Fracture Grouting in Theory: Modelling of Fracture Grouting in sand

Abstract

The construction of a new metro line in the city centre of Amsterdam, the so-called Noord/Zuidlijn, is a challenging job as many buildings along the route are sensitive to settlement and angular distortion. To prevent damage, settlement criteria are very strict and mitigating measures are necessary at several locations to meet the criteria. One of the measures that will be taken is the use of Fracture Grouting. A full-scale Compensation Grouting Trial showed that Fracture Grouting in sand underneath pile foundations can be used to control the vertical movement of wooden pile foundations. Nevertheless, the grouting efficiency fluctuated considerably during the trial. It can be concluded that hydraulic fracture initiation and fracture propagation in sand are theoretically not well understood. Consequently, the hydraulic fracture dimensions in sand are not actively controlled. For this reason the grouting efficiency of Fracture Grouting in sand is suboptimal. Objective of this study was to find methods that can be used in order to actively control fracture growth. To enlarge the understanding of hydraulic fracture initiation and propagation, a literature study has been performed. The results were used to study fracture initiation and to develop an analytical model of fracture propagation in sand. The fracture propagation model has consequently been used to study the effect of changing input parameters, like variation of the grout properties. Additionally, several laboratory experiments were done to find realistic values for several input parameters of the model. Conclusion Fracture Grouting in Theory It can be concluded that three constituent processes affect the fracture process: 1. The friction of propagating grout (related to rheological parameters and resulting shear stress) 2. Dehydration of injected grout (related to permeability) and accompanying development of a dehydrated layer at the outer ring of the fracture. 3. The response of surrounding soil Considering these processes it seems possible to influence the output of the fracture propagation process in order to increase grouting efficiency. Grouting efficiency is increased whenever injected grout can be retained under the foundation of a building. It is favourable to limit the fracture length and to maximise the fracture thickness. There are two theoretical methods to influence the output of the fracture process and to increase grouting efficiency: Reducing the Water Solids Factor (WSF) or increasing the amount of angular aggregate material in the fracture grout, increases the shear stress. This results in decreasing fracture length and increasing thickness. By changing the amount of bentonite in the fracture grout, the moment of propagation stagnation can be affected. The grouting efficiency can be affected indirectly when taking the foundation configuration into account. Stimulating propagation stagnation reduces the fracture length. Injected grout will be concentrated under the foundation piles. Consequently, more of the injected grout contributes to the objective of fracture grouting: creation of heave of a building. In other words, the amount of bentonite can theoretically be used to tune the fracture length to the foundation configuration. Results of this research are used to continue the research on Fracture Grouting in sand. The TU Delft and COB (Netherlands Center for Underground Construction) are preparing several laboratorial experiments in which it will be tried to actively control the hydraulic fracture dimensions in sand.