【作者】 孙训海；

【导师】 闫明礼；

【作者基本信息】 中国建筑科学研究院 ， 岩土工程， 2010， 博士

【摘要】 本文运用室内大型模型试验并结合现场试验研究了基础刚度、褥垫厚度对刚性桩复合地基桩、土承载性状的影响,并提出了多桩型复合地基变厚径比设计方法。模型试验分为两个部分：模型试验Ⅰ是四种不同基础刚度下单一桩型四桩复合地基静载试验,基础形式分别为碎石基础、碎石夹土工格栅基础、碎石夹双层钢丝网、足够刚度基础。模型试验Ⅱ是对几组不同桩长、不同桩径、不同褥垫层厚度组合的多桩型复合地基的静载试验。在本文试验基础上得到以下主要结论：基础刚度对刚性桩复合地基承载性状影响较大,同一荷载水平下随着基础刚度的增加桩土应力比逐渐增加,桩的发挥系数也逐渐增加,土的发挥系数逐渐减小,基础的沉降也逐渐减小。试验还表明,随着荷载水平的增加,柔性基础下刚性桩复合地基桩土应力比n在2左右,最大不超过5,类似于碎石桩的桩土应力比。桩荷载分担比较小,刚性桩承载力高的特点没有得到充分体现。因此,柔性基础下复合地基采用刚性桩是不经济的,若采用刚性桩,应加大基础刚度。褥垫厚度对足够刚度基础下多桩型复合地基桩土承载力的发挥起着至关重要的作用。本文试验条件下,即褥垫厚度与桩径之比为0.45～0.55时,桩间土都能充分发挥,而主辅桩承载力发挥系数均小于1,虽然进一步减小主辅桩褥垫厚度,有可能提高主桩和辅桩的承载力的发挥,但是由于褥垫层过薄,一方面会出现桩顶应力集中,不利于基础底板的抗冲切,另一方面,当基础承受水平荷载时,褥垫过薄会增加桩顶水平应力集中,不符合桩在复合地基中主要传递竖向荷载的基本原则。因此调整褥垫厚度的目的不是使主、辅桩都百分之百发挥,或者说,为保证不发生桩对基础的冲切、不产生桩顶过大的水平应力集中,过分追求桩承载力发挥系数达到1是不适宜的。足够刚度基础下,刚性桩多桩型复合地基主桩承载力发挥系数γ1、辅桩承载力发挥系数γ2、桩间土承载力发挥系数β的取值与主、辅桩的褥垫铺设方法有关,不同的褥垫铺设方法,γ1、γ2、β的取值范围不同。等厚径比设计时(厚径比0.45～0.55),三者关系为β>γ2>Y1,且γ1在0.55-0.7之间,γ1有进一步提高的空间。变厚径比设计是保证桩间土承载力充分发挥(桩间土的褥垫厚度一定,厚径比0.45-0.55)的前提下,调整主、辅桩桩顶褥垫厚度,达到提高主、辅桩的承载力发挥系数,从而提高复合地基承载力的目的。本文试验表明,减少主桩桩顶褥垫厚度,可提高主桩承载力发挥,但主、辅桩承载力发挥系数仍都小于1,桩间土承载力发挥系数β≥1。更多还原

【Abstract】 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.更多还原