Model tests on soft marine clay layers without foundation elements showed signiﬁcant ampliﬁcation of earthquake motion by soft marine clay layers. The ampliﬁcation ranged from 2 to 6 depending on the peak ground acceleration of the input motion. For large input acceleration having peaks of about 0.1g or higher, the ampliﬁcation was about 2. However, for smaller input acceleration with peaks of about 0.02g or less, the ampliﬁcation increased to about 6. In all cases, the maximum ampliﬁcation occured within the frequency band ranging from 0.4 Hz to 0.8 Hz. This means that ampliﬁcation is higher for smaller tremors. For regions where the norm is to design for relatively small earth tremors, the effect of site ampliﬁcation is therefore correspondingly more signiﬁcant. The above results were obtained from tests on Singapore marine clay which is a high-plasticity clay. Tests on kaolin show a different trend, with ampliﬁcation increasing slightly with input acceleration. This can be explained by the difference in damping characteristics between the two soils. Hence, while the general trend of ampliﬁcation remains unchanged, detailed features of the site response are highly dependent upon soil type and earthquake motion.
The model results were compared against ﬁeld data recorded by seismological stations in Singapore. The ﬁeld data was recorded at rock and stiff soil sites as well as soft soil sites. Analysis of the ﬁeld data showed that, for the recorded earthquakes, all of which have peak ground acceleration well below 0.02g, the typical ampliﬁcation by soft soil sites is between 5 and 6. This agrees well with the model test results.
The current phase of the study is focused upon the interaction between soft soil strata and pile foundations during seismic shaking. This is an important issue since many tall buildings in Singapore are supported on pile foundations. The results obtained to-date show that there are signiﬁcant soil-pile effects which cause the resonance periods of the ground motion and pile-raft structure to differ from their respective uncoupled resonance periods. Secondly, for the case of short piles in soft clays, ground surface motions may not be representative of pile-raft motion. Furthermore, a pile in soft clay under earthquake excitation behaves quite differently from the same pile loaded dynamically from the pile top. In the latter case, the soil around the pile has a restraining effect on it. In the former, the centrifuge model results show that the soil actually applies an inertial loading onto the pile. In view of this, it seems highly questionable whether dynamic or cyclic pile load tests will shed any light on the response of such piles to earthquake loading. Longer and more ﬂexible piles are currently being investigated.