Research interest in reliability and sensitivity analyses of concrete dams has grown in recent decades, yet existing studies remain largely limited to gravity and arch dams and to case studies focused on earthquake events. To address this gap, this study aims to evaluate the reliability and sensitivity of concrete buttress dam monoliths using a simulated population based on existing Swedish designs with varying geometric properties such as height and width. Three failure modes, sliding, overturning, and combined sliding and overturning, were assessed under ice load and overtopping conditions. Reliability indices and local sensitivity measures were first estimated using first-order reliability analysis, after which polynomial chaos expansion metamodels were constructed to facilitate global sensitivity analyses. The results show that sliding has the lowest reliability, whereas overturning exhibits very high reliability and is generally not governing. Local sensitivity analyses indicate that rock-concrete interface friction dominates sliding and the combined failure mode, while concrete density is most influential for overturning. Global sensitivity analyses show that sliding reliability is primarily affected by front plate inclination, while overturning is mainly relevant for low monoliths or those with near-vertical front plates, and the combined mode is mainly influenced by asperity and front plate inclinations.

Shearing of rock joints is a critical failure mode in rock masses. The shear strength of rock joints must, therefore, be considered in the design of structures in rock masses. Several criteria for prediction of shear strength have been proposed over the years. However, the possible effect of scale on shear strength is an issue. One possible reason for this is that previous experimental studies on the scale effect contain various sources of uncertainties (mixed test methods, multiple testing of same specimen, application of results to other materials than tested, and omitted handling of statistical dispersion). In this paper, the results from a uniquely comprehensive experimental laboratory program, that handles these uncertainties and also extends the range of previously tested conditions, is presented. 46 direct shear tests on two joint types, natural and tensile induced granite rock joints, have been performed under the constant normal stress and the constant normal stiffness boundary condition at 5 MPa initial normal stress applied over three specimen sizes (35 mm × 60 mm, 70 mm × 100 mm and 300 mm × 500 mm). Analysis of variance shows no effect of the specimen size on the shear strength, whereas the joint type and boundary condition has. Quantitative estimates of the influence of the joint type and boundary condition on the shear strength are presented. A consistent approach for determination of shear strength from the point of time associated with a shear stiffness change of the test system is also presented.

AbstractThe stability of concrete dams is commonly evaluated in terms of sliding, overturning, and bearing capacity. Although these failure modes are simple to evaluate analytically, they may not accurately reflect a dam’s actual failure behavior because of inherent idealizations and assumptions. Previous studies have shown that the failure mechanism for dams often involves both sliding and overturning, particularly in dams with uneven rock-concrete interfaces and large-scale asperities. This article presents results from 1:5 scale model tests of a buttress dam, conducted to investigate the behavior and failure mechanism of dams featuring large-scale asperities in the rock-concrete interface. Each of the twelve scale models incorporated a specific combination of interface geometry, rock bolts, reinforcement, and rock joints. The results were compared with FEA and analytical estimates for sliding, overturning, and combined sliding and overturning failure. The scale models failed through a combination of sliding and overturning, with significant strain concentrations observed at the asperities’ upstream faces and the toe at peak load, indicating reliance on these regions for force transfer. The presence of rock bolts and a rock joint altered the load capacity but generally did not affect the overall failure mechanism. The results indicate that dams with irregular interfaces may exhibit significant overstrength compared to current assessment practices in dam safety guidelines.

Tunnels passing through faults and weakness zones require more extensive support. To secure the tunnel from collapse during excavation, pre-support known as spiling is a frequently applied solution. For more disintegrated rock mass conditions, the preferred technical solution is often thicker steel pipes (hereby referred to as pipe spiles). When pipe spiles are used as pre-support in disintegrated rock masses, a potential failure mode is progressive fall-out of smaller rock blocks between the spiles. In this paper, the critical block size for which pipe spiles can be applied without progressive failure was studied. Small-scale laboratory tests were conducted with the use of a specially manufactured box made of steel plates and plexiglass panels.The width, depth and height of the box was approximately 0.7 m, 0.7 m and 0.6 m respectively. Steel tubes, representing pipe spiles, were inserted into the box through holes drilled in the plexiglass panels. Gravel, representing the surrounding rock mass, of various particle sizes between 2 and 32 mm was filled in and compacted. The locking mechanism was then released, and the subsequent re-arrangement of granular material was observed. The results of the tests are presented and discussed with respect to the required spacing between pipes to prevent this mode of failure. For the specified testing conditions, the critical limit of the ratio between the pipe spacing and block size was determined to be between 3.7 and 4.6 for a rougher steel-rock interface and between 2.8 and 3.2 for a smoother one representing a pure steel-rock interface.

Load capacity assessment of concrete dams often includes verification of the stability for multiple separate failure modes, such as sliding and overturning. However, in the case of dams, the underlying failure mechanism for these failure modes may be too idealized, and the analysis could yield inaccurate results. Previous research has, for example, shown that regular rigid-body sliding failure analysis provides inaccurate load capacity estimates for dams with uneven interface geometries. This article discusses the behavior of such dams and presents a failure mode that combines the traditional sliding and overturning failures. The failure mode is termed combined sliding and overturning and serves as an intermediate to the traditional failure modes. It allows for the assessment of concrete dams with uneven interface geometries, whose behavior is not expected to be fully represented by only sliding or overturning. To estimate the load capacity for the presented failure mode, an analytical formulation based on simple force and moment equilibrium is provided. The formulation is compared with finite element simulations and previously reported results from experimental scale model tests and is shown to accurately predict the load capacity.

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.

The partial factor method is the most common approach to verify structural safety in Eurocode 7. Given the ongoing discussion on the European level to include also underground excavations in rock in the scope of the Eurocodes, there is a clear need to investigate the applicability of partial factors to the design of rock tunnels. However, implementing fixed partial factors, in accordance with the suggestion in the current Eurocode 7, may not be appropriate to account for the large uncertainties and variable conditions prevalent in rock engineering. This paper studies the critical characteristics and underlying assumptions of different reliability-based partial factor formats. The suitability of the analyzed partial factor formats to evaluate safety is analysed and discussed with reference to a design example of a rock-shotcrete interaction system for support against block failure in an underground opening. The results show that reliability-based partial factor methods outperform the traditional partial safety format suggested in the Eurocode in terms of accuracy and consistency.

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.