range of asperities sizes. Three behaviours were observed depending on the applied normal stress: (i) at low normal stress the larger wavelengths asperities cause dilation since they are not sheared off; (ii) at normal stress over 40% of the UCS value, tensile and/or slip cracks were observed around those asperities, leading to their crushing and beheading; (iii) at normal intermediate stress, the two mechanisms were conjointly observed. In the second case (ii) joint closure is observed and the permeability increases in the surrounding matrix. Those results implies that the UCS/σ_n ratio plays an key role in fault shear behaviour, off-fault damage propagation and fluid circulation.