The A2 tunnel in Maastricht has been built within a dry building pit with a maximum depth of 22 meters with sheet pile walls, suspended in a cement bentonite trench supported by struts at 2 to 4 levels. Dewatering is done by deepwells. The excavation reaches into Limestone layers so the maximum mobilized passive resistance of the sheet pile wall is depending on the strength parameters of these layers. In the design stage of the project uncertainties occurred about both strength parameters and permeability of the Limestone for a large part of the project. These uncertainties could only partly be reduced by additional soil investigation and in-situ tests, but they were vital for the design of the retaining walls. To combine economical implementation of the project with a very favourable risk profile the “Observational Method” was adapted. This article deals with the background behind the variations in soil conditions and the residual uncertainties. The elaboration of the Observational Method is explained, including the measurement results and the decisional system during the construction. Finally the results of the adaption of this method in the project will be evaluated. In May 2014 the building pit was fully excavated, so final conclusions are drawn.