Study on Groundwater Table Variations Induced by Earthquake in the Choushi River Alluvial Fan.
Date Issued
2010
Date
2010
Author(s)
Chen, Bo-Hung
Abstract
In 1999, Chi-Chi earthquake hit Taiwan, causing civilian casualties, house collapses, and landslides, also caused many liquefaction cases in the alluvial plain of western Taiwan. Groundwater level changes during earthquakes were recorded widely. The groundwater monitoring network established by the Water Resources Bureau to the west side of Chelungpu fault recorded the groundwater level changes during the Chi-Chi earthquake.
In this research, records of six wells were chosen for simulation from Jhuoshuei River alluvial fan study area. They are Hao-Hsiu, Wen-Chang, He-Hsing, Kang-Hou, Chiu-Lung and Tien-Yang. A simplified stratigraphic column profile was used for the water-rich strata in the earthquake simulation. Results were comparing with the data of groundwater monitoring network for validation of water level changes using different modes of pore water excitation for the seismic analysis.
In order to understand the seismic behavior of water-rich strata, this research uses SHAKE program for computing deconvolution function as the base motion of groundwater monitoring wells. Then FLAC was used with the simplified stratigraphic profile and input acceleration history to obtain analysis results for simulating the mechanism between earthquakes and groundwater level changes.
Conceptual model of pore water pressure seismic response is constructed using the finite difference program, FLAC where both Finn and Biot modes of pore water pressure generation were used for analysis. Analysis showed that simulation of Finn mode is better than Biot mode, Biot mode is not recommended to be used in this analysis. There are two types of groundwater level changes, step-like and oscillating changes in response. The simulated water pressure curve from the Finn mode shows reactions of step-like changes, and Biot mode exhibits oscillating changes. In addition, Finn mode triggered a large amount change of the pore water pressure, while in Biot mode the pore pressure change is much smaller.
Comparing numerical results with the observed data, the results of input E-W ground acceleration is better than input N-S acceleration. This is related to the east to west direction of the thrust action of Chelungpu fault. Therefore, the direction of seismic waves is more consistent in east-west direction, so the simulation results with E-W ground acceleration is better.
Generally speaking, in Finn mode simulation results, the simulation of Aquifer 2 is fairly good and is quite consistent with the overall trend; while Aquifer 4 is the worst. This may be because the deeper part of the models closer to the bottom boundary, which constrains the horizontal and vertical directions motion at the bottom. In addition, in Aquifer 1, there are differences between the simulation results in Finn mode and the recorded groundwater level changes. But the variations of simulated results with the epicentral distance, fault distance and peak ground acceleration are quite consistent with the overall trend. Biot mode results in ground water changes in a small range, and only Aquifer 1can obtain good simulation results, when the depth increasing, the simulation of pore pressure change has not changed much.
Subjects
earthquake
groundwater level
dynamic pore pressure
co-seismic water level change
total stress analysis
effective stress analysis
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