Stability Study Of Super High Fills Subjected To Multi-Layered And Multi-Directional Loadings Over V-Shaped Gully

The employment of high fills over gullies at mountainous or reservoir areas makes good use of road or tunnel excavations which otherwise pose a big issue for the disposal of large quantities of earthwork. The advantages are numerous:the saving of construction cost, the saving of construction time for the short hauling distance, simultaneous compaction and road cutting or tunnel boring on both sides, thus construction organization is facilitated. It can be seen obviously that high fills over gullies prove more economical than bridges. However, the stability of the high fills is worth in-depth research due to the special topographical, geological and hydrologic conditions at the gullies. The gullies at the mountainous or reservoir areas have generally undergone years of formation, generally steep at the bottom. The hydrologic conditions at the gullies are complicated, affected by the seepage flow from the high ground and rainfall, as well as the seasonal fluctuation of water levels. To make the matter worse, the huge weight of the high fill tends to deform the whole slope, so if not properly treated, settlement or slide of the high fill might occur, which will seriously disrupt the safe operation of the highway.An analysis of the characteristics of a super high fill over a V-shaped gully indicates that the stability of the high fill is affected by a couple of factors, either positive or negative. For each factor, the influencing degree varies due to the specific local conditions. Where these various factors combined together, the stability of the high fill deteriorates drastically, and even fails, so retaining structure is needed at the toe of the fill. The above analysis shows that study on the overall stability of high fills over V-shaped gullies are necessary and meaningful. This research firstly gained the subgrade stability factors of a typical cross section at the gully bottom in order to verify the reliability of the calculating method. Secondly, the stability factors and stability evolution rule of a typical slope section were gained with different stability influencing factors taken into consideration:three-dimensional (3D) effect, multi-layered loadings, fluctuated water levels, and surface seepage flow due to rainfall, respectively. Finally, the appropriate type of retaining structure for the high fill subjected to multi-directional loadings, combined with all the negative stability influencing factors, was discussed.With the above research, the results show that:①The V-shaped gully imposes obvious confining effect on the high fill, and the gully assists the stability of the high fill. So3D rather than traditional2D stability analysis should be utilized upon high fills over V-shaped gullies. The traditional2D method tends to be conservative and thus does not properly reflect the stability of the high fill. If the traditional2D method is used because3D method is too complex, difficult or timing-consuming, or can not be realized, the2D stability factor thus gained needs to be modified, by multiplying a3D influencing factor.②Multi-layered loading has impact on the stability of the super high fill. In the stability analysis, assuming that the loading on the fill was uniform; the mid-slope loading helped stabilize the upper part of the slope and the effect of the top-slope loading upon the lower part of the slope was ignored, the results indicate that the fill is most instable when loadings is applied both on the top and mid slope. Where the vehicles are overloaded, the slope will become weaker and even fail if overloading becomes more serious.③Rising water levels impose high pressure on the slope, so the water will infiltrate into the slope. With continuous rising of water levels, the saturation degree of the slope soil will increase, which will finally lead to deeper seepage into the slope and higher pore water pressure. All of these factors help stabilize the high fill. The higher the water level, the more stable the fill. On the contrary, the receding of water is dangerous to the slope. When the water level drops, water flows outside the slope, and water head pressure on the slope disappears, which will eventually result in overturning tendency of the slope, so the stability factor of the slope decreases. The slope becomes most instable when the water level drops to the lowest degree.④The influence of surface runoff due to long lasting, heavy rainfall upon the slope stability of the high fill over a V-shaped gully is significant. Generally, short and mild rain will not affect the stability of the slope. However, the addition of either the amount or duration of the rainfall will make the water seep deeper into the slope and thus the added weight of the slope, and the changed stress-strain relationship of the fill materials, so the stability factor of the slope decreases. Regional failure rather than overall slide of the slope might occur under such condition.⑤The gully’s restraining effect helps stabilize the high fill, but vehicle load, decreased water level and runoff seepage into the slope will produce large negative effect. In some cases these factors are so influential that the embankment will not be stable enough for safe operation of vehicles above. To ensure the safety of the high fill, retaining structure is needed at the toe of the slope. Where the cross section of the retaining wall is given, curved wall is better than the linear wall in terms of deformation and distress. Assuming that the maximum tensile stress of the concrete fill is1.27MP, the ideal radius of the wall is calculated to be70m in circular arc. As the fill height adds from both sides of the gully to the center, the earth pressure also increases to the center of the gully. Thus it is concluded that the thickness of the retaining structure can be reduced gradually from the center to the sides of the gully when the maximum tensile stress of the materials1.27MPa is met. So the retaining structure could be formed by transformed section, thickest in the middle and thinner on both sides.By focusing on the overall stability study, the stability evolution rule and selection of appropriate retaining structure for super high fills constructed over V-shaped gullies subjected to complicated topographical and hydrological conditions, as well as multi-layered and multi-directional loadings, this research provides an economical and reliable design for highways built over gullies in mountainous or reservoir areas, and especially offer theoretical and technical assistance to the structure of high fill plus retaining dam over gullies in mountainous or reservoir areas.