Field Comparative Tests for Studying the Vertical Loading Performance of Piles Due to Water Level Periodical Fluctuation in Foundation Soils

Abstract

The water level in the reservoir of the Three Gorge Dam rises from 145m to 175m since the dam is in full operation. The periodical fluctuation of the water level changes the water environment of the reservoir banks. The water saturation and drying cycles affects the strength and stiffness of original rock and soil mass. These changes have led to problems such as the instability of ancient landslides and the safety of buildings supported on pile foundations whose bearing stratums are affected by the changing water levels. Seven 8-story height buildings supported on pile foundations in Fengjie county in the Chongqing Municipality had been facing such threats. A series of field direct shear tests and pile loading tests were conducted to study the influence of capacity of foundation soils due to water level fluctuations. The results indicated that after water soaking, the cohesion strength of the foundation soil decreases significantly, while the internal friction angle is relatively stable. The weaken ratio of the internal friction angle which is the ratio of strength parameters before and after water soaking into the gravel soil is mainly between 0.78-1.0 and the weaken ratio of cohesion strength is around 0.4 to 0.8. The capacity of pile decreased when the water submerged the pile tip. Field calibration and indoor calibration have proved the availability of the O-cell boxes designed and produced by the authors in pile static loading tests. The observations provided important support for evaluating the safety of seven 8-story height buildings.