Study On The Effect Of Cement-soil Mixing Piles On The Reinforcement Of Collapsible Loess Foundation

Because of its wide adaptability,low cost and good reinforcement effect,cement soil mixing pile composite foundation has been widely used in the reinforcement of soft soil foundation.The bearing capacity of mixing pile composite foundation is one of the most important indexes to evaluate its reinforcement performance.In engineering practice,the corresponding construction methods are usually selected according to the results of field investigation,and the construction parameters are determined and the reinforcement effect of composite foundation is analyzed by testing piles and monitoring the bearing capacity of piles and composite foundation in the process of pile testing.This paper first analyzes the development trend and engineering application of cement soil mixing pile composite foundation at home and abroad,summarizes the concept,main types and utility of composite foundation,analyzes the reinforcement mechanism of pile composite foundation,and the calculation theory and method of bearing capacity and settlement characteristics of cement soil mixing pile composite foundation.On this basis,taking the loess foundation of Taigu section of Taijiao railway as the research background,according to the field investigation and test results,the characteristics and collapsibility of loess in this section are analyzed and evaluated.Finally,by means of field monitoring,numerical analysis and theoretical calculation,the bearing capacity and settlement characteristics of cement soil mixing pile composite foundation in this section are calculated and analyzed,and the following main research conclusions are obtained:(1)It is found that the integrity of the core sample is good and the pile body is uniform through the inspection of the appearance and dimension of the pile body and drilling core.The unconfined compression test results of each core sample are all greater than 1MPa,which shows that the integrity of the pile body and the strength of the pile body can meet the design requirements.(2)With the gradual increase of load,the load settlement curve of composite foundation develops slowly downward,and the proportion limit and ultimate load are not obvious;under the vertical load,the characteristics of compaction,local shear failure and complete failure of foundation are not obvious,which indicates that the bearing capacity of composite foundation is not up to its ultimate state.(3)Through static load test,theoretical calculation and numerical simulation,the characteristic values of bearing capacity of single pile composite foundation with cement soil mixing pile are 190 kpa,315kpa and 238 kpa respectively.It can be seen that the theoretical calculation results of bearing capacity eigenvalue of composite foundation are larger than the field test results,mainly because the test load does not reach the limit state of composite foundation,and the bearing capacity is not fully developed;the numerical simulation results are slightly smaller than the field static load test results,but the difference value is small.(4)The results of field monitoring,numerical simulation and theoretical calculation are21.17 mm,26.14 mm and 31.25 mm respectively.The results of numerical simulation are larger than that of field monitoring,because the density and elastic modulus of foundation soil usually increase with time,but in this model,the density and elastic modulus are taken as fixed values.