An approach to evaluation of field CPTU dissipation data in overconsolidated fine-grained soils: Discussion

Abstract

Discussions 1397 A proper interpretation of pore-water pressure dissipation behavior in fine-grained soils is of great interest in evaluat ing the in situ permeability during geotechnical site investi gations and estimating the time rate of consolidation around driven piles. The Authors provide a categorization scheme for the types of “nontextbook” decays that are observed during piezocone dissipation tests in stiff clays and silts, thus complementing the prior review by Robertson et al. (1992). Because of the importance in evaluating the horizontal coefficient of consolidation c h and the horizontal coefficient of permeability k h , the Discussers offer the following remarks and advice. Type I curves are truly characteristic of the soil behavioral response for midface (u1) pore-pressure elements, whereas type II curves actually represent a procedural operation of the test. Thus, with their well-recognized expertise in cone testing, perhaps the Authors could recommend that the rod pressure be maintained during dissipation testing, or alternatively, that the load be released. Pore-water dissipation at the shoulder (u2) element can follow a type I, III, IV, or V response, thus the Authors have suggested a number of separate curve-shifting or replotting techniques to obtain a representative t50 value for analysis, where t 50 is the time to a 50% reduction in excess pore pressure. While perhaps convenient from a commercial testing viewpoint, the Discussers believe that the focus on t 50 , in fact, is misleading because it is only a single measure towards defining the value for the coefficient of consolidation. Using five different methods, Gillespie and Campanella (1981) showed that the piezocone-estimated value of c h varied by a factor of 5 as the curves were fitted to dissipation degrees ranging from 20‐80% complete. These studies did not validate that time factors taken at 50% provide the best evaluations of c h . It can be shown that the apparently different response types are actually representative of the same mechanisms, if one considers the measured excess pore-water pressures (Δu) to consist of two components: Δu measured = Δu octahedral + Δu shear The octahedral component can be represented by a plasticized zone that extends several diameters away from the location of the filter element measurement, whereas the shearinduced zone extends only a few millimetres away (Burns and Mayne 1995). The octahedral component is always positive and decays monotonically with time, whereas the shear com