| With the development of modern cities,urban rail transit can effectively alleviate the problem of traffic congestion.In the construction of urban subways,shield construction technology has the advantages of short construction period,high degree of automation,good safety and little impact on the surrounding environment.It is the most widely-used method in the construction of subways at the present.Lanzhou has a unique geographical location and presents a long and narrow distribution.The Yellow River passes through the city,and urban construction begins along the west coast.Lanzhou Rail Transit Line 1 crosses the east-west direction.The shield tunnel passing through the Yellow River section(from the Olympic Sports Center to Century Avenue and from Yingmentan to Matan Section)has become the key control project of the whole line.For the shield tunnel which penetrates the pebble stratum of the Yellow River rich water sand,the shield tunnel segment loading mechanism and the ordinary shield tunnel are quite different,and the mechanical characteristics are complex;The pebble content is large,the high strength has great wear on the cutter;the soil has large permeability and is easy to be separated,the slurry is easy to be diluted with water during the construction process,the gelation time is long,the early strength is low,and the grouting effect is difficult to evaluate;There are many risk factors in the construction process.For example,the risk level is unclear,and the corresponding countermeasures are not easy to come up.The main contents of the research are as follows.(1)Through the analysis of the wear mechanism of the shield cutter and the cutting mechanism of the cutter,the composite cutterhead should be selected for Shield Tunneling of water-rich sandy gravel layer condition in yellow river down-traversing Section of Lanzhou Subway.In the sand-pebble stratum,the rock-breaking method is mainly based on rolling rock breaking and cutting rock breaking supporting as auxiliary rock breaking method.The factors affecting the cutter torque are summarized and the mathematical model of the cutter torque is obtained.(2)The finite element model is established for the scraper,and the rock breaking effect under different cutting speed,cutting depth,cutting angle and knife spacing is analyzed.The research shows that the cutting speed and the knife spacing have little effect on the cutting resistance;The horizontal cutting force is increasing with the increasing cutting depth;as the cutting angle increases,the equivalent stress of the pebbles increases slowly,and the rock breaking is best when the cutting angle is 20°,which has good cutting and rock breaking effect.The finite element model of the hob is established,and the rock breaking effect under different hob spacing is analyzed.The hob spacing is between 12 cm and 16 cm,and the rock breaking effect is the best.According to the tool wear calculation formula,the tool wear coefficient is derived.The allowable wear value of the hob is 20 mm.It is recommended that the pressure inspection should be carried out once every 220 m,the tool with large wear should be replaced and every 770 m a stop of reinforcement point for inspection and replacement of all tools should be set.(3)Through the model hypothesis,the circumferential force of the shield segment is analyzed,and the mechanical analytical model of the joint is established.Through the force characteristics of each stage of the segment,the circumferential joint of the segment under different force characteristics is derived.The mechanical expression is then established.The finite element model is established by ANSYS for the tube joint,and the rotation angle and opening amount of the ring joint of the segment under different bending moments and axial forces are calculated,and the mechanical analytical solution is verified.The results show that the boundary conditions are the same,the mechanical analytical model is consistent with the calculation law of the finite element model.Under the same axial force,the joint angle and the bending moment are positively correlated,but the bending stiffness decreases with the increase of the bending moment,and finally becomes stabilization.The curve is completely degraded into an approximate straight line.(4)Strain gauges and seam gauges are installed in the Yingmentan-Matan section tunnels to monitor the strain and joint displacement of the shield lining segments and it is found that the strain and joint displacement of the monitoring point increase differently with the increase of the load.The increase is due to the fact that after the grouting,the segment is subjected to the grouting pressure and the surrounding soil load,and the segment is deformed;with the advancement of the shield and the condensation of the slurry,the strain and joint displacement at each monitoring point are then gradually reduced,as the grouting pressure is gradually reduced due to the escape of the slurry to the soil,resulting in a decrease in the grouting pressure acting on the segment.As the forward segment of the shield machine leaves the shield tail,the strain and joint displacement of each monitoring point suddenly increase,and then the shield machine gradually reduces the influence of the shield machine on the segment as the shield machine moves away from the monitoring section.The strain and joint displacement of each characteristic point are also correspondingly reduced.After 60 days,the strain and joint displacement of each monitoring point tend to be stable.(5)For the Lanzhou sand-pebble water-rich stratum,the permeability coefficient is large,and the traditional synchronous grouting material is used.After the grouting,it will be diluted by the groundwater,and the strength will not be reached in the later stage.The slurry type is selected as a single liquid slurry by comparison.The cement,clay,bentonite,fly ash,fine sand and admixture are selected as raw materials,and the appropriate slurry mixing ratio is obtained through laboratory test;the mixing ratio is compared with the actual slurry ratio on site to conduct field test,and the geological radar is used for detection.The combination is superior to each index and worth promoting.(6)Modeling with the finite element software Plaxis,the different depths of the shield tunnel and different grouting pressures are calculated,and a reasonable grouting pressure of0.4 MPa is obtained.When grouting in sand pebble formation,grouting pressure should be used to control synchronous grouting.With reference to previous projects,the injection rate is controlled between 130%and 180%,each ring The grouting amount is 4.3m~3 to 6m~3.(7)According to the expert survey in the field of shields(including the scientific research personnel of the institutions of university and the on-site management and construction personnel of the China Railway Sixteenth Group Co.,Ltd and the China Railway fourteenth Group Co.,Ltd),the questionnaire was calculated and the risk index system of the shield tunnel was established.Using the analytic hierarchy process and the fuzzy evaluation method,it is acceptable to pass the Yellow River risk of the shield tunnel of Lanzhou Rail Transit Line1 and the risk can be controlled. |
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