The Study On Shaking Table Of Liquefiable Sand Soil Improved By Multiple Composite Piles

In various research subjects of geotechnical engineering field, the saturated sand earthquake liquefaction is always the focus on the important subject of engineering. In recent years, the frequent earthquake has brought great damage and loss of life and property, while the liquefaction has intensified the occurrence of this disaster. So it is very necessary to study on liquefaction mechanism and the liquefied foundation reinforcement for engineering construction and disaster prevention and mitigation. The long-short-pile composite foundation is widely used to reinforce the soft soil to improve the bearing capacity and decrease the settlement. It is little research or used for reinforcement of liquefied foundation. Gravel pile composite foundation is considered to be an effective way of liquefied foundation reinforcement, and it is also needed further research on the function mechanism that the gravel pile of the multiple composite pile foundation reinforced. Therefore, it has great engineering sense that further study on the effect and function mechanism of liquefied foundation comprehensive multiple reinforcement by multiple composite piles, and the function of the gravel pile in the pile reinforcement liquefied foundation.In this view, under the support of National Science Fund Project "The experimental study on mechanism of piles improving liquefiable soil" (No.50578104), the research object focus on long-short multiple composite pile comprised of gravel pile and cement pile, by comparing the difference of non-improved soil and the soil improved with long-short cement pile, long cement pile-short gravel pile, long gravel pile-short cement pile, the shaking table tests were designed and carried out to get the macroscopic phenomena, forces of piles and soil schedule, excess pore water pressure-time curve and pore water pressure ratio-time curve in different depth improved by different ways. By analyzing the test obtained data, the results are as follows:(1) On the macroscopic phenomena of models:the non-improved soil completely liquefied under the vibration load. The long-short cement pile reinforced foundation in the process of earthquake appears a sandblasting water phenomenon on the soil surface and around the pile. After the vibration, the displacement of long pile is not apparent, while the short pile has more sinking. During or after the vibration, it is obviously observed the drainage effect of gravel pile in the long cement pile and short gravel pile composite reinforcement foundation, it also made the pore water pressure lifts and the pore pressure ratio decrease. In the process of earthquake, water has been discharging from the gravel pile in the long gravel pile and short cement pile composite reinforcement model. After period of time, water will surface covered soil, water depth is about2cm.(2) On the Soil pressure curve of models:The soil pressure value are all small in everywhere of the non-improved foundation. The long-short cement pile can inhibit vibration of the influence of the soil pressure mutations, and reduce pressure peak. But it is not obvious to improve the soil strength, and to increase the force binding of the pile from around soil. Long cement pile and short gravel pile composite reinforcement, long gravel pile and short cement pile composite reinforcement, both them Obviously increase the middle and deep soil strength between pile, and increase the force binding of the pile a lot which can enhance side friction in order to decrease subsidence. Including long cement pile and short gravel composite reinforcement can also play a role of mutations that inhibit the peak pressure and reduce the stress.(3) On pore water pressure ratio-time curve of models:No matter in what parts the pore water pressure of non-improved foundation is greater than1, and larger than other reinforcement model. Which proves that liquefaction whole happened from shallow to the deeper soil. The long-short cement pile reinforcement foundation has the function of inhibit liquefaction in shallow and middle soil, its mechanism mainly is the soil partitioned by pile, which play a "the grille" effect, but it can’t play the deeper soil liquefaction inhibition. A gravel pile in the pile foundation reinforcement can significantly inhibit the liquefaction occurrence of deep and middle of soil, which explain that the good drainage channels forming by pile can quickly dissipates pore water pressure, and have good role to reduce the liquefaction.To sum up, the short gravel pile would effectively restrain the liquefaction in deep and middle of soil, good lift pore water pressure, and improve soil strength of the deep and the middle. So it has excellent project characteristic, and is the best and effective reinforcement method among the several composite pile reinforcement foundation.