5.3 collapse susceptibility map based on microgravity results.
Susceptibility maps for construction collapse may be based on geographic information system (GIS) (yilmaz,2007), geomorphic factors (papadopoulou-vrynioti et al., 2013) or sinkholes (Galve et al., 2009b). It is less highlight areas where collapse might occur owing to the presence of voids.
In this case, the residual anomaly map was considered together with the forward models of the mine galleries to arrive at a collapse susceptibilty map underlining key areas related to the mine galleries (figs.y and 7).
The extension and position of the galleries along the profiles determine the anomaly contour (fig.7a) -that of 0.15 mGal (marked with red line in (fig.7) fits the mine limits and retricts the high collapse susceptibility area. Further, shallow voids in the hill define a lower collapse susceptibility area (0.05 mGal, yellow line in fig.7).
These contour can be highlighted over the orthophot to verify the marked areas (fig.7b). The mine is seen to lie mainly E of the construction area. However, on the W and NW sides of the hill the red line is located over some buildings. The areas marked with purple circles in (fig.7b), match the red line and constructions, indicate major collapse and deconstruction on the hill.
6. Conclusion
The present of a disused mine beneath Candado hill originated construction collapse along the W side of the hill. The gypsum mine comprised several galleries, but the lack of outside access impedes determination of its morphology and position. Accurate microgravity research to the hill provided new data for assessing its collapse susceptibility. Density analysis, applied to yhe terrain correction and Bouguer anomaly, revealed that the optimal values of the host rocks is close to the standard 2.67 g/cm3. Two profiles were modeled crossing each other to derive the approximate galleries morphology, dimensions and depth.
Altogether, our results produce a collapse susceptibility map supported by the residual gravity anomaly contours, highlighting areas where shallower and smaller voids might also originate local collapse. The area undergoing the principal construction collapse is underlined, matching the highly susceptible area.
This study serves to confirm that gravity research is an adequate tool for detecting voids and eraborating collapse susceptibility maps that condition future construction in areas where subsoils may contain anthropic or natural voids.
Acknowledgement
We thank Jeans Sanders for reviewing the English grammar. Our research was supported by projects CGL2010-21048, P09-RNM-5388 and RNM148. In addition? We are grateful with the positive comments and reviews made by the two anonymous reviewers who significantly helped to improve the manuscript.