ABSTRACT

The Minimal Flow Criterion is a potential substitute to the RTGC stop criterion. It signals the target overrun independently of the delays that are either generated by uncertainties or deviations from the grouting conditions. In this note, MFC is inserted into RTGC, preserving its main features. The insertion key is the flow rate limit that is related to the stage target beyond which the injection is stopped. Indications are given on how to calculate the flow rate limit when the distribution of the fractures does not correspond to the hypothesis of RTGC. Indications are also given for the injection of the residual fractures with a Newtonian mix. The Minimal Flow Criterion is generally referred to as Refusal that has always been used in the sealing of rock fractures.

INTRODUCTION

Grouting to refusal is a controversial subject although it has been used since the early days of rock fracture grouting. Its detractors allege that there is no rock mass refusal and that it is a matter of injection pressure. Other mentions the impossibility to achieve absolute refusal. They are right and this is proved scientifically. On the other hand, the injection pressure and the flow rate limit that are the basic ingredients of grouting to refusal are hot matters. The refusal pressure is a controversial subject and the flow rate limit is never clearly specified, if not empirically. But it should be recognized that there is something that is fundamentally correct in grouting to refusal because it is still used. It is even used by its detractors. For example, the Norwegian guideline (NTS, 2011) ignores it, but it was used at the Follo Line project in Oslo. It is also ignored in Sweden, but it was used with its complementary criterion at the City Line project in Stockholm (Brynjolfsson, 2014; Holmberg et al., 2015). The complementary criterion is a visual criterion based on tightness and water ingress. The minimal flow criterion is the modern version of grouting to refusal. Grouting is stopped when the flow rate is less than a predefined flow rate limit at a given injection pressure during a predefined time period. It remains to specify on a scientific base the relation between the flow rate limit and the stage target. It will be shown how it can be done within the frame of RTGC the acronym of Real Time Grouting Control model.

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