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Study on Stress Characteristics and Load Reduction by TriAx Geogrid Reinforcement for High Embankment Culverts

Author: MaQiang
Tutor: ZhengJunJie
School: Huazhong University of Science and Technology
Course: Geotechnical Engineering
Keywords: high embankment culvert earth pressure geogrid field test theoretical analysis numerical simulation
CLC: U449
Type: PhD thesis
Year: 2011
Downloads: 41
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Abstract


High embankment culverts are widely used in constructions of expressway in mountainous and loessial regions. Large loads are usually exerted in the crown of the high embankment culverts, and the mechanism of the fill-culvert interaction is complex, it has a lot of influencing factors, including not only installation conditions of the structure(embankment installation or trench installtion), but also heights of embankment fill, terrain conditions, geological conditions, properties of filling material, sizes of structure and many others. Because the influencing factors of the earth pressure are not clearly understood, there are frequent occurrences of cracking on the crown of the high embankment culverts. TriAx geogrid has unique physical and mechanical properties, when it is used as reinforcement material for imperfect trench method, it can reduce the earth pressure on the crown of high embankment culvert and obtain good economic and social benefits. In this dissertation, the load reduction process of imperfect trench covered with TriAx geogrid is studied systematicly on the basis of the investigation of the earth pressure and its influencing factors on high embankment culvert, which mainly contains:(1) The variation of vertical earth pressures on the crown of the high embankment culvert is studied by field tests. Numerical simulations are also carried out to investigate the influencing factors on the vertical earth pressure and displacement of culvert. The results show that the vertical earth pressure increases nonlinearly with the height of embankment. Asymmetric installation of the culvert causes different earth pressures on both sides. The width of the trench which effectively impacts the culvert is approximately three times of the width of culvert, and the slopes angles of 15┬░or 75┬░result in less vertical earth pressures. Larger moduli of the subgrade result in more severe stress concentration. Compared with the moduli, the internal friction angles and cohesions of the filling material have much less influences on the earth pressures and the displacements of the culvert. The types of culvert foundations have significant effects on the displacements and earth pressures, thus the appropriate choice of the foundation type should be made in accordance with the requairements of the earth pressure and the displacment. The earth pressures on wide-and-short culverts are less than those on narrow-and-tall culverts. The settlements and earth pressures are less affected by the curvatures of the arch of the culvert.(2) Considering the interaction between culvert-soil, the mechanism of load reduction for high embankment culvert is analyzed. Numerical simulations are carried out to investigate the effect of different load reduction methods by studying the vertical earth pressures and displacements of the culverts. The results show that the compressible materials can be filled within a certain area above the crown of the culvert, or the embankment fill above the culvert can be compacted loosely in the center to alleviate the concentration of the vertical earth pressure on the crown of the culvert. Excavation after filling method can be also used to reduce the vertical earth pressure, the width of the excavation should be eaqual to that of the culvert and the slopes should be vertical in order to achieve a better load reduction effect. In ground improvement, a relatively flexible composite foundation is preferred and the improvement width should extend a certain range wider than the culvert foundation, thus it can alleviate the pressure concentration on the crown of the culvert effectively.(3) Field tests are carried out to study the distribution and the variation of earth pressures with the increase of the embankment height. The variation laws of the displacement of the culvert with load reduction treatment are also investigated by field monitoring. The results show that the earth pressure on the crown of the culvert underneath an imperfect trench covered by a geogrid layer is transferred to the lateral soil prisms, so that the purpose of load reduction is achieved, and the load transfer efficiency is increased by the "tensioned membrane effect". The load reduction efficiency is related to the height of the load reduction ditch, a higher ditch results in lower vertical earth pressures.(4) The calculation model of the culvert underneath an imperfect trench covered by a geogrid layer is established by theoretical analysis, considering the support of the compressible fill below the geogrid, the analytical expressions of the earth pressures on the upper surface of geogrid layer and the crown of the culvert are derived. The anchorage forces at each end of the geogrid layer are obtained as well. Parametric analyses are performed to investigate the influencing factors of the vertical earth pressure. The results show that the vertical earth pressure on a culvert underneath an imperfect trench covered by a geogrid layer can be accurately calculated by the proposed method. The modulus of the compressible material in the trench has greater impact on the vertical pressure, the modulus is suggested within a appropriate range. The geogrid has significant effect on further reducing the vertical earth pressure, only if the geogrid with sufficient stiffness. Compared with the single layer of geogrid, the increase of efficiency of load reduction with multilayer geogrids is neglectable. The cohesion of the material in the trench has little effect on the vertical earth pressure, while the internal friction angle has a greater impact.The achievements of this research can provide valuable references for the design and construction of high embankment culverts.

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