Study On Mechanical Characteristics And Damage Evolution Of Water-Bearing Sandstone Under Cyclic Impact Loading

In deep geotechnical engineering,a series of frequent disturbances such as cyclic blasting of rock masses,presplitting and pressure relief by multi-point blasting,and multiple aftershocks from earthquakes can cause rock masses to be in a dynamic cyclic impact state.In addition,due to the widespread action of water on deep rock masses,when rock masses are subjected to the combined action of water and the frequent disturbances mentioned above,the dynamic mechanical properties of rock masses will undergo significant changes,making it difficult to determine their stability,posing significant risks to the safety of underground geotechnical engineering.Therefore,in this thesis,a separate Hopkinson compression bar test platform was used to conduct cyclic impact tests on fine sandstone under natural and saturated conditions at different impact velocities,and the dynamic mechanical properties,energy dissipation,failure modes,and damage evolution laws of natural and saturated fine sandstone during cyclic impact at different impact velocities were studied.The main conclusions are as follows:(1)Under low speed impact(1m/s),the compression compaction effect of natural and saturated sandstone during the early stage of cyclic impact is strengthened,and the dynamic peak stress shows a trend of first increasing and then decreasing.However,when the impact velocity is 2 m/s and 3 m/s,it is not affected by this effect,and the dynamic peak stress continues to decrease.In addition,under the three impact velocities,the average strain rate and dynamic peak strain increase as the number of cycles increases,while the dynamic elastic modulus gradually decreases as the number of cycles increases.The greater the impact velocity,the more obvious the weakening effect of water on the dynamic mechanical properties of sandstone.Under the same impact velocity,the dynamic mechanical parameters of natural sandstone are inferior to those of saturated sandstone,and the degradation effect of water increases with the increase of cyclic impact times.(2)In the total energy of sandstone at various cyclic impact stages,the energy absorbed by the rock accounts for the largest proportion of incident energy,followed by transmitted and reflected energy.When the internal damage of the sandstone is large enough,the reflected energy will also be higher than the transmitted energy The energy absorption rate of natural sandstone increases,decreases,and then increases with the number of cyclic impacts when the impact velocity is 1 m/s,and increases with the number of cyclic impacts when the impact velocity is 2 m/s and 3 m/s;When the impact velocity of water-saturated sandstone is 1 m/s or 2 m/s,the energy absorption rate increases and decreases with the increase of the number of cycles of impact.Under different impact velocities,the cumulative unit volume absorption energy of both natural and saturated sandstones increases with the increase in the number of cyclic impacts,while the energy transmittance gradually decreases with the increase in the number of cyclic impacts,and the overall energy reflectance increases with the increase in the number of cyclic impacts.Under the same impact velocity,the accumulated energy absorption per unit volume and energy reflectance of saturated sandstone are always higher than those of natural sandstone,while the energy transmittance is lower than that of natural sandstone.(3)Under low speed impact(1m/s),there is no significant difference in the degree of fragmentation between natural and saturated sandstones,but as the impact velocity increases,the degree of fragmentation of sandstones becomes higher and higher,and the degree of fragmentation of saturated sandstones is higher than that of natural sandstones at the same impact velocity.Microscopically,natural sandstones are prone to transgranular failure,while saturated sandstones are mostly destroyed along initial cracks or pores.(4)Based on statistical damage theory and regression analysis,a damage constitutive model was established,and the overall fitting effect was good.The damage variables calculated based on this model increased with the increase of the number of cycles of impact at impact velocities of 1 m/s and 2 m/s,while at impact velocities of 3 m/s,they tended to decrease first and then increase.At the same impact velocity,the damage degree of the saturated sample is significantly greater than that of the natural sample.The comparison between the modified damage calculation based on elastic modulus and the dynamic statistical constitutive calculation shows that the formula can better predict the damage of samples during cyclic impact failure,and can be used to predict the stability of water-bearing rock masses in geotechnical engineering such as frequent blasting.Based on geological conditions,the parameters for presplitting and pressure relief of roof blasting were designed and a double hole blasting simulation was conducted.The results showed that the adjacent rock fractures between holes in natural and saturated rock layers were connected after blasting,but the saturated rock layer had a better effect on rock fragmentation between holes compared to natural rock layers,with a larger range.The thesis has 44 figures,5 tables,and 93 references.