Negative skin friction for cast-in-place piles in thick collapsible loess

Abstract

Large-scale field tests were performed on cast-in-place piles installed in self-weight collapsible loess with a thickness of over 35 m. The pile diameter was 0.8 m and the pile length was 40 m. The piles were constructed in a water immersion pit of 30 m diameter. Two types of piles were tested; a frictional end bearing pile and a suspension pile. Two kinds of measuring methods were adopted; a steel bar stress gauge and a slippage measurement micro-gauge. The results indicate that when the suspension pile is much shorter than the thickness of the collapsible loess layer, the measured negative frictional resistance values are small. However, the results obtained for the frictional end bearing pile penetrating through the whole collapsible loess layer are more appropriate for the real situation. The real measured negative resistance is far higher than the negative resistance values suggested by the loess codes. The values of negative frictional resistance have no apparent corresponding relationship with the form or magnitude of loess collapse at the site. Analysis of the test data from in-situ pile tests from several large-scale engineering projects shows that the neutral point positions exceed the reference ranges provided by the technical specifications. As a result, this research project can provide an important reference for the design of future similar piles and modification of the loess codes.