Groundwater and drawdown in a large earth excavation

Abstract

In deep excavations below the water table, the transient and steady state drawdown conditions are often of interest. Particularly in moderately pervious soils, the stability of the slopes, the excavation methods, and equipment are affected by the groundwater response in the excavation. The extent of the drawdown in the area beyond the excavation must also be determined.For a site under construction by Ontario Hydro, a test excavation to determine the drawdown response in the irregularly stratified soils is described. Analyses of the data by the finite element method and by the theory of aquifer tests are presented. The effect of recharge on the steady state and transient drawdown is discussed.For the main excavation, approximate methods to determine the rate of drawdown, the quantity of seepage, and the steady state drawdown position with recharge from infiltration or deep seepage are outlined.