Quantitative mechanisms of groundwater level fluctuations under harmonic disturbance: stress–strain evidence in a confined sand layer
摘要
Oscillations, step-like changes, and sustained changes in groundwater levels induced by earthquakes have long been investigated. However, most existing mechanisms explaining these changes are semi-quantitative or qualitative, due to incomplete explanations of the response process. This study presents a comprehensive and quantitative analysis of groundwater level fluctuations under controlled laboratory conditions. A metal experimental box was constructed, containing a layer of water-saturated sand beneath a layer of clay and three fully penetrating “wells.” The experimental box was fixed on a shaking table to test the effects of harmonic disturbance on the horizontally confined sand layer. Eight shaking table tests were conducted at frequencies ranging from 0.5 to 2 Hz and acceleration amplitudes from 0.05 to 0.45 g (1 g = 9.81 m/s2). Real-time changes in acceleration, pore-pressure, and water level were observed by sensors installed in various parts of the experimental apparatus. Shear stress and shear strain behaviors manifest the characteristic relations of sediment deformation during progressive deformation and water level fluctuations. The findings show that shear strain thresholds trigger pore pressure evolution, resulting in variations in water level. Specifically, shear strains between 10–6 and 10–5 initiate oscillation, while those ranging from 10–5 to 10–4 induce monotonic changes. Nonlinear relationships between shear stress and shear strain indicate complex deformation in the water-saturated sand layer. The experimental data provide insights into the quantitative understanding of coseismic water level changes.