| Due to the complex geological conditions, there are some occasions that bearingstratum in each single pile of pile group foundation varied in conditions or the soillayer interface inclines greatly, which forms differential pile group. Since the rockmass and soil-body differ in pile shift and tip of each single pile, there is a huge gap inmechanics mechanism between differential pile groups and ordinary pile groups. Thus,the current code for the design calculation method and the displacement computingmethod of group pile is not suitable for those differential group piles which havedistinctive geological conditions. Therefore, in view of the engineering characteristicsof differential group piles in bridge pile foundation, the study of bearing capacitycharacteristic and settlement calculation method of differential group piles has becomea hotspot of pile foundation research under complex geological conditions. This paperanalyzed the working mechanism; put forward a complete set of design and computingideas, which is of great theoretical and practical significance to guide the design andconstruction of differential pile groups under complex geological conditions.First, pile foundation was reviewed in this paper. Then through the existingliterature both at home and abroad, a comprehensive analysis and research wasconducted on the bearing capacity characteristics and force mechanism on end bearinggroup piles and friction group piles. Then based on the characteristics of theengineering geological condition, and puts forward the concept of differential pilegroup which is considered a mixture of the end bearing group piles and friction grouppiles. Also, the mechanical characteristics and load transfer mechanism of differentialgroup piles have been delved, considering that the settlement of differential grouppiles can be calculated as the superposition of friction piles and rock-socketed piles.The load transfer model of the skin resistance and base resistance was determined: theSaturo Sato’s ideal elastic-plastic model was employed on the pile shaft soil; abilinear model was used on pile tip. What’s more, those commonly used methods forcalculating the pile displacement was researched, evaluating their respectiveadvantages and disadvantages and applicability scope, etc. And the equivalent piermethod and the total sum method of equivalent effect fail to consider the influence ofthe compression deformation of soil between piles. Thus, they are not suitable to grouppiles whose pile spacing were more than six times that of pile diameter, and they are only suitable for friction group piles. The value of two important parameters of the soil:compression modulus and Poisson ratio was discussed. And then based on theproposed settlement calculation ideas of differential pile group, the calculation of thepile-soil system and the pile-rock system was normalized—the skin resistance modelof both was employed as the ideal plastic-elastic model through the comparison of both.The bilinear resistance model of pile-soil and pile-rock can be used provided notconsidering the influence of sediment. The equivalent load coefficient, exerted onthe pile top of friction piles, was put forward to differentiate between the friction pileand the end bearing pile. This coefficient is used to consider the additional settlementdue to the pile group effect According to the load transfer method, the foundation pileload transfer analytical solution was obtained considering the characteristics of the soillayer. And then based on the varied transfer function, the single pile settlementcalculation formula was obtained under different loads on the pile top. This methodcan make full consideration of lateral soil resistance and resistance of the pile tip soil,both suitable for friction piles and end-bearing piles. Finally finite element analysis,Adina, was used to verify the settlement of differential pile group. A case studyanalyzed the applicability various methods and verifies the accuracy of the designmethod. |
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