Integrated Geophysical Methods for Investigations of the Ayat Real Estate Foundation: An Implication for Engineering Work Suitablity, Northeastern Addis Ababa
No Thumbnail Available
Date
2020-08-06
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
An integrated geophysical investigation was carried out at a site near Yeka-Ayat condominium and just in area about the middle of Ayat square and Tafo square, northeastern Addis Ababa, Ethiopia. The principal objective of this research is to map the subsurface geology of the site and discuss its suitability for the construction of large infrastructure. The method employed are the electrical resistivity tomography Vertical Electrical Sounding (VES) and Seismic refraction. Four VES points distributed along one profile line, four dipole-dipole survey lines spread and seismic survey on two profile lines were conducted to characterize the foundation materials within the study site. Qualitative and quantitative interpretations of the acquired data were made by comparing the results with borehole log data available from nearby areas. The results have revealed that the subsurface geology consists of three to four distinct units that reflect different physical properties (resistivity and seismic wave velocity). The upper most layer is characterized by low resistivity (3.03-25Ω-m) and low P-wave velocity (<0.42km/s) are associated with the response of the upper black cotton soil, composed of highly plastic clay and silty clay materials. The thickness of this layer is found to vary from 1.6m to 5.7m. Underlying the top layer, is a relatively thick (depth variation between 1.6m-12.4m) and its resistivity varying from 30-42Ω-m and P-wave velocity(<1.79) this response is due to light gray, medium to highly plastic clay, which is found inclined from north to the southern direction of the study area. In order to the result of 2D resistivity models, geo-electric section of VES and seismic velocity model of the third layers are the response of slight to moderate ignimbrite. It represents the bedrock of undulating morphology in the study area and is characterized by increment in depth (varying from 12.3m to 37.6m) in a direction from north to south of the study area. Its resistivity is varies from 38 to 65Ω-m and P-wave velocity less than 2.4km/s. Whereas the bottom layer mapped with the survey is a fine and slightly weathered bed rock, clearly observed under the first 20-60m distances of the study area. Its depth is found to vary from north to south direction and its resistivity from 86 to 206Ω-m. From the result of ERT profiles and VES profile around 50m and 70m distances there is hidden structure/fracture in the subsurface, which is oriented from NW to SE. Which is similar to seismic refraction around 20m and 40m distance of the profile south to north direction. Generally, the study result shows that the entire study area is characterized by low resistivity, which is less 206Ω-m and this resistivity is a response of a mostly saturated layer with those higher resistivity to be responses of weathered and fractured formations of low moisture content. Areas of potential weakness have been mapped. Utilization of the site for, development of engineering structures around this site requires proper measures, including proper drainages, compaction, stabilization, removing and backfilling of the top soil and filling by other well graded material that enable to make the structures with stand soil expansion and the impacts from natural geo-hazard events.
Description
Keywords
Site Characterization, Rift Margin, Dipole-Dipole Tomography, Seismic Refraction, Seismic Wave Velocity, Ethiopia