The Influence Of Large And Deep Excavation On The Bearing Behavior Of Uplift Piles

With the rapid development of urban construction, a large number of underground structures are built in coastal areas, the large and deep excavation are more and more. On the other hand, the underground water level is high in coastal areas, the underground structures are under great buoyancy, and the uplift piles are used widely in the project. Limited by the construction procedure, piles are completed before excavation, because of the large area of unloading, it is very complicated to determine the behavior of uplift piles. Under the physical and mechanical properties of soil layers and the shearing properties of pile-soil interface, to calculate the behavior of uplift piles before and after excavation is an important issue in geotechnical engineering.Based on the above background, the pile-soil interaction during large and deep excavation is studied. At first, In order to realize the ultimate uplift capacity of model piles under excavation in sand, model tests are conducted on different excavation diameter, depth, and different pile length under the bottom of pit, several result are obtained. The ultimate uplift capacity of piles become smaller after excavation. Under the same excavation depth, the larger the diameter of excavation is, the lower the ultimate uplift capacity of piles will be. When the excavation diameter is all of the container, the sand is overconsolidated after excavation, the ultimate uplift capacity of piles are bigger than the piles in the normally consolidated sand.The sand (different relative density), saturated silt and silty clay K0 loading-unloading tests are conducted, using the JCY consolidation apparatus, then theσv'～σh'relationships during the tests are described by K0 loading-unloading model. The influence of the physical and mechanical properties of soil on the parameters of K0 loading-unloading model are discussed.The big soil consolidation apparatus is made, the uplift pile can be placed in it. When the soil around the pile are sand and saturated silt, the ultimate bearing capacity of uplift piles are tested when the soil around the piles are under K0 loading-unloading condition. Then the ultimate bearing capacity of uplift piles are tested when the OCR of the soil are different. The result indicates that the OCR has a great influence on the ultimate lateral friction of uplift pile. The bigger the OCR is, the bigger the ultimate lateral friction of uplift pile is. The influence of stress path of soil on the ultimate lateral friction of uplift pile can be calculated by K0 loading-unloading model.Based on the element experiment and model test, combined with the shearing properties of the pile-soil interface, the one dimensional model and FEM model are conducted to study the influence of the large and deep excavation on the bearing behavior of uplift single pile. An example is calculated, the calculated result of one dimensional model is similar to the FEM model. The normal stress of the pile-soil interface becomes smaller after large and deep excavation, this cause the ultimate bearing capacity and the bearing stiffness of uplift pile smaller. The soil rebounds after excavation, then drive the pile rebounds, the initial lateral friction will be produced after excavation, it will influence the load transfer process of uplift pile.The behavior of group piles after large and deep excavation is studied using ABAQUS, the rebounding displacement and pull force of piles influenced by the shielding effects are studied. Then the shielding effects of different number and pile spacing of group piles are discussed. The results indicate that pull force of center pile is smallest after excavation. The smaller the pile spacing and the larger the number of piles are, the greater the shielding effects are.The behavior of uplift group piles after excavation are calculated, the result indicates that when the uplift group piles reach the ultimate bearing capacity, the displacement are larger than the single pile. With the same pile spacing, the group efficiency is smaller when the number of piles increases. With the same number of piles, the group efficiency is larger when the pile spacing increases.At last, some research results are used in the design and calculation of Shanghai 500kV underground substation.