Grouting is widely used in tunnel construction as a measure to reduce water seepage through rock fractures. Fresh cement-based grout often comes into contact with flowing water after being injected into rock fractures, especially in post-excavation grouting scenarios in rock tunnels or pre-excavation grouting in deep tunnels and remedial grouting in dam foundations. The flowing water can cause erosion of the fresh grout and viscous fingering in the grout, which reduces the efficiency of the grouting. In the present study, experimental tests using a simulated fracture were carried out to investigate grout erosion and viscous fingering in the time period after the injection stops until the grout has gained sufficient strength. The aim of the tests was to evaluate the validity of the existing criteria used to determine grout erosion and viscous fingering. The test results showed significant grout erosion and viscous fingering caused by the flowing water despite these behaviors not being expected according to the existing criteria. The reduction in the grouted area was up to 50% after 10 min and up to 64% after 60 min. Based on these results, the mechanism of grout erosion and viscous fingering between water and grout is discussed with respect to grouting design strategy. The present study provides a deeper understanding of grout erosion and viscous fingering after the grouting is completed, indicating complex mechanisms of these behaviors and oversimplification in the existing criteria. The results are useful for the design of grouting in fractures with flowing water.

Rock grouting is a common measure to reduce the seepage through conductive fractures in the rock mass around tunnels. Two types of grouting are normally carried, pre-excavation grouting and postexcavation grouting. Pre-grouting, commonly applied in Scandinavian tunnels, is used to seal the conductive fractures around the tunnel before the excavation of tunnel sections. In post-excavation grouting, which is dedicated to seal the remaining leakage in the excavated tunnel sections, the injected grout often encounters large seepage in rock fractures. Previous experiments have shown that the grout can be washed out easily when the grout is fresh even though the injected grout has initially sealed the fracture. One of the most significant phenomena for the water to “break up” the grout is viscous fingering. Viscous fingering occurs when certain conditions enable interface instability between the water and the cement-based grout. In this paper, the authors aim to evaluate if viscous fingering can be avoided under pre- and post-grouting conditions. For this purpose, computational fluid dynamics (CFD) simulations using the software Ansys Fluent is carried out. The simulation results demonstrating viscous fingering between water and cement-based grout are analyzed and discussed. Based on the results, suggestions on the grouting strategy with respect to pre- and post-grouting are provided to deal with the potential issues related to viscous fingering.

Grout curtains are commonly constructed under dams to reduce the seepage through the rock foundation. In the design of grout curtains, empirical methods have mainly been used since the introduction of dam foundation grouting. Although empirical methods have been used with success in several projects, they have their limitations, such as poor control of the grout spread, only an indirect consideration of the threat of internal erosion of fracture infillings in the grouted zones, and the risk of hydraulic jacking. This paper presents a theory-based design methodology for grout curtains under dams founded on rock. In the design methodology, the grout curtain is designed as a structural component of the dam. The risk of erosion of fracture infilling material is explicitly accounted for along with the reduction of the hydraulic conductivity of the rock mass, and an optimization of the total uplift force. By applying the proposed design methodology, engineers can create a design better adapted to the prevailing geological and hydrogeological conditions in the rock mass, resulting in more durable grout curtains. The proposed methodology also enables cost and time estimates to be calculated for the grout curtain’s construction. Applying the principles of the observational method during the grouting execution also allows the design to be modified via predefined measures if the initial design is found to be unsuitable.

Grouting has long been implemented as a ground improvement technique to reduce the seepage through the rock mass. Grout curtains are usually constructed under dams as a barrier to prevent leakage from the reservoir. So far, the grout curtains under dams have mainly been designed by using an empirical design approach. However, the empirical approach has its limitations. Generally, the usage of “rules of thumbs” makes the design highly dependent on the experience of the designers. Lack of experience can result in insufficient or over-conservative grout curtains. For example, the stop criteria for the grouting process adopted by the empirical approach can lead to long grouting time and thus becomes inefficient. In addition, high grouting pressure may cause unexpected deformations of the rock and open up new leakage paths.

To deal with these limitations, a theory-based design methodology has been developed. Theories on rock grouting developed in recent decades are used to build up the design methodology. In the theory-based design methodology, the grout curtain is treated as a structural component in the dam foundation. The geometry and location of the grout curtain is first designed with respect to three requirements: (i) the hydraulic conductivity reduction, (ii) prevention of erosion of fracture infillings and (iii) optimization of uplift reduction. Grouting work is then designed to obtain the designed geometry of the grout curtain. In the design of the grouting work, analytical calculations are implemented to determine the grouting pressure, grouting time and grout hole layout. The stop criteria are based on the grouting time, which is believed to obtain better efficiency. The principles of the observational method are implemented to deal with the uncertainties involved in the grouting process. 

One of the main limitations with the proposed methodology is the limited research on the erosion process of fracture infilling materials in flowing water. To study this issue, coupled numerical analyses are performed to better understand the initiation of erosion of fracture infillings. The results show that the Hjulström and Shields diagram are not appropriate to be used to estimate the incipient motion of fracture infilling materials. Instead, a previous equation derived under laminar flows shows better agreement with the results.

Injektering har länge använts som en metod för att förstärka grunden och reducera vattenflödet i en bergmassa. Injekteringsridåer uppförs ofta under dammar som en barriär med syfte att förhindra läckage av magasinets vatten. Hittills har injekteringsridåerna i huvudsak dimensionerats baserat på empiriska metoder. De empiriska metoderna har emellertid sina begränsningar. Användandet av olika tumregler resulterar i att dimensioneringen i hög utsträckning är beroende av ingenjörens erfarenhet. Brist på erfarenhet kan resultera i en ineffektiv, eller en för konservativ, utformning av injekteringsridåerna. Till exempel kan de stoppkriterier som tillämpas i det empiriska tillvägagångssättet leda till en för lång injekteringstid och därmed bli ineffektiv. Utöver detta kan höga injekteringstryck leda till oväntade deformationer i bergmassan och nya läckagevägar. 

För att hantera dessa typer av begränsningar har en teoribaserad dimensioneringsmetodik utvecklats. Teorier för berginjektering som utvecklats under de senaste decennierna används för att bygga upp metodiken. I metodiken utformas injekteringsridån som en strukturell komponent i berggrunden under dammen. Den geometriska utformningen och läget för injekteringsridån bestäms med hänsyn till tre kriterier: (i) erforderlig reduktion av den hydrauliska konduktiviteten, (ii) förhindrande av erosion av sprickfyllnadsmaterial, och (iii) optimering av reduktionen i upptryck under dammen. Injekteringsarbetet utformas därefter i syfte att eftersträva den erforderliga utformningen. I utformningen av injekteringsarbetet används analytiska beräkningar för att bestämma injekteringstryck, injekteringstid och hålavstånd. Stoppkriterier baseras på erforderlig injekteringstid, vilket bedöms uppnå en mer effektiv injektering. Principerna för observationsmetoden används för att hantera de osäkerheter som kvarstår.

En av de huvudsakliga begränsningarna med den föreslagna metodiken är den begränsade kunskapen som idag finns om erosion av sprickfyllnadsmaterial vid flödande vatten. För att studera denna fråga genomfördes kopplade numeriska analyser för att bättre förstå processen kring initiering av erosion av sprickfyllnadsmaterial. Resultaten visar att Hjulströms och Shields diagram inte är lämpliga att använda. Istället visar en tidigare framtagen ekvation för laminära förhållanden en bättre överensstämmelse. 

Fine-grained infilling materials in rock fractures cannot be penetrated by cement-based grout, while high water velocities in the unfilled parts of the fracture can impose erosion of the infilling materials. Understanding the erosion process of the infilling materials is, therefore, essential for the design of grout curtains. In this paper, the incipient motion of infilling particles in a three-dimensional rock fracture is predicted by a coupled Computational Fluid Dynamics (CFD)-Discrete Element Method (DEM) approach. A fracture model is built based on highresolution optical scanning data of a natural rock joint surface. Infilling particles are modelled as non-cohesive spheric particles in a range of fine-sand sizes. The motion of particles is produced by coupling the CFD, solving the Navier-Stokes equations, with the DEM, prescribing the contact forces between particles. The model could capture the particle-particle and particle-fluid interaction behaviours during particle movement. Simulation results of the fracture model are compared with a parallel-plate model, which shows that the fracture geometry significantly affects the transport and distribution of the infillings. The dimensionless critical shear stress of the fracture model for the studied fracture is 11% larger than the values obtained from the parallel-plate model. Furthermore, the simulations are compared with the Hjulstro center dot m and Shields diagrams, showing that the use of these two diagrams to predict the infilling erosion in the fracture results in a significant discrepancy. In contrast, a previous equation derived from flume experiments under laminar flows agrees better with the simulations. The present study visualises and quantitatively analyses the erosion process of the fracture infillings, which provides a reference to predict the threshold of the infilling erosion.

To reduce the water leakage and the uplift pore pressure in the rock foundation, grout curtains are oftenconstructed under dams. However, the design of grout curtains has long been based on empirical knowledge.When designing the grout curtain, focus was mainly given on closing the curtain and limiting the leakage,without considering the thickness of the curtain. However, a too thin grout curtain could lead to high gradients,which with time could result in internal erosion of degraded grout and fracture infilling materials. This couldlead to poor performance and durability problems, potentially jeopardizing the dam safety and increasingthe cost for remedial measures. In this paper, three design criteria are discussed for determining the requiredthickness of the grout curtain, including water loss reduction, uplift pore pressure reduction and internalerosion of degraded grout and fracture infilling materials. Analyses on the required thickness are made fora typical concrete gravity dam. The results show that the required thickness is significantly affected by theinternal erosion of fracture infilling materials. These results show the need for a more refined design approachfor the grout curtains based on the suggested criteria.

Grout curtains are commonly constructed under dams to reduce the seepage through the rock foundation. In the design of grout curtains, empirical methods have mainly been used since the introduction of dam foundation grouting. Although empirical methods have been used with success in several projects, it has its limitations, such as poor control of the grout spread, indirect consideration of the threat of internal erosion of fracture infillings in grouted zones, and the risk of hydraulic jacking. This paper presents a theory-based design methodology for grout curtains under dams founded on rock. In the design methodology, the grout curtain is designed as a structural component of the dam. The risk of erosion of fracture infilling material is explicitly accounted for along with the reduction of the hydraulic conductivity of the rock mass, and an optimization of the total uplift force. By applying the proposed design methodology, engineers can create a design better adapted to the prevailing geological and hydrogeological conditions in the rock mass, resulting in more durable grout curtains. The proposed methodology also enables cost and time estimates to be calculated for the grout curtain’s construction. Applying the principles of observational method during the grouting execution also allows the design to be modified via predefined measures if the initial design is found to be unsuitable.

Fine-grained infilling materials in rock fractures cannot be penetrated by cement-based grout, while high water velocities in the unfilled parts of the fracture can impose erosion of the infilling materials. Understanding the erosion process of the infilling materials is, therefore, essential for the design of grout curtains. In this paper, the incipient motion of infilling particles in a three-dimensional rock fracture is predicted by a coupled Computational Fluid Dynamics (CFD)-Discrete Element Method (DEM) approach. A fracture model is built based on high-resolution optical scanning data of a natural rock joint surface. Infilling particles are modelled as non-cohesive spheric particles in a range of fine-sand sizes. The motion of particles is produced by coupling the CFD, solving the Navier-Stokes equations, with the DEM, prescribing the contact forces between particles. The model could capture the particle-particle and particle-fluid interaction behaviours during particle movement. Simulation results of the fracture model are compared with a parallel-plate model, which shows that the fracture geometry significantly affects the transport and distribution of the infillings. The dimensionless critical shear stress of the fracture model for the studied fracture is 11% larger than the values obtained from the parallel-plate model. Furthermore, the simulations are compared with the Hjulström and Shields diagrams, showing that the use of these two diagrams to predict the infilling erosion in the fracture results in a significant discrepancy. In contrast, a previous equation derived from flume experiments under laminar flows agrees better with the simulations. The present study visualises and quantitatively analyses the erosion process of the fracture infillings, which provides a reference to predict the threshold of the infilling erosion.