| In the northern area of Anhui province, low liquid limit silt and silt-sand-basedsoil deposition are widely distributed. The characteristics of the silt from this area arelisted as follows:(1) it has uniform particle size, poor stability of water, developedcapillary and are frozen sensitive.(2) it also has high coefficient of permeability,strong consolidation, drying form of powder and can be shaped only in conditionswith certain water content, etc. Due to the special physical and mechanical properties,the silt from this area is not suitable to be used as the material of soil sub-grade.However, with the rapid development of highways in Anhui province, more and moresilt filling has been used for embankment construction.The liquefaction for saturated silty foundation can be usually incurred underdynamic loading, such as vehicular loading and earthquake. The silt soil liquefactioncaused by earthquake is mostly the direct reason for the foundation invalidation andthe structure collapse.Therefore, it is very important to set up a series of mechanical method and finiteelement method to deal well with the liquefaction and anti-liquefaction characteristicsof silt soil. The analytical and finite element results of the anti-liquefaction by usinggeo-grid gravel pile composite foundation can provide some reference to the similarpractical engineering.In this paper, in accordance with the practical engineering of Si-Xv highwayunder construction in Anhui Province, the liquefaction behavior of the silt foundationand anti-liquefaction characteristic of the silt gravel pile composite foundation areanalyzed by using the dynamic tri-axial test and the finite element method FLAC3D.Meanwhile, the validity and feasibility of anti-liquefaction of the gravel pile isanalyzed and verified by the analytical and finite element method.Based on the experimental and numerical research, the main following contentsand conclusions have been obtained:(1) To analyze the liquefaction resistance and obtain the corresponding maximumliquefaction depth of saturated silty soil from Anhui province under different highseismic shock, a series of dynamic tri-axial tests and finite element simulation arecarried out under the given density of remolding saturated silt with different confiningpressure conditions varying from1m to15m depth. Compared to the experimentalresults and finite element results, the maximum liquefaction depth is that there is no liquefaction incurred under the seven seismic shock, there is11meters of liquefactiondepth incurred under the eight seismic shock, and there is more than15meters ofliquefaction depth incurred under the nine seismic shock.Besides, the increasing law of liquefaction resistance of the remolding saturatedsilt, dynamic intensity and the process of excess pore pressure are obtained from thedynamic tri-axial test and finite element simulation research. And one new increasinglaw of the process of excess pore pressure of the saturated silt in this area is provided.The maximum liquefaction depth and the dynamic deformation properties of thesilt in Anhui province can provide scientific data to further make engineeringmeasures such as gravel piles preventing the silty foundation from liquefying to someextent.(2) In order to obtain the relationships of vibration frequency and dynamic stress,dynamic tri-axial test is performed. The experimental results show that at theprophase of the vibration (about500cycles), the increasing law of the dynamic strainis linear with the number of cycles increasing, after the prophase, the increasing law isnonlinear, and the increasing increment is reducing and the dynamic strain is beingsteady at last. The test results also show that the larger dynamic stress applied to thesilt soil, the more quickly the dynamic strain being steady.The relationship between the dynamic stress and the dynamic strain is one closedhysteresis loops, with the given dynamic stress. The obliquity and the shape ofstress-strain hysteresis loops are almost the same, but the interval is quite different. Atthe prophase of the vibration (about500cycles), the interval is large, however,interval is being reducing with the number of cycles increasing. Besides, the larger thedynamic stress applied, the more quickly the interval being reduced.(3) Based on the research of the increasing law of the excess pore pressure, onenew kind of analytic solution of pore pressure is proposed and deduced in accordancewith the drainage capacity of gravel drains, which can actually describe thecontributions of gravel drains to the generation, diffusion and dissipation of theexcess pore pressure. Besides, the effect of drain permeability of the gravels on rateof excess pore water pressure dissipation is discussed using the proposed formulation.It shows that the larger the KP/KS, the more quickly the excess pore pressure ratiobeing reduced, which means the saturated silt soil reinforced with gravel pile is hardto be liquefied.Meanwhile, the geo-grid can obviously strengthen the anti-shear of the gravel pile and ensure the drainage is expedite.(4) In this paper, the anti-liquefaction of the silt gravel pile composite foundation isanalyzed and calculated using the finite element procedure FLAC3D. The analyzedresults show that the excess pore pressure of the silt soil near the gravel pile is lowerthan that of the silt soil far away from the gravel pile. That is to say, the excess porepressure close to the gravel pile is dissipating more quickly, the anti-liquefaction ismuch stronger. On the other hand, the excess pore pressure is dissipating more slowlywith the increase of the depth. And the excess pore pressure ratio is bigger at themiddle of the gravel pile composite foundation.To sum up, it is quite effective and feasible to adopt the geo-grid gravel pile toprevent the saturated silt from liquefaction. The geo-grid can not only dissipate theexcess pore pressure quickly, but also strengthen the shear resistance of the gravel pileto ensure the drainage is expedite. |
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