| The stratum with salient hard bedrock often occurs in the scope of excavation section during the construction of shield tunnel.The drilling and blasting on the ground has obvious advantages in dealing with this problem.However,this technique has two difficulties that the complex operating environment and the blasting result is difficult to detect.In this paper,combining with the rock blasting engineering in silt-rock stratum from Jingrongdao to Hengqin shield zone in Zhuhai City,the action mechanism of explosive shock wave in silt-rock stratum,the action mechanism of detonation gas in silt-rock stratum,the dynamic response characteristics of rock in silt-rock stratum blasting under different charge conditions,the vibration characteristics of rock blasting in silt-rock stratum,the vibration propagation characteristics of rock blasting in silt-rock stratum under different thickness of stratum are carried out by the combined methods of theoretical calculation,numerical simulation,laboratory experiment and field test.The main research contents and conclusions are as follows:(1)The action mechanism of explosive shock wave of rock blasting in silt-rock stratum are investigated.Based on the theory analysis,the process and characteristics of the detonation wave is produced by explosive and the explosive shock waves pass through the water in the blasthole,rock around the blasthole and silt under water coupling charge are studied.It is obtained that the suitable decoupling index for blasthole charge is 0.56,the charge weight per blasthole is 5.55kg,the radius of cleavage district caused by explosion stress wave is 0.46m under this charge structure when the explosive is at 30m of water depth in the blasthole.(2)The action mechanism of detonation gas of rock blasting in silt-rock stratum are investigated.1)Based on the theory analysis,the process of producing cleavages in blasting with water-copuling,the quasi-static stress field intensity with water-copuling and the propagation rule of micro cracks under the action of water wedge are studied after the explosive stress wave acts on the rock.It is obtained that the range of cleavage district and elastic vibration district under the combined action of explosion stress wave and water wedge,the blastholes are arranged in plum blossom pattern on the plane,the spacing of blastholes,and the array pitch of blastholes are also studied.2)Based on the action of detonation gas,and testing the the bulk density of the gravel and the external friction coefficient between the gravel and PVC board in laboratory,it is concluded that the bulk density of the gravel 1-1.5cm in diameter is 1370 kg/m~3,the external friction coefficient between the gravel1-1.5 cm in diameter and PVC board is 0.42,the optimum stemming length is 0.16 m when using the PVC pipe 7.5 cm in diameter to protect the wall of blasthole and the gravel 1-1.5 cm in diameter to stem the blastholes of rock blasting in silt-rock stratum.(3)The dynamic response characteristics of rock in silt-rock stratum blasting under different charge conditions are studied.The dynamic response characteristics of rock under three conditions that the water-coupled charge and the rock is subjected to silt pressure,the air-coupled charge and the rock is subjected to silt pressure,the water-coupled charge and the rock is not subjected to silt pressure are investigated by the combined methods of field test and dynamic finite element numerical simulation.It is concluded that there is a linear relationship between the peak particle velocity of vector superposition and the blasthole depth,and there is a linear relationship between the peak element pressure and the blasthole depth too,the peak particle velocity of vector superposition and the element pressure will decrease with the increase of blasthole depth under the conditions that the water-coupled charge and the rock is subjected to silt pressure,the water-coupled charge and the rock is not subjected to silt pressure.The peak particle velocity of vector superposition and the element pressure are not affected by blasthole depth under the condition that the air-coupled charge and the rock is subjected to silt pressure.(4)The vibration characteristics of rock blasting in silt-rock stratum are analysed.Through testing and analysing the blasting vibration on-site,and the measured data are analysed by wavelet analysis and dimensional analysis,the conclusions are as follows:1)the silt has higher structural strength,the duration of the acceleration vibration waveform is about 0.4s,and the main frequency of vibration is higher during the first blasting.After the disturbance of blasting load,during the second blasting,the duration of acceleration waveform is prolonged by 0.4 s,and the main frequency of blasting vibration velocity is decreased,which is very close to the natural frequency of the buildings and is harmful to the surface buildings.At the interval of 10 days,during the third blasting,the duration of the acceleration waveform is close to that of the first blasting waveform,and the residual vibration of pulsation is weakened.2)The dominant frequency band of ground vibration energy is close to the natural frequency of buildings,and has a great influence on vibration damage of surface buildings.3)The maximum number of energy in the vertical direction is the largest when the distance from the explosion source is 30-60m,followed by the horizontal direction which pointing to the explosion source.The blasting vibration energy in vertical direction should be mainly analyzed when the distance from the explosion source is 90-180m.4)The mathematical prediction model considering the depth of explosion source can better reflect the influence of rock blasting vibration in silt-rock stratum on the ground surface.(5)The vibration propagation characteristics of rock blasting in silt-rock stratum under the different thickness of stratum are investigated.Based on the combined methods of field test and finite element numerical simulation,the vibration propagation characteristics of rock blasting in silt-rock stratum under 5 working condition are studied,it is concluded that the peak vibration velocity of working condition 1(the artificial filled soil thickness is 3m,the silt thickness is 30m)is larger than that of other 4 working conditions,and the attenuation velocity is the fastest,the blasting vibration monitoring should be strengthened.The maximum number of peak vibration velocity of surface is 180 degrees between the direction and the tunnel axis on the silt-rock stratum is the largest under 5 working conditions when the distance from the center of the circle is 10 m and 15 m,which need to focus on maintenance monitoring.The peak vibration velocity suddenly increase at the interface between silt and artificial filled soil,then reduce in the range of 2-4 m from the vertical distance of the explosive source under 5 working conditions on the vertical direction from surface to tunnel.The thickness variation of artificial filled soil thickness and silt above the shield zone has little effect on the peak vibration velocity on the axis direction of tunnel. |
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