Research On Behavior Of Pile Shaft Compression And Settlement Deformation Of Pile Group Foundation Under Vertical Load

In recent years, the pile foundation field has a rapid development due to the scale enlargement of engineering construction in China. More and more types of long pile foundations, meanwhile, are adopted in many construction projects. Static testing results of long piles in different strata figure out the essential behaviors of long piles under vertical loading, e.g. load transfer mechanism and compression behavior. Generally, the loading behaviors of long piles in hard soil ground are similar to super-long piles in soft soil and the total compression settlement of pile body is so great that it should not to be ignored for the compression calculation of long piles in practice. At the present stage, in-situ static testing is still one popular and authentic method to acquire compression behaviors of long piles. Consequently, in order to investigate the elastic compression behaviors of pile group foundation, the field tests are designed and carried out. Based on a series of large scale model tests of single pile and pile group, the settlement behaviors of pile group foundation are observed and the load transfer mechanisms are analyzed by the corresponding test data. Furthermore, the pile group effects of shaft resistance, point resistance, soil resistance beneath cap, and load-bearing capacity under vertical load are revealed. The principal conclusions may be summarized as follows.1. The load transfer mechanism of single pile and group piles under vertical loads are analyzed by large scale model tests. The distribution modes of axial force and lateral resistance along pile body differ with the location within the pile group. Analysis results show that the point resistance of pile group has an obvious hardening effect of settlement, which means that the point resistance increases distinctly with pile group settlement. On the other hand, the increase magnitudes of point resistance are different in the cases of small and large intervals between two adjacent piles. Popularly, the point resistance has a greater magnitude in a smaller interval condition than that one in a larger condition.2. The engineering static load testing results show that pile elastic compression settlement appear at the initial stage of loading process and then keep existing and developing in the whole process. Generally, the settlement behaviors of long piles in hard soil ground are similar to the super-long piles in soft soil and the total compression settlement of pile body is so great that it should not to be ignored for the compression calculation of long piles in practice.3. By the analysis of soil settlement behavior within group piles, the largest settlement appears at the top position of pile closed to the cap, and the settlement decreases with the burial depth. Meanwhile, the settlement covers two kinds of deformations, i.e. shear and compression.4. Under the same loading conditions, it is disclosed that the soil settlement below pile tip decreases with the burial depth by the analysis results of soil settlement at different depths within pile groups. Furthermore, one conclusion may be obtained that the compression settlement of soil mass mostly occurs in a certain range of soil closed to the pile tip. Actually, the compression settlement of per unit thickness of soil mass closed to the pile tip is greater than that one with a larger distance away the tip; the settlement value of soil mass increases with the load magnitude at the same burial depth.5. For a certain average loading magnitude on each pile within pile groups, the total compression settlement at the tip is greater in a smaller interval condition between two adjacent piles than that one with a larger interval. On the other hand, the settlement is smaller in the case of less numbers of piles than the case of more piles. This observation result may be connected with the pile group effect, which means that at a certain interval between two adjacent piles within pile groups, the settlement-increase effect of pile group becomes sharper with the increase of pile number.6. The thickness of compressed layer and total settlement behavior of soil ground below the tip of pile group both differ with the intervals between two adjacent piles within pile group. The field test results show that at the loading condition of P=Pu/2, the total settlement and thickness of compression soil layer both are smaller in a case of lager intervals between two adjacent piles within pile group than that one with an opposite case. These results agree with the previous field tests in silt and soft soil.7. Based on the analysis results of the pile elastic compression settlement and the lateral resistance distribution pattern, three linear equation of compression coefficient, which considers the difference of lateral resistance distribution pattern, are proposed as follows.ξe正三角=0.33α+0.67ξe矩形=0.50α+0.50ξe倒三角=0.67α+0.338. By the analysis of settlement mechanism of pile group foundation, one equation to calculate the foundation settlement of pile groups in the less interval cases is obtained. In the equation, the critical factors influencing pile foundation settlement, such as compression behavior of pile body, pile number, geometrical characteristics of pile group, lateral resistance distribution pattern, and resistance ratio of tip to lateral, etc., are considered completely and the detailed format, which is more reasonable than current methods, is showed as the follow.9. Based on the assessment method of compression soil thickness in current code and the observation and analysis results in this dissertation, the author take more efforts to optimize assessment scheme for compression soil thickness in the cases of pile group and one more reasonable calculation formula is acquired finally.10. According to the research achievement of vertical stress coefficient solved by Mindlin method considering the influence of pile diameter, numerical solutions for the mean vertical stress coefficient of pile group with small pile interval are proposed and listed in one table. These results in the table are convenient for engineer to calculate the compression settlement of pile group foundation by hand.