In design of a cut-off wall at the transition from bed rock to a softer stratum the wall gains high flexural deformations k, which will automatically lead to high strains, and respective stresses. To sustain the function as barrier, the wall must retain its low permeability which it has been designed for. The necessary high deformation capacity of a cementitious material like a cement–bentonite slurry, plastic concrete or a soil–cement mix can only be achieved with a low strength of the filling material. But, with the conventional design concept using an elastic–plastic stress–strain behaviour for the filling material, in a worst-case scenario, will automatically lead to stresses exceeding the nominal strength of that low-strength material. The way out is the allowance for ‘plastic deformations’. Although it’s obvious by its term that ‘plastic concrete’ is made for large deformations, it needs acknowledged codes of practice to prove the feasibility of the design of a cut-off wall, and to prove the suitability of the specified filling material. The German Association for Water, Wastewater and Waste (German abbreviation: ‘DWA’) is working on and soon publishing a new Guideline on ‘highly deformable cut-off wall filling materials’. This shall give relevant recommendations on the design of cut-off walls with such materials, highlight the need for a minimum strength for erosion resistance and in particular present a design approach. The key is the non-elastic part of the stress–strain curve, beyond the elastic strain, which applies to a ‘reduced deformation modulus’ E* which can be used for a simplified, but more realistic design. Considerations are made on why and how it is made sure (safe) that actual strains remain in a non-critical stage without risking wide cracks respectively an irreversible deterioration of the material that would end with an uncontrolled loss of its basic sealing function. This paper presents the scope and content of the Guide which is in near-end progress including design formulae and typical mix designs governing respective mechanical properties. In the final section, the Guide manifests potential for sustainable use of resources by limitation of cement consumptions.