The Emergency Cooling Reservoirs of the Trillo Nuclear Power Plant, with a capacity of approximately 84.000 m3, have been excavated in a soft sedimentary rock formation. The fact that they fall under Seismic Category Class I as they form part of the emergency cooling system, has required a study and control of movements during the excavation, construction and operation phases.

This article describes the tasks performed for the design of these reservoirs, based on a geotechnical exploration. and the hypothesis and calculations used to evaluate vertical movements of an elastic nature produced by excavation and filling of the reservoirs, and possible non-elastic movements, caused by a hypothetical increase in the ground moisture content.


Spanish tertiary formations frequently contain clayey marl levels of an appreciable consistency, which may alter significantly when discharged during excavation. or when an increase or reduction of moisture content is produced in them on modification of their natural conditions during construction or subsequent operation of the facility.

At the Trillo Nuclear Power Plant, the proximity of the bottom of the Emergency Cooling Reservoirs to a layer with the characteristics mentioned above, has called for in-depth analysis of possible movements and the installation of a surveillance system to validate the design hypotheses "a posteriori".

The particular geometrical location of the underlying materials has conditioned the final structural arrangement adopted for the construction design, making the most of favorable factors involved for its stability and minimizing the risk of a possible alteration of materials which could cause undesired movements during the useful life of the facility.

The following sections contain a brief description of the plant site and its general geological-geotechnical characteristics. with special emphasis on the Emergency Cooling Reservoirs. They also describe the available monitoring, theoretically expected movements and real movements measured. The evaluation of these data leads to the summary and recommendations given at the end of the article.


The Trillo Nuclear Power Plant is located at the top of a small hill about 120 m high, on the River Tajo, some 3.5 km to the west of the village of Trillo in the province of Guadalajara (Fig. 1).

(Figure in full paper)

Except for a shallow quaternary covering, the subsoil explored consists of soft rock dating from the Upper Paleogene-Lower Miocene, characterized by the alternation of clays and marls with sandstone, and also a certain amount of conglomerates whose stratification is appreciably horizontal.

Geotechnical reconnaissance of the foundation area of the plant buildings and structures was done through various "in situ" exploration campaigns and from laboratory-tested samples.

The analysis of all this information showed the existence of a regular spectrum of materials among the clays which, after alteration when located near excavation slopes or bottoms, could contain from dry densities of about 1.40 g/cm3 to conglomerates with densities greater than 2.40 g/cm3, with no clear band separating clays from marls or sandy marls from sandstones or the latter from conglomerates (Fig. 2).

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