Field Test And FEM Analysis For Composite Foundation Of Cement Mixing Pile Under Embankment Load

The soft ground foundation under the embankment load, treated by the cement-soil mixing method, is a typical flexible-piled composite foundation under a flexible loading. It has been successfully employed in soft ground foundation treatment for high-grade railway and highway gradually in recent years.Now, methods for calculation or estimation of the bearing capacity in terms of strength and deformation of the flexible-piled composite foundation under the function of the flexible foundation have been developed. But due to the insufficient knowledge on the load transfer rule and the deformation features of this kind of composite foundation, there exists big differences between the calculation result and the actual case. What 's more, there is also potential safety and economy risks. Therefore, carrying out the tests on the flexible-piled composite foundation to study and analyze the load transfer rule and the deformation features will be of help to the theory development and help make use of it more rationally in the construction.In this paper, based on the large-scale field tests of the cement mixing end of the soft-foundation treatment section in the new-built Dazhou-Chengdu Railway, dedicated to both passengers and goods transportation with a designed speed of 200km/h in which a large amount of testing elements were buried, a series of testing observation on the lateral movement of deep layer foundation, strength of the cement-soil mixing piles, settlement of foundation, pore water pressure, pile and earth pressure, tensioning force of geotechnical grille of reinforcing cushion etc. Meanwhile, with the help of professional ANSYS software, a preliminary numerical analysis on the bearing capacity and deformation features of the flexible-piled composite foundation under the embankment load has been done, acquiring the results on deep layer foundation horizontal movement, pile axis force, foundation settlement and distribution of loads etc. Comparing and analyzing the results gained both from field tests and the calculation, basic and main conclusions have been reached as follows:(1) The additional stress on the cement-soil mixing pile body shall be along the depth direction of the foundation, increasing first, then decreasing. Maximum additional stress is at the middle section of the pile.(2) The additional stress on the soil between piles in the reinforced section is small, but increases rapidly at the bottom-laid section.(3) According to the actual survey, the compressed curve of the soil between piles presents a K shape, bigger at both top and bottom sections and smaller in the middle. The compression section with minimum amount is near the pile where the maximum axis force lies.(4) When the embankment load reached to a certain degree, a turning point of the earth pressure between piles presents. Afterwards, part of the additional stress will be transferred to the cement-soil mixing piles gradually, leading to a further increasing of the add-on stress on the pile head.(5) The change of the excess static pore water pressure reflects the filling speed of the embankment construction. If the speed is fast, the excess static pore water pressure is bigger; the speed slow, the pressure is smaller.(6) There is a close relation between the foundation settlement, the tensioning of geo-grids and the depth of the soft ground foundation. The deeper the soft foundation is, the bigger the settlement and tensioning are; the shallower the foundation is, the smaller the settlement and tensioning are. After the embankment deposit period, due to the the tensile creep, the tensioning force of the geo-grids will decrease.(7) The actual measurement of the deep layer horizontal movement of the foundation is close to the calculation result, but the distribution state, to a certain extent, is different.