Model Test And Numerical Analysis Of The Behavior Of Inclined Pile Supported Loess Foundation Pit

At present,the main forms of foundation pit support are cantilever type,pile anchor type,and internal support type.The above three types of support structures all have certain shortcomings,such as poor stability of cantilever support structural stability and small excavation depth;The pile anchor support structure has a significant impact on the surrounding environment and is not suitable for poor soil quality;The internal support structure has high cost,long cycle,and a large amount of solid waste.In recent years,new unsupported support systems such as inclined piles have received widespread attention in the engineering community.Related research and engineering practice have shown that the inclined pile support structure has relatively small stress and deformation,high stability,large ultimate excavation depth,and self supporting effect.At present,there is insufficient research on the deformation laws,failure modes,and bearing mechanisms of inclined pile support structures.This article studies the support performance of cantilever straight piles,double row piles(double row straight piles and double row inclined piles),and inclined piles(outer inclined straight,inner inclined straight,and combination of inner and outer inclined piles)in loess foundation pits through indoor model tests and numerical simulations.The stress and deformation characteristics of the support structure,as well as the stability of the foundation pit,are analyzed,and some regular understanding is obtained.The following research work is specifically carried out:(1)Through model experiments,the variation patterns of pile top displacement,pile bending moment,and surface settlement outside the loess foundation pit were analyzed for cantilever straight piles,double row straight piles,outer inclined straight and inner inclined straight combination support structures during excavation.The ultimate excavation depth of the foundation pit under different support structures was obtained.The results show that the performance of the inner inclined straight support structure is better than that of the outer inclined straight support structure,but both support performance is better than that of the cantilever straight pile and double row straight pile support structure.The ultimate excavation depth of the inner inclined straight support structure is increased by 40% and 16% compared to the cantilever straight pile and double row straight pile,respectively.(2)Conduct indoor rainfall experiments using self-made artificial rainfall equipment to analyze the impact of rainfall on the stability of foundation pits and support structures.The results indicate that the stress and deformation of the support structure during the rainfall stage are significantly greater than those during the excavation stage of the foundation pit,and the overall stability of the foundation pit is also reduced,leading to soil collapse outside the pit and surface cracking of the soil inside and outside the pit.Therefore,in practical engineering,various possible disasters should be anticipated in advance,and corresponding measures should be taken in advance to avoid causing foundation pit accidents.(3)The numerical simulation of the entire process of loess foundation pit excavation was conducted using the numerical software PLAXIS 3D.Based on the experimental results,inclined double row piles and internal and external inclined composite support structures were added,and the changes in pile displacement and axial force were further analyzed.The numerical simulation results are consistent with model tests.Among the inclined pile support structures,the performance of the inner and outer inclined support structures is the best,followed by the inner inclined straight combination,and the outer inclined straight combination is the worst.The ultimate excavation depth of the inner and outer inclined combination support structure is about1.6 times that of the cantilever straight pile,1.2 times that of the double row straight pile,and 1.1 times that of the double row inclined pile.The force and deformation of the outer inclined straight combination and the double row inclined pile support structure are similar.(4)Based on an actual engineering site in Xi’an,numerical software was used to analyze the stability of the foundation pit supported by cantilever straight piles,inclined piles,and double row piles,and further research was conducted on the impact of rainfall on the stability of the foundation pit.The results show that the support performance of inclined piles and double row piles is significantly better than that of cantilever support structures,among which the inclined single(double)row piles with inner row piles as inclined piles have the best support effect;under the action of rainfall,the safety coefficient of the foundation pit decreases,ranging from 0.3 to 0.5.The decrease in safety coefficient is closely related to the initial safety coefficient of the foundation pit,and the larger the initial safety coefficient,the smaller the decrease.