Hydrogeological Methods in Geotechnical Engineering: Applied to settlements caused by underground construction
Improvement in the infrastructure in urban areas generally involves underground construction with tunneling and deep excavations. One important consideration when constructing in cities founded on soft soil is settlement resulting from groundwater drawdown. The potential cost of excessive settlement in these projects is immense. Despite the strict requirements regarding inflow rates into tunnels and maintained groundwater levels around excavations, groundwater pressures will from time to time be altered as a result of construction activity.
The overall objectives of this thesis are to assess and summarize some hydrogeological issues important in geotechnical engineering, to determine the key parameters and assumptions influencing the problem and to evaluate the reliability of the analyses. The importance of different parameters is studied by performing sensitivity analyses using a closed form solution for estimating the consequences of tunnel inflow, thus enabling prediction of groundwater drawdown as well as settlement. From this analysis it is concluded that the transmissivity of the lower aquifer, T, is the most important soil parameter for predicting the drawdown and settlement response. Naturally, the preconsolidation pressure of the clay, σ´c, is crucial to making reliable estimates of the resulting settlement.
The evaluation of the transmissivity from pumping tests is studied in order to assess the effect of all the assumptions associated with the analysis. It is concluded that a well-planned and conducted test can provide a reasonable estimate of the effective transmissivity of the aquifer and information about the boundaries of the aquifer.
Two case studies, a bedrock tunnel and a deep excavation in soft clay, are reviewed to assess the potential and limitations of different groundwater models. Analytical models generally give an upper limit of the expected drawdown, following from the assumptions regarding the aquifer conditions. Using numerical tools, some of the crude assumptions in the analytical models can be overcome, although the uncertainty in the assigned boundary conditions, inflow rate and T influence the predictions considerably. A monitoring and mitigation program is therefore necessary in underground construction projects to minimize the risk of excessive settlement. Finally, it is concluded that the ratio of the inflow to the excavation or tunnel and the transmissivity, Q/T is necessary to predict the resulting drawdown from a leakage situation. Consequently, σ´c and T need to be determined to set the requirements for inflow rates. The area influenced by the leakage is difficult to estimate and strongly dependent on the recharge to the aquifer.