Investigation on Physicomechanical Properties and Constitutive Model of Tuff in Mila Mountain Tunnel under Dry and Saturated Conditions

Many tunnel engineering accidents are generally caused by water softening tuff of high porosity. Experimental and theoretical analytical methods, including rock ultrasonic testing, X-ray diffraction (XRD), microscopic observation, uniaxial compression test, and scanning electron microscope (SEM), are employed to analyze the physicomechanical properties of tuff in Mila Mountain tunnel under dry and saturated conditions. Then, the mechanism of tuff softening in water is explained. Finally, the statistical damage constitutive model of tuff is established. It was revealed that the tuff compositions were dominated by quartz and clay minerals accounting for more than 90%, and clay minerals, anhydrite, and pyrite were mainly soluble minerals. After being saturated with water, the soluble minerals in the tuff are dissolved, and the porosity and wave velocity are increased; however, the elastic modulus and peak strength are decreased, indicating that water softening was distinct. Water softening after saturation was due to the mineral compositions and microstructure characteristics of tuff in Mila Mountain tunnel; specifically, as the tuff characterized by high porosity was conductive to water absorption, the soluble minerals in the tuff were corroded and swelled by water, dissolving, loosening, and softening the tuff structure; then, its mechanical behavior was degraded. It was demonstrated by the experimental results consistent with theoretical results that the model can be employed to express the constitutive behavior of tuff in Mila Mountain tunnel under dry and saturation conditions. The findings provide insights into macroscale deterioration of tuffs and theoretical knowledge for the tunnel excavation and support of Mila Mountain tunnel.

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