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本帖最后由 三T上人 于 2015-11-8 15:18 编辑 <br /><br />Long-term set-up is defined as an increase in bearing capacity over time that takes place
after the dissipation of excess pore pressures induced from the pile driving. In sand, for
instance, the dissipation can be expected to take place within a few hours. Long-term setup
of driven piles in non-cohesive soils can, in many cases, be substantial, as several
studies in recent years have shown. Nevertheless, the underlying mechanisms of set-up are
not clearly understood. Long-term set-up in non-cohesive soil can roughly be divided into
two main time-dependent causes, based on the hypotheses presented by Schmertmann
(1991) and Chow et al. (1996):
1. Stress relaxation (creep) in the surrounding soil arch, which leads to an increase in
horizontal effective stress on the shaft.
2. Soil ageing1, which leads to an increase in stiffness and dilatancy of the soil.
Both these mechanisms start directly after pile installation and are, to a certain degree, also
a part of the short-term set-up that takes place during the dissipation of excess pore
pressures. It is, however, unclear which one of these mechanisms is predominant under
dissimilar conditions, and also how long this process continues.
In a practical sense, the problem is predicting the size of the set-up at a specific site and its
behaviour in the long-term, which would enable maximum use of the phenomenon. To
make this possible, a deeper understanding is needed of the mechanisms involved and their
effects under different conditions. Furthermore, reliable field investigation and testing
methods for determining the set-up characteristics are needed
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