Study On Antifriction Grouting Mechanism And Optimum Design Of Super-large Rectangular Section Pipe Jacking

As a trenchless underground pipeline construction method,pipe jacking technology has been widely used in urban underground space development projects in recent years due to its low cost,small environmental impact and short construction period.However,at present,China has less experience in the construction of super large rectangular section pipe jacking with shallow overlying soil under complex geological conditions in coastal areas,and most of the construction is done with small breaks in deep overlying soil.The rectangular pipe jacking construction experience is based on.Based on the rectangular pipe jacking project of Baima Road in Fuzhou City,this paper studies the mix ratio of friction reducing mud and construction parameters of super large rectangular pipe jacking project in shallow overlying soil under complex geological conditions in coastal area by combining indoor test,numerical simulation and field measurement.The main research contents and results are as follows:(1)Through the mud performance test of different mix proportion of friction-reducing mud,the influence rule of bentonite and polyacrylamide on mud performance is obtained.Combining with the pipe jacking project of Baima Road in Fuzhou,the model test of mud mix proportion optimization and friction-reducing effect is carried out,and the high-quality friction-reducing mud for pipe jacking construction in mud stratum under the condition of engineering site is obtained.The ratio is bentonite: water = 1:9,and the optimum proportion of anti-friction mud in sand layer is bentonite: polyacrylamide: water = 1:0.00625:11.494 and bentonite: polyacrylamide: water = 1:0.0125:11.487.(2)On the basis of summing up predecessors' experience in numerical simulation of pipe jacking construction,the method of finite element simulation of pipe jacking construction is optimized.Combined with pipe jacking construction without grouting,the mechanism of friction reduction grouting is analyzed.By analyzing the law of soil disturbance,it is found that the curve of surface settlement trough is normal distribution,and the influence of pipe jacking construction can be neglected beyond 4 times of pipe diameter.(3)Three-dimensional finite element numerical simulation of pipe jacking construction process under different face support force,different grouting characteristics and different grouting pressures is carried out.The following conclusions are drawn: 1)The gradient surface load on the face of the face can effectively reduce the disturbance of pipe jacking construction to the soil,and when the support pressure on the face of the face is less than the lateral static earth pressure,the face of the face of the pipe jacking construction can effectively reduce the disturbance of the soil.When the support pressure is greater than 1 times of the static earth pressure,the soil uplift mainly occurs in the 2B(B is the larger height or width of rectangular pipe jacking)area in front of the palm,and the maximum uplift is 5 meters in front of the palm;2)the friction resistance on the outer wall of the bottom of the pipe jacking is the largest,the friction resistance on the top of the pipe jacking is the smallest,and the average resistance on both sides of the pipe jacking is the smallest.Friction resistance is greater than the friction resistance above the pipe jacking;3)When grouting pressure is less than the overlying soil weight,the settlement of the soil above the pipe jacking is larger,especially the soil near the excavation area,but if the grouting pressure is greater than 1.1 times the overlying soil weight,the surface will appear larger uplift deformation.(4)Based on the monitoring data analysis of the rectangular pipe jacking project on Baima Road in Fuzhou City,the settlement caused by pipe jacking construction is not completely symmetrical under the action of vehicle driving load;the excessive earth pressure set in pipe jacking construction will cause the uplift of the front pavement,and the excessive small earth pressure will lead to the settlement of the front pavement;the construction of pipe jacking will be stopped and started for a long time.The trough will lead to increased resistance,difficult jacking and greater soil disturbance when subsequent re-jacking.The feasibility and validity of the optimized finite element numerical simulation are verified by comparing the numerical simulation results with the field monitoring data,which verifies the correctness of the optimization design and construction parameters proposed in this paper.