| This paper take the section two,line 6,Wuhan Metro,as the main research object,which is located between Qian-jin village and Ma-ying road in Han-yang,or " Qian-ma section " for short.It is aim to explore the deformation of segment structure in subway shield tunnel,which is through the sandy soil stratum on the top of the underlying karst.Combined with on-site grouting treatment test and site monitoring results of segment structure deformation,countermeasures for karst collapse treatment are put forward in study area.The study consists of three main parts.First,on the basis of summarizing and analyzing the geological situation of typical subway tunnel crossing Stratigraphic subsidence area,the author analyses the mechanism,distribution,development and collapse mechanism of karst geology in that area.Second,the author further studies the collapse mechanism of karst cave in special geological structure,which is the limestone karst contacting with sandy soil directly.Based on that,by changing the karst cave’s roof thickness d,diameter D,span w and filling degree ω,the author uses FLAC3 D software to analyze the karst cave’s axial force,shear,bending moment and its deformation of tunnel segment structure.According to the characteristics of sand,which has low strength and poor self-stabilizing ability,by means of combining theoretical analysis with numerical simulation,the author analyses the safe thickness of the karst cave roof and the horizontal safe distance between the lower cave and the tunnel.Finally,by means of field test and numerical simulation,the author explores the key treatment technologies in karst geologic subsidence area.Such as: karst water channel cutoff technology,cave filling and reinforcement technology,etc.The author also verifies the effectiveness of the treatment measures.The specific work and conclusions are as follows.(1)From multiple aspects,such as: topography,geomorphology,geological structures and stratigraphic lithology,this paper introduces the general situation of regional engineering geology in Wuhan area.The author not only explores the recharge,runoff,drainage and dynamic change of karst fissure water,but also the aquifer(group)development conditions and groundwater chemical characteristics.Based on that,the paper analyses the hydrogeological conditions in Wuhan area,hoping to provide foundations for subsequent analysis of karst development.By using physical and chemical test,the paper also analyses the chemical composition and mineral elements ofsoluble rock.Combined with the relevant geological data in Wuhan,the paper reveals the effect of tectonic movement on karst.(2)This paper also analyzes the effect of groundwater on the influence of karst formation.By detecting the underground water level,the paper finds that there is a relationship between karst water and sand layer pore water from the same set of observation hole.For the fact that the main indexes,such as: chemical composition,ph.Value,total hardness,mineralization,of the two are very close.It comes to the conclusion that pore water and karst water have the same supply sources or have a close relationship in hydraulic.In addition,by using geophysical CT and drilling methods,we know the development scale of karst cave,filling characteristics,the elevation of characteristics of the space and the distribution features of the bedrock surface distance in Wuhan metro 6.(3)By analyzing the geological conditions of karst distribution in subway crossing area,this paper divides the geological structure into five different types.What’s more,according to the subsidence deformation characteristics of the soil,we divide the area where the collapse may occur into three kinds of collapse mode.That is sand,soil hole leakage and mud flow pattern.With the consideration of karst collapse history in Wuhan and trigger factor of different karst collapse,we divide the collapse mechanism of I degree geological structure into five types.That is internal erosion,losing support,mechanical penetration,overload and compound causes.(4)Within a certain range,the horizontal convergence value of shield segment structure is positively correlated with the distance d between karst cave and bedrock surface.That is to say,the horizontal convergence deformation of the segment is inhibited by underlying carst cave to some extent.(5)Along with the increase of karst cave diameter(D),the settlement of tunnel dome increases and the displacement of arch uplift decreases gradually.When D is less than a certain value,the caves has little impact on the deformation of shield tunnel segment,and also on the internal force of every characteristic spots of segment.Comparing to other characteristic spots,the arch bottom suffers a smallest change at its axial force and shear force values,and also gain least effect when the cave diameter D is changing.When cave diameter D increasing,the maximum segment internal force transfers from the arch foot area to arch bottom,however these changes takes place within the area of 1/2 D.(6)Along with the increases of the cave width(W),the horizontal displacement of vault and arch bottom is gradually increasing too.The same as the vertical displacement of certain characteristic spots.Under the same cave width(W),the bending moment internal force,axial and shear force in the arch bottom of segment are greater than that in vault of segment and both side of vault of segment.The axial and shear force of cave segment are negatively related with the cave width.When W decreases,the place where suffer the largest axial and shear force would move from the the arch foot area to arch bottom,however these changes takes place within the area of 1/3 D.(7)When the filling degree increases from ω = 0(no filling)to ω = 0.75,the increment of horizontal convergence and vertical deformation of the tunnel segment is much smaller than that when the filling degree increase from ω = 0.75 to ω = 1.0.That is to say the cavern grouting treatment must be filled completely.With the increase of filling,the axial force,shear force and bending moment internal force of segment is increasing too.The area where suffer the largest inner force would expand haunch to bottom,but the range are still in the area below haunch.(8)This paper analyses the safety of shield tunnel structure under special conditions(underlying karst through sand layer).Beyond that,through the calculation method of structure mechanics and discrete element theory,we analyze the safety thickness of tunnel structure.Finally,we obtained the safe thickness of roof cave calculation formula by orthogonal experiment.(9)After studying the case of Wuhan metro line 6 and its special stratum,we try to calculate its "horizontal safe distance".Combine with the geometric collapse model,we analyze the horizontal safe distance between the tunnel and the karst cave after karst leakage passage is formed.By using FLAC3 D software,it is concluded that the horizontal safe distance is L0 = 20.0 m.(10)Through on-site Grouting Monitoring,combined with the core drilling,standard penetration test was carried out to test the quality of grouting reinforcement effect.Under the action of 0.3~0.5MPa grouting pressure,the strength of the stone body after the consolidation of the pure water slurry has reached the desired requirements.On-site monitoring shows that the karst of K12+536 section has not been found in the preliminary investigation,the deformation rate is larger.After the timely grouting treatment,the deformation rate of the segment slowed down,the final deformation is in the allowable range.The simulation shows that if the cave is not treated,the segmentstructure of the tunnel will be destroyed.Thus,it can be seen that the grouting reinforcement effect of karst cave is good.(11)According to the results of grouting treatment of the cave at the construction site,after the grouting reinforcement,the maximum settlement of the tunnel segment is3.8 mm.And the sedimentation value is reduced by 94% compared with that before grouting,which is within permissible deformation value of the segment lining vertical deformation.It can be seen that the treatment of karst grouting reinforcement is very well.Comparing the supporting effect of single,double-row pile and three-row pile,we suggest the 6 m of outer contour line of tunnel should be reinforced by three-row jet grouting pile.In that condition,the maximum stress is 256.36 kPa,which is lower than the design value of shear strength of high spray wall.When the cave is completely collapsed,the three-row jet grouting pile can play a good supporting role and without damage. |
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