Geomechanics and Engineering, Vol. 5, No. 5 (2013) 447-462 DOI: http://dx.doi.org/10.12989/gae.2013.5.5.447 447 Copyright © 2013 Techno-Press, Ltd. http://www.techno-press.org/?journal=gae&subpage=7 ISSN: 2005-307X (Print), 2092-6219 (Online) Improvement of tip analysis model for drilled shafts in cohesionless soils Yit-Jin Chen 1 , Hao-Wei Wu 1a , Maria Cecilia M. Marcos 1,2b and Shiu-Shin Lin 1c 1 Department of Civil Engineering, Chung Yuan Christian University, Chung-Li , 32023, Taiwan 2 Department of Civil Engineering, Adamson University, Manila, Philippines (Received September 24, 2012, Revised April 10, 2013, Accepted May 18, 2013) Abstract. An analysis model for predicting the tip bearing capacity of drilled shafts in cohesionless soils is improved in this study. The evaluation is based on large amounts of drilled shaft load test data. Assessment on the analysis model reveals a greater variation in two coefficients, namely, the overburden bearing capacity factor (N q ) and the bearing capacity modifier for soil rigidity (ζ qr ). These factors are modified from the back analysis of drilled shaft load test results. Different effective shaft depths and interpreted capacities at various loading stages (i.e., low, middle, and high) are adopted for the back calculation. Results show that the modified bearing capacity coefficients maintain their basic relationship with soil effective friction angle (ϕ ¯ ), in which the N q increases and ζ qr decreases as ϕ ¯ increases. The suggested effective shaft depth is limited to 15B (B = shaft diameter) for the evaluation of effective overburden pressure. Specific design recommendations for the tip bearing capacity analysis of drilled shafts in cohesionless soils are given for engineering practice. Keywords: drilled shafts; cohesionless soils; tip bearing capacity; friction angle; effective shaft depth 1. Introduction Tip resistance is an essential source of drilled shaft capacity under axial compression loading. The contribution of tip resistance is relatively significant for shorter shafts or when the shaft penetrates through layers of soft soils where side resistance is limited. Various soil-bearing capacity equations from the literature can be applied to estimate the tip capacity. Researchers (Hansen 1970, Vesic 1975, Kulhawy et al. 1983) have continually improved the general solution of the ultimate soil bearing capacity (q ult ). Often, to evaluate the ability of an analysis model to predict shaft capacity, field load tests are used (Zhang et al. 2006, Schneider et al. 2008, Cai et al. 2009, 2012, Ching and Chen 2010) because they provide a convenient way of comparison between predicted and measured capacities. Recently, Chen et al. (2009), based on a wide range of compression field load test data, performed an extensive evaluation of tip bearing capacity of drilled shafts from a representative analysis model. The examined model (Kulhawy Corresponding author, Professor, E-mail: yjc@cycu.edu.tw a Master Science b Ph.D. Student, Instructor, E-mail: g9802203@cycu.edu.tw c Associate Professor, E-mail: linxx@cycu.edu.tw