A case history in Barcelona is described where a tunnel was excavated by traditional methods below an active railway line through a formation with lenses of water-bearing granular material. To avoid the possibility of sudden collapses a massive jet-grout treatment was applied. The treatment took several forms. Subvertical double and triple-fluid injection was applied whenever possible. Sub-horizontal monofluid canopies and slabs executed from within the tunnel were however required in zones where no vertical access was possible. This communication focuses on the later type of treatments and gives an overview of the design tools that were applied. These comprised the execution of several large trial fields and the systematic application of a probabilistic framework for design.