Experimental Study Of Mechanical Properties Of Soft Rock With Water

Water plays a dominant role in affecting short-term or long-term stability ofgeotechnical engineering. What’s more, it is concerned gradually that geologicaldisasters are caused by water. Water-rock interaction influences the strength anddeformation of rock, resulting in deterioration of rock characteristics, which has asignificant impact on the stability of geotechnical engineering. However, based onprevious studies, combing with relevant practical engineering, mechanisms ofmechanical characteristics of sandstone caused by water-rock interaction are studiedsystematically through laboratory testing methods. All in all, the main work andconclusions including:(1) Tri-axial compression experiment is carried out to investigate dry and saturatedred sandstone specimens. The results show that the effects of water on strengths anddeformation characteristics of red sandstone are obvious. Comparing with driedspecimens, the strength and deformation parameters of saturated specimens, includingstrength, elastic modulus and deformation modulus, decreased in different degree.Elastic modulus and deformation modulus increase with confining pressure. The changetrend of elastic modulus and deformation modulus under different situations issignificantly different. Mogi-Coulomb criterion fits experimental results better thanMohr-Coulomb criterion in comparison. The fracture of dry specimens under lowconfining pressure is controlled by tensile-shear mixed mode fracture, which iscontrolled by shear fracture as confining pressure increases; while the saturatedspecimens rupture by shear fracture.(2) Several saturated sandstone specimens have been tested with meso-mechanicstesting system. The result shows that the crack initiate at the pre-existing hole whenstress of dry and saturated sandstones levels off at the peak stress, before propagating tothe entire specimen and forming a crushed zone. By contrast, the loads of saturatedspecimens which are required for crack initiating are smaller. Moreover, water-rockinteraction weakens rock brittleness. The failure mode of sandstones is mainly shearfailure whether initiating cracks are tensile cracks or shear cracks. What’s more, thecracking process is more complicated and its rupture form is multiple because ofwater-rock interaction. As for the strength, its decreases dramatically comparing withdry specimens.(3) A series of tri-axial tests under loading and unloading conditions are operated on dry and saturated sandstone specimens. It is shown that there are some differencesamong deformation behavior, mechanical properties and fracture mode under loadingand unloading conditions. By contrast，cohesion of saturated sandstone under unloadingcondition increases to some extent, while friction angle declines. Meanwhile, As for thesaturated sandstones, their confining pressure under loading and unloading conditionsare less than that of dry sandstones. Moreover, comparing with the loading text, thefailure process under unloading condition is more complicated and initiates immediately,indicating that brittle characteristics are obvious.(4) The split tests on sandstone which was in drying, wetting and long-termimmersion after cyclic saturation-drying have been carried out in order to explore thelong-term effect of water-rock interaction. The influences of water-rock interaction onrock tensile strength were analyzed. Test results show that, comparing with thespecimens of natural state, the tensile strength of the specimens after cyclesaturation-drying decreased by various extent, and the decrease degree become lessgradually with increasing cycles. The strength of the specimens saturated with a longtime decreased significantly during the first few days, but did not present obviouslychange along with the immersion time. The damage of cyclic saturation-drying on therock tensile strength is greater than the long-term water-saturation.