Use of fall cone test as measurement of shear strength for soft clay materials

A fall cone test (FCT) was carried out to obtain the undrained shear strength (su), and to measure the sensitivity (St) of intact and remolded samples from several sites in Japan. The value of su from FCT was compared with half of the unconfined compression strength (qu/2), which is a standard way to evaluate su for practical design in Japan. It is found that su from FCT is neither related with qu/2 nor used directly as a design value. St was also obtained by field and laboratory vane tests. A comparison study revealed that St by FCT is as much as 10 times larger than that by the field vane. The large difference in St measured by different testing methods can be attributed to remolding methods. That is, the remold condition for FCT was created by kneading the sample in a plastic bag by hand, while in the field vane, the ground was remolded by 30 turns of the vane blade. This indicates that the degree of the reduction in strength caused by the turn of the vane blade is less significant than that caused by remolding by kneading.