| 英文計畫摘要 | 1、Perface
To clarify the mechanism of ground subsidence in the Yunlin area. This project involves geologic drilling, laboratory testing, and numerical analysis to investigate the compression mechanism and compressibility parameters. Field data revealed that compression in aquifer layers is the primary source of ground subsidence. In addition, compression of saturated granular soils due to fluctuations in pore pressure is essential in ground subsidence behavior. Based on the K0-compression testing results, the proposed procedure for the evaluation of subsidence is helping to evaluate the relationship between groundwater pressure fluctuation and compaction in aquifer layers. The shakedown parameters a, b, and Nstable values are estimated to describe the induced vertical strain by groundwater fluctuations and the stabilization time in each aquifer in the test area. By incorporating the hydraulic computations, these values could benefit a plan of regional groundwater usage.
2、400m stratum drilling and HQ sampling
In the 400-meter drilling sampling part, the project completed a total of 400-meter HQ full core sampling, 31 thin-tube sampling and 100 sets of soil physical tests to establish a core histogram in this area. From the composition characteristics and sequence of the accumulation layer, it can be subdivided into 63 layers from the surface to 400 meters underground. It is an accumulation layer dominated by sand and clay, mainly including sand (66%), silt and mud and clay layer (31.8%), a few are composed of gravel layer (2.3%).
The result of core identification shows that the depth of GL.-332~-371.1 meters is sand. Therefore, 1.5 inches of 375 meters deep PVC pipe is placed in the drill borehole, and the screen is opened at GL.-351~-363 meters., The upper and lower water blocking layers are all sealed with bentonite. An electronic piezometer is installed in the pipe, groundwater measuring equipment is installed, and an automatic transmission system is established to measure the groundwater level below 300 meters.
3、Mechanism of ground subsidence
(1) Behavior of subsidence in the Yunlin area
Significant ground sunsidence in cetral-western Taiwan has drawn attendtions for decays. To facilitate the ground water and subsidence analyses, Central Geologic Survey (CGS) (1999) proposed the hydrogeological framework of the Choushui alluvial fan. The alluvial fan is divided into three aquitard layers and four aquifers layers. Hung et al. (2012) summarized the field monitoring results during the period of 1997 to 2010 and concluded that the contribution of aquifer compression is about 83% of the total subsidence and the aquifer compressions are closely related to the long-term groundwater pressure fluctuations. Therefore, it is necessary to clarify the compression mechanism of the aquifer soil. The consolidation behaviors are mainly for low permeability soils such as silt and clay and the Terzaghi's 1D consolidation theory can be applicable. For granular materials, the compression is the consequence of shakedown response caused by repeated effective stress variations from pore water pressure fluctuations. The shakedown results can be used to quantativaly predict the future subsidence as well as control the subsidence rate.
(2) Laboratory apparatus and testing methods
This project adopts the subsidence simulator proposed by Chang and Chou (2019). The simulator adoptes the Rowe’s cell concept to maintain the K0 condition and controllable hydraulic and mechanical boundaries. The simulator is used to mimic the stress conditions of long-term groundwater fluctuations on the soil element ,to investigate the compression mechanism and prediction parameters.
(3) Testing results
Labotaory tests were conducted to verified the aquifer compression induced by field groundwater pressure histories. The results of the laboratory compression tests are consistent with the field magnetic ring data of layered compression. Under the applied pore water pressure fluctuations, the aquifer material exhibits the shakedown compression behavior as mentioned by Johnson (1986). The compressibility increases with the increase of the groundwater fluctuation amplitudes. To complete the subsidence analyses, the magnitude of consolidation for fien-grained soils based on the 1D consolidation theory and the contributuions of compressibility of coarse-grained soils from the shakedown theory shall be included. In this year, laboratory test program including 14 sets of constant-head permeability tests in the field stress conditions, 10 sets of constant rate of strain (CRS) 1D consolidation tests, 10 sets of shear wave velocity measurements, and 13 sets of K0 repeated and periodic compression tests were performed. The test results provide the required parameters for detailed evaluations of subsidence compaction and hydrological analysis.
(4) Evaluation of compressibility parameters of aquifer layers
The in-situ void ratio of the sandy soil in the aquifer layer is one of the the significant factors for the compressibility evaluation. Other factors arethe amplitude of the groundwater fluctuation and the number of cycles. The compressibility parameters of aquifer soil are estimated by periodic K0 repeated compression teststo determine, the values of the shakedown model parameters under different void ratios (e) and groundwater fluctuation amplitudes (Δu). Specifally, the shakedown parameters a, b, and Nstable values are estimated to describe the induced vertical strain by groundwater fluctuations and the time of stabilization in each aquifer in the test area.
(5) Proposed procedure for evaluation of subsidence in aquifer layers
Based on the shakedown concept in aquifer layers, a procedureis proposed to predict the induced subsidence due to assumed future groundwater fluctuations. Take the aquifer three in the test area as an example, the maximum subsidence and stable time in this layer can be computed. Furthermore, the available amount of groundwater can be evaluated based on the acceptable subsidence criteria in this area by incoperating the hydraulic computations. These values could benefit for plan of regional groundwater usage.
(6) One-dimensional viscoelastic-plastic numerical
One-dimensional viscoelastic-plastic numerical model were built, and at the same time, the required soil layering data is calculated using the latest 400-meter drilling sampling results of the project. The data of groundwater level and stratum subsidence are verified by calibration of parameters. The results show that considering the visco-elastoplastic deformation mechanism of the soil layer, it is possible to more accurately simulate the change of the ground subsidence. Later, it can be matched with the groundwater level data to predict the change of the land subsidence, grasp the settlement of the subsidence problem, and provide warning messages.
4、Installation of new monitoring equipment
This project will cooperate with the research of the Ministry of Science and Technology to implement the installation of new monitoring equipment under the conditions of not affecting the construction period and risks of the drilling project and the installation of groundwater level measuring equipment. However, the comprehensive evaluation results show that the characteristics of the 400-meter drilling well diameter and the embedded water pressure gauge of Xiutan Elementary School are not suitable for the installation of other related monitoring instruments. After further discussions with the Ministry of Science and Technology team, this year's monitoring wells can regularly cooperate with them to conduct groundwater temperature measurement operations. Our team cooperated with the team of the Ministry of Science and Technology to conduct groundwater temperature measurement operations on September 15, September 29, and November 12, respectively. |