Land subsidence prediction for a new urban mass rapid transit line in Hanoi

Abstract

The rapidly increasing population, urbanization, and industrialization of Hanoi city in the last few decades have necessitated the urgent development of a new underground public transport infrastructure such as the urban mass rapid transit (UMRT) system that is now under construction. Simultaneously, concerns on the possible effects of land subsidence on the to-be-constructed and operated UMRT lines must be addressed. One of the difficulties while assessing land subsidence in Hanoi is the insufficient subsidence monitoring data. Therefore, a good geotechnical characterization of the subsoil profile and proper numerical analysis can help predict groundwater pumping-induced subsidence adequately. The UMRT line No. 2 that was selected for this study is special as it runs across the old quarter downtown, located at the very heart of Hanoi city. In this paper, the Hanoi aquifer system, groundwater extraction, and subsidence monitoring network of Hanoi are reviewed comprehensively. Soil data from 30 investigation boreholes were analyzed to characterize the subsoil profile underlying a segment of the UMRT line No. 2 that consists of both elevated and underground sections. Seven soil layers were identified up to a depth of 50 m, including a compressible soft clay layer 10–15 m thick that is supposed to be most vulnerable to subsidence by groundwater pumping. Based on the long-term groundwater monitoring data, one-dimensional finite element method (FEM) consolidation analyses were carried out for three drawdown boundary conditions of 5, 10, and 15 m and yielded a total subsidence of 0.15, 0.35, and 0.5 m, respectively, after 25 years with 1994 as the reference year. The results of this study confirmed that land subsidence in particular should be considered in the construction and maintenance of the UMRT lines in Hanoi, in particular for underground sections that are founded on soft to medium clay layers. Keywords: Hanoi aquifer system, Groundwater pumping, Land subsidence, UMRT line, Geotechnical characterization