This paper concerns the methodological aspects of investigations carried out in weak rock, for the design of a large inland navigation canal where the rock particularities, demanded the adaptation and introduction of techniques and methodologies, some of them which could be considered unusual.
The sedimentary nature of the rock mass of almost unknown geomechanical behavior, as well as the characteristics and large dimensions of the Pereira Barreto Canal, demanded the establishment of very peculiar investigation programs, for the characterization of mass rock and the development of the design.
One of the main aspects of the investigation was the characterization of the rock, which was homogeneous in appearance, by adopting as property index the uniaxial compressive strength, standardized for samples in the natural moisture content condition.
This property allowed the definition of the rock classes, to which were associated almost all other properties and to which were programmed the tests on whose results the design was based on.
During the development of the Basic Design, the establishment of a more precise geomechanical model for the sandstone rock mass was found to be important in order to allow more reliability in the stability analyses, and to achieve the final geometry of the excavated slope under an appropriete technical economical equilibrium.
In this sense, the determination of the existance of not of large discontinuities, as well as the determination of the initial stress state acting in the rocks mass and its extreme boundaries, constituted a very important activity during the geotechnical investigation phase.
In the same way, the lack of knowledge of its behavior when faced with weather action, associated to the large areas to be exposed in the future, represented relevant aspects concerning the decision for adopting protections on the excavated slope surfaces, mainly in the water level variation zone.
Other aspects related to excavation methods and drainage design, not perfectly defined at that time, were observed and measured by means of "in situ" tests carried out in a large experimental excavation test where, in addition to the compartimentation and weatherability, fractures of the rock mass were analysed.
Pereira Barreto Canal is part of a global hydroelectric generation and navigation system involving the already existing Ilha Solteira Hydroelectric Plant (3.200 MW) and the under construction Três Irmãos Hydroelectric Plant (1.280 MW).
The canal will interconnect both reservoirs, and will be 9 km long. The width at the botton will range from 50 to 70 m, and the highest slope will reach 61 m; the total excavation volume being estimated in 18,5 × 106 m3.
The job has just started and it's being done in a very weak sandstone, showing uniaxial compressive strength values lower than 120,0kg/cm2. This sandstone, so called Bauru sandstone, belongs to the cretaceous age and has a fluvial-lacus. trine origin.
Bauru sandstone is essencially formed by a fine quartz sand slightly feldspathic cemented by both clayey fractions of the montmorillonite group and ferrugineous fractions in variable degrees, and it shows a homogeneous appearance along the extent of the canal.
