Running Limit and Refusal in Rock Fractures Grouting

Absolute refusal is in practice impossible to achieve and is substituted by the minimal flow criterion. Grouting is stopped when the flow rate is less than a predefined quantity known as the flow rate limit. The majority of the grouting models and procedures do not mention how to calculate the flow rate limit if they do not specify empirical values. A parametric relation for the flow rate limit is obtained in this note. It binds the flow rate to the target, effective pressure, fracture opening, mix rheology and borehole. It is a flexible relation from which it is deduced that there is no need to use either the highest injection pressure or a constant pressure to attain the target. The monitored pressure can be used transforming the flow rate limit into a Running Limit. A practical case will illustrate the use of the running limit for a back-analysis of the log data of a grouting stage at La Yesca hydroelectric project. Subsequently, the minimal flow criterion will be inserted in three of the main grouting models RTGC, GIN and ALT, which will require special handling to preserve the specifications of each model.