| Pile composite foundation can make good use of the bearing capacity of soil and pile,which is widely used in Chinese northwest loess area. However, the theory of pile compositefoundation is far from sufficient, hindering its long-term development.In this paper, relying on the National Natural Science Foundation of China“Optimization Mechanism of Multivariate Pile Composite Foundation and TopologyOptimization Methods for the Pile Layout”, an indoor model test of pile composite foundationin loess area was conducted from the analysis perspective of settlement, pile axis stress, sideresistance and pile-soil stress ratio, to explore the common working mechanism and variationsof each bearing stage. Besides, the settlement of single pile composite foundation wascalculated by using modified tangent modulus method, and the result was compared with theexperimental data. The main results of this paper are as follows:（1） Through conducting indoor model tests of single pile, single pile compositefoundation and group piles composite foundation, the law of settlement was revealed. Both insingle pile and single pile composite foundation, Q-s curves showed a trend of “elastic toelastoplastic to plastic”, accompanied by the appearance of plummeting point. Influenced bythe pile-group effect, the Q-s curve of group piles composite foundation showed aslow-varying trend without obvious breakdown point. Besides, the settlement of group pilesfoundation was larger than that in single pile and single pile composite foundation.Furthermore, compared with single pile composite foundation, the bearing capacity of grouppile foundation indicates a significant increase.（2）Pile axis stress increased with the growth of upper load. In the model test of singlepile, pile axis stress decreased gradually as pile became deep. Whereas both in single pile andgroup piles composite foundation, at the beginning of loading, the pile axis stress indicatedsuch a distribution that stress on both ends of pile were smaller than that in the middle of pile.When reaching a certain load, the location of the biggest pile axis stress transferred to the piletop, and the pile axis stress decreased gradually as pile became deep.（3）The side friction resistance in the static load test of single pile was always positive.However, in the composite foundation of single pile and group piles, neutral point M appeared.Accordingly, in the upper side of neutral point, side resistance was negative, whereas side resistance was positive under the neutral point. Besides, neutral point M moved up graduallyas upper load increased. When reaching a certain load, side resistance became positive in thewhole pile. Side resistance in the pile toe increased substantially with the growth of upperload. This is due to the fact that a closed circular plastic zone forms in the pile toe, whichbrings about the increase of compaction and normal stress. In this case, the side frictionresistance strengthens.（4）The pile stress was larger than the soil stress, and pile-soil stress ratio in the depthof12cm changed with upper load. More specially, the pile-soil stress ratio declined firstlythen increased with upper load. In the loess composite foundation, the soil’s role in sharingload was obvious, which leaded to the decline of pile-soil stress ratio; with the growth of load,pile came to bear more loads, which caused the increase of pile-soil stress ratio. Meanwhile,saddle-shaped foundation pressure resulted in the distribution that side pile and soil stresswere higher than that of middle pile and soil.（5）Modified tangent modulus method could better simulate the p-s curve in the elasticand elastoplastic stage. However, when the upper load was too larger, elastomer assumptionwas no longer fitted with this method, which leaded to a relatively big error between thecalculated outcome and experimental data. |
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