Pile-soil Interaction Behaviour During The Surrounding Soil Unloading

Unloading effect of the deep excavation would cause the decreasingof pile’s bearing capacity and increasing of the tensile in pile shaft. Manypile engineering accidents caused by the deep excavation have beenreported in recent years. To evaluate and predict the performance of pilefoundations, both the pile-soil interactive behavior during deep excavationand the pile bearing mechanism influenced by the deep excavation shouldbe studied. In this paper, the interaction between pile and soil during thesurrounding soil unloading is analyzed by model tests, numerical analysis,and case study. The focus of this study is the characters of the pile-soilinterface under complex stress state.(1) A new test system, which can control the soil stress state, isdeveloped to test the pile-soil interface behavior during unloading. In thisnew system, the normal stress on the pile-soil interface is applied by the airpressure in the triaxial cell. The soil stress along the pile axis and themovement of model pile are controlled by the double piston. The porepressure and water flow in the soil sample is adjusted by the two watercannels on the test cell. With the monitoring part of the system, the totalshear force on the interface, the relative movement of the pile, thedeformation of the soil layer can be measured during tests. Therefore, thisnew test system can be used to test the interactive behavior of pile-soilinterface under complex stress state, which has been approved by themodel tests. The triaxial interface tests and numerical back analysis showthat the limit friction resistance and friction coefficient are not only impacted by the confining pressure but also influenced by the soil stressalong the pile axis.(2) The pile performance during surrounding excavation and the pilebearing capacity after excavation are studied by the indoor model tests.Then the pile-soil interface behavior under the low stress can be analyzedand compared with that under the high stress, which is obtained from thetriaxial interface tests. Model tests show that the pile bearing capacitydecreases while the pile settlement increases after surrounding excavation.Both the limit friction resistance and friction coefficient are reduced by thesurrounding excavation, but the influence depth is limited. Measurementsof the pile internal force during the excavation show that the tensile stressin the pile shaft increases with the unloading, and the depth of maximumtension is located at the2/3pile length.(3) Based on the model test results, the pile performance duringsurrounding excavation and the pile bearing capacity after excavation areanalyzed numerically with two cases in Shanghai. Numerical results forHongqiao hub project show that the pile bearing capacity is reduced by thedeep excavation and the decrement is affected by the parameters of thepile-soil interface. The tension in pile and the depth of maximum tensionincrease during deep excavation. Numerical results for Guxiang Buildingshow that the displacement of pile is impacted by the friction coefficient.The dewatering during the deep excavation could reduce the tensile forcein pile, movement of pile and soil heave.