AbstractsEarth & Environmental Science

3D subsurface modelling and geotechnical risk analysis for the tunnel construction of the A2-project Maastricht:

by P.E.D.M. Kouwenberg

Institution: Delft University of Technology
Year: 2009
Keywords: limestone; Maastricht; tunnel
Record ID: 1250879
Full text PDF: http://resolver.tudelft.nl/uuid:615dfe85-b3a5-441a-9523-d83d56df41a6


To reduce traffic problems and to improve the social climate in Maastricht a tunnel will be constructed for the A2 motorway. The tunnel will be construction with the cut-and-cover method. Before the constructions starts first a ground improvement campaign is performed. Afterwards sheet pile walls will be installed in cemented slurry trenches and the building pit will be excavated dry wherever possible. The subsurface in Maastricht consist of a top soil of sand and clay and a thick gravel layer on top of limestone. The water level in Maastricht is generally high. Goal of this report is to determine the geotechnical and geological risks related to the tunnel construction. The Formation of Maastricht consists of weak porous limestone with flint and hardground layers. The limestone can be affected by dissolution, weathering by groundwater flow, erosion due to the Meuse River or be faulted and reduced to carbonate sand. Problems with excavation, leakage of slurry, lack of bearing capacity and passive resistance and uplift of the building pit floor or high groundwater inflow are expected. In this report the extensive preliminary site and laboratory investigation is described. This investigation had the goal to assess geohazards and to identify the most promising techniques for further investigation phases. Various databases were exploited to map the fluctuations of the gravel-calcarenite interface. Characteristic horizons made it possible to make a member division for the Formation of Maastricht. Vertical displacement of members between adjacent boreholes revealed the presence of a fault. The cores did not only contain rocks but also carbonate sands. Pocket penetrometer test performed on cores at close spacing allowed an objective mapping of the spatial distribution of carbonate sands, very weak and weak calcarenite along the tunnel alignment. These results have been correlated to the UCS results. Beside that a correlation between carbonate sand and fault zones has been detected. At the fault location, analyses of groundwater chemistry confirmed the connection between deep and shallow limestone layers. Many geophysical test methods were performed in a pilot zone, above the detected fault. The seismic reflection conducted with a low energy airsound source was found to be useful to image the fault. It revealed the presence of a complex fault zone rather than a single fault. The north east orientation of the fault zone was interpreted based on regional fault trends and correlated to fractures measured in boreholes with the help of a borehole camera. Estimating in situ permeability and deformability remained difficult. Primary permeability was found to vary over 3 orders of magnitude, up to the in situ permeability measured with the Lugeon test. In the fault zone a higher permeability is expected. For each member the distribution of material properties is studied and the flint content per member is estimated based on the borehole logs. Of the stratigraphy, lithofacies and some material properties a 3D model has been…