Fractal contact and asperities coalescence of rock joints under normal loading: Insights from pressure-sensitive film measurement

Direct measurement of the real contact area of rock joints under normal loading is crucial for comprehending the subsurface geological processes. However, measuring this phenomenon quantitatively at site-scale or laboratory-scale is challenging. Here, we investigate the evolution mechanism of the real contact area in rock joints by conducting closure tests on artificial and saw-cut sandstone joints under normal stresses up to 50 MPa. Geometrical shapes of contact patches are quantified by the pressure-sensitive film using the adaptive threshold method. An extensive range of contact stress within contact patches is innovatively measured by integrating the results from multi-type pressure-sensitive films. Experimental results demonstrate that the real contact area increases with the increasing normal stress hyperbolically. Such a nonlinear contact evolution behavior can be attributed to the coalescence of adjacent contact patches. The fractal dimension of composite surface governs the geometrical shapes of contact patches and the distribution of contact stress. The relationship between patch areas and bearing loads follows the Hertzian theory when the patches are small, while it gradually becomes linear with the increasing patch size. A power model with exponential cut-off is proposed to predict the size distribution of contact patches. This work can provide new insights for estimating the patch-dependent seismic nucleation length and slip stability of subsurface joints.