Abstract
A three-dimensional, transient groundwater model was developed to determine the rate, volume, and number of pumping wells required to estimate the dewatering of three deep excavations at a large coastal construction site in the Middle East with a shallow groundwater table. There was limited site-specific hydrogeologic information for the site. A calibrated MODFLOW-based groundwater model was developed using the “the model-independent parameter estimation and uncertainty analysis” (PEST) software with pilot points and regularization mathematical techniques. Simulated heads were fitted against the monitoring well heads along extrapolated site groundwater head contours by estimating the hydraulic conductivity at each pilot point. Model-calibrated hydraulic conductivities obtained were within the range of medium to fine sand with silt values and matched closely with the subsurface material descriptions obtained through site geotechnical investigations. Dewatering of the three pits, each with approximate dimensions of 10 by 8 m and a depth of 20 m, was simulated through a series of sensitivity analyses to determine the number of wells, discharge rate, time duration to dewater the pits, and the volume of discharge water per pit to be diverted. Conclusions from the dewatering simulation estimations were as follows: (1) sensitivity analysis showed that the range of dewatering from each pit was dependent on the selected hydraulic conductivity and storage values, (2) storage was most sensitive to achieve the dewatered groundwater elevation depths, and (3) a one order-of-magnitude decrease of storage resulted in a shorter duration to dewater a pit. In summary, model simulations showed that site-specific pumping tests should be performed to optimize the design of a dewatering well system, specifically in low hydraulic conductivity soils where using large capacity wells is not feasible. The use of a numerical transient groundwater model is warranted for dewatering estimations as site-specific conditions are complex.
Key words: Middle East, construction dewatering, groundwater modeling, MODFLOW, PEST.
