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where ρi and hi is the resistivity and thickness of ith layer.
Finally, a comprehensive interpretation of the R2D, R3D and the SSR data can lead to define five sites lines of cracks or chinks (Figure 15); these chinks are illustrated from south western part to north eastern part over all the study area.
The chinks come out as a result of the irrigation water (used in garden). This water seeps into the subsurface layers. Clay minerals in the second and third layers “Marley Limestone” are saturated with water in the winter and they stay nearly stable. When the hot weather of summer appears, the subsurface layers drive out water by evaporation. This operation, in addition to the loads of buildings, produces the chinks features which cause splits in the buildings.
Figure 5. (a) R2D of the study area with over measured surface shows the geoelectrical cross sections along profiles 1, 6, and 8; (b) R3D of the study area shows the geoelectrical cross sections along the study area.
Figure 6. Velocity estimation using the slope method calculation with shot point and geophones array in field (after Basheer, 2003).
Figure 7. Time-Distance curves along profile “1”.
Figure 8. The lithological layers interpreted from the seismic data of the profiles.
Figure 9. The P-waves distribution along profiles in two dimensions.
Figure 10. The SH-waves distribution along profiles in two dimensions.
Figure 11. The Standard Penetration Test “N-values” distribution along profiles in two dimensions.
The present study has been conducted mainly to detect the site of the chinks and provides an interpretation of the data in terms of the foundation rock materials and parameters.
The study involves carrying out three main geophysical techniques integrately; the first technique embraces executing two- and three-dimension of electrical resistivity imaging survey in the form of eight parallel profiles. These profiles are distributed over a suggested portion of the study area where the bad effects on building are noticeable. The Wenner’s electrode arrangement with maximum spread of 72 m, 1 m between the electrodes and 0.5 m interline distance is used utilizing the SYSCAL R2 system. The second techniques depend on the same last techniques with another processing and presentation; three-dimension electrical resistivity imaging gives a formed shape. The third technique of the study depends on the analysis of the acquired seismic refraction data using eight shallow seismic refraction profiles spread over the same sites of R2D and R3D profiles. In the survey, the velocities of the P- and SH-wave have been specified with interpreted standard penetration test (N-values). Most interpretations of seismic profiles have been made on normal and middle shooting position types to avoid the effect of blind layer that causes the attenuation of waves. Through the R2D and R3D survey, the penetrated depth reached about 30 m while the penetrated depth using the SSR varied from 22 m under geophone No. 16 of profile 5 to 23.3 m under geophone 22 of profile 2.
The integrated results, obtained from the interpretation of the R2D, R3D, the SSR records and calculated N-values, can be classified lithologicaly into three layers invaded with five structural features as subsurface cracks or chinks (Figure 15). These layers from the top to the bottom (according to geo-electrical values, shallow seismic refraction waves’ values, interpreted N-values, and the known geological column in the area) are classified as follows:
1) Top soil layer consists of weathered limestone (mainly Dolomite); its thickness varies between about 3.5 m and about 5 m.
2) Wetted Marley limestone with thickness varies from 10.3 m to 10.33 m.
3) Semi-wetted Marley limestone.
Figure 12. The distribution of the P-waves in three dimensions profiles and maps (Logarithmic scale).
Figure 13. The distribution of the SH-waves in three dimensions profiles and maps (Logarithmic scale).
Figure 14. The standard penetration test “N-values” distribution along profiles in three dimensions profiles and maps (logarithmic scale).
Figure 15. Direction and spread of cracks lines over the studied area.
Figure 16. (a) Crack between two sides of building in the study area; (b) The water pool used in the irritation of garden between buildings in the study area.
Five chinks or fractures have been outlined by the interpretation of these three techniques. These features spread all over the study area. They have also been organized and noticed by eyes in the buildings near the study area (Figure 16). It is recommendable to remove the gardens between buildings to avoid the effect of irrigation water (used in garden’s watering) and keep it away.
I would like to thank the personnel in the authority of 15th May City, because they considered the results of this research seriously by evacuating residents of the two buildings No. 13, and 14 and removing these buildings (depending on the results and the recommendations of this work). Great thank to Dr. Nihal Adel for her language review in this research.