D 7 Quality Assurance Observation documents the installation of the induction plates, which will fasten the Below Grade Waterproofing system to the tunnel wall. The specified Hydro-Ultra Mat B series product is a polypropylene mat filled with sodium bentonite granules. Sodium bentonite is a natural clay material that expands when wet. Once the Hydro-Ultra Mat B is hydrated by water, it will swell to form an impervious waterproofing membrane.