Investigations within World Heritage Monument Zones of Kathmandu after the 2015 Mw 7.8 Gorkha Earthquake

This paper presents the results of an extensive study carried out to better understand the causes of collapse of heritage structures within the UNESCO-listed World Heritage Monument Zones of the Kathmandu Valley during the 2015 Gorkha Earthquake. The investigation comprised a field reconnaissance survey, followed by geotechnical investigations (field and laboratory) with the aim of assessing and demonstrating the performance of five exemplar historic monument sites. The investigations include evaluating the liquefaction potential of the sites and understanding the influence of local site effects by performing free-field ground response analysis. The geotechnical investigations indicated the presence of sands and silts at all sites, whereas clay strata were present at the Jaisidewal and Bhaktapur locations. The compositional criteria and the stress-based approach for liquefaction assessment of the bore log data indicated liquefiable soil strata at the Pashupati, Bhaktapur, Jaisidewal, and Hanuman Dhoka sites located within sedimentary basin, though no evidence of liquefaction was recorded during the survey. The free-field ground response analysis indicated a higher maximum spectral response for the Hanuman Dhoka and Bhaktapur sites compared to the other sites, which corroborated the outcome of the field survey, where a greater level of damage was observed within Hanuman Dhoka Durbar Square, compared to Changu Narayan located on hills, and the Jaisidewal and Pashupati sites. Such damage was mainly due to the combined effect of deteriorated structural members, loosened masonry joints, and the local site amplification effects while considering the free-field condition. The findings of the present study will support the planning of accurate geotechnical investigation and analysis within the monument zones and effective action plans to safeguard them against earthquake hazards in Nepal.