To install a pipeline underneath an interstate, a horizontal directional drill was used. However, the drilling discovered that the area consisted of large voids / karst zones. The pipeline itself needs to have a solid foundation to rest on. To fill the voids, polyurethane grout was injected into the problem areas. The grout used was Hyperflex, manufactured by SealGuard, Inc. Hyperflex reacts with the moisture in the ground which creates an expanding foam to provide a solid base for the pipeline. Hyperflex is NSF/ANSI/CAN 61 for contact with potable water. The grouting was done by Sub-Technical, Inc.