Study On Composite Foundation Improved With Long-short Piles Under Subgrade Loading By Using Indoor Physical Model Tests And Numerical Simulations

Viewing from the engineering application, the technology of composite foundation hasbeen widely used in soft soil area. With the continuous development of the technology andtheory, load transfer mechanism, deformation control, stress field and displacement fieldcharacteristics of piles in composite foundation are being studied deeply, compositefoundations of the combination of different pile type (or piles of same type but differentlength) have emerged, among which the combination of different length of piles is thelong-short-pile composite foundation.Long-short-pile composite foundation is a new type of foundation treatment methoddeveloped in recent years, because it has a good effect on controlling settlement of compositefoundation, especially uneven settlement, and can produce good economic and social benefits,it has been widely applied in the engineering project of civil building, high grade highway andrailway in soft soil area. But the theoretical research of long-short-pile composite foundationis not much, and there is a lack of experience in engineering practice. In order to make thelong-short-pile composite foundation technology be better used in the construction of highgrade highway and railway in soft soil area, there is a need for a more in-depth study of thecharacters of long-short-pile composite foundation. The main works of this paper are asfollows:Firstly, summarize the research status of long-short-pile composite foundation; study thecalculation theory of settlement of existing long-short-pile composite foundation, compare theadvantages and disadvantages of these methods, attention should be paid to the applicationscope.According to the prototype of long-short-pile composite foundation under flexible loadof high speed railway, combined with the similarity theory, indoor model test is designed andconducted to study the working character of long-short-pile composite foundation underflexible load effect of subgrade. The influence of the length of short pile on long-short-pilecomposite foundation is analyzed, the bearing capacity, settlement, axial force of long pileand short pile, stress ratio between pile and soil, load sharing ratio between pile and soil oflong pile and short pile are respectively analyzed under different length ratio n between longand short pile to study the optimal length ratio under flexible load.When n=0.5and the loadis100kPa, long pile bears55.7%of the whole load, short pile bears15.4%of the load, the soil between the piles bears28.9%of the load. When n=1and the load is100kPa,short pile bears27.5%of the load,long pile bears45.3%of the load, the soil between the pilesbears27.2%of the load. The length of short pile has an optimal value.Model of long-short-pile composite foundation under subgrade flexible load isestablished by MIDAS/GTS, in order to consider more comprehensively, in numericalsimulation, short pile of different rigidity, existence of shallow bearing layer and other factorsthat influence the character of long-short-pile composite foundation are analyzed.Thesettlement value under different conditions is71.13mm,62,22mm,57.03mm,46.11mm,28.94mm,23.17mm. Through the analysis of different conditions, it is obtained thatthe shallow bearing stratum and the stiffness of short pile have great effect on settlementof composite foundation.Long-short-pile composite foundation finite element model is designed, throughnumerical simulation, the bearing capacity, settlement changing law, stress ratio between pileand soil of long and short pile, settlement difference between pile and soil of long and shortpile, horizontal displacement of long-short-pile composite foundation are analyzed.