Building the tunnel would have been complex, boosting the cost of the work. The geology of the area features poor quality rock that is heavily fragmented as well as a high water table. The tunnel would have needed extensive support measures as well as waterproofing to keep it strong and dry. Meanwhile, complex water control and separation methods would have been required to ensure that the water table would have been undisturbed and that no contaminants would reach the local water courses.
This time it seems as if the project has been cancelled largely on economic grounds (once again). The new Labour Government has said that there are insufficient funds for the project to go ahead, with other national priorities such as health and education to be addressed first given the disastrous financial mismanagement of the previous administration.