Mechanism And Design Method Of Strengthening Weak Surrounding Rock In Shallow Section Of Tunnel With High Pressure Jet Grouting Pile On Ground

With the rapid development of traffic tunnels in China,in recent years,it is common to see tunnels crossing mountainous areas,river valleys and other terrain,and the occurrence frequency of shallow buried sections of tunnels is increasing.Because the shallow buried section of the tunnel is prone to geological disasters such as collapse and water gushing,the shallow buried section of the tunnel often becomes a key section of tunnel construction and usually requires reinforcement.The reinforcement method of high pressure jet grouting pile on ground has been paid more and more attention gradually because of its good reinforcement effect and wide application range in the reinforcement of soft surrounding rock in shallow buried section of tunnel.However,due to the late start of related applications,there are relatively few applications and studies at present,and its reinforcement mechanism is still unclear.With the deepening of China’s "One Belt And One Road" strategy and the gradual implementation of the "14th Five-Year Plan",the development of tunnel engineering will be more and more prosperous,and the method of high pressure jet grouting pile on ground has broad application prospects.The in-depth understanding of the jet flow and grout effect of the high-pressure jet grouting pile is an important prerequisite for the correct study of its reinforcement mechanism,and the understanding of the macro-reinforcement mechanism directly affects the rationality of the design method.Based on the above understanding,under the premise of in-depth analysis of the high-pressure jet grouting pile spray flow and slurry action,the paper uses theoretical deduction,numerical simulation,indoor test,on-site monitoring and other means to analyze the mechanism and design method of strengthening weak surrounding rock in shallow section of tunnel with high pressure jet grouting pile on ground.It is of great significance to promote the rationalization of relevant engineering design ideas and design parameters.The main research work and results are as follows:(1)Combined with the theoretical analysis and the summary of the existing research on the high-pressure jet grouting pile,the destruction effect of the jet flow and the extrusion effect of the slurry are analyzed.The destruction effect of jet flow includes displacement effect and stirring effect on rock and soil,and slurry soil forms mixed consolidated body under the two kinds of action.The compaction of the slurry on the soil can make the soil more compact.For gravel soil,broken sandstone and other highly permeable formations,through the analysis of the established theoretical model and laboratory tests,the calculation formulas for the range of grout infiltration,the increase of soil strength in the seepage area,and the decrease of the soil permeability coefficient in the seepage area are deduced.(2)Combined with the existing research,the process of high pressure jet grouting pile slurry and the improvement of the formation of piles are analyzed.The high pressure jet grouting pile is formed with the solidification of the slurry under the damage of the jet stream;the formation of the pile body can improve the integrity,strength,and impermeability of the formation,and can reduce the level of the surrounding rock before the tunnel excavation(3)Combined with numerical simulation and on-site monitoring,the cross-section force mode and longitudinal section force mode of the high pressure jet grouting pile reinforcement area after tunnel excavation are studied.In the cross section of the surrounding rock,as the tunnel is excavated into an arched section,part of the piles and the soil between the piles bear the force together,and jointly exert the characteristics of the arch structure;the other part of the pile body can reduce the loosening range of the soil layer,and its function is similar to that of the retaining pile.In the longitudinal section of the surrounding rock,the pile can block the extrusion of the soil to the open surface,thereby reducing the soil displacement of the tunnel face and reducing the surrounding rock pressure on the supporting structure near the tunnel face.In general,the existence of piles improves the force mode of surrounding rock after excavation,and the self-supporting capacity of surrounding rock is improved.Through on-site monitoring and analysis of the pore water pressure in the reinforcement area,it is found that when the pile spacing is small,even if the pile bodies are not occluded,the pile groups have a macroscopic water blocking effect,and they can play the function of multiple rows of "defective curtains".It is difficult for groundwater to penetrate into the reinforced area.Based on the research on the various improvement effects of high pressure jet grouting piles in the shallow buried section of the tunnel,the mechanism of macroreinforcement performances such as reduction of surrounding rock pressure,reduction of supporting structure displacement,reduction of water seepage in the tunnel,and reduction of tunnel face displacement are analyzed.(4)Combining the macro-mechanism research and the numerical simulation analysis of various design conditions,the design methods of strengthening weak surrounding rock in shallow section of tunnel by high pressure jet grouting pile on ground without special antiseepage requirements are suggested: For the design of the vertical reinforcement range,when the buried depth is greater than 10 m,the pile is only used to exceed the tunnel excavation contour line by 0.6 times the buried depth;and when the buried depth is greater than 6 m and less than 10 m,the pile will only be constructed to 6 m beyond the excavation contour line of the tunnel;and when the buried depth is less than 6 m,apply to the surface.For the design of the lateral reinforcement range,construct 2 or 3 side piles on both sides of the contour line.For the pile diameter design,the value is within the range of 0.6 m ～ 0.9 m,and the smaller the buried depth is,the smaller the value is.For the pile spacing design,the pile diameter shall not be higher than 2 times the pile diameter,and 1.5 to 2 times the pile diameter can be selected if there is no higher anti-seepage requirement.According to the analysis of the numerical simulation results under different designs,the proposed design methods for the four key parameters can be applied at the same time.