Research On Efficiency Factor Of Rigid-flexible Long-short-pile Composite Foundation's Bearing Capacity

A new composite foundation form, kNown as rigid-flexible long-short-pile composite foundation,has emerged in recent years. This foundation type has been widely used in actual engineering, such as the foundation engineering of multistory and high-rise buildings and the treatment engineering of high fill sub-grade. It can be founded in Technical Code for Building Foundation Treatment that in the calculation formula of the vertical bearing capacity eigenvalue of rigid-flexible long-short-pile composite foundation,the efficiency factor of bearing capacity of rigid single pile, flexible single pile and soil between piles needs to be calculated. Only an approximate value range is specified in the Code, while there is no much explanation to the bearing capacity of piles in different locations in actual engineering when other parameters change. As a result, a great deal of practical engineering experience needs to be used for reference in practical engineering design. Because of this reason, the application of rigid-flexible long-short-pile composite foundation in practical engineering has been limited.On the basis of laboratory testing, this paper set up a corresponding computation model to analyze the impact of a change in such parameters as thickNess-radius ratio, flexible pile length, soil modulus in bearing stratum, soil modulus in reinforcing area and pile spacing on the axial force of the rigid and flexible piles, the pile-soil stress ratio and its exerting degree in different locations of a rigid-flexible long-short-pile composite foundation.(1) When an equal-thickNess cushion is laid, the bearing capacity of a rigid pile is exerted more fully than that of a flexible pile; when the thickNess-radius ratio equals to 0.6, the bearing capacity of soil between piles is exerted the most fully. In order that the bearing capacity of rigid piles, flexible piles and pile-between soil should be fully exerted, different thickNess-to-radius ratios ought to be given to rigid andflexible piles in a rigid-flexible long-short-pile composite foundation in actual engineering. In other words,when there is an increase in the thickNess-to-radius ratio of flexible piles within a reasonable range, a reduction in the thickNess-to-radius ratio of rigid piles can help make the thickNess-to-radius ratio of flexible piles less than that of flexible piles.(2) When a rigid-flexible long-short-pile composite foundation is designed, attention should be paid to the ratio?between the length of flexible piles and that of rigid piles. When?-(28)75.05.0, the bearing capacity of rigid piles, flexible piles and adjacent soil is exerted well.(3) A change of soil modulus in bearing stratum has great impacts on the exerting degree of rigid long piles, but little impact on the mobilization factor of flexible piles, so the tip of rigid piles should be put on a soil mass with greater bearing capacity as far as possible in actual engineering.(4) Rigid long piles, flexible short piles and soil between piles are all affected by the change of soil modulus in reinforcing area. As far as efficiency factor of bearing capacity is concerned, a combination of rigid-flexible long-short piles is more suitable to reinforce the soil masses with a weak upper part.(5) When the ratio between the length of rigid piles and that of flexible piles equals to 2, and the thickNess-to-radius ratio equals to 0.4, a change in pile spacing has great influence on rigid-flexible long-short-pile composite foundation. When pile spacing increases, the efficiency factor of bearing capacity of rigid piles basically remains at the same level, and that of flexible piles increases accordingly,while that of soil between piles, however, decreases, so the bearing capacity of piles and soil between piles can be exerted well if pile spacing is 3 times as much as pile diameter.