TECHNICAL NOTE Determination of Coefficient of Radial Consolidation Using Steepest Tangent Fitting Method J. S. Vinod • A. Sridharan • Buddhima Indraratna Received: 14 January 2010 / Accepted: 15 April 2010 / Published online: 27 April 2010 Ó Springer Science+Business Media B.V. 2010 Abstract Barron (Trans ASCE 113:718–724, 1948) theoretical relationship between degree of consolida- tion (U r ) and time factor (T r ) is generally used to determine the coefficient of radial consolidation c r . Several researchers have proposed different labora- tory techniques for obtaining the value of c r . How- ever, the usual approach is to compare some salient features of the theoretical U r –T r relationship with the compression, d and time, t based on the laboratory data. In this paper, rapid consolidation test procedure originally proposed by Su (J Soil Mech Found Div Am Soc Civ Eng No. 95: Proc. Pap. 1729 (1–9), 1958) to determine the coefficient of vertical consolidation (c v ) is extended to evaluate Barron’s coefficient of radial consolidation c r for the case of equal strain. The characteristic feature of the proposed method is the identification of the steepest tangent from the d–log t plot. The slope of steepest tangent is independent of the drain spacing ratio (n) with a value of 0.847. This approach is rapid, and it predicts more reliable values of c r as it is less influenced by the secondary compression. In addition, this method is more versa- tile and can be applied for consolidation trends which do not always exhibit the typical S-shape. Keywords Consolidation  Radial drainage  Clays  Coefficient of radial consolidation 1 Introduction It is well known that a reasonable estimate of coefficient of radial consolidation (c r ) is required for the economic design of sand drains and prefabricated vertical drains. Coefficient of radial consolidation is generally determined from the classical work of Barron (1948) on the radial flow toward a drain well and the resulting soil consolidation. In the recent past, several methods have been proposed for the reliable estimate of coefficient of vertical consolidation (c v ) from laboratory tests (Olson 1986). However, very few methods have been found in literature for the determination of c r from the laboratory d–t curve (e.g., McKinlay 1961; Berry and Wilkinson 1969; Sridharan et al. 1996; Robinson 1997, 2009). Mckin- lay (1961) and Berry and Wilkinson (1969) developed a procedure for determining c r for the condition of free strain and this is not discussed here due to brevity. Sridharan et al. (1996) extended Taylors Ht method for the determination of c r for the equal strain J. S. Vinod (&)  B. Indraratna School of Civil, Mining & Environmental Engineering, University of Wollongong, Wollongong, NSW 2522, Australia e-mail: vinod@uow.edu.au B. Indraratna e-mail: indra@uow.edu.au A. Sridharan Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India e-mail: asridhran@yahoo.com 123 Geotech Geol Eng (2010) 28:533–536 DOI 10.1007/s10706-010-9330-8