STUDY ON CHARACTERISTICS OF PILE END BEARING CAPACITY ON THIN BEARING LAYER (PART 2): INVESTIGATION ON BEARING CAPACITY MECHANISM BY MODEL PILE TEST

Abstract

 The pile end bearing capacity is reduced when the pile is constructed on the thin bearing layer overlying clay layer. This paper focuses on the bearing capacity mechanism based on the results of the model pile test. The soil tank of the test is semi-cylindrical. The side of the tank is constituted by a semi-cylindrical steel plate and a flat acrylic board. The ground displacement can be observed through the acrylic board. In addition of the pile load and settlement, the ground displacement as photograph and the horizontal earth pressure in the thin bearing layer were recorded. These records were shown in the following figures. (1) distribution of the ground vertical displacement (2) distribution of the ground maximum shear strain (3) pile load and the settlement relationship and the horizontal earth pressure and the settlement relationship.

 The figures (1) and (2) lead to the recognition that there are two types of the bearing capacity mechanism. The one occurs if H_s/d is 3 or more (H_s is the thickness of the bearing layer. d is the pile diameter.), and is that the shear strain zone stays around the area below the pile end and does not develop. The other occurs if if H_s/d is 2 or less, and is that the shear strain develops downward vertically from the pile end outside to the clay layer. The latter is punching failure that the cylindrical shear strain zoon makes a columnar ground which moves downward with the pile. The bearing capacity of the punching failure is the sum of the shear resistance force of the columnar ground side surface and the base resistance force from the clay layer.

 The figure(3) shows the phenomena that the pile load reduction and horizontal earth pressure reduction occur at almost the same settlement. It can be explained by the punching failure model because of the strong relation between the shear resistance force of the columnar ground side surface and the horizontal earth pressure. The explanation enhances the existence of the punching failure.