A Study on the Evaluation of Pile Bearing Capacity Factor and Adhesion Factor in IS 2911

Bearing capacity factor (Nq) and adhesion factor (α) are the key parameters for analysing the load carrying capacity of pile foundations embedded in cohesionless and cohesive soils, respectively. Numerous models are available in the form of charts or equations for the computation of these parameters and, hence, have been adopted by most of the international and national standards. Indian Standard for the Design and Construction of Concrete Pile Foundations, IS 2911 Part 1 (2010), also makes use of charts detailed in its Annexure B. However, the sources of these charts have not been specified clearly, making it difficult to back-refer to the actual models to assess their basis for resolving any critical scenarios encountered during design. The present study aims at acquiring a distinct understanding of the development of these charts in order to bring more clarity to the design process of pile foundations. The given charts have been compared with various models specified in the existing literature and international standards for the calculation of Nq and α for both driven and bored piles and, hence, the basis of Indian Standard charts has been identified. Moreover, it is evident that the use of these charts complicates the process of automation of analysis and design of pile foundations as well as the associated optimisation studies. Additionally, the manual entry of data especially, from the logarithmic graph of Nq, escalates the chances of error thus, making it a critical concern for design offices. Therefore, this paper also presents a non-linear regression model for Nq and α, developed using NCSS software for both driven and bored piles. Through multiple iterations, the value of coefficient of determination for Nq and α has been found to reach greater than 0.995. The developed equations can be simplistically used for both manual and automated analysis of pile foundations.