Method and Parameter Reliability of In-situ Direct Shear Test on Vibroflotation Gravel Piles

Abstract

Vibroflotation gravel piles can improve the antisliding stability of soft soil foundations. However, in-situ direct shear tests have rarely been conducted on vibroflotation gravel piles, and the understanding of the test methods and the reliability of the derived parameters varies. Three groups of in-situ direct shear tests on vibroflotation gravel piles for a reservoir bank treatment project are conducted. The design diameter of the piles is 100 cm, and the pile filler is artificial sandstone gravel with a maximum particle size of 15 cm. The specimens are formed by applying a steel square frame to encase the top section of the piles. For the specimens of groups S1 and S2, only the section above the shear plane is encased with the frame. The shear surface of S1 contains a small amount of soft soil while that of S2 contains no soft soil. For group S3, two pile sections above and below the shear plane are encased with the frame, and a steel support is set upon the lower frame in the opposite direction of the shear force. The shear failure of the piles is a type of plastic shear contraction failure mode with more than 4.5% shear strain and no peak shear stress. The internal friction angles and cohesion values are 20.8° and 20 kPa for S1, 30.5° and 21 kPa for S2, and 35.0° and 30 kPa for S3. Test results show that the restraint of the steel frame enhances the bite force between the gravel particles in the shear process. Moreover, the rigid shear displacement constraint set upon the lower frame makes the test pile cut along the control shear surface, thereby increasing the shear strength. Therefore, the test parameters are higher than those under working conditions. To ensure consistency between the test conditions and the working conditions of the piles, this work proposes the following suggestions for improving test measures: 1) the pile section should not be encased below the shear plane and should be kept buried in soil; 2) some soil should be retained around the pile in the steel frame above the shear plane.