The Impact of Fracture Orientation on the Choice of Grout Fan Geometry - a Statistical Analysis

University essay from KTH/Jord- och bergmekanik

Abstract: Water ingress into rock tunnels is a problematic phenomenon – especially in urban areas – as a lowered groundwater table may cause harmful settlements. Furthermore, too much ingress can be an incentive for the environmental court to halt the tunnel process, in order to protect the nature as part of a national interest.Water ingress is normally lowered by injecting a water and cement mixture into boreholes in the rock mass – a process called rock grouting – thus sealing the rock fractures. Very little information and research has been on the subject of how the rock fracture orientation interact with the orientation and geometry of the grouting holes. The purpose of this thesis is to investigate whether or not it is possible and feasible to select a grout fan geometry that will have the most intersections with the rock fractures, based on fracture information gained in an early pre-investigation stage. The suitability of different grout fan geometries will be determined by analyzing the amount of fracture intersections that each geometry has in a discrete fracture network, generated based on data obtained from rock cores in the Stockholm Bypass project. The assumption is that more fracture intersections means a higher chance of sealing the rock mass. The results show that there is no clear difference in number of intersections between the analyzed grout fan geometries, indicating that focus should not be on analyzing the grout fans as whole units, but rather on the scale of individual grouting holes and fractures. This thesis also highlights the importance of monitoring according to the observational method.  

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