GPR BASED SUBSURFACE GEOTECHNICAL EXPLORATION

Abstract

The Seismic refraction technique (SRT) and Electrical resistivity technique (ERT) have long been in use in geotechnical exploration. A relatively recent technique is Ground penetrating radar (GPR). The study presented in this paper is on GPR-aided geotechnical subsurface exploration. The usual method of exploration is drilling, which gives much-needed site-specific information, but is expensive and restricted to a few point locations. The possibilities of non-invasive investigation offered by GPR make it useful for supplementing geotechnical investigations. The present work describes GPR survey at a construction site in Mumbai. The objective was to derive subsurface logs from GPR signals. Conventionally, subsurface logging is done using boreholes. First, the extracted soil and rock samples are examined visually. Second, additional information such as Core recovery ratios (CRR), Rock quality designation (RQD) and Standard penetration test (SPT) N values are collected and strata are demarcated. In comparison, the amplitude variations of GPR signals may not correspond directly to variations of these physical properties with depth. However, the study shows that fairly good correlations do exist with the subsurface stratification and transformed signals. Method and materials The geology of the study area is composed of Mesozoic rocks of volcanic origin. Known as the Deccan trap, the main rock type is basalt (Geocon, 2011). The rock exhibits weathering of varying degrees and the overlying soil is known locally as murrum. The GPR study area is covered by survey grids that are 7 m x 26 m in size. The subsurface conditions, particularly, depth to rock, varied significantly between boreholes, raising possibility of a sloping bedrock surface and arousing stability concerns for the proposed tall building. Six boreholes were drilled at site and borehole logs were available. GPR has been used to obtain supplementary information particularly between the boreholes. Two GPR frequencies (40MHz and 20 MHz), were employed. A typical survey grid is presented in Figure 1. GPR data using a 40 MHz antenna was collected in 2D point mode scanning from left to right, bottom to top, with Scans/m = 5. In L1, 9 X lines of 7m length & 3 Y lines of 26 m length each were traversed The 20 MHz survey was conducted close to and parallel toY2. The initial GPR data gathering settings are given in Figure 2.