During construction of a tunnel in TriassicMercia Mudstone near Leicester a full face tunnel boring machine did not behave asexpected on the basis of the pre-construction ground investigation. When the machine was operated in open mode the spoil often reduced to a slurry as anticipated. However, when it was operated in closed mode the spoil changed to a plastic clay which clogged the machine. Further investigations were carried out after the tunnel had been constructed. Mineralogical analyses revealed that the Mercia Mudstone formation through which the tunnel was driven contained highly plastic swelling smectite clays which had not been identified in the pre-construction ground investigations. Samples from the tunnel and from elsewhere in the Mercia Mudstone Group were examined in special laboratory tests designed to replicate the conditions during construction. These tests demonstrated that under the influence of the tunnel boring machine in closed mode the spoil could have disaggregated releasing swelling smectite-rich clay. The results of these tests provided explanations for the behaviour of the tunnel boring machine.