Mechanism of strength degradation and energy evolution of semi-saturated sandstone in cold regions under freeze-thaw action

Abstract:

To address the strength degradation of slope rock masses in cold regions under freeze-thaw action， the bluish-grey sandstone from the Yulong Copper Mine slope was taken as the research object. Through indoor freeze-thaw cycles， SHPB dynamic impact tests， and nuclear magnetic resonance （NMR） pore analysis， the dynamic mechanical degradation laws， energy evolution characteristics， and pore structure evolution laws of semi-saturated sandstone under freeze-thaw action were systematically investigated， and a freeze-thaw strength degradation model based on absorbed energy was established. The results show that as the number of freeze-thaw cycles increases， the dynamic peak stress of sandstone decreases from 65.45 MPa to 51.8 MPa， exhibiting a nonlinear degradation trend of rapid initial decline followed by slow attenuation. The dynamic stress-strain curve sequentially undergoes an elastic stage， a strain-softening stage， and a rapid unloading stage. The strain-softening stage gradually shortens with increasing freeze-thaw cycles， accompanied by the phenomena of stress drop and strain rebound. Both incident energy and transmitted energy generally show a decreasing trend with increasing freeze-thaw cycles， whereas the reflected energy increases significantly at 20 freeze-thaw cycles. High freeze-thaw cycles promote the migration of moisture into larger pores， expand the pore distribution range， and enhance the response characteristics of mesopores and macropores. The research findings reveal the damage evolution mechanism of semi-saturated sandstone under freeze-thaw cycles， providing a theoretical basis for the stability evaluation and protection design of slope engineering in cold regions.