| Abstract | This dissertation is a comprehensive study, dealing with development of a computer-aided database for subsurface characterization, groundwater modeling, analysis of land subsidence and its effect to infrastructure development of Ho Chi Minh city, the biggest commercial and economic center in Vietnam, under the strong urban development and climate change conditions. The dissertation is structured in five chapters as seen in the following table of contents. The major works and outputs of the dissertation include: i) Construction of a computer-aided subsurface database for HCM city; ii) Application of new subsurface database in groundwater modeling and analysis of land subsidence and its effects on HCM city’s infrastructure. The detailed research works and outputs are summarized below. Construction of a computer-aided subsurface database for HCM city: More than three thousands hydrogeological and geotechnical boreholes were collected to construct a computer-aided subsurface database using Rockworks16 platform. As a result, 2D and 3D hydrogeological and geotechnical models can be generated in any direction over the city area, and soil parameters at a location can be automatically retrieved for geotechnical analyses. Subsurface database can be linked to and displayed on Google Earth so that one can easily access and check the information of subsoil profile either by computer or smart phone. Hydrogeological characterization of a six-aquifer system up to the depth of 290 m and geotechnical characterization of a twelve-soil layer profile up to the depth of 140 m could be done. The multi-aquifer system underlying HCM city consists of six aquifers and six aquitards. Contour maps of aquifer top and bottom elevations and those of the aquifer thickness were automatically generated and plotted. The uppermost layer of this aquifer system is a thick soft clay layer, whose compression due to groundwater extraction can be the main cause of subsidence. Application of the new subsurface database in groundwater modeling: Groundwater model was constructed and run using Visual Modflow to simulate groundwater drawdowns for 10 years from 1999 to 2009 due to groundwater exploitation from the Second, Third, and Fourth Aquifers of HCM city with help of the newly constructed subsurface database, groundwater observation and pumping test data. Notably was the automatic generation of the geometric model of the aquifer system and exporting it to Visual Modflow for construction of the groundwater model as this could save a lot of works, not only for this study but also other groundwater analyses in future for HCM City and the Mekong delta area. In this study, an area of 168,00 km2 was selected for groundwater modeling covering the Dong Nai river basin and the lower part of Mekong delta basin. The model grid was separated into two meshes, the coarser mesh for the outer area (5.0 x 5.0, 10 x 10, and 20 x 20 km) and the finer mesh (2.0 x 2.0 km) for the center area of HCM city, where most of groundwater extraction boreholes are located. The geometric model of the analyzed aquifer system was constructed based on the top and bottom elevations of each layer, which can be exported from the constructed subsurface database in Rockworks to the groundwater flow model in Visual Modflow. More than 700 pumping wells with a total rate of 350,000 m3/d were input for simulation of groundwater drawdowns. Input hydraulic parameters from pumping test data were checked and calibrated by comparing the computed drawdowns with the observed ones from 1999 to 2010. As results, groundwater depression cones for ten years were found, for all three pumped aquifers, to be located right at the center of the city, where the population is densest, with the maximum drawdowns of 20 m, 30 m, and 28 m in the Second, Third, and Fourth aquifer, respectively. The computed drawdown contour maps could be used as the boundary condition for further land subsidence analyses. Analysis of land subsidence and its effects on HCM city’s infrastructure development: the land subsidence predictions were made using an 1D consolidation FEM analysis code. Computed drawdowns in the pumped aquifers, geotechnical properties of the clay layers, retrieved from the subsurface database, were used as the input data for land subsidence analyses, which were conducted for a few selected locations that have shown signs of well protrusion and also for a number of points picked from a PSINSAR-based subsidence map by a previous study. In particular, prediction of land subsidence was conducted for the first Metro line (MRT line No. 1) in HCM city. A location at Binh Tan district (BTA-72) with drawdown of 7.5 m in the first aquifer underlying the uppermost soft clay of 24-m thick was selected for the first analysis that gave a subsidence of 0.44 m with a rate of 4.4 cm/yr for the first 20 years since the drawdown occurred and a subsidence of 0.98 m with an average rate of 2.45 cm/y for a 40year period. The second analysis was made to predict subsidence along a 19.6 km long MRT line with drawndown varying from 1.0 to 5.5 m and the thickness of very soft to soft clay layer varying from several meters to more than 30 meters. The results showed that a settlement up to 0.8 m could occur at some places, especially at the junction between underground and elevated train segments, where the soft clay deposit is thick. Comparisons of results obtained by 1D FEM consolidation and PS-InSAR analyses were carried out. The estimated for a location in District 8 (D8-46) and the land subsidence rates for the period from 2006 to 2010 along a selected SW-NE cross-section showed a good match between two methods. A map of estimated subsidence rate in the central area of the city from 2006 to 2010 showed similar distribution and magnitude for both methods. To help assessing the effect of land subsidence on the city infrastructure development, land subsidence was predicted up to 2020, 2040 and 2100 for the central area of the city. By consolidation due to groundwater extraction alone the maximum subsidence can be 63.8 cm, 85.2 cm, and 97.6 cm for 2020, 2040 and 2100, respectively. If taking into account of the effect of sea level rise as outlined by the climate change analysis scenarios, estimated about 12 cm, 23 cm and 75 cm corresponding to 2020, 2040 and 2100, the total sinking in the central area of HCM City will be 75.8 cm, 108.2 cm, and 172.7 cm, respectively. The results of this study showed that the subsoil in HCM city with thick soft clay layers existent in the aquifer system is vulnerable to the groundwater extraction. The subsoil database developed by this study in combination with a 1D FEM consolidation program could provide a useful tool to estimate land subsidence of HCM City when a conventional land subsidence monitoring network has not yet been fully operated and the pore pressure, settlement data are not yet available. The analysis results indicated that the city infrastructure development plans need to take into account both effects of subsidence and climate change, in particular the sea level rise, in its fast growing infrastructure development in the coming years. |