Further studies on the repeated loading of piles in sand.

The work presented in this thesis concerns the behaviour of
isolated piles subjected to repeated loading and placed at
various depths in a medium dense sand upon which either static or
cyclic surcharge acted. The piles, which were of laboratory scale,
were instrumented by strain gauged load cells located along the
inner surfaces of the pile shafts. The behaviour of tension as
well as compression piles was examined. It was found that the
behaviour of the pile was governed to a large extent by the repeated
load level, the number of load cycles and the initial boundary
stress conditions existing along the pile shaft. In compression,
the pile life-span decreased when the embedment depth increased
while the reverse trend was observed,for tension piles. The
movement of both tension and compression piles decreased when the
surcharge pressure was increased or was cycled, and it was of a
minimum value when the upper repeated load acted in-phase with the
higher surcharge pressure. For tests performed with static
surcharge pressure, repeated loading was found to decrease the
bearing capacity and the pulling resistance of the pile. The higher
percentage of reduction was recorded for the tension pile. In
contrast, after cyclic surcharge tests the pile capacity always
increased.
At any depth of embedment or surcharge pressure,' as the nun-her of
load cycles was increased the shaft load of a compression pile
increased up to a peak value then decreased gradually until it
reached a limiting value. This limiting value increased when the
load level, the pile depth or the surcharge pressure was increased
and it was independent of the pile loading history.
iii
For a tension pile the shaft load decreased progressively as the
number of cycles increased until failure occurred.