Research On Energy Adjustment And Dynamic Effect Of Deep Surrounding Rock Based On Excavation Unloading Rate

Excavation of deep rock is often involved in hydropower projects,underground storage of strategic energy and deep mining of mineral resources et al.With the rock mass being crushed and thrown,and the forming of newly excavation face,the strain energy stored in the original rock mass would adjust and release rapidly,,however,the unloading rate and strain energy adjustment of surrounding rock under different excavation methods are significantly different,which would inevitably lead to different catastrophic failure phenomena.Therefore,study on the energy adjustment and catastrophic effect control of deep rock mass has important guiding significances for deeply understanding the failure modes of deep rock,optimizing the excavation program and controlling the engineering geological disasters.The main objective of this dissertation is to research the energy adjustment and catastrophic effect of deep rock mass under different unloading rates(different excavation unloading methods),and combined methods of theoretical analysis,numerical computation and in situ test are adopted.The main research results are as follows.The faster the unloading rate of stress(the smaller the unloading time),the more obvious the dynamic fluctuation effects of strain energy during deep rock excavation.When the unloading time is very small(drilling and blasting excavation,transient unloading),the strain energy in the surrounding rock will undergo a dynamic adjustment process,it firstly decreases and then increases to the finally steady state.When the unloading time increases to the magnitude of quasi-static level(Tunnel Boring Machine(TBM)excavation),the transient process of rock strain energy is very smooth after excavation.And compared to the initial state of the rock mass,for the propagating of unloading stress waves from the excavation face to the deep rock,the near-zone and far-zone of excavation face will the work reciprocally,which causes the rock strain energy in the vicinity of excavation face to adjust and redistribute.There are big differences in dynamic release of rock strain energy and transformation law of rock vibration energy under different unloading rates.The faster the unloading rate of rock mass,the greater the vibration energy of surrounding rock induced by excavation unloading will be.When the unloading rate is in a limiting case(unloading time is 0),50%of the work far-zone rock doing to near-zone rock transfers to seismic energy,and the rest 50%converts to near-zone to increase the strain energy of surrounding rock(strain energy accumulation).When the unloading rate is infinitely small,nearly half of work far-zone rock doing to near-zone rock is consumed to overcome the work done by excavation load on unloading surface,while the remained half is used to increase the strain energy of surrounding rock,and there is no seismic energy produced.The measured blasting vibration signal has two excitation sources obviously during drilling and blasting excavation,For the MS1 detonation delay,the difference between the two excitation sources is about 5 ms on the time scale,namely,the lag time between in situ stress transient unloading(strain energy transient release)and the explosion load unloading is approximately ms.For TBM excavation,the strain energy releases steadily and slowly,surrounding rock is relative stability,the strain energy is mainly consumed in the deformation of surrounding rock,and almost no seismic events are produced.The measured vibration waveform of the rock mass is simple,it can be considered as single-frequency vibration,which is mainly caused by cutter cutting rock mass under hydraulic pressure.The seismic energy transformed from the rock strain energy is roughly equal to that coming from explosive energy during drilling and blasting excavation,while the total strain energy released and transferred is about 1/10 of the total explosive energy.The plastic-damage radius is closely related to the mechanical parameters of rock mass,and it is proportional to the exponential power of the excavation radius after deep tunnel excavation.That is,the larger the radius of tunnel,the greater the zone of plastic-damage.Excavation unloading results in the accumulation phenomenon of strain energy in the surrounding rock,the rock strain energy shape showing hump-like,and outside to the inside shows low?high? low distribution.The accumulation characteristics of rock strain energy and the plastic-damage extent of the rock mass are obviously different under different unloading rate,and depth of the rock damage caused by the transient adjustment of strain energy is approximately 2?3 times to the quasi-static adjustment of rock strain energy.The larger the unloading rate of the rock,the smaller of the accumulation peak of rock strain energy will be,while the accumulation zone of the rock strain energy is deeper,the excavation disturbance is greater,and the depth of rock damage is positively correlated with the accumulation depth of rock strain energy.With the increase of excavation footage,the disturbance of surrounding rock becomes larger,so as to the unloading rate of the rock mass,the rock damage extent and the accumulation depth of rock strain energy,however,the accumulation peak becomes smaller.The more obvious of the post-peak brittleness of the rock mass,the larger range of the rock damage will be,the accumulation depth of rock strain energy will be deeper,and the accumulation peak will be greater.Under different confining pressures,the deep marble shows brittle-ductile-plastic transfer characteristics obviously,these characteristics have huge impacts on the occurrence of rock burst under different unloading rates.With the increase of the unloading rate of the surrounding rock,the range of rock burst will gradually increase,while its intensity(e.g.the ejection speed of the rock)would gradually decrease for the same level of rock burst.For drilling and blasting excavation(high-speed unloading),the strain energy of surrounding rock transient releases,which reduces energy source of rock burst,but due to the thickness of brittle rock is thicker,the burst depth is generally 2.0 m,high-level rock bursts can be morefound.The disturbance of surrounding rock is small during TBM excavation,the stored energy near the excavation face is higher.As the thickness of brittle rock is thinner,the rock burst grade may be smaller.The burst depth is generally in range of 0?1.0 m,and mostly less than 0.5 m.In addition,with reinforced brittle characteristics of rock mass,the occurrence of rock burst will be more frequent.However,with the increase of rock strength,the potential risk of rock burst is suppressed.Through the simulating of parallel cutting,spiral cutting,wedge-shaped cutting of two pairs and three pairs during cutting blasting of deep rock,it can be found that the methods of spiral cutting and wedge-shaped cutting of two pairs can effectively control the vibration of surrounding rock caused by blast load and strain energy transient release,besides,the two methods can also be used to reduce the unloading rate of the rock,control the release process of rock strain energy and maintain the stability of construction.For contour controlled blasting of deep rock,there are strong accumulation phenomena of rock strain energy under different line charge density,and the strain energy accumulation zone will increase significantly with the increasing of line charge density.The accumulation zone of rock strain energy increases relatively slowly,and it linearly changes with line the charge density increasing for single blasting crater.However,the accumulation zone of rock strain energy increases rapidly under multiple blasting craters.The accumulation of strain energy and the failure of rock mass can be effectively controlled by adding blast holes and reducing the dose of single-hole.With the reducing of excavation footage,the effect of excavation unloading on energy release becomes smaller gradually,the release of rock strain energy becomes more stable and the energy release rate becomes smaller and smaller.The larger the excavation footage,the more obvious the release effect of the rock strain energy,the disturbances and the catastrophic failures of the rock mass will be higher.