Effect Of Foundation Rigidity And Cushion Thickness On Rigid Pile Composite Foundation Bearing Capacity

By employing the large-scale model tests in combination with field tests, the characteristic of pile-soil bearing capacity of composite ground and the effect of the cushion thickness on the bearing capacity of long-short pile composite foundation is analyzed and studied. Changing thickness-diameter ratio design method for multi-pile composite foundation is proposed.The model tests in this study can be divided into two types. In the model test I, composite foundations with four different rigidities are selected. The types of foundations are: the one consisted of stones; the one consisted of stones with geogrid laid inside; the one consisted of stones with double layers of steel wires put inside and those with enough rigidity respectively. Then static loading tests are conducted for the composite foundation. In the model test II, static loading tests are carried out for multi-type-pile composite foundation with combination of different pile length, pile diameter, and cushion thickness. On the basis of tests in this study, some primary conclusions can be drawn:The rigidity of foundation has a great effect on the bearing capacity of composite foundation. Under the same loading level, pile-soil stress ratio gradually increases with the foundation rigidity. The mobilization coefficient of the pile gradually increases and that of the soil gradually decreases. The settlement of the foundation also gradually decreases. Test investigations also indicate that with the increase of loading level, flexible foundation rigid pile composite foundation pile-soil stress ratio n is about 2, most no more than 5, similar to the gravel pile of pile-soil stress ratio. Pile load sharing is relatively small, the characteristics of high rigidity pile has not been fully reflected. Therefore, the flexible use of rigid composite foundation pile is not the economy, the use of a rigid pile, foundation stiffness should be increased.The cushion thickness is of key importance for the mobilization of pile-soil capacity of composite foundation. In the stress conditions of this study, i.e. the ratio of cushion thickness to pile diameter is between 0.45 and 0.55, the shear strength of soil can be fully mobilized, while the mobilization factor of dominating and assistant piles are both smaller than one. Reducing the cushion thickness of dominating and assistant piles furthermore will probably increase the mobilization of bearing capacity of them. But if the cushion thickness is too small, on the one hand, stress concentration will occur on the top of the pile, which will reduce the resistance of foundation base to pushing shear; on the other hand, when the foundation is subject to horizontal loads, such thin cushion thickness will increase the horizontal stress concentration on the pile top, which doesn't accord with the rule that piles transmit the vertical loads in the composite foundation. Therefore, the object of adjusting cushion thickness is not to make the mobilization factor of dominating and assistant piles to reach one. In the other word, it isn't suitable to excessively pursue the full mobilization factor of pile bearing capacity in order to make sure that the pushing shear of pile to foundation and excessive horizontal stress concentration at pile top do not take place.For the foundation with enough rigidity, the values of bearing capacity mobilization factor of dominating pile, assistant pile, and pile-soil for rigid multi-pile composite foundation are relevant to the laying method of cushion of dominating and assistant piles. For different laying methods of cushion, the range ofγ1,γ2 andγ3 values are different. For constant thickness-diameter ratio, there existsβ>γ2>γ1. andγ1 is between 0.55～0.7 and it can be increased furthermore. According to the design method of varying thickness-diameter ratio, after making sure that the bearing capacity can be fully mobilized (By maintaining the cushion thickness of pile soil a constant and thickness-diameter is 0.45～0.55), the cushion thickness at the dominating and assistant pile top can be adjusted to increase the mobilization factor of bearing capacity of dominating and assistant pile, which in turn will increase the bearing capacity of composite foundation. The tests in this study show that, reducing the cushion thickness can increase the mobilization of bearing capacity of dominating pile. But mobilization factor of dominating and assistant piles are both smaller than 1 and mobilization factor of pile soil bearing capacityβ≥1.