Highway Subgrade Widening Reconstruct Mechanical Behavior And Typical Engineering Countermeasure Research

The key point of successfully broadening the highway subgrade is to control the differential settlement of the new and old subgrade top surfaces within permissible range on the premise of ensuring the safety and stability of subgrade. For this reason, the research on the mechanical behavior of plain soft soil area and mountain highway subgrade while broadening and rebuilding as well as the representative engineering countermeasures are conducted by means of numerical simulation.Expressway embankment widening may produce instability and different settlement on soft ground with high under ground water level. The existed research focus on a certain kind of widening type. Based on elastic-plastic plane strain finite element analysis using PLAXIS, Biot consolidation theory and shear strength reduction method were comprehensively applied to study the mechanical behaviors comparisons of four widening types. Some critical responses, such as potential slip surface, safety factor, excess pore water pressure, vertical displacement and lateral displacement were compared. The analysis results show that broadening subgrade in bilateral symmetry style is advantageous to improving the stability safety factor and gets good results for diminishing the differential settlement of the new and old subgrade top surfaces and the lateral displacement of foundation. The unilateral broadening however leads to the large differential settlement.For broadening subgrade in the way of unilateral matching, the pile-net structure is adopted to consolidate foundation. The refined finite element model used to simulate the filling process of pile body, geogrid and new subgrade in layers is formulated to study the characteristics of structural deformation and soil mechanical responses. Results show that pile-net structure could significantly decrease the post construction settlement and differential settlement at the top of old and new embankment, effectively slow down the old embankment lateral disturbance impacted by new embankment, geosynthetic tension reach peak on pile top edge, critical slip surface changes to old embankment from new embankment, stability safety factor keeps relatively high in the full process, excess pore water pressure peak and dispersing time significantly decrease. Pile-net structure can be used in the soft ground old road widening project quick safety treatment.With consideration of the landform of mountain area, the retaining wall is applied for the mountain-area secondary highway subgrade broadening and rebuilding project. Non-linear finite element method was conducted to simulate the dynamic construction of balance weight retaining wall building and new embankment fractional filling considering the mountain region highway embankment widening particularity. The effects of new embankment geosynthetic reinforcement and fill compaction condition on the embankment stability, slip surface behavior, settlement and retaining wall displacement were discussed. The dynamic coupling relationship between embankment settlement and retaining wall was investigated combined with the geotechnical centrifuge model tests which simulate the retaining wall different displacement modes in mountain region embankment widening project. The retaining wall displacement and embankment settlement, differential settlement are intercoupling. The analysis results show that the embankment differential settlement will increase for the insufficient compaction under the equilibrator. The retaining wall leaning toward will induce additional embankment differential settlement. The pavement structural design should consider the magnitude modification of differential settlement induced by new embankment self-weight.According to the engineering practice on subgrade broadening and rebuilding for the existing highway, the basic techniques for the project are put forward initially. The countermeasures to control the differential settlement of the new and old subgrade top surfaces including the technical essentials and scope of application of related projects are emphasized and summarized. More specifically, the foundation treatment method includes composite foundation, consolidation by dewatering, settlement separating wall, etc. And the subgrade treatment method includes junction surface, subgrade filling, compactness, reinforced materials and retaining wall and so on. The whole research can provide a reference for the following highway subgrade broadening projects.