Blasting Failure Mechanism And Safety Criteria Of Cement-grouted Rock

Cement grouting is a construction technology commonly used in engineering to reinforce fractured rock masses,and has been widely used in the field of national economic construction such as water conservancy and hydropower,mining engineering and underground projects.After grouting,the cement-based grout is tightly bonded to the rock along the interface,resulting in the formation of a composite structure,which will henceforth be referred to as ‘cement-grouted rock’.During construction,it is inevitable that blasting operations will be performed near the grouted area owing to the need for rock excavation or demolition of concrete structures.The shock and vibration effects induced by blasting can seriously affect the safety and stability of cement-grouted rock.Therefore,a detailed analysis of the blasting failure mechanism of cement-grouted rock has important theoretical significance and engineering application value in reducing blasting damage of grouted rock,optimizing tunnel advanced grouting design parameters,and ensuring the safety of major national infrastructure.Aiming at the scientific problem of blasting failure mechanism and safety vibration of cement-grouted rock,the thesis carried out a series of researches by combining theoretical analysis,model test and numerical simulation.The main research contents and results are concluded as follows:(1)According to the mechanical properties of cement-grouted rock,the distribution characteristics of cement stones and the bonding mechanism between cement stones and rock were analyzed.During grouting process,cement paste was filled in the joint and harden into cement stone by hydration reaction.The cement stone was tightly bonded to the rock mass and mechanical properties were improved.Based on the results of strength testing and prediction models,it was known that the mechanical properties of cement-grouted rock were influenced by rock mass,cement paste,grouting age,grouting pressure and other factors.And the mechanical properties of the bonding interfaces were the weakest component of cement-grouted rock.(2)In view of the safety impact of blasting excavation and demolition of structures near the grouting area on the grouted rock,an analytical model was presented to describe the stress wave propagation in cement-grouted rock mass and to investigate the failure modes at the bonding interfaces.The models to calculate the safe vibration velocity(SVV)that induced by incident P wave and SV wave were then proposed to study the effects of the incident angle,in-situ stress,and bonding strength on cement-grouted rock.The results showed that a normally incident wave was identified as the most dangerous for cement-grouted rock;thus,the SVV was the minimal in that case.It was more appropriate to evaluate the safety effect of blast vibrations on cement-grouted rock using the SVV of the incident P wave,particularly given that the difference in the obtained SVVs for P and SV waves was less than 1 cm/s.(3)Aiming at the impact of tunnel blasting on the surrounding rock in advance grouting,propagation characteristics of shock wave and strong stress wave in cement-grouted rock were analyzed.Considering the in-situ stress environment,the stress field of cement-grouted rock under the coupling function of blasting load and redistributed in-situ stress was studied.Based on the dynamic strength failure criterion,the SVV calculation model under the action of shock wave and strong stress wave were established,and the safety impact zone(SIZ)was determined.Results showed that perimeter hole played a more important role in SIZ of cement-grouted rock in the near-field of blasthole.(4)The physical model test was carried out to study blast vibration resistance of cement-grouted rock mass.Mechanical properties of the grouted rock before and after blast vibration were analyzed through the relationship between shear strength,acoustic wave velocity and the vibration strength.The experimental results indicated that cement-grouted rock has strong blast vibration resistance and can withstand PPVs faster than 20 cm/s.(5)The numerical simulation technique was used to analyze the effect of blasting vibration and shock stress wave on cement-grouted rock.Considering the blasting load,rock type,grouting age,relationship between vibration intensity and blasting damage of cement-grouted rock in the middle-far field was determined.PPV of 14 ～ 16 cm/s can cause the cement-grouted rock at the damage threshold value of 0.19.In the near field of blasting source,SIZ was influenced by grouting age,grouting material and in-situ stress.The numerical result of SIZ for cement-grouted rock was 1.38～2.89 m.In a certain degree of in-situ stress,SIZ was decreased.When the in-situ stress level was high,SIZ was increased.(6)Proposed safe blast vibration velocity for grouted area was put forward based on comprehensive consideration of the effects of cement-grouted rock structure,grouting age,and natural environment on the resistance of grouted rock to blast vibrations.The recommended SVVs for 3-day,7-day,and 28-day-old cement-grouted rock were 6 cm/s,9 cm/s,and 13 cm/s,respectively.If in-situ stress was considered,the SVV can be further increased.