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Detection of Rock Boulders by Electroresistivity

Geoelectrical imaging


The main objective of the survey was to verify the possible occurrence of blocks of rock (boulders) in the subsurface in order to provide additional information for the implementation of the foundations of housing projects. 


During the operation of excavation and installation of foundations, prior knowledge of the subsurface terrain can avoid major difficulties. 

In order to minimize the unknown variables, the most usual is to conduct a campaign of mechanical probing in order to provide designers and civil engineers the rock types, their contacts, and some of the physical properties of the subsurface materials. 

However this information is specific to one point. 

The geoelectrical imaging was used in order to fill the gaps left by mechanical polls giving as final result, sections of geoelectrical imaging (SIG) as shown in figures below.


Figure 1 - GIS implemented on where a block of rock was detected by probing. In this case it was possible to estimate the volume of the rock block (Figure 2).



Figure 2 - Detail of the anomalous region where is possible to estimate the size of the rock block.

Another case study was in another housing project where six towers were built. During the process of implementation of the first tower foundations and piling the cuttings, some of them found blocks of bedrock.

Thus, in order to predict the presence or absence of blocks of rock, and when they were there, try to map them, a detailed geophysical investigation was planned and executed (Figure 3).


Figure 3 - 14 SIGs in the NE and 12 in the SW direction were executed.

In Figure 4 five SIGs are shown, where values ​​above 2,500 ohm m were delimited by a dashed line, as they were correlated with the presence of stone blocks.



Figure 4 - Illustration of five parallel GIS, where high values ​​of resistivity were surrounded by a dotted line and were correlated to the presence of stone blocks.

In order to synthesize all the information of SIGs held "cuts" in six different altitudes and compiled-Contour maps showed Electrical resistivity of the following depths 1.1m, 2.8m, 4.9m, 7.8m 13.2m 10.2me. These maps were prepared individually in Figure 5 may thus provide a better visualization of the anomalous areas correlated to the presence of stone blocks.



Figure 5 - Contour maps showed electrical resistivity for different depths made from made of SIGs cuts. The areas in warm colors are correlated to the presence of stone blocks.

During the excavation process for lowering the elevation of the terrain it was proved the presence of rocky material are as shown in Photo 4. It was also verified that where the highest values of electrical resistivity were recorded there was a strong correlation with the core of bedrock.



Figure 6 - Excavation at the site of the foundations of Tower 2. Through the survey by geoelectrical imaging it was possible to predict the locations of higher incidence of stone blocks.

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