Application of the wave equation analysis to friction piles in sand: Discussion

Abstract

In their paper the authors present an interesting study on the application of the wave equation analysis to instrumented and production piles. The results are especially interesting to the discusser because, charged by the German Pile Committee, he is investigating the applicability of loading tests by impact instead of static loading. In addition to the wave equation analysis (in its most recent form developed by Goble et al. 1975), the discusser has also studied a similar method developed by the Di~tcll Institute TNO for Building Materials and Building Structures at Delft. For the prediction of the ultimate bearing capacity of piles these methods require the measurement of force and motion at the pile top. Simple test equipment has been developed for in situ tests. The discusser considers these measurements to be a prerequisite for a precise prediction of the bearing capacity. The authors present calculations of the ultimate bearing capacity of precast piles driven in a homogeneous sand deposit at the Saint-Charles River site (see Table 2 of their paper). In these calculations the wave equation was applied, based on blow counts only. They show that there is only a poor correlation between the computed and measured ultimate bearing capacity Q,, and Qfl,, (regression coefficient r = 0.3). This is due to a nonlinear relation between Qf,,. and Q,,,,. A much better correlation can be achieved by a regression of Qflll on 7 = Qfn/Qc1,, (correlation coefficient Y = 0.78, Fig. I ) . It can be seen in Fig. 1 that the prediction of the ultimate bearing capacity tends to be on the safe side with increasing pile bearing capacity. In addition to the regression lines the confidence interval on the 95% level for the prediction of Qc,,, from 7 is shown in Fig. 1. Of course this relationship is restricted to the piles driven into the sandy layers of the Saint-Charles River,