Peat stratigraphy mapping using ground penetration radar and geotechnical engineering implications

Abstract

A Combined Land and Marine Ground Penetrating Radar (GPR) surveys was carried out in Southwestern part of Lagos, Badia, Nigeria with a view of delineating the subsurface peat stratigraphy that would aid geotechnical engineering design of the appropriate soil stability processes. The GPR study was conducted along eight parallel traverses trending East-West, using the Geophysical survey system incorporated (GSSI) SIR-3000 200MHZ Monostatic shielded antenna. Geographically, the study area is approximately between latitude 715000N and 716000N and longitude 538600E and 540300E of the universal traverse Mercator. The antenna was present with three gain points in order to improve the scans during data acquisition while 33 scans per meter were taken (representing 3cm station spacing) with a sampling window of 400ns with offset of +25ns.The topography is generally flat with localized undulations due to sand heaps from canals dredging and sand filling activities in the area. The peat depth was found to vary from 1.5m to 6m and the thicknesses from traverse one (1) to eight (8) on chainage 625m to 1100m. The area belongs to Dahomey Basin which is also known as Dahomey Embayment or Benin Benin or as West Nigeria Basin. The mineralogy, micro-fabrics and Morphology of the delineated stratigraphy was determined using the Scanning Electron Microscope (SEM) and X-Ray diffraction (XRD) methods. The results obtained revealed the presence of five subsurface (Topsoil/ sand filled Layer, Silty Clay, Peat, sandy Clay and Silty Sand) geological Layers, distinct geomorphological features, and high, moderate and low amplitudes, to continuous and discontinuous planer relax Facies structures. SEM and XRD analyses of the field samples obtained showed the dominance of Kaolinite, illite and quarts minerals in the clay/peat mapped. The derived engineering parameters suggest that the peat found in the study area are “fibrous peat” with low strength and medium to low bedding stress. It is observed that the peat generally depict high compressibility value, low internal frictional angle, they exhibit low elastic properties such as low shear modulus, low bulk modulus and young modulus values. The findings confirm the efficacy and relevance of GPR technique for pre-construction engineering investigation.