Delineation of Water Seepage Flow Path in the Underground Metro Rail Tunnel Using Cross-Hole GPR Tomography: A Case Study

Abstract

AbstractConstruction of infrastructure development projects in urbanized environment suffers with obstructions like lack of surface free space, underground utilities, traffic and shallow drainage system. These obstructions hamper the work progress as well as major factor for project delay and cost overruns. Obtain timely solution to these obstructions helpful in meeting the deadlines and reduce the inconvenience to the public. One such case was discussed here, where Mumbai Metro Rail Corporation Limited (MMRCL) proposed to construct a 33.5 km long underground metro rail tunnel between Colaba and SEEPZ. TBM was deployed to make 5.8 m dia tunnel at a depth of 18 m. After 265 m of tunnel excavation, water seepage was encountered with a high discharge flow rate of 60,000–80,000 L/h at Marol Naka station box area. To plug this flow path, Identification of source location or the direction of flow path is the prime concern. Surface investigations to identify this flow path were not possible, due to heavy traffic, shallow underground utilities and congested area. Ground Penetrating Radar (GPR) investigations through boreholes were proposed to delineate the probable water seepage flow path. Five bore holes were drilled over the station box area up to a depth of 25 m. Five sets of tomography investigations were carried out to map the high/low attenuation zones based on the radar wave attenuation characteristics of the subsurface medium. The Frequency Domain Attenuation Tomography (FDAT) analysis was done to characterize the attenuation pattern of observed radar wave signal in the dry as well as in the saturated medium. 3-D subsurface image was generated based on attenuation characteristics to demarcate the probable water seepage flow path in the study area. KeywordsGround Penetrating RadarCross-hole tomographyWater seepageAttenuationUnderground metro tunnel