Shih Jung Wang

Estimating poromechanical properties using a nonlinear poroelastic model

AbstractA physics-based method is proposed for simultaneously obtaining the hydraulic conductivity, Young’s modulus, and Poisson’s ratio of soil materials using the uniaxial consolidation test. A nonlinear poroelastic model is presented, and the settlement data from consolidation tests are fitted to the model at each load step with the least-squares error method to inverse the parameters. The model results perfectly fit the experimental data in the initial load steps but slightly deviate from the data in later load steps as a result of secondary settlement and a largely increased Young’s modulus. The inversed parameters are compared with those calculated from the uniaxial consolidation test and those found in the literature. The comparison results demonstrate that the inversed parameters are reasonable. The proposed method provides both an estimation of parameters and the parameter-change information during a consolidation test. The method is simple, efficient, and versatile for obtaining poromechanical p...

Evaluation of Hydraulic Properties of Aquitards Using Earthquake-Triggered Groundwater Variation

The hydraulic properties of aquitards are not easily obtained because monitoring wells are usually installed in aquifers for groundwater resources management. Earthquake-induced crust stress (strain) triggers groundwater level variations over a short period of time in a large area. These groundwater anomalies can be used to investigate aquifer systems. This study uses a poroelastic model to fit the postseismic variations of groundwater level triggered by the Chi-Chi earthquake to evaluate the hydraulic properties of aquitards in the Jhoushuei River alluvial fan (JRAF), Taiwan. Six of the adopted eight wells with depths of 70 to 130 m showed good agreement with the recovery theory. The mean hydraulic conductivities (K) of the aquifers for the eight wells are 1.62 × 10-4 to 9.06 × 10-4 m/s, and the thicknesses are 18.8 to 46.1 m. The thicknesses of the aquitards are 11.3 to 42.0 m. Under the isotropic assumption for K, the estimated values of K for the aquitards are 3.0 × 10-8 to 2.1 × 10-6 m/s, corresponding to a silty medium. The results match the values obtained for the geological material of the drilling core and those reported in previous studies. The estimated values were combined with those given in previous studies to determine the distribution of K in the first two aquitards in the JRAF. The distribution patterns of the aquitards reflect the sedimentary environments and fit the geological material. The proposed technique can be used to evaluate the K value of aquitards using inverse methods. The inversion results can be used in hydrogeological analyses, contaminant modeling, and subsidence evaluation.

Combination GM(1,1) and Stochastic Poroelastic Theory to Prediction the Land Subsidence in Yunlin and Jiayi Area, Taiwan

In this study, the grey system GM(1,1) and the geostatistical method are initially used to predict the land subsidence of the Yunlin area in the next five years. The Yunlin County Yuanchang Element School is taken as the site example when the stochastic poroelastic model is used in the study because there is a lack of data related to land subsidence. The verification results and prediction data of the stochastic poroelastic model and the grey system are displayed. The results show that the land subsidence in the next five years as calculated based on the data obtained from the monitoring wells is 0.3 m, while 0.31 m and 0.33 m are the prediction results from the stochastic poroelastic model and grey system model, respectively. This indicates the high precision of both models in predicting land subsidence. In order to simulate the effect of climate change on territorial planning, a prediction is made on the possible land subsidence for 2030, in this study. The prediction results are shows that by 25 January 2030, the stochastic poroelastic model shows a land subsidence of 1.01 m, while it is 1.68 m for the Grey System model. Because only the Changyuan Element School is taken as an observation station for the stochastic poroelastic model, the Grey System model is used to predict land subsidence for 2007 by the geostatistical method. The results show that land subsidence will mainly occur in the mid-western area of Yunlin and the western coastal area of Jianyi. To simulate the possible situations in the year 2030, four scene simulation models are proposed in this research, that is, adding 10% discharge, adding 20% discharge, subtracting 10% discharge, and subtracting 20% discharge. The results show the prediction on land subsidence for the year 2003 when add 10% discharge, subtract 10% discharge, add 20% discharge, and subtract 20% discharge, respectively. Land subsidence will occur mainly in the mid-western area of Yunlin and the western coastal area of Jiayi. The maximum land subsidence could reach up to 150 cm or so.