| The construction of subgrade in high-speed railway resting on soft ground requires not only the stability of embankment, but also the strictly control for the post-construction settlement. The geogrid-reinforced and pile-supported embankment (herein after for GRPSE) is a new type of composite foundation to control the settlement, and the current researches are basically focused on normal embankments with a relatively small cross section resting on a specific commonplace soil foundation. The GRPSE used to support a station yard of a high-speed railway resting on oversize-deep-soft soil have never been studied, and no relevant research or engineering principles can be used for reference. In this dissertation, a construction section with relatively deep soft layers and a large cross section was chosen as the testing site, and the corresponding data is analyzed in detail to investigate the bearing behavior and settlement mechanism of GRPSE resting on oversize-soft-soil by using field test and three-dimensional numerical simulation methods. The main results obtained are listed below.1. Based on the specific strata distribution of oversize-deep-soft soil in the Chaoshan station and physical and mechanical properties of soil, the three corresponding main characteristics (rheological properties, thixotropic properties and high organic matter content) and their impact on the control of subgrade settlement are analyzed in detail. The stratigraphic distribution of the field test section and the soft soil characteristics, and the engineering significance are investigated specifically. Furthermore, the physical and mechanical properties of soft soil, details of sections stratigraphic distribution and cross section of field test section are introduced.2. According to the in-situ soft soil handling segment of Chaoshan station, a monitoring section are selected, and the corresponding monitoring instruments consists of layered deposition tubes, inclinometer tubes, earth pressure cells, flexible displacement sensors, stress gauges and pore water pressure gauges are arranged. The process of pore water pressure in the upper part of the embankment during construction period, the surface subsidence, the deep part of layered settlement, the horizontal displacement, the stress on the top of piles and soil among piles and so on are under observation to process the analysis of the settlement characteristics of the composite foundation, stress distribution and the pile-soil stress ratio. So that the reinforced effect of tube pile on soft ground can be estimated and verified. 3. By regulating and analyzing the monitoring data, the following conclusions can be drawn:①In the early stage of filling, the earth pressure on the soil between the piles and the top of the pile increase drastically, however the rate of change of the earth pressure on the top of the pile is larger than that in the soil between the piles. When the filling reaches a certain value, the maximum earth pressure occurs in the soil between the piles. The pile-soil stress ratio increases with time and loading, and it fluctuates under the constant adjustment of the cushion.②The pore water pressure increases slightly with the filling height because the load borne by the soil between the piles is relatively small.③Both the axial force and the skin friction of the piles increase with time and with load. In addition, the force and the friction are closely related to the properties of the soil layers. In the silt layer and the silty soil layer with low strength, the skin friction is relatively small, and the axial force can be quickly transferred.④The rate of change of the layered settlement is directly proportional to the filling speed. The settlement increases abruptly when filling is conducted in a short time; the settlement then develops gradually and decreases after a certain time period.⑤The stretching of the geogrid increases with the filling height, and the stretching in the soil between the piles is larger than that on the top of the pile. The stretch rate and tension of the geogrid located in the soil between the piles are both larger than the corresponding values on the top of the pile.⑥As the fill height and the consolidation of soil gradually increase, the lateral displacement of the embankment increases accordingly. The rate of change of the lateral displacement during the filling period is larger than that during the stable period after filling. The lateral displacement of the embankments varies in different soil layers under loading, and the lateral displacement of the embankment in soft soil layers is relatively large.4. Using the finite difference three-dimensional software of FLAC3D, the full monitoring sections are simulated to analyze the axial force, shear, bending moment, skin friction of piles as well as soil pressure on the top and the bottom of the pile body, the variation properties of pore water pressure and pile-soil stress ratio. The relationship of negative skin friction from the filling height, the pile-soil differential settlement, the pile-soil stress ratio, the length of pile and the distributions of underlying soil layers are investigated, and the connection between the location of neutral point and the pile spacing, the size of pile cap, the elastic modulus of the cushion and so on are discussed correspondingly. Furthermore, the influence of the negative skin friction on the settlement and deformation of embankment and the bearing capacity of piles are studied. In addition, under consideration fluid-solid interaction, the variation law of the surface subsidence of soil among piles, the settlement of pile top, the differential pile-soil settlement and the lateral displacement are analyzed. At last, by extracting the monitoring data from the center pile-soil subsidence, settlement of pile top and embankment surface settlement considering the additional loading stage and the rheological phase of soft soil, a relevant settlement prediction is made. |
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