Behavior of microcontacts in rock joints under direct shear creep loading

• The mechanical behavior of fractured rocks induced by shear or tensile mechanism is different.
• The failure mode in microscope of fractured rocks depends on the fracture formation.
• Shear fracture and tensile fracture display a distinct contact configuration under creep shear loadings.
• The creep shear strength of fractured rocks closely relates to the contact areas of the fractured rocks.

Two types of fractures were induced in sandstone by means of modified shear and indirect tension. The fractured rock samples were assembled following their initial formation and direct shear tests were performed under a constant normal force and stepwise increased shear creep loadings. Using computerized tomography (CT) and laser scanning, the micro-contact mechanism of the fractured rock surfaces were observed and measured before, during, and after the shear tests. Under a moderate normal force, dilation tends to occur in the shear fractured rocks; with the increase of normal force, the peak shear strength of tensile fractured rocks grows faster than that of shear fractured rocks and eventually gives rise to a higher value. A few micro-contact modes and fracture phenomena of the rough surfaces in contact are recognized through CT and laser scanning images, such as shear-off of micro-asperities, friction between macrowaved rough surfaces, migration of worn debris, propagation and healing up of hybrid cracks in subsurface. The long-term shear strength of fractured rocks is mainly composed of two mechanisms: one is the shear resistance raised from interlocked microasperities in the scale of roughness in tensile fractured rocks, the other is the frictional resistance produced between macroasperities in the scale of waviness on the shear fractured rocks. The microcontact configuration and contact area are observed and measured by slice cutting of the mated rough surfaces in contact. Two distinct patterns of contacts in rock joints are recognized, term as coastline pattern and islands pattern. The shear strength of fractured rocks is closely related to the maximum contact area and the surface roughness.

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