Forensic Geotechnical Investigation of the Slope Failure at Qila Bala Hisar With A view to propose its remediation

Abstract

Qila Balahisar is one of the noteworthy places of Peshawar, Khyber Pakhtunkhwa. The construction of the fort dates back to 18th century and was constructed on filled ground. It was renovated by various rulers of the time like Mughals, Sikhs, British and the current occupants. The fort is located in the seismic zone 2B with Peak horizontal ground motions varying from 0.16g to 0.24g. Recently, on 27th October 2015, a major earthquake of magnitude 7.3 struck northern Pakistan. Due to this earthquake the upper part of south western wall of the fort collapsed. The collapse of the wall was attributed to the failure of retained slope. This research was undertaken to characterize the slope material, study causal factors of failure and evaluate remedial strategy. The research involved forensic geotechnical investigation including conventional field and lab testing of the slope and geophysical investigation using electrical resistivity technique to evaluate the nature of stratum. Also, x-ray diffraction and scanning electron microscopy was included to study the slope material at a molecular level to evaluate existence of swelling potential or otherwise. The analysis of the forensic level investigation has shown that excessive seepage of water caused by the poor maintenance of runoff and sewage drains is the causal factor triggered by the seismic event of magnitude 7.3 during October 2015. A remedial strategy involving soil nails and improvement of the surface drainage was evolved. The authors believe that the research will enhance the understanding of the behavior of forts built on filled grounds and findings of the research would contribute towards the perseverance practices of heritage sites.