Early evidence of land subsidence in Kathmandu Valley

Land subsidence in Kathmandu Valley has been recorded since 2003. The major cause of the Kathmandu valley subsidence is still unidentified and the subsiding depth or layer is not clear yet. Published research work has revealed the positive correlation between subsidence and sediment thickness of the valley. Kathmandu Valley, with a heterogeneous sediment thickness and different depths of bedrock, creates favorable conditions for differential settlement.This study is focused on understanding and analyzing the early evidence of land subsidence in different parts of Kathmandu Valley, with direct field observations. Developing empirical relationships to better understand the underlying causes of land subsidence, primarily, multiple deep tubewells (DTW) sites across the valley were observed with the aim of analyzing changes in tubewell head and the impact of subsurface geology and tubewell lithology being studied. Tubewell constructed over bedrock shows the upheaval of the tubewell head in UN Office, Pulchowk, Jagdal Gulma, Chhauni, Prime Hotel, Thamel, where depth to bedrock is 126, 100, and 210 meters respectively. Tubewell upheaval has been recorded since 2017 in Pulchowk and Chhauni and since 2020 in Thamel with the rate of tubewell upheaval is similar to land subsidence recorded from InSAR and DGPS surveys. A similar pattern of tube well upheaval is visualized in and around the surrounding area, where tubewells are constructed over bedrock. In the same area, tubewells constructed on soil subsurface with relatively shallow depth remain the same. A 550 meter long horizontal surface crack around the Manbhawan area indicates the differential settlement of ground surface due to land subsidence. However, no strong evidence is recorded around the central part of the valley floor, where the depth of bedrock is deep.The evidence recorded by tubewell upheaval at Pulchowk, Chhauni and Thamel validates the regional compaction of sediment layers deposited over the bedrock and the surface crack around the Manbhawan area validates the differential settlement due to change of subsidence rate over the same area. No change of tubewell head in shallow tubewell in the same area validates the deep aquifer compaction due to groundwater extraction and sediment load.