Discussion. From Theory to Field Experience With the Non-Destructive Vibration Testing of Piles.

Abstract

The vibration test is more than an integrity test because it also gives a measurement of soil-pile interaction. Of particular importance is the measurement of pile head stiffness E', which is a dynamic measurement of the initial load settlement characteristic of the pile. It is useful to plot pile head stiffnesses of a population of piles of similar dimensions on a site where the geological profile is fairly uniform. Fig. 18 shows what was done on a site at Rossano de Calabro in southern Italy where 730 piles were tested. They varied in length from 14 m to 17 m and they had a ,diameter varying between 520 mm and 540 mm. These were cast in situ end bearing piles passing through soft alluvium and founded in a conglomerate.The water/cement ratio of 0.33-0.40 was rather lower than one would use in the UK and this, without doubt, accounted for some of the troubles that were experienced in installing these piles. The range of pile head stiffnesses was quite considerable. The average value was 0.60 MN/mm and the standard deviation was 0.23.72. It seems that for the first time one has a means of obtaining information enabling one to select piles intelligently and rationally for load testing. The problem until now has been that when a pile has been selected for load testing there has been no way of knowing whether or not that particular pile is an average pile, a good pile or a defective one. The load test results therefore could give no indication of the relative load carrying characteristics of the other piles. On the site in Italy it would have been possible to select one or two piles having a stiffness of 0.25 MN/mm knowing that 97% of the population of piles had a stiffness greater than this. Dr DavisSince the Paper was written, two modifications have been made to the equipment. First, it is no longer necessary to ensure that there are no extraneous vibrations on site, because an extra item in the recording circuit smooths out any spurious signals. Second, it is now possible to test raker piles using a special harness to position the vibrator in the line of the pile axis. Inclined piles up to a rate of one in four have similar values for pile stiffness as vertical piles on the same site.74. One of the major restrictions of the method is the unlikelihood of receiving a reflected signal from the bottom of the pile when the pile length to diameter ratio is too great. A likely limiting upper ratio of 30 to 1 was suggested in the Paper. Subsequent testing in the Thames Estuary has shown that good results have been obtained for ratios up to 50 to 1, where a very soft lateral soil has resulted in little damping, and the pile toe was founded in a stiff stratum. This is not the case for piles founded in stiffer lateral soils such as London Clay.