Study On The Law Of Ground Settlement When The Shield Passes Through The Heterogeneous Soil Layer

In the past 40 years of reform and opening up,China's urbanization has developed rapidly,a large number of people have gathered in urban life,and the pressure of public transport has increased year by year.Under this background,it is imperative to develop underground rail transit.Shield method is the most important construction method in underground railway.Due to its safety,high efficiency and little influence on surrounding environment,shield tunneling is widely applied in metro construction.Although the shield construction technology has been mature,the complexity of geological conditions,shield tunneling will cause surface subsidence more or less,and the surface subsidence will cause damage to surrounding buildings and pipelines.Therefore,it is necessary to study the surface settlement of the shield excavation in different strata.This paper based on the background of Wuhan Metro Line 7 Hongkong road to Sanyang road right line project,by analysis of the field data and monitoring data,combining with the engineering case,using geotechnical engineering finite element software PLAXIS to numerical simulation,study on the regularity of surface subsidence when the shield excavation in different soil,the relativity between grouting pressure and buried depth,grouting pressure and soil pressure on the change of surface subsidence are optimized,draws the following conclusion:(1)When the shield passes through the fine sand layer,the maximum settlement of the ground surface is the 10 ring of the monitoring point of the excavation face on the axis of the tunnel.The maximum settlement is about 22mm,and the silt sand layer,the maximum settlement is about30mm.The influence radius of silt sand layer is greater than that of fine sand layer.But the maximum surface settlement,settlement of shield,stable time required is less than the excavation in silty sand.(2)By numerical simulation,obtained that the buried depth is 3:1,the surface subsidence is minimum when excavating silt sand layer,the minimum value is 22.5mm,the influence radius is25.8m,when the depth is 2.5:1,the surface subsidence is minimum when excavating fine sand layer,the minimum value is 31.3mm,Combined with engineering case,it is concluded that the most reasonable buried depth ratio of Hongkong road is three 3:1,and the surface settlement is controlled within a safe range,the grouting pressure is proportional to the ratio of buried depth.(3)Combining with the construction parameters,grouting pressure and earth pressure were optimized,concluded that when the grouting pressure from 170kN/m~2 to 190kN/m~2,the surface total settlement is in an inverse proportion with the grouting pressure,when the grouting pressure reaches190kN/m~2,the surface total settlement stabilization.The earth pressure is increased from 180kN/m~2 to240kN/m~2,the surface settlement decreases with the increase of the earth pressure,the earth pressure increases gradually as with the surface settlement.And the numerical simulation are basically consistent with the values obtained by theoretical calculation of earth pressure value.Through the comparative analysis of 5 different conditions,obtained the best excavation strata grouting pressure is190kN/m~2,the best earth pressure is 240kN/m~2,in this condition,the cumulative maximum surface settlement quantity is 11.5mm,4.5mm is reduced about accounting for about 28.2%than before optimization.