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 in the dam foundation to reduce the water seepage through the foundation. Specifically for concrete dams, the grout curtain together with the drainage system reduce the pore pressure in the foundation and the uplift pressure acting on the dam body, thereby reducing the risk of sliding failure. Previously, an analytical method was developed to estimate the reduction of uplift pressure from grout curtains under concrete dams. In this paper, a case study was carried out with the aim of presenting a systematic review on this analytical method. The case study focuses on a Swedish concrete dam which is under reconstruction after more than 100 yearsin service. As part of the reconstruction, a new grout curtain was constructed to reduce the seepage inthe foundation and reduce the uplift pressure on the dam body to enhance the dam safety. This case study provides an opportunity to validate the analytical method. In the case study, the results from geological and hydrogeological investigations, as well as the initial grouting design and grouting procedure during the construction was summarized. Based on this input, calculations were performed using the new analytical method to estimate the reduction of uplift after grouting. As a part of the dam’s monitoring system, and a validation measure of the calculation, four piezometers were placed in the rock foundation after the grouting was completed, two on the upstream side of the grout curtainand two on the downstream side. The readings of all the piezometers were recorded to show the actual reduction on the uplift pressure. Results from the calculations and the piezometer readings were compared and discussed. As an additional comparison, numerical analyses on the studied dam using software COMSOL Multiphysics® were performed. The numerical analyses took the influence from the drainage system on the pore pressure into consideration, which could isolate the effect from the drainage system from the grout curtain. The comparison of the grout curtain and the drainage system with respect to the uplift pressure reduction is further evaluated and discussed. Finally, the authors provide some practical suggestions on future dam foundation design with respect to uplift pressure reduction.

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.

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.

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.

Remedial grouting under existing dams founded on rock is often conducted under unfavorable hydraulic conditions with high gradients and fast-flowing water in the rock fractures. The fast-flowing water may push the grout downstream, change the shape of the grout spread, and erode the fresh grout. All these issues can jeopardize the outcome of the grouting work. To deal with the fast-flowing water, drilling pressure relief holes upstream of the grout holes can be one of the alternatives. These pressure relief holes function as additional drainage. They can reduce the velocity of the water in the fractures near the grout holes, thus reducing the risk of erosion of the grout. However, the usage of pressure relief holes in remedial grouting practices has not been widely documented, and different opinions still exist on their effectiveness. Meanwhile, few studies have analyzed the effectiveness of the relief holes. In this paper, the usage of pressure relief holes in remedial grouting of rock foundations under existing dams is analyzed. The effectiveness of the relief holes is evaluated using both analytical calculations and numerical simulations. The analysis results show that the pressure relief holes can reduce the downstream hydraulic gradient and facilitate the remedial grouting. Based on these results, discussions are made on their effectiveness together with suggestions on how to deal with excessive groundwater flow in remedial grouting.