Airi SUGAWARA

Numerical Analysis of Regional Subsidence by Winter Pumping of Groundwater Based on Repeated One-Dimensional Consolidation Tests

Satoru OHTSUKA


One of the factors causing ground subsidence is the cyclic load acting on the ground due to fluctuations in the groundwater level. In Niigata Prefecture, snow-melting pipes that use groundwater to remove snow during the winter are widely used, and the annual fluctuation of the groundwater level is extremely large because of the use of groundwater for snow removal, causing continuous land subsidence. The land subsidence problem in the Joetsu area was caused by the increased use of groundwater for snow removal during the heavy snowfall years of 1984 and 1986, which resulted in the greatest land subsidence in Japan, prompting the development of various land subsidence countermeasures with an emphasis on snow removal, rather than conventional regulations for water supply and industrial use. Securing roads during the winter is essential for the livelihood of citizens and economic activities. It is obvious that excessive pumping of groundwater will cause land subsidence, and unless the government and citizens create a snow removal system that does not depend on groundwater, it cannot be said to be a fundamental measure to prevent land subsidence. However, in reality, there has been no case in the past to estimate the period of subsidence convergence in response to groundwater fluctuation, and even if the ground becomes overconsolidated, there is concern that subsidence may occur in response to repeated changes in groundwater levels.
In this study, the consolidation characteristics of the local ground were clarified using a one-dimensional consolidation test in which the groundwater table fluctuation is taken into account in the stress cycle of loading and unloading of the consolidation load, the analytical parameters were determined from the results obtained from the consolidation test, and a fully coupled analysis of ground subsidence was conducted. The settlement behavior of the soft ground layer under cyclic changes in groundwater was investigated by numerical analysis.
In the analytical investigation, ground deformation analysis due to repetitive changes in groundwater level was conducted using the field surveyed ground as the model ground to simulate groundwater pumping. The Subloading tij model and the modified Cam Clay model were used to simulate the groundwater recharge period from spring to fall with water pumping in winter. The modified Cam Clay model cannot describe plastic deformation in the overconsolidation region, while the subloading tij model can. Comparison of the numerical results with both models suggests that subsidence continues even when the groundwater table repeatedly changes in a steady state and ground stresses remain in the overconsolidation region. Since snow removal using groundwater can cause land subsidence, it is necessary to develop a new sustainable snow removal system that can deal with fluctuations in the groundwater table without causing a sudden drop in the groundwater table during the winter.