Engineering Site Characterisation Using 2-D and 3-D Electrical Resistivity Tomography

Abstract

Land reclaimed from water bodies or its equivalent (swamp, flood zones and abandon river channel) usually contain certain mechanically unstable geological formations (e.g. peat and clay) which are inimical to the foundation of engineering structures. Concerted effort must therefore be ensured to determine depth to competent layer across the proposed site as against conventional geotechnical test which is a point test.Complementary non invasive geophysical methods must be carried out for any engineering construction so as to ensure stability of building construction and avoid the devastating effects of building collapse rampant of late. Thus, electrical resistivity tomography and induced polarisation methods, constrained by percussion drilling, were used for the present study. The results of the 2-D and 3-D DC resistivity tomography using 64 multi-electrode systems with gradient electrode configuration along five parallel profiles with smallest electrode separation of 4.0m on a total length of 252 m was interpreted using Earth Imager Inversion software. The subsurface was characterised by Peat/Clay sandwich between upper and lower Clayey-sand/Sandy-clay and Sand. The presence of Peat and Clay were correlated to the cause of chronic differential settlements of some engineering structures within the area. The depths to the sand material were observed between 17-47 m. At shallow depths, the sand materials were underlain by mobile clay/peat materials, which correlates with the depth to competent layer (Sand) achieved by percussion drilling in the range of 20-24 m. This lithological sequence in which the near surface sand deposits are underlain by clay/peat saturated with water, gives a sense of instability to the overlaying sandy layer. To buttress this finding, residential and other public buildings surveyed in the entire area are marked with various degrees of distress, ranging from multiple cracks, sinking of building, partial collapse to complete collapse. It should be noted that the depth to the sand layer achieved during the research varied along the respective profiles. Thus application of 2-D resistivity tomography has revealed both the lateral and vertical variation in depth to competent sand layer within the study area as opposed to point information from CPT or Percussion drilling. This provides a useful guide to the site engineers in designing appropriate foundation structure.

LiDAR and 2D Electrical Resistivity Tomography as a Supplement of Geomorphological Investigations in Urban Areas: a Case Study from the City of Wrocław (SW Poland)

“…Geophysical measurements were not conducted in the direct vicinity of power lines, gas and sewage networks, in private areas and developed areas with a hardened surface. A review of available literature confirms that except for isolated examples (e.g., PAPADOPOULOS et al 2009;BOUDREA-ULT et al 2010;AYOLABI et al 2013) the ERT method is rarely used in city centres. All ERT profiles were marked by high accuracy GPS (Trimble GeoXH) with real-time corrections from EGNOS (European Geostationary Navigation Overlay Service).…”

Electrical resistivity mapping as a tool for post-reclamation assessment of subsurface condition at a sand-filled site in Lagos, southwest Nigeria

“…Electrical resistivity is considered to be a suitable technique in geotechnical site characterization and environmental studies due to economic factors i.e. reduced cost of investigation and efficiency in terms of speed and reliability [28,[37][38][39]. Several workers have established the use of electrical resistivity techniques as a very effective and practicable geophysical tool in geotechnical and environmental investigation of the condition of subsurface earth materials.…”

Hydrostratigraphic Characterisation of Shallow Coastal Aquifers of Eastern Dahomey Basin, S/W Nigeria, Using Integrated Hydrogeophysical Approach; Implication for Saltwater Intrusion

“…The measurement proceeds automatically and is controlled by a computer/laptop. Details of this method can be found in the work of [31][32][33][34].…”