Retaining wall in groundwater

Positive side or negative side

The positive side is the one water pushes towards: the outer face of the wall, in contact with the soil. Waterproofing there is always the technically better choice, because the system works in compression and water never reaches the concrete.

It is only possible, though, while the excavation is open. Once the structure is finished and backfilled, only the negative-side option remains, from the inside: the system works in tension, must have adhesion to the substrate greater than the pressure it is subjected to, and must be chosen with far more care.

The three families on the positive side

Polymer-modified bituminous thick coatings (PMBC) are the classic solution for ordinary water-load classes: ready-to-use single-component, or reactive two-component versions where resistance to pressurised water and radon-tightness are required.

Mineral cementitious slurries, rigid or flexible, work by adhering to the substrate and remain the only family compatible with damp substrates without a primer and with a subsequent cementitious layer applied on top.

Bentonite geocomposites are active sheets: the sodium bentonite enclosed between two geotextiles hydrates on contact with water, swells in a confined way and seals. They are the typical choice for base slabs, diaphragm walls and structures in permanent groundwater, where the mechanical adhesion of the subsequent concrete pour to the geocomposite makes the difference.

The choice starts from the water load

The DIN 18533 standard classifies water loads on below-grade structures, from soil moisture to pressurised water, and each class corresponds to a family of permitted systems. This is the design criterion: first the class is established, then the product is chosen — never the other way round.