Agreat part of the Channel Tunnel Rail Link (CTRL) is constructed through the Chalk and the ground investigation for the CTRL has yielded a mass of new information on allaspects of the Chalk. A precise bed by bed lithostratigraphy obtained from cored boreholes has aided engineering description, classification, decision making on design of machines and construction methods/specifications. Correlation of Chalk marker beds between boreholes drilled for the tunnels beneath London illustrated the influence of sub-Tertiary erosion and of faulting onpreservation of different stratigraphic levels in the Chalk. These different stratigraphic levels affect materials through which the tunnels will be constructed andconsequently tunnel vertical alignment or choice of construction method. The marker bed stratigraphy in the Thames Tunnel boreholes has enabled the same stratigraphic levels to be identified in local quarries and detailed analyses to be carried out for the design of Tunnel Boring Machines. Using individual marker beds, a detailed ground profile was constructed for the North Downs Tunnel which allowed fault zones to be predicted accurately and the different rock mass character of the Chalk formations to be delimited for numerical modelling, design zones and construction monitoring. The project also provided the opportunity to evaluate the CIRIA Chalk Grading scheme.