| Tunnel engineering is one of the key projects in the construction of high-speed railway and high-grade highway in Loess area.For large-section loess tunnel of high-speed railway,the permeability and water content change of surrounding rock not only affect the strength and deformation characteristics of the face soil,but also affect the stability of surrounding rock and surface subsidence.It is directly related to the excavation method of tunnel construction,as well as the reasonable advance support,initial support and secondary lining support.Therefore,the study on the permeability characteristics of surrounding rock and the monitoring of water content are of great significance for determining the initial supporting time,selecting supporting measures,ensuring the stability of surrounding rock and construction safety.Based on the tunnel engineering in the Loess Plateau area of Yinxi High Speed Railway,this paper systematically studies the seepage characteristics of tunnel surrounding rock and its influence on tunnel stability under four typical strata,i.e.soft plastic loess,collapsible loess,red clay and paleosol,by means of theoretical analysis,field monitoring and numerical simulation.The main research work and achievements of this paper are as follows:(1)Summarize the stratum structure and groundwater distribution law of tunnel surrounding rock in Loess Plateau area,and the engineering characteristics of different strata;analyze the influence of four typical strata in Loess Plateau area on tunnel construction safety,and select the tunnel test section on this basis.(2)The water content of the surrounding rock strata presents obvious two-stage change trend of "growth period" and "stationary period".The growth period generally lasts about 2weeks,and then enters a stationary period.The water content of surrounding rock at the arch top and waist of tunnel is generally less than that at the arch foot and the inverted arch bottom;the water content of surrounding rock at the inverted arch position is the largest in the whole section during the stable period,and the water content of surrounding rock at the arch foot and inverted arch position is higher than that at the arch waist position during the growth period;for large cross-section tunnel in Loess Plateau area,inverted arch should be applied in time,as well as "diligence" The construction management concept of measuring water content,paying attention to waterproofing and drainage,and strengthening initial expenditure is put forward.(3)During the construction period,the stress of the steel arch truss increases nonlinearly at first,then fluctuates and finally tends to be stable.Before the initial support and the second lining,the steel arch truss plays an important role in bearing the surrounding rock pressure and ensuring the stability of the surrounding rock.According to the monitoring results ofwater content change of soft plastic loess surrounding rock and its influence on construction,the growth period of steel arch frame’s stress and bearing capacity is generally 3-4weeks,which is slightly longer than that of surrounding rock’s water content by 1-2weeks.(4)The growth rate of surrounding rock pressure shows that it increases sharply at first,then slowly and finally tends to be stable.The growth of surrounding rock pressure slows down after initial support closure.The surrounding rock pressure of the tunnel side wall position is larger,followed by the vault,the surrounding rock pressure of the vault bottom is the smallest.The settlement of tunnel vault shows a trend of rapid growth at first and then slow growth,which is similar to the growth law of water content in surrounding rock.Among the four typical strata in the Loess Plateau,collapsible loess tunnel vault settlement is the largest,followed by red clay and collapsible loess,ancient soil tunnel is the smallest.(5)Influenced by rainfall seepage,the surface subsidence in the contour line of loess tunnel increases by about 30%.The surface subsidence in the middle line of tunnel is most affected by rainfall,while the surface subsidence on both sides of the middle line is less affected.The maximum influence range is about 10 m from the excavation side line.The stable period of surface deformation during rainfall is 2 days larger than that without rainfall.(6)The basic principles and equations of seepage and seepage-stress coupling calculation are summarized and introduced,which provides a theoretical basis for the coupled numerical calculation of stress field and seepage field during tunnel construction in Loess Plateau area.By using Midas/GTS finite element analysis software,the stability of tunnel under seepage is calculated and analyzed,and the calculation results are compared with the field measured values to verify the rationality of the numerical simulation.Under seepage-stress coupling conditions,the settlement of vault increases by 24%,the uplift of vault bottom increases by 21%,and the horizontal convergence value increases by 17%.Seepage has a significant impact on the extrusion displacement of upper step face,palm.Subsurface extrusion displacement increased by 34.7mm,an increase of 16.2%.Under seepage-stress coupling condition,the maximum tension stress of the initial supporting structure appears at the arch foot of the tunnel,which is 0.42 MPa higher than that of uncoupled supporting structure;seepage effect has no obvious influence on the stress state and magnitude of the bolt.Setting up drainage tunnel in large section tunnel of water-rich loess can not only reduce the water pressure of surrounding rock,improve the seepage field of surrounding rock during excavation,but also improve the uplift of tunnel basement. |
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