INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int. J. Numer. Anal. Meth. Geomech. 2009; 33:1203–1225 Published online 25 November 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/nag.763 Bearing capacity factor N c under  = 0 condition for piles in clays Vishwas N. Khatri ‡ and Jyant Kumar ∗, †, § Civil Engineering Department, Indian Institute of Science, Bangalore 560012, India SUMMARY Bearing capacity factor N c for axially loaded piles in clays whose cohesion increases linearly with depth has been estimated numerically under undrained ( = 0) condition. The study follows the lower bound limit analysis in conjunction with finite elements and linear programming. A new formulation is proposed for solving an axisymmetric geotechnical stability problem. The variation of N c with embedment ratio is obtained for several rates of the increase of soil cohesion with depth; a special case is also examined when the pile base was placed on the stiff clay stratum overlaid by a soft clay layer. It was noticed that the magnitude of N c reaches almost a constant value for embedment ratio greater than unity. The roughness of the pile base and shaft affects marginally the magnitudes of N c . The results obtained from the present study are found to compare quite well with the different numerical solutions reported in the literature. Copyright 2008 John Wiley & Sons, Ltd. Received 10 June 2008; Revised 30 July 2008; Accepted 18 October 2008 KEY WORDS: axisymmetry; bearing capacity; limit analysis; optimization; pile foundation; plasticity 1. INTRODUCTION Experimental studies [1] on saturated normally consolidated and lightly overconsolidated clays indicate that the cohesion of soil mass under undrained condition increases almost linearly with depth. A few theoretical studies have been proposed in the literature to incorporate the variation of cohesion with depth in order to compute the ultimate bearing capacity of strip and circular footings. The solutions for strip footings have been obtained with the use of (i) the limit equilibrium method [2, 3], (ii) an upper bound limit analysis with the assumption of a collapse mechanism [4], and (iii) the method of characteristics [5–7]. On the other hand, for circular footings, the solutions have been determined primarily by using the method of stress characteristics [6–11] in which case as per Harr–Von Karman hypothesis it is always assumed that the magnitude of the hoop stress (  ) ∗ Correspondence to: Jyant Kumar, Civil Engineering Department, Indian Institute of Science, Bangalore 560012, India. † E-mail: jkumar@civil.iisc.ernet.in ‡ Research Scholar. § Associate Professor. Copyright 2008 John Wiley & Sons, Ltd.