Identification of Karstic Features with Extensive Geophysical Investigation for Underground Infrastructure - South of Wakrah Pumping Station and Outfall (SWPSO) Project

Abstract

There has been a significant increase in the number of underground infrastructure projects built in a short period because of Qatar's goal of effective mass transit and robust urban drainage infrastructure, as well as the country's recent fast urbanization and harsh weather events. The Doha Metro, the Inner Doha Re-sewerage Implementation Strategy (IDRIS), and most recently the Doha South Terminal Pumping Station (DSTPS) and the Musaimeer Pumping Station and Outfall (MPSO) projects faced difficulties in identifying and quantifying the Karstic features and the remedial approach for underground works to ensure the completion of the projects on schedule and within budget due to the unknown ground conditions. One of the most important risks for underground construction in Qatar is the presence of karst environments, which are concentrated mainly within the limestone, dolomite, gypsum, and anhydrite horizons of the Eocene strata. The proper assessment of Karstic features requires an appropriate selection of geophysical methods and a high level of skills and expertise to interpret the data. To do so, a comprehensive geophysical survey campaign was deployed at the early stages of the project's execution to identify the Karstic features of concern associated with subsurface conditions at shallow and deeper depths up to 80 meters below ground level. This paper presents an example of a recently developed geophysical investigation program for the pre-tender design of the South of Wakrah Pumping Station and Outfall project. The project team carried out a comprehensive geophysical study to identify the Karstic features of concern along the tunnel alignments, shaft locations, and at the pump station area. Most of the identified Karstic features are categorized as low risk, but there was a high-risk anomaly (cavern) identified, and the extent of this cavern was verified with well-known geophysical methods like Multichannel Analysis of Surface Waves (MASW), seismic refraction, and 3D crosshole seismic tomography.