| With the development of China’s economy,the number of road tunnels and urban subway tunnels has grown rapidly.Among them,the construction of shallow clay tunnels is greatly affected by geological conditions.When the surrounding rock is weak and the self-stability is poor,the bearing capacity of the surrounding rock itself can’t usually meet the safety requirements of the project,resulting in the collapse of the tunnel,affecting construction progress,and even causing casualties,it is necessary to use pre-support methods to improve the stability of tunnel surrounding rock.Grouting small conduit technology is a kind of advanced pre-supporting form which is widely used in engineering practice.It has good economy and flexibility,and also has bolt,grouting and other reinforcement effects,which is suitable for excavated tunnel of complex geological conditions and has good surrounding rock reinforcement effect.In order to study the effects of different types of grouting small ducts on the deformation mechanism,settlement and stability of shallow tunnels,the following work was carried out:(1)the Acutronic 661 centrifuge was used to simulate the single-hole circular tunnel under seven small conduit support conditions,and the resin composite material was used to simulate the injection,small ducts.The displacement transducer and the digital image processing technology were used to collect the stratum movement information.Through observation and analysis of the test results,it is found that the arch waist is a key area for grouting small duct support.As the arch support range expands toward the arch waist,the wedge-shaped sliding area at the end of the support shifts downward and appears as springback of arch foot;the stability of the tunnel increases with the increase of the support range;when the range of the annular support zone is the same,the arrangement of the grouting small conduit is the key factor affecting the surface settlement;the stability coefficient is sorted from large to small as:FP5、FP8、FP6、FP2、FP4、FP3、FP7.(2)Analytical solution was used to analyze the unreinforced tunnel and the seven supporting conditions in centrifugal test.It was assumed that the soil settlement curve is an ideal Gaussian curve,and the stress balance equation of the upper soil of the dome was established.The critical collapse support pressureσ_t of the tunnel under different volume loss rates was calculated,and the stability coefficient of the tunnel when the volume loss rate is 10%was calculated.The results show that the method is suitable for supporting the area above the arch waist.the stability coefficient is sorted from large to small as FP8、FP2、FP4、FP3 and FP7,which are consistent with the results of the centrifugal test.(3)Using finite element analysis software,ABAQUS,to model and analyze the tunnel,and using the strength reduction coefficient method to calculate the stability coefficient of the tunnel.The results show that when supporting the vicinity of the arch waist,obvious plastic slip surfaces are formed on both sides of the tunnel;the plastic area gradually decreases with the increase of the support range,and shifts to the foot of arch,and the deep soil begins to bear;The effective support area is the key to affect the support effect.The same support range,the effect of double-sided support is better,and the surface subsidence can be reduced by not supporting the arch roof and the arch bottom;the stability coefficient is sorted from large to small as FP5、FP6、FP8、FP2、FP4、FP3、FP7,which are consistent with the results of the centrifugal test.;combining with the above conclusions,an optimized small-caliber support scheme is proposed,which is a ring with a central angle of(α+β)on both sides of the tunnel arch waist,α≦60°,β≦45°,andα﹥β. |
