Closure to "Stress-strain degradation response of railway ballast stabilized with geosynthetics" by Buddhima Indraratna and Sanjay Nimbalkar
The authors propose to employ t-z functions for estimating the load-movement response of a pile subjected to a static loading test. Judging by the authors’ Fig. 1, the proposed t-z functions are springslider relations, that is, they model the shaft and toe resistances as bilinear with an elastic initial part followed by a plastic part representing the ultimate resistance (capacity). The authors state that the capacity is mobilized at a pile-head movement of 5% of the pile diameter, which they also apply as the definition for the pile-toe load-movement response. First, the discusser would expect that the movement should be the movement at the pile element considered—a pile does not have to be very long before a substantial portion of the movement at the pile head is attributable to pile axial compression. It is not logical to assume that the length of the initial elastic response decreases with depth, all other factors being equal.