ABSTRACT:

Investigations were been performed to verify the competency of rock foundation of nuclear building founded in rock. In order to confirm that the in situ rock over the whole of the foundation excavation correspond to the conditions determined from the original subsurface investigations and used in the foundation design, the techniques were employed: visual observations, surface geologic and structural mapping, shallow cross-hole seismic measurements and laboratory tests. In this paper's are presented the results of the investigations performed and we concluded that cross-hole is one the most pratical precise tests of elastic in-situ rock parameters.

1 INTRODUCTION

The analysis of foundations and of soil-structure interactions in the design of nuclear buildings founded in rock require precise in-situ values of the dynamic elastic moduli of the foundation materials. These values are particularly important for wave propagation and finite element calculations, and where soil-structure interaction is a dominant factor in the response of the structure to dynamic loadings (Ballard (1972)).

In order to verify that the dynamic properties of the in-situ rock over the whole of the foundation excavation correspond to the properties determined by the original subsurface investigations that are used in the foundations design, techniques such as visual observation, field tests and laboratory tests are employed.

Ries et al (1980) concluded that visual inspection by experienced personnel is a most important factor in establishing the competency of a rock foundation. They also State that these further investigations and measurements provide the quantitative information required to substantiate the geologist's and engineer's opinion and serve to identify possible geological anomalies.

2 CHARACTERISTICS OF THE ROCK FOUNDATION

Structural and lithological types and relationships were determined at an engineering site in Brazil through detailed site investigations comprising surface geologic and structural mapping, inspection and analysis of all subsurface data and detailed petrographical work.

The principal and most widespread rock type at the site is a foliated augen gneiss. It is a leucocratic rock of granitic composition containing large megacrysts of feldspar elongated in the direction of the gneissic foliation.

Aplites and diorites occur as small to medium sized intrusions in the gneiss, and some small pegmatitic dikes also occur. In general the contact between the different rock types is abrupt and welded, with no physical discontinuity between them.

The gneissic foliation defined by the parallelism of the platy minerals and feldspar laths is oriented in a northeast direction. All other major structures such as joints, faults, lineations occur also oriented in the NE-SW trend. The dip of the structure is 70° to the north.

Gneissic banding is locally common, parallel to foliation, where the rocks present parallel bands of layer of different minerals textures and colors.

Light-colored bands of felsic composition alternate with dark-colored bands of mafic composition the former rich in alkalic feldspar and quartz and the latter composed predominantly of biotite.

Although the rock is comprised of several lithological types, in general, all contacts are welded and the rock mass is fresh and little fractured (less than one fracture/m).

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