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Features such as foliation, folds, faults and fracture systems deserve special attention as they are associated with weakness zones of the rock mass. In many places geological, geomorphological and urban occupation conditions hinder exposure and identification of these features on the surface.

In this context, researchers developed new technologies to facilitate access to the structures mentioned above. Among these innovations is the optical televiewer, which consists in obtaining images of the borehole walls, providing what could be considered a virtual log.

This technique has growing application in studies and investigations of major infrastructure works in Brazil. A methodology has been proposed in Oliveira et al. The data obtained from 17 boreholes, with the total of meters of filmed rock, was massively used to characterize the rock mass and its discontinuities.

It was created statistic graphics with the discontinuities characteristics, considering parameters such as fracturing frequency, aperture, roughness and weathering conditions of the walls. This paper show not only this suggested methodology for optical televiewer investigation, but also how this technique could be applied for underground excavation project design. Introduction The optical televiewer OPTV is a technology introduced since in Brazil, having its considerations about this method been included only recently in the ABGE1 Drilling Manual , without, however, presenting detailed specifications of a systematic interpretation of the data.

On the other hand there was a lack of suggestions of how these data could be applied for infrastructure site design.

On the specific case of urban underground excavation, such as tunnelling projects for subway lines, it seems essential to obtain as much ground data as possible, optimizing the stabilization and contention of structures, reducing costs and decreasing risks. So taking in consideration the suggested method from ISRM , already used as standard guidelines for rock mass characterization, it has been has proposed guidelines to achieve same standardization for optical televiewer images of borehole walls.

Optical Televiewer Proposed Methodology The suggested parameters for the rock mass description possible to be observed in the optical televiewer images are lithology, structure, texture, mineralogical composition and weathering degree.

It is suggested the use of a spreadsheet indicating the various sections defined for the rock mass, as indicated in Table 1. In the rock mass characterization is essential to describe the penetrative structures such as foliation, schistosity, banding, layering and veins. Table 2 presents the description of these penetrative structures.

Regarding the weathering stage of the rock, it is only possible a limited characterization, where it might be noticed variations in brightness and mineral colours Figure 1.

To describe the weathering degree is suggested Table 3. Table 1 — Rock mass characterization table. II biotite. Slightly All the rock material may be discolored by II weathered weathering. Moderately Less than half of the rock material is de- III weathered composed and or disintegrated to a soil. All parameters can be described and presented in a table, as suggested in Table 4.

For the wall roughness parameter it was only possible to observe the roughness on a scale of detail in the optical televiewer data. The roughness profiles suggested by Barton , showed at Figure 2, has been modified by separating the profiles of the smallest detail, being possible to characterize the discontinuities only in rough, smooth or slickensided Figure 3.

For the weathering conditions of the wall is possible to deduct the weathering degree of a discontinuity taking into account the brightness of the mineral along the features.

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The weathering conditions of the walls and the presence of filling can be characterized using the Table 5, modified from IPT Table 4 — Example of table of characterized discontinuities in a virtual log. Table 5 — Filling and wall weathering conditions characterization table, modified from IPT D2 Wall to wall contact, rigid rock material filling calcite, quartz, epidote.

D3 Walls with incipient weathering, slightly discoloured, rust staining, filling absent.

D4 Weathered walls, filling absent, presence of clay coating, opened discontinuity. D5 Weathered walls, discoloured, rock fragments as filling. D6 Weathered walls, discoloured, clay, silt and or sand filling, describe.

Weathered walls, discoloured, filling not recognized, describe as detailed as D7 possible. For the fracturing degree, here expressed in terms of discontinuity planes occurring per each meter, defined for homogeneous fracturing parts, it has been used Table 6. The discontinuity frequency degree shall be indicated as shown in Table 4, using the parameters of Table 6. It should be only considered relevant discontinuity, which most probably would involve breaks in the core along this plane.

And by then the spacing can be directly calculated as a function of the distance between each fracture.

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Table 6 — Discontinuity frequency degree, modified from IPT Finally, the Phase 3 stretch would add 2. It is included the following constructive unities: main tunnel track, two stations, two ventilation and emergency exit shafts and four access shafts, besides several access tunnels. The entire massif is characterized by gneissic and migmatitic rocks, foliated and strongly conditioned by a structural component influenced by major shear zones.

Together with the banded structure it creates an irregular rock top, besides a vertical and horizontal heterogeneity, being that certainly a challenge for the prevision of the ground behaviour due to the excavations. The discontinuity walls weathering degree, aperture and discontinuity roughness, as well as fracturing degree data were described as discontinuity parameters and were considered for the pie chart diagrams definition, indicating the percentage of each parameter that occurred along the filmed portion.

Each pie chart for each borehole was considered and plotted on a map of the line stretch, using ArcGIS Software and Google Earth images, so it was possible to understand how each parameter varied along the line. Figure 4 shows the pie charts considering the discontinuities walls weathering degree variation as main parameter, having as classification base the Table 5.

Figure 4 - Discontinuity walls weathering degree variation along the line. The point-load strength test measures the mechanical strength, and the chemical measurements are designed to detect weathering.

Rock materials The studied samples represent four different sequences of basalt flows within the Itaipu Dam region.

The flows were assigned local denominations during construction of the dam E, D, C, and B and based on their respective lithotypes: dense basalt the central part of the flow , vesicular-amygdaloidal basalt the top of the flows , and basaltic breccia the transition zone between the flows. The following materials were obtained from the excavation fronts during the construction phase and are representative of the main construction materials used during the construction of the dam: concrete aggregate, filters, transitions, rockfill, and rip-rap ITAIPU, ; Figure 1 shows a vertical geological profile with a general description of each studied rock group associated with the various basalt flows.

Methods The point-load strength test To determine the mechanical strength of the rock, a point-load strength test was performed according to the recommendations of the ISRM for blocks of irregular geometry.

The value for IS 50 Point load strength index for 50 mm diameter core is determined with the equivalent core diameter of the specimens.

Weathering degree The degree of weathering was classified using visual-tactile analysis according to the classification described by ISRM , a digital microscope with X magnification, and chemical weathering tests. The pH and electrical conductivity measurements characterize levels of weathering that are very subtle but strongly affect the quality of rock materials granites in the study by Shalkowski et al. To perform the test, in each rock group, four samples of different particle size ranges were evaluated: Sample 1 2.

Different particle sizes from each rock group were obtained by grinding the rock. Immediately after sample immersion, the solution is homogenized on a magnetic stirrer for five minutes. Then, the pH and electrical conductivity are read over 10 minutes of testing at intervals of one minute which theoretically measures the weathering state; subsequently, a reaction occurs with the ions released by the sample.

Figure 2 presents the device developed for this study using pH and conductivity meters and a container with a lid adapted to accommodate the electrodes, thermometer, and sample.

System calibration was performed in two stages: a an experiment with ml of pure Milli-Q water the neutral experiment and b an experiment with a bag of polyester and no sample the control experiment. According to Shalkowski et al. Therefore, the difference between a and b was subtracted from the test results. The solution may interact with atmospheric gases or dissolve in the Milli-Q water; however, because of the controlled laboratory conditions and the water used, these interactions were taken into consideration in the calibration reactions.

To reduce the effects of the external environment on the test, the container holding the solution was protected with a plastic film. Results and duscussion Figure 3 presents the images obtained with a X microscope from the basaltic matrix of each rock group studied. In these images, the plagioclase grains are highlighted light gray minerals. The observation of the plagioclase grains present in the dense basalt matrix allowed for the verification of different behaviors associated with mineral weathering as indicated by the degree of discoloration, especially of the larger grains.

The results of the point-load strength tests are shown in Table 1 , illustrating that the different rock types exhibited different values of point-load strength. The dense basalts had point-load indices of 8. Sample 4D, a dense basalt with clay mineral content, exhibited a high strength, with an IS 50 value of 8. The vesicular-amygdaloidal basalt showed point strengths of 4. In terms of weathering, the point-load test combined with analyses at X provided more detailed characteristics of vesicular-amygdaloidal basalts and basaltic breccias.

For the dense basalts, mineralogical variations related to weathering were not evidenced by point compression. In the chemical measurement test, which was conducted to evaluate the degree of weathering, the initial assessment indicated that of the four particle size ranges tested, the one that best represented the ion dissolution of the rock was 0.

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The particle size range of 0. By analyzing the relationship between the pH and EC parameters, the results could be individualized according to the different lithotypes, as shown in Figure 4.

In this graph, the dotted lines are illustrative and do not represent the actual variation among rock types. The results confirmed that the presence of secondary minerals tends to increase the sample's electrical conductivity and pH value. This graph should not be considered for samples with high calcite concentrations because it biases the result toward high pH values. Comparing the pH and EC parameters 0. However, the EC x pH data Figure 4 showed good potential for use with dense basalts.

For the vesicular-amygdaloidal basalts and breccias, the pH tended to increase with decreasing strength Figure 5B. There was no correlation of EC with point compression for these lithological types.

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The laboratory results indicated significant differences within basaltic lithotypes, which are difficult to detect with visual-tactile evaluation aimed at determining the degree of weathering or by the point-compression test. However, sample 10B presented significant variation in terms of pH.

The point-load test was not effective in the evaluation of weathering in these dense basalt groups, and only a partial association of weathering with EC was found. Conclusion One of the greatest challenges in the evaluation of rock materials for different uses is the diagnosis of incipient weathering that cannot be visually observed.

The results of the present study support the view that pH measurements combined with microscopic analyses appear promising for the assessment of the degree of rock weathering. In dense basalts, based on microscopic analyses, pH values above 8 are indicative of the early stages of weathering. In vesicular-amygdaloidal basalts and basaltic breccias, the mechanical strength has no correlation with electrical conductivity; however, there is a decreasing trend in point-load strength with an increase in pH.

Although the low strength of these basaltic lithotypes is associated with the presence of vesicles and amygdales, a relationship can be verified between point-load strength and the level of weathering: the higher the pH, the lower the mechanical strength. Incipient weathering in the plagioclase can be seen in these groups, but the increased pH is closely associated with the presence of secondary minerals, such as clay and calcite.The authors are thankful to the engineer Ivo Teixeira for the final English revision of the text.

Geologia Aplicada… Table 5 — Filling and wall weathering conditions characterization table, modified from IPT So taking in consideration the suggested method from ISRM , already used as standard guidelines for rock mass characterization, it has been has proposed guidelines to achieve same standardization for optical televiewer images of borehole walls.

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NGI Publication , Features such as foliation, folds, faults and fracture systems deserve special attention as they are associated with weakness zones of the rock mass. On the specific case of urban underground excavation, such as tunnelling projects for subway lines, it seems essential to obtain as much ground data as possible, optimizing the stabilization and contention of structures, reducing costs and decreasing risks. Will be grateful for any help!

Table 6 — Discontinuity frequency degree, modified from IPT So finally all the support design was proposed considering the presence of these blocks, also suggesting that during the station implementation these blocks should be recognized and stabilized.