Investigations on groundwater dewatering by using vertical circulation wells: Numerical simulation method development and field validation

Abstract

Construction dewatering is a common engineering problem encountered at construction and mining sites. Successful site dewatering requires proper design and implementation of groundwater lowering techniques depending on excavation dimensions, soil types, and local environmental regulations. Among the dewatering techniques, different types of pumping wells are usually the applied methods of choice. The conventional pumping wells require groundwater abstraction from aquifers and, consequently, the discharge. Environmental problems as a result of massive groundwater abstraction and foreseeable disposal costs are the known consequences. In contrast, the implementation of vertical circulation wells (VCWs) is an innovative approach, which enables dewatering without any net discharge. A VCW consists of an abstraction and an injection screen in the upper and lower part of a single borehole, respectively. The successful application of this new dewatering technique requires a sufficient knowledge of the influencing factors on the induced groundwater flow patterns and the water table drawdown. Since the groundwater flow near a VCW is very complex, traditional methods neglecting the vertical flow are not sufficient anymore. Therefore, the systematic investigation of the groundwater flow near a VCW and consequently the achieved drawdown is the main focus of the thesis. The investigation includes the development of a comprehensive simulation method, the identification and evaluation of relevant hydrogeological parameters, and eventually the performance of dewatering tests at a field test site. A novel simulation approach, coupling the arbitrary Lagrangian‐Eulerian (ALE) algorithm and the groundwater flow equation, is presented. The obtained results are compared and verified with several analytical solutions. The developed numerical model is suitable for simulating groundwater flow near VCWs, since it is not restricted in considering the vertical flow component. As a result, especially in unconfined aquifers, the position of the groundwater table can be precisely estimated. After calibrating the model with observations from several field tests, it is applied to assess the sensitivity of relevant parameters on the groundwater flow and the drawdown. The obtained results show that the drawdown is proportional to the flow rate, inversely proportional to the hydraulic conductivity, and almost independent from the aquifer anisotropy in the direct vicinity of a VCW. Further, the position of the abstraction screen has a stronger effect on drawdown than the position of the injection screen. The circulation field and thus the extension of the influenced area depend on the screen separation length, but mainly on the anisotropy. To investigate the effects of aquifer layering properties on groundwater flow, the layer structures were characterized in detail with various field methods including several direct‐push tests, pumping‐, injection‐, and circulation flow tests as well as grain‐size analysis in the lab. The employed field methods in combination with numerical simulations provide an appropriate base, to further investigate the role of the aquifer layer structure on the drawdown. The gained insight from this study provides an important contribution and gives practical implications for the future design and operation of VCW for groundwater lowering in unconfined aquifers. Eventually, the thesis highlights the potential of this new dewatering technique as an alternative to conventional dewatering methods.