ABSTRACT
This paper presents an approach to grouting in a highly water bearing ground, by means of a hybrid grout. A cement suspension is mixed with a variable ratio of polyurethane and depending on the pressure development during grouting, the percentage of cement suspension and polyurethane are correspondingly adjusted. Both the selection of the cement type, of the ratio water/cement and of the polyurethane system potentially allow for a wide range of application. The method was first applied in 2019, during excavation of the new headrace tunnel at Ritom HPP, in Switzerland. An unexpected strong water inflow in a highly fractured rock mass called for the selection of a suitable grouting technique to seal the rock without being washed out. Hybrid grouting proved to be very efficient in stopping the water inflow, and economical. Following the application at Ritom HPP, a laboratory test campaign was conducted to study the short- and long-term properties of the hybrid grout. The effect of adding polyurethane is to significantly reduce the setting time. It also affects some of the mechanical properties, e.g. compressive strength and Young’s modulus. Additional laboratory testing aimed to optimize the grout design with respect to specific applications, and large-scale projects will help to refine and further improve the efficiency of the hybrid approach.
