A detailed geological mapping of the stratigraphy, the techtonic and the structure of the cavern for La Muela power plant has been performed during the excavation works. This has been helpful in completing and refining the data from previous investigations. Fifty seven stratigraphic logs have been made following the excavation process, while the main discontinuities -faults and fractures- have been mapped topographically and other minor accidents have been mapped with compass. The result of this work is a substantial amount of data which can be very useful to support further actions. At the same time convergence and extensometer measurements have been taken on the site. Before excavating the main cavern a series of microsismic refraction and up-hole tests were performed to establish the elastic parameters of the rock. The results are analyzed and a comparison is made with those deducted from previous investigations.
While the process of construction for the cavern of the hydroelectric pumping plant at La Muela. (River Jucar, Valencia, Spain) was going on geological data were Compiled, with a view to both assisting the excavation work and completing the information acquired in the preliminary study and project phases.
At the same time a monitoring process in the different areas of the cavern was carried out by means of convergence and extensometer measurements.
As a task prior to the excavation, micro-seismic refraction studies and up-hole tests took place from the different galleries that make up the underground complex; these made it Possible to know the elastic Characteristics of the area in which the cavern is situated.
The cavern, aspects of which have been dealt with on other occasions (Navalón et alt., 1979, Navalón and Alcazar, 1987, Navalón and López Marinas, 1988), is located in a massif made up of limestones and marly limestones with an isoclinic structure of 5° dip into the hill. Of the stratigraphy as a whole, it is worth mentioning that there is one layer with worse characteristics than the others which consists of sandstones grauwackes and marly clays whose exact position it was possible to define.
The geophysical work was carried out by "La Sociedad de Reconocimientos GeofÍsicos, S.A. (1984)" and consisted of four different types of studies.
Registering of celerity of vibrations
These tests have made it possible to define the thicknesses of the decompressed areas and the elasticity characteristics of both these and the sound rock areas. The have been carried out in the main access tunnel (devices 2 and 3), Group III Busbar Gallery (device 4), auxiliary traverse gallery, and Group III ventilating tunnel (device 5). The location of these tests can be seen in Figure 1. As can be observed, the data are rather homogeneous. The thickness of the decompressed rock is not excessive and the velocity measured in the sound rock is of good value. Considering that the average density is 2.65 g/cm3 and the dynamic coefficient of Poisson 0,25