Thermal image and spectral characterization of roadway failure process in geologically 45° inclined rocks

Abstract

Large-scale geomechanical model test was conducted in order to investigate stability of an un-supported tunnel with rectangular cross section embedded in 45° inclined alternating strata of sandstone, mudstone and coal seam. The loading path consists of two groups: cases A–G with overburden depths from 296 to 948m and small loading rate, and cases H–M with overburden depths from 1126 to 2047m and fast loading rate. Infrared thermography, incorporated with image processing and Fourier transform, was employed to characterize the rock responses. Averaged temperature field, 〈IRT〉, represents energy release rate, oscillating at stick–slip pattern with different periods and amplitudes. Overburden depth and loading speed have a significant impact on 〈IRT〉 curve, i.e. small overburden depth and loading speed corresponds to long period and small amplitude; whereas, great overburden depth and fast loading speed to short period and high amplitude. The processed thermal image best represents rock behavior by two major IRT distribution modes. For loading cases A–G, the coal strata were over stressed indicated by high IRT while the mudstone strata were less stressed represented by low IRT, corresponding to the static interlayer friction. For loading cases H–M, the mudstone strata were over stressed indicated by high IRT while the coal strata were less stressed indicated by low IRT, corresponding to the dynamic friction. Fourier spectra and spatial frequency were employed to characterize the infrared sequence. Ultra-high spatial frequency component is a precursor for predicting the imminent dynamic event. Low spatial frequency component may be served as an indicator of the tunnel-wide sphere of influence that the stress redistribution extends.