Development of an algorithm to detect hydraulic jacking in high pressure rock mass grouting and introduction of the PF index

Abstract

Pre-excavation rock mass grouting is a common procedure for reducing water ingress into tunnels during construction in Norwegian tunnelling. The level of grouting pressure is a disputed subject and the knowledge of how the rock mass responds to the high pressure and how this inflicts on the grouting results is sparse and little investigated. For this reason, it is of interest to use data from high pressure grouting performed in Norwegian projects to investigate these matters. This paper presents the development of a method for identifying hydraulic jacking during rock mass grouting and the making of an algorithm to perform computerized detection of hydraulic jacking in grouting logs. The algorithm is the base for a larger study, where screening for hydraulic jacking is performed in over hundred grouting rounds, distributed on several Norwegian tunnels excavated in rock mass. The relation between grout flow and grouting pressure has shown to be vital for the understanding of the grouting progress and events occurring during the grouting. Interpretation of pressure and flow as two separate variables, which are affected by aliasing, caused by low and irregular sampling frequency is a challenging task, and it was found to be helpful to create a parameter to represent this relationship, named the PF index (Pressure Flow index). This parameter has also shown to be useful in other applications such as monitoring the grouting progress on site.