ABSTRACT
The collapse deformations have been observed in most of rockfill dams (and also in the gravel to a lesser extent) during the impoundment of the dam upstream shoulder by the first filling of the reservoir. In some cases, less frequent, there have been episodes of deformation associated with the collapse of the rockfill upon wetting by rainfall. One of the first and most significant examples of both rain-induced collapse and time-dependent deformations is the Cogswell Dam, also known as San Gabriel No. 2. It was built with dumped granite gneiss rockfill without compaction. In the New Year’s Day of 1934, when 80% of the rockfill volume was already placed and the crest of the fill was 60 m high, a storm dumped 380 mm of rain on the structure. The result was a settlement of 4% (of the height of the fill), accompanied by some bulging of the upstream slope in the lower half of the dam (Bauman, 1960). As a result the concrete face, whose construction was carried out simultaneously with the rise of the fill, was severely damaged. After this episode, holes were drilled in the concrete face and, for several months, water was injected into the rockfill in an attempt to complete the process of deformation. The result was a further increase in settlements up to a maximum of 6% of the height of the fill and an average of 4.5%. The damaged concrete face was then removed and replaced by a temporary wooden screen in order to allow the
1 INTRODUCTION
Rockfill is a building material with singular features. Usually obtained from a quarry or as a waste from rock excavation works, “particles” of this material have sizes ranging from a few centimeters to a meter or more. Particles are angular in shape, with sharp edges and, in the case of foliated rocks, they tend to tabular shapes. The use of rockfill as a building material was not common until the midnineteenth century, when the discovery of gold in California promoted the construction of dams for water supply to the mining industry. The availability of crushed rock as a byproduct of mining favored the construction of small dams with a body of dumped rockfill and a wooden screen as sealing element. From the beginning of s. XX, rockfill began to be used in the construction of large dams with either a concrete face on the upstream slope or with clay core (Penman, 1992). The operation of such large dams, brought about the measurement and recording of their deformations as safety routine, yielding the first data related to the mechanical behaviour of rockfill.
