An Experimental Study On Composite Foundation Of Compacted Cement-soil Pile And Its Numerical Simulation

The composite foundation of compacted soil-cement pile (herein for short CCP) developed is one of ground improvement methods recent years. It is used widely in the northern regions where underground water level is lower such as Beijing and Hebei province. Presently, engineers in situ design and calculate the composite foundation according to their experiences in most of time. Therefore, the theory and the criterion of soil improvement need to be perfected. In fact, the compacted soil-cement pile is one of the flexible piles. Although it's engineering chacrateristics is similar to soil-cement piles, its making technics process is specific, so it's load-bearing and deformation properties is unique. The research on its load-bearing and deformation properties and constitutive model will provide theoretical evidences for evaluating and predicting bearing capacity and settlement of the composite ground. According to the theory of modern soil mechanics and rockmass mechanics and engineering geology, the characteristics of compacted soil-cement strength and the load-bearing and deformation of CCP have been studied systematically by means of spot determination and lab test. The deformation mechanism of the CCP has been discussed from the point of microstructure by the combination of microstructure and macro mechanic property. And the constitutive model of CCP has been built forward. The engineering properties of CCP have been simulated. The following conclusions have been obtained. (1) The soil of test ground consists of silt and silty clay. It is alluvium sediment in front of Taihang Mountain. The comparison of mainly physical and mechanic indices of soil and the soil microstructure around the pile fore-and-aft making pile presents every index is scattered. This phenomenon reflects that the making pile process influence for the characteristics of the soil around the pile is small. This is one of the creative cognition. (2) The research of soil-cement microstructure helps to reveal the mechanism of distortion and failure of CCP. The feature of soil-cement microstructure presents the crystal of cement hydration after the hydration and hydrolyzes of cement powder, which makes the dispersive soil to be the agglomeration. The crystal of cement among soil grain which cements grains comes into being the gridding structure with curing time. Therefore, the contact force among soil grains is so increasing that the strength of the soil-cement is obviously higher than of natural soil. (3) The strength of the compacted soil-cement relates to its dry density, the kind of mixed soil, cement ratio and curing time. The strength of cement-soil is linearly increasing with the augment of cement ratio and the extension of curing time. Under the condition of single axis compression, tensile failure occurs in the cement-soil. The result of compressed pile test in spot shows perpendicular cleave fracture upper the pile (≦3~4 pile diameter). Whether single-pile, single-pile composite foundation or four piles composite foundation displays the characteristics that the pile shaft occur yield first, and then the composite ground occur failure. So the bearing capacity of CCP should be controlled by the yield of the pile shaft. This is one of the creative cognition, too. (4) The result of static loading test shows that the pile shaft and the soil among pile bear the load together when the composite ground stand perpendicular loading. However, the quality of the pile is better than the soil, so the shared load amount of the pile shaft is higher more than of the soil. The stress concentration of the pile is obvious. The shared load of the pile shaft in four piles composite foundation is larger more than of single pile. When single-pile composite foundation is used, the pile-soil stress ratio is 1.5~6.1, whereas the stress ratio of four piles is 9.6~25.7. Comparing with the single-pile composite ground, the composite foundation of four piles favors the exertion of bearing capacity, and the quality of pile shafts needs to be better than of single pile. This is also one of the creative cognition. (5) The curve pattern of load-settlement of the single-pile is steep fall; However, the curve pattern of the composite ground is from steep to gentle fall. The curve change is related with the cushion thickness. The step pattern of curve shape of four piles reflects the load-bearing property of every pile in CCP is complex and non-synchronous. This is one of the creative cognition. The load-settlement curve shape of the single-pile composite foundation is from steep to gentle with the thickness of the cushion. Under the reasonable thickness of the cushion, the load-settlement curve of the pile peak is all steep. The test result reveals that brittle deformation occurs in the shaft when the pile is compressed. Nevertheless, the settlement of the soil among the pile is sharply decreasing with depth. The deformation of the soil mostly occurs within the scope of less than 1.0m depth. (6) The thickness and character of the cushion have an important influence on the loading and deformation characteristics of composite foundation. With increasing of the cushion thickness, the loading ratio of the pile is decreasing, but the ratio of the soil is increasing. The pile-soil stress ratio reduces with the increasing of the cushion thickness. On the other hand, the more the character of the cushion is strong, the stronger the ability of adjusting deformation, the thinner the cushion thickness. The test result of deferent thickness and types of cushion shows that the best thickness is of 0.2~0.3m. (7) According to the deformation properties of the pile, the composite foundation and the cement-soil, the constitutive model has been built. The loading and deformation characteristics of the composite ground under various conditions have been simulated by the test parameters of the pile and soil. The simulated result is corresponding nearly with the result of loading test. The constitutive model built has been proved to be a good application. Although the simulating stress condition has some errors with practice, the whole trend is satisfied, and the model needs to be improved in the future research. Because of the test condition limited, the static loading test had been carried out only under constant cement ratio, dry density and length of the pile. It is necessary to study the engineering feature, the law of loading transfer and pile-soil stress ratio of CCP under various lengths and strength of the pile.