| The rectangular box jacking has the advantages of green,cost-saving,environmentally friendly,and safe.In recent years,it has been widely used in urban underground infrastructure construction in China,and has shown the characteristics of jacking in sandy soil layers,shallow burial depth,and large section.Accurate estimation and reasonable control of the jacking force in rectangular box jacking is of great significance to construction costs and safety.Due to the interaction between the box-soil-slurry during dynamic jacking,the jacking force is affected by factors such as excavation face support force,box normal pressure,and frictional mechanics characteristics.However,the jacking standards and previous research on jacking force calculation methods have not considered the above factors,and lack the calculation model of jacking force under different mud lubrication conditions.The methods,model experiments,in-situ tests,shear friction tests,numerical simulation,and theoretical analysis,are adopted to systematically study the jacking force factors of excavation face support force,box normal pressure,and frictional mechanics characteristics from the perspectives of qualitative regularity description and quantitative model characterization.The study analyzes the stress characteristics and influencing mechanism under different lubrication conditions,and the following research results can be obtained:After the appearance of a triangular shear band in front of the rectangular excavation face,the support force rapidly decreases to P1.With the emergence of soil arching,the support force reaches Pmin.After the soil arch fails,the support force slowly rises,and a new arch-shaped shear band appears on the outside of failed soil arch and gradually developing towards the surface.Finally,a vertical shear band that penetrates the surface is formed,and the support force reaches a stable value Ps,with a trapezoidal settlement surface appearing on the ground.Meanwhile,the soil density and the ratio of burial depth have a greater influence on the stable value Ps,failure angleα,and critical settlement amount Scr,which have little influence on the settlement rate and equilibrium arch angleβ.The support force calculation model proposed in this study considers the three-dimensional soil arch and produces more accurate results than the traditional wedge-shaped model and the jacking standards.The contact pressure around the box at the same depth varies in a similar way.The pressure at the lower part of the box wall is higher than that at the top of the box,and the contact pressure and its change rate go through the stages of rapid increase,rapid decrease,and slow decrease in turn over time.Since the slurry buoyancy,the rectangular box is suspended,with the box top in contact with soil and the other three sides in contact with slurry.This study proposed a modified Terzaghi soil pressure calculation model and a contact pressure calculation model during slurry filtration.The field data show that this calculation model can more accurately predict the change range of contact pressure.In contrast,the traditional Terzaghi model does not consider the action of slurry and underestimates the soil pressure.The type of soil and lubrication conditions have a significant effect on the magnitude of the shear stress at the box-soil-slurry contact interface,while the influence of moisture content,shear velocity,and normal pressure is relatively small.The box-soil friction resistance decreased by 60%～80%for sand and 90%for clay when the slurry was used.The box-soil friction coefficients for different lubrication conditions were obtained by inversion method.Furthermore,the study established the friction resistance calculation models based on the contact status,friction coefficient,and friction force type.Under the condition of without slurry lubrication,the full box-soil interface have a high friction coefficient,and the jacking force increased linearly with the jacking distance.Under the transitional condition of slurry lubrication,the full box-soil contact gradually transformed into partial contact（including three-sided box-soil contact and two-sided box-soil contact）,and the friction coefficient decreased,leading to a significant decrease in jacking force.Under the condition of effective slurry lubrication,the one-sided box-soil contact was formed,and the friction coefficient reached the lowest value.The jacking force increased linearly again with the distance of jacking,but the increasing slope was significantly smaller than that under the without slurry lubrication condition.Through the analysis of two rectangular box jacking projects and numerical simulations,it was proved that the jacking force calculation model proposed in this study has a accurate prediction results.In addition,this study found significant differences between rectangular box jacking and circular pipe jacking in terms of excavation face instability characteristics,contact pressure distribution patterns,and pipe-soil contact status,which lead to the differences in the jacking forces calculation models.From four perspectives including excavation face support force,contact pressure,frictional mechanical characteristics,and jacking force influence mechanism,this study proposes recommendations for reasonable control the jackig force of large-section rectangular shield tunnels buried in sand stratum. |
