Civil Insight: A Technical Magazine Volume 2 | Page 23

CIVIL INSIGHT 2018 TEACHERS’ SECTION 23 Incorporating Stress-Dilatancy Effects for Evaluating Skin Friction of Tapered Piles using Cylindrical Cavity Expansion Theory Dr. Eng. Suman Manandhar Assistant Professor, Geodisaster Research Center, Central Department of Geology, Tribhuvan University, Visiting Faculty, Department of Civil and Geomatics Engineering, Kathmandu University [email protected] Abstract The mechanism of tapered piles and results obtained from small scale model tests showed that a slight increase in tapering angle affected in the determination of skin friction. The increment in the tapering angle of the pile not only increases the skin friction with compared to straight conventional pile but also increases the lateral stress. In this paper, a closed form solution is proposed by applying cylindrical cavity theory inserting stress-dilatancy relationship to evaluate the skin friction of tapered pile. In general, either the angle of internal friction or dilatancy angle was assumed to be constant. In this research, both properties are considered to compute the skin friction through an iterative process in the load transfer method. The test results and proposed models show that a slight increase in the tapering angle of the pile results in higher skin friction compared with conventional straight piles on different types of sands at different relative densities. The proposed models are then checked with different types of prototype and real type pile resultsand parametric studies were carried out to evaluate the predicted skin friction. Keywords: Skin friction; Cylindrical cavity expansion theory; Tapered pile; Stress-dilatancy relationship INTRODUCTION A tapered pile is characterized by the difference in axial diameters of the pile at its top and bottom. A slight increase in tapering angle of the pile results in higher skin friction as compared to the conventional straight piles. Consequently, the skin friction and the radial stress increase due to the difference in axial diameters at its top and bottom. The mechanism of tapered piles also has good pressing effects towards the depth of penetration (Manandhar et al., 2009). Nevertheless, the accurate model to derive the axial capacity of the tapered pile in context of deep foundation is not well identifi ed. Researchers and practitioners generally apply empirical solution, fi nite element method and constitute modeling to interpret the behavior of skin friction of tapered piles. In this context, the most closed form solution for large straining condition and complete solution for cylindrical cavity expansion in ideal elastic-plastic model in non-associated fl ow rule has been proposed by Yu and Houslby (1991). In this paper, effects of stress-dilatancy have been adopted in cavity expansion theory. In general, one of the soil parameters (angle of internal friction or dilatancy angle) is assumed to be constant for the ease in computation. Although, the stress-dilatancy relationship is interdependent on the confi ning pressure, relative density and angle of internal friction. Therefore, the new concept is proposed to unify the idea proposed by Kodikara and Moore (1993), and cavity expansion theory to estimate the skin friction of