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  • 1.
    Ansell, Anders
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Sjölander, Andreas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Statiska beräkningar och analyser avseende betongplatta i Spendrups höglager2015Report (Other academic)
  • 2.
    Bennerstedt, Patrik
    et al.
    WSP.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. WSP.
    Bond, Håkan
    WSP.
    Halvarsson, Andreas
    WSP.
    Bond, Håkan
    WSP.
    Konstruktion av avbördningsanordningar: En jämförelse av nuvarande, nya, äldre och internationellt använda normer - Energiforskrapport 2019:5722019Report (Other academic)
    Abstract [en]

    The current RIDAS application guidelines for concrete dams, updated in 2017, refers to Eurocode for cross-sectional design. This report presents an investigation of the consequences from using RIDAS application guidelines for concrete dams (TV 7.3) for the design of spillway gates. A comparison between how different design guidelines for hydraulic structures treats loads are performed together with calculations that compare the different guidelines. Besides this, a review of international guidelines for spillway gates has been performed with the focus on countries which apply Eurocode. In addition, a historical presentation of the design guidelines used in Sweden during the time period when a majority of today's gates were built is presented. The international comparison shows that partial coefficients applied in Sweden result in steel members with lower overall safety factor than all other studied guidelines. Eurocode applies for the design of spillway gates in Finland, Germany, and Norway. The countries have separate and different approaches on how to apply loads and load combinations. Older Swedish guidelines for steel design (StBK 70 and Järnbestämmelser, 1931) indicate a higher coefficient of safety than RIDAS TV 7.4 (BSK) and RIDAS TV 7.3 (Eurocode) without the corrective coefficient. Design calculations for major members of a segment gate were conducted according to BSK/BKR, to RIDAS TV 7.3 (Eurocode) and to DIN 19704 (Eurocode). The conclusion from comparing the results was that applying RIDAS TV 7.3 results in similar overall safety as BSK/BKR but lower than DIN 19704. This study concludes that Eurocode should be applied for the design of spillway gates and load combination should be performed similarly as for cross-sectional design of concrete dams. Whether a corrective coefficient should be included in the guidelines to achieve an overall safety which is not substantially lower than internationally should be further investigated.

  • 3.
    Eriksson, Daniel
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Assessment of frost damage in hydraulic structures using a hygro-thermo-mechanical multiphase model2019In: Sustainable and Safe Dams Around the World / [ed] Jean-Pierre Tournier, Tony Bennett & Johanne Bibeau, 2019, Vol. 2, p. 332-346Conference paper (Refereed)
    Abstract [en]

    This paper presents an extension of a novel hygro-thermo-mechanical multiphase model for simulation of freezing of partially saturated air-entrained concrete on the structural scale to account for the effect of damage in the material. The model is applied in an example which investigates the extent and severity of frost damage caused by extremely cold climate conditions in a typical concrete wall in a waterway constructed with air-entrained concrete. The results were concluded to comply with observations made in experimental work and testing of freezing air-entrained concrete under exposure conditions similar to those in hydraulic structures. Furthermore, the results indicate that the effect of short periods of time with high rates of freezing was rather small on the obtained damage. Additionally, increasing the depth of the boundary region with an initially high degree of water saturation on the upstream side had also a rather small effect on the damaged zone.

  • 4.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    A systems approach to ice loads on concrete dams2022Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Dams are mainly used for the storage of water to electricity production and irrigation, or for river regulation. Continuous work to assure a high dam safety level is a prerequisite to minimize the risk for the uncontrolled release of water. An essential part of the safety evaluation of concrete dams is to understand the loads they are exposed to and the expected response of the dam. Under normal conditions, the behaviour of concrete dams is, to a great extent, governed by the ambient variation in temperature and water level. For concrete dams in cold climates, the large variation in ambient temperatures between summer and winter is particularly significant. In addition, these dams may be subjected to a pressure load from the expansion or movement of an ice sheet on the reservoir. The current guidelines for these ice loads are based on the dam's location and state that concrete dams must be designed for a line load of 50-250 kN/m. Thus, the ice load constitutes a significant part of the total load, especially for small dams. Despite its relatively significant impact, the knowledge about ice loads is insufficient, and the magnitude and return period of ice loads constitute one of the greatest uncertainties during stability evaluations of concrete dams. Furthermore, an apparent contradiction is that measurements and models indicate that ice loads are higher than the recommended values. Simultaneously, there are no reported dam failures where the ice-load has been addressed as the reason for the breach.

    To increase the knowledge about ice loads and the structural behaviour of concrete dams, this thesis applies an approach where the ice and the dam are parts of a structural system. The thesis contains six studies investigating the dam's, the ice's or the system's response to external loads.  Studies of the dam are aimed at increasing the understanding of the normal behaviour of concrete dams. Studies of ice loads include measurements, and a major contribution from this project is the development of a 1$\times$3 m$^2$ ice load panel, the installation of the panel on a concrete dam, and subsequent measurements during six winters. In addition, a systematic review and meta-analysis of previous measurements have been performed. The studies of the different parts have been connected through two studies of the entire system. The first is a parameter study where the static interaction between ice and dam is simulated to quantify how geometric variations of the reservoir and ice affect the mechanical ice loads. The second study searches for detectable influence from the ice load in the measured behaviour of concrete dams.

    The results show that the structural behaviour of concrete dams under normal conditions is primarily governed by the variation in water level and temperature. For the studied dams, these effects are significantly larger than the influence from damage and degradation. Ice load measurements and simulations show that ice loads varies significantly along the dam. This variation makes it difficult to quantify the impact of external factors on the magnitude of ice loads. Despite this difficulty, three independent analyses show that ice thickness, water level change, and the dam's properties have an evident effect on the magnitude of ice loads. If current guidelines are to be updated to consider local conditions at the dam, these three parameters should be included. Ice loads of the magnitudes measured and specified in the current guidelines should have a notable impact on the behaviour of a dam during normal operation. However, such an impact has not been found in the eight dams studied within this project. This result indicates that the ice loads measured locally do not necessarily represent the global ice load that acts on the entire structure.

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  • 5.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Condition assessment of concrete dams in cold climate2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Dams in many countries are approaching their expected service life. Proper assessment of the aging dams structural health increase the knowledge of the current safety, and allow for better planning of renovation and rebuilding investments. The behavior of concrete dams is, to a great extent, governed by the ambient variation in temperature and water level. In cold regions, the ice sheet formed in the reservoir may subject a pressure load on the dams. Theoretically, this load has a significant impact on the structural behavior of dams. Despite this, the maximum magnitude, as well as the seasonal variation of the ice load, constitute the most considerable uncertainty in the safety assessment of dams.

    This thesis presents research that examines how to model the expected behavior of dams in cold climate. The underlying problem is to predict the response of dams due to variation in the external conditions. Since the ice load is such a vital part of the external conditions in cold climate, the understanding and modeling of ice loads have been given extra attention. Models suitable to predict the long-term behavior of dams can be divided between theoretical, data-based, and hybrid. Prediction accuracy is essential to set alert thresholds, and in that regard, the data-based models are generally superior.

    The major contribution of this thesis is the design and installation of a prototype ice load panel with direct measurement of the ice pressure acting on a dam. The panel is attached on the upstream face of the dam and is large enough so that the whole thickness of the ice sheet is in contact with the panel. The predicted ice load from the best available model that includes loads from both thermal events and water level changes did not correspond to the measured ice loads. As there are no validated models or measurement methods for ice load on the dam, continued research is necessary, both through further measurements to increase knowledge and development of models.

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  • 6.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Enzell, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Estimating the ice loads on concrete dams based on their structural responseManuscript (preprint) (Other academic)
    Abstract [en]

    In the assessment of concrete dams in cold climate, it is common that the theoretical stability becomes insufficient for load cases that includes ice loads. However, the magnitude and return period of these ice loads have a high degree of uncertainty.  This study estimates the magnitude of ice loads on eight concrete dam monoliths using measurements of their displacement from 29 winters. In the displacement signals, events are identified and assumed to be caused solely by ice loads. The observed displacement during an event is interpreted as an ice load using a load-displacement relationship derived from FE simulations of each dam. These simulations show that ice loads of the magnitudes given in design guidelines and recorded in previous measurements would significantly affect the structural response of the studied dams. However, only small traces of ice loads can be found in the observed responses of the studied dams. The estimated ice loads are significantly lower than the ice loads recorded in traditional ice load measurements. These results indicate that the average magnitude of ice load on an entire monolith is significantly lower than the measured local pressures. This would imply that ice loads may be a smaller concern regarding the dam safety than previously believed.

  • 7.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Enzell, Jonas
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Estimating the Ice Loads on Concrete Dams Based on Their Structural Response2022In: Water, E-ISSN 2073-4441, Vol. 14, no 4, article id 597Article in journal (Refereed)
    Abstract [en]

    In the assessment of concrete dams in cold climate, it is common that the theoretical stability becomes insufficient for load cases that include ice loads. However, the magnitude and return period of these ice loads have a high degree of uncertainty. This study estimates the magnitude of ice loads on eight concrete dam monoliths using measurements of their displacement from 29 winters. In the displacement signals, events are identified and assumed to be caused solely by ice loads. The observed displacement during an event is interpreted as an ice load using a load-displacement relationship derived from FE simulations of each dam. These simulations show that ice loads of the magnitudes given in design guidelines and recorded in previous measurements would significantly affect the structural response of the studied dams. However, only small traces of ice loads can be found in the observed responses of the studied dams. The estimated ice loads are significantly lower than the ice loads recorded in traditional ice load measurements. These results indicate that the average magnitude of ice load on an entire monolith is significantly lower than the measured local pressures. This would imply that ice loads may be a smaller concern regarding dam safety than previously believed.

  • 8.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Fluid structure interaction2015In: International Water Power and Dam Construction, ISSN 0306-400X, no 8, p. 40-45Article in journal (Refereed)
    Abstract [en]

    The aim of this study is to investigate how Fluid-Structure interaction may beincluded in numerical earthquake analyses of dams. The basis for this project is theme Afrom ICOLDs 12th international benchmark workshop on numerical analysis of dams,which was held in October 2013. The focus of theme A was on how to account for fluidstructure interaction in numerical earthquake analyses of dams. In this study, parametricnumerical analyses have been performed where the purpose was to isolate someimportant parameters and investigate how these influence the results in seismic analysesof dams. These analyses were performed through the use of the finite element methodare the choice of Rayleigh damping parameters, reservoir boundaries and waveabsorption in the foundation-reservoir interface. The use of acoustic elements has proven to be a powerful approach for FSI analyses of adam-reservoir-computation time, while allowing for more advanced features such as bottom absorptionand non-be a challenging task, where it has a significant impact on the results. The methodproposed by Spears and Jensen has a physical meaning in the sense that this methodexcites the same e ective mass for the Rayleigh damped case as for the modal dampedcase. If a constant modal damping is desired or prescribed in a standard, this methodprovides a reasonable and sound method to choose the Rayleigh damping parameters fora complex structure. A more straightforward method is to choose the two frequencies insuch a way that the span between the frequencies covers about 80% of the e ective mass.The choice of reservoir boundary conditionsparameter showed to be the one that least a ected the results in the time-history analysis.results and this coe cient should be used carefully.

  • 9.
    Hellgren, Rikard
    et al.
    WSP Bridge and Hydraulic Design.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures. Vattenfall Engineering, Stockholm, Sweden.
    Influence of fluid structure interaction on a concrete dam during seismic excitation2015Conference paper (Refereed)
    Abstract [en]

    The aim of this study is to investigate how Fluid-Structure interaction may be included in numerical earthquake analyses of dams. The basis for this project is theme A from ICOLDs 12th international benchmark workshop on numerical analysis of dams, which was held in October 2013. The focus of theme A was on how to account for fluid structure interaction in numerical earthquake analyses of dams. In this study, parametric numerical analyses have been performed where the purpose was to isolate some important parameters and investigate how these influence the results in seismic analyses of dams. These analyses were performed through the use of the finite element methodare the choice of Rayleigh damping parameters, reservoir boundaries and wave absorption in the foundation-reservoir interface. The use of acoustic elements has proven to be a powerful approach for FSI analyses of adam-reservoir-computation time, while allowing for more advanced features such as bottom absorption and non- be a challenging task, where it has a significant impact on the results. The method proposed by Spears and Jensen has a physical meaning in the sense that this method excites the same e ective mass for the Rayleigh damped case as for the modal damped case. If a constant modal damping is desired or prescribed in a standard, this method provides a reasonable and sound method to choose the Rayleigh damping parameters for a complex structure. A more straightforward method is to choose the two frequencies in such a way that the span between the frequencies covers about 80% of the e ective mass. The choice of reservoir boundary conditionsparameter showed to be the one that least a ected the results in the time-history analysis. results and this coe cient should be used carefully.

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  • 10.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Gasch, Tobias
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Influence of fluid structure interaction on a concrete dam during seismic excitation2015In: Dam Engineering, ISSN 0958-9341, Vol. XXVI, no 2, p. 1-15Article in journal (Refereed)
    Abstract [en]

    The aim of this study is to investigate how fluid-structure interaction (FSI) may be included in numerical earthquake analyses of dams. The basis for this project is Theme A from ICOLD’s 12th International Benchmark Workshop on Numerical Analysis of Dams[5], which was held in October 2013 in Austria. The focus of Theme A was on how to account for fluid-structure interaction in numerical earthquake analyses of dams. In this study, parametric numerical analyses have been performed, where the purpose was to isolate some important parameters and investigate how these influence the results in seismic analyses of dams. These analyses were performed through the use of the finite element method, where the reservoir was modelled with acoustic finite elements. The studied parameters are the choice of Rayleigh damping parameters, reservoir boundaries, and wave absorption in the foundation-reservoir interface.

     

    The use of acoustic elements has proven to be a powerful approach for FSI analyses of a dam-reservoir-foundation system. The acoustic finite elements provide reasonable computation time, while allowing for more advanced features such as bottom absorption and non-reflecting boundaries. The choice of Rayleigh damping coefficients has proven to be a challenging task,

    where it has a significant impact on the results. The method proposed by Spears & Jensen[1] has a physical meaning in the sense that this method excites the same effective mass for the Rayleigh damped case as for the modal damped case. If a constant modal damping is desired, or prescribed in a standard, this method provides a reasonable and sound method to choose the Rayleigh damping parameters for a complex structure. A more straightforward method is to choose the two frequencies in such a way that the span between the frequencies covers about 80% of the effective mass. The choice of reservoir boundary conditions was based on the assumption of an infinite reservoir. A conservative approach is to use a fixed boundary condition, where the pressure waves are reflected at the upstream boundary of the reservoir. However, this parameter proved to be the one that least affected the results in the time-history analysis. The reflection coefficient describing bottom absorption showed to greatly influence the results, and this coefficient should be used  carefully.

  • 11.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Johnsson, Lars
    WSP.
    Portin, Hanna
    WSP.
    Andersson, Patrik
    WSP.
    Halvarsson, Andreas
    WSP.
    Utökning av avbördningskapacitet: RAPPORT 2021:7832021Report (Other academic)
    Abstract [en]

    This report compiles experiences from projects completed during 2000– 2020 where Swedish dams' discharge capacity has been increased or secured. The report consists of two parts, a quantitative part with data from a survey answered by Swedish dam owners and a qualitative part summarizing discussion and conclusions from interviews and a workshop.During the recent decades, several projects have been undertaken in Sweden to increase or secure existing dams' discharge capacity. In some cases, new spillways were built adjacent to the existing spillways or in a new location. In other cases, one or more of the existing spillways were reconstructed, or inactive spillways were recommissioned. Additional dams will be rebuilt to meet modernrequirements of increased and available discharge capacity in the future. Although each project is unique and the technical solutions must consider local conditions, much can be learned from completed projects.The survey results describe the typical project with info regarding background, selected measures, and discharge capacity data. Extra emphasis is placed on issues related to concrete in a hydropower context. Appendix A presents a list of all projects with metadata, and Chapter 3 present an aggregate compilation of the data. The most common method is to modify an existing spillway or to build a new spillway. Such projects often involve significant interventions in the dams and are technically complex. It is prevalent that this type of project involves one or more types of complicated castings.A significant part of the project was spent planning and hosting a workshop for representatives from the Swedish dam owners. The workshop consisted of varied topics, and each presentation with accompanying discussions is presented in Chapter 4. These presenters exemplify the summarized data from Chapter 3 and show the issues' breadth and complexity in all the project phases.Chapter 5 presents an in-depth study of four projects carried out by the same dam owner and project organization. Interviews were conducted with the pre-study project manager, the dam safety expert, and the project manager during the construction. These different interviews create an overview of the projects from different perspectives, focusing on similarities, differences, and lessons learned.

  • 12.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    A parametric numerical study of factors influencing the thermal icepressure along a dam2020In: Proceedings of the 25th IAHR International Symposium on Ice (Trondheim, 2020), Trondheim, 2020, article id 8563Conference paper (Refereed)
    Abstract [en]

    The uncertainty regarding size and variation of the ice pressure constitute one of the mainsignificant sources of concern for condition assessment of concrete dams in cold regions. Inmost current dam safety guidelines, the design ice load is determined solely from thegeographic location of the dam. However, factors such as variation in temperature and waterlevel, the slope of the banks, wind etc. may also influence the size of the load. Previousmeasurements indicate that the pressure on a dam varies along the dam line during the sametime and that the average ice pressure decreases as the area of the ice-structure interface isincreased. This paper presents numerical studies on how the ice pressure varies along the damwall. A finite element model that includes several dam monoliths, the ice and the beaches ofthe reservoirs are used to simulate thermal ice loads. The pure elastic load caused by therestrained expansion of an ice sheet subjected to a change in thermal gradient are greater thanthe design ice load in current dam safety guidelines and the loads measured. The simulationsshow that the ice load vary significantly along the dam, and the variation in cross-sectionstiffness along the dam greatly influence the magnitude of the total ice pressure. Theparameter study shows that the elastic thermal ice load increases with an increased slope ofthe banks, reservoir length, and ice thickness and decreases as the angle of the connectionbetween the dam and bank increases. However, the difference in ice load between theindividual monoliths with the same geometry and temperature change are in several cases ofthe same magnitude as the variation in the external factors.

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  • 13.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Stability assessment of concrete dams with non-linear FEA2017In: Nordic Concrete Research Publications / [ed] Marianne Tange Holst, 2017, p. 299-302Conference paper (Refereed)
    Abstract [en]

    In dam safety assessments, it must be shown that concrete dams are safe against sliding and overturning failure modes. This is typically performed with analytical calculations based on force equilibrium of a rigid body. In more complex cases, these analyses are time-consuming and have to be based on assumptions regarding the dam’s behaviour or simplifications. By using non-linear finite element analyses (FEA), more detailed and realistic analyses can be performed where it is possible to include cracks, degradation, material strength, dam-rock interaction etc. A method to perform these analyses is presented, based on the same safety concept as for the classical methods.

  • 14.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Systematic literature search and meta regression of measured static ice loads on concrete damsManuscript (preprint) (Other academic)
    Abstract [en]

    This study presents a systematic literature review of ice load measurements on dams. Several hypotheses about the relationship between the maximum ice load and external variables are tested using regression analysis on the data collected from the literature. The performed tests show that ice thickness, water level change category, and dam height are factors that have a significant and relevant relationship with the magnitude of all measured ice loads. The ice thickness is the only tested variable that also shows a significant and relevant relationship with differences in ice load between winters at one dam. The variation in recorded ice load from several sensor positions at one dam during the same winter is considerable. Generally, the difference between the sensor area and the structure-ice interaction area is large, resulting in extensive extrapolation and uncertainties regarding the representativeness of the measured results.

  • 15.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Systematic Literature Search and Meta Regression of Measured Static Ice Loads on Concrete Dams2024In: Journal of cold regions engineering, ISSN 0887-381X, E-ISSN 1943-5495, Vol. 38, no 2, article id 04024002Article in journal (Refereed)
    Abstract [en]

    This study presents a systematic literature review of ice load measurements on dams. Several hypotheses about the relationship between the maximum ice load and external variables are tested using regression analysis on the data collected from the literature. The performed tests show that ice thickness, water level change category, and dam height are factors that have a significant and relevant relationship with the magnitude of all measured ice loads. The ice thickness is the only tested variable that also shows a significant and relevant relationship with differences in ice load between winters at one dam. The variation in recorded ice load from several sensor positions at one dam during the same winter is considerable. Generally, the difference between the sensor area and the structure–ice interaction area is large, resulting in extensive extrapolation and uncertainties regarding the representativeness of the measured results.

  • 16.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Performance of data-based models for early detection of damage in concrete dams2020In: Structure and Infrastructure Engineering, ISSN 1573-2479, E-ISSN 1744-8980Article in journal (Refereed)
    Abstract [en]

    A failure of a massive concrete dam could cause catastrophic consequences. The purpose of monitoring is to detect anomalies and damage at an early stage to prevent failure. Data-based models for anomaly detection are based on the hypothesis that the behaviour of an undamaged dam will follow an expected pattern, and deviation from this pattern is an indication of damage. In this study, simulations were used to create time series for an undamaged dam and three different damage scenarios at three different locations in the dam body. Three common data-based models were used to predict a dams crest displacements, both on the generated artificial data and the corresponding measurements from the dam. Prediction bands for future measurements were created, and the ten time-series were used to test the ability to detect damage. All models could detect instantaneous damage but struggle to detect progressive damage; the Neural network outperforms the two regression models. The choice of the mathematically optimal threshold limit leads to a large number of false alerts. Requiring three consecutive values outside the threshold before an alert is issued, increases the possibility to receive an early alert compared to the standard approach where observations are classified individually.

  • 17.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Performance of data-based models for early detection of damage inconcrete damsManuscript (preprint) (Other academic)
    Abstract [en]

    A failure of a massive concrete dam could cause catastrophic consequences. Thepurpose of monitoring is to detect anomalies and damage at an early stage to preventfailure. Data-based models for anomaly detection are based on the hypothesis thatthe behaviour of an undamaged dam will follow an expected pattern, and deviationfrom this pattern is an indication of damage. In this study, simulations were usedto create time series for an undamaged dam and three different damage scenariosat three different locations in the dam body. Three common data-based modelswere used to predict a dams crest displacements, both on the generated artificialdata and corresponding measurements from the dam. Prediction bands for futuremeasurements were created, and the ten time-series were used to test the ability todetect damage. All models could detect instantaneous damage but struggle to detectprogressive damage; the Neural network outperforms the two regression models. Thechoice of the mathematically optimal threshold limit leads to a large number of falsealerts. Requiring three consecutive values outside the threshold before an alert isissued, increases the possibility to receive an early alert compared to the standardapproach where observations are classified individually.

  • 18. Hellgren, Rikard
    et al.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Progressive Failure Analysis of a Concrete Dam Anchored with Passive Rock Bolts2020In: Infrastructures, ISSN 2412-3811, Vol. 5, no 3, article id 28Article in journal (Refereed)
    Abstract [en]

    Passive rock bolts are commonly used to anchor concrete dams, and they may have a significant impact on stability-evaluations. However, these bolts are often omitted from dam safety analysis due to uncertainties regarding their condition and the size of displacements required in the dam-rock interface to mobilize significant bearing forces in the passive rock bolts. This paper address the latter question by studying the failure process of a small concrete dam anchored with rock bolts. Failure simulations were performed with the increased density method in a finite element model consisting of a dam, the corresponding part of the rock and rock bolts. Two types of approaches are used to simulate the anchorage of the rock bolts; a method where the anchorage to the rock is simulated using a fixed boundary condition; and a method where the anchoring of the bolts are modelled using springs. Depending on the method of analysis, the rock bolts contribute with 40–75% of the load-carrying capacity of the dam. The rock bolts increase the load-bearing capacity of the dam, partly through anchorage forces, but also by keeping the contact surface between rock and concrete together and thereby increase the shear capacity of the interface.

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  • 19.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Eriksson, Daniel
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Modelling of the ice load on a Swedish concrete dam using semi-empirical models based on Canadian ice load measurements2019In: Sustainable and Safe Dams Around the World / [ed] Jean-Pierre Tournier, Tony Bennett & Johanne Bibeau, 2019, Vol. 2, p. 3068-3080Conference paper (Refereed)
    Abstract [en]

    In cold regions where the water surface of a river or lake freezes during the winter, concrete dams may be subjected to a pressure load from the ice sheet. This pressure load may constitute a large portion of the total horizontal load acting on a small dam. From a dam safety perspective, it is important to determine the design value of the ice load. In February 2016, a prototype of an ice load panel was installed on a Swedish concrete dam. The 1x3m2 panel measures the ice pressure with three load cells. In this paper, the ice load measured on the Swedish dam is predicted using a Canadian empirical model, previously developed from a 9-year field program to estimate the ice loads caused by thermal effects and variation in water level. The predictions from the model could not accurately predict the measured ice loads. Since the current understanding of ice load is limited, it is not possible to determine whether the measurement, the model or both are inaccurate.

  • 20.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Fransson, Lennart
    Luleå University of Technology.
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Westberg Wilde, Marie
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Measurement of ice pressure on a concrete dam with aprototype ice load panelManuscript (preprint) (Other academic)
    Abstract [en]

    This paper presents the development and installation of a prototype ice load panel andmeasurements of ice load from February 2016 to February 2018 at the Rätan hydropower damin Sweden. The design of the 1x3 m2 panel enabling direct measurement of ice pressure on theconcrete surface is based on previous experience from similar measurements with sea ice.Important features of the design are sufficient height and width to reduce scale effects and tocover the ice thickness and variations in water level. The Rätan dam was chosen based onseveral criteria so that the ice load is considered to be reasonably idealized against the damstructure.For the three winters 2016, 2016/2017, 2017/2018, the maximum ice load recorded was 161kN/m, 164 kN/m and 61 kN/m respectively. There were significant daily fluctuations duringthe cold winter months, and the daily peak ice loads showed a visual correlation with the dailyaverage temperature and with the daily pattern of operation of the power station with itscorresponding water level variations

  • 21.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Fransson, Lennart
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Luleå, Sweden.
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Westberg Wilde, Marie
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Measurement of ice pressure on a concrete dam with a prototype ice load panel2020In: Cold Regions Science and Technology, ISSN 0165-232X, E-ISSN 1872-7441, Vol. 170, article id 102923Article in journal (Refereed)
    Abstract [en]

    This paper presents the development and installation of a prototype ice load panel and measurements of ice load from February 2016 to February 2018 at the Rätan hydropower dam in Sweden. The design of the 1 × 3 m2 panel enables direct measurement of ice pressure on the concrete surface is based on previous experience from similar measurements with sea ice. Important features of the design are sufficient height and width to reduce scale effects and to cover the ice thickness and variations in water level. The Rätan dam was chosen based on several criteria so that the ice load is considered to be reasonably idealized against the dam structure.

    For the three winters 2016, 2016/2017, 2017/2018, the maximum ice load recorded was 161 kN/m, 164 kN/m and 61 kN/m respectively. There were significant daily fluctuations during the cold winter months, and the daily peak ice loads showed a visual correlation with the daily average temperature and with the daily pattern of operation of the power station with its corresponding water level variations.

  • 22.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Ríos Bayona, Francisco
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Pull-out tests of 50-year old rock bolts2016In: Proceedings of the International Symposium on Appropriate technology to ensure proper Development, Operation and Maintenance of Dams in Developing Countries, Johannesburg, 2016, p. 263-272, article id 258Conference paper (Refereed)
    Abstract [en]

    In this paper, detailed numerical and analytical calculations are presented with the aim to study the effect of rock bolts and the influence of degradation on the dam safety. In addition, results from inspections and pull-out tests are presented from a case study with rock bolts that has been installed for 50 years. All tested bolts have with margin withstood higher stresses than the maximum allowed stresses according to Swedish standards. At visual inspection, no bolts have shown any major signs of degradation and all investigate bolts are in very good condition.

  • 23.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Modeller för övervakning av betongdammar: Energiforskrapport 2019:5802019Report (Other academic)
    Abstract [en]

    In the field of dam surveillance, alert and alarm values are used for error detection to notify about the discovery of a potential dam safety risk. Alarm values are used to notify that a dangerous behavior have been reached. Alert values (warnings) are used to notify that the measured data is out of the expected range. Dam surveillance can thereby be considered as a classification problem where every measured response should be classified as safe or unsafe, or alternatively as expected or unexpected. Dangerous behavior implies that the safety of the dam may be compromised (e.g. the coefficient of safety may be surpassed), while an unexpected value occurs when the dam no longer acts according to the predicted behavior based on current conditions. Such a discrepancy does not necessarily mean that the dam safety is compromised, but could indicate that the dam is damaged.In order to determine the expected behavior, some type of prediction model is required that can predict the dam behavior based on ambient conditions. In this report, the prediction models are defined as either theory-based or empirical. The theory-based models are based on physics with correlation between ambient conditions such as water level and temperatures and the response of the dam. In the empirical models, this coupling is purely empirical without any physical meaning.There are several types of sensors that can be used for dam surveillance. In this report, the instruments commonly used within the field of dam engineering and their expected accuracy are discussed. In addition, the use of different sensors based on their purpose in the surveillance program is discussed where sensors are denoted as detectors or support instruments. The detectors are considered as sensors suitable to be used to monitor a potential failure mode while the purpose of the support instruments is to provide additional information about the dam response or the ambient conditions.Every type of model has unique properties with different pros and cons. It is therefore important to choose the type of model based on the need and purpose of the monitoring or evaluation of the dam response. A finite element model gives good possibilities to interpret the results and find the physical meaning of a specific behavior, but is not as good to give exact predictions. The most advanced empirical models are defined without any physical meaning, but are capable to give precise predictions of the expected response. To choose one type of model over another can thereby be described as choosing between understanding and performance. Luckily, the use of one model does not exclude the use of others. The models based on physics helps us understand and interpret the dam behavior and will therefore always have a use in the dam safety work. It is also the type of model that best can be used to study scenarios during dam failure and thereby to define alarm values. As a compliment to these, the empirical models can be used to give early warnings when the dam behavior is out of the ordinary and is therefore most suited for definition of alert values. These models generally perform better and has smaller deviation than the corresponding theoretical models.The case studies presented in this report shows that the simple empirical models with some physical meaning gives very good predictions of the expected behavior.

  • 24.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Persson, Anton
    Klasson Svensson, Emil
    Estimating the effect of ice load on a concrete dams displacement with regression models2017In: Proceedings ICOLD 2017 International Symposium: Knowledgebased dam engineering, Prag, 2017Conference paper (Refereed)
    Abstract [en]

    In colder climates, the ice load is an important factor to consider when assessing the dam safety. The size of the ice load is associated with uncertainties, regarding both its maximum size and seasonal variation. There is today no reliable method to determine the ice load acting on a dam, since available measurements and theories results in a large scatter regarding the maximum ice loads.

    In this project, the possibility to determine the ice load based on traditional measurements of a dam behavior is investigated. It’s evaluated if the ice thickness is a significant predictor for the displacement of a Swedish arch dam. A case study on inverted pendulums was performed where dynamic regression models were used to calculate the displacement of the dam.

    The model showed significant results which could explain the displacements. The ice thickness shows an effect on a 5 % significance level. The results indicate that the seasonal ice load is large enough to be traceable, and could be determined from conventional pendulum measurements. Since pendulums are installed on a large number of dams, a method that estimates the ice load from those measurements could vastly reduce uncertainties regarding ice loads on dams.

  • 25.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Petrich, Chris
    Arntsen, Bård
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ice load measurements on Rätan concrete dam using different sensor types2021In: Cold Regions Science and Technology, ISSN 0165-232X, E-ISSN 1872-7441, p. 103425-103425, article id 103425Article in journal (Refereed)
    Abstract [en]

    Concrete dams in cold regions are designed to withstand loads from the ice sheet on top of the reservoir. However, the ice load's magnitude and return period are among the most considerable uncertainties in safety assessments of concrete dams. In a previous study, the development and installation of a 1 × 3 m2 prototype ice load panel attached at the upstream face of a concrete dam was presented. The panel is large enough for the ice sheet's cross-section to remain in contact with the panel as the water level varies, and it measures the total ice load without interpolation. This paper presents measurement results from the load panel from winters 2018–19 and 2019–20, an update to the measurement design, and additional ice pressure measurements with traditional stress cells. The panel measured seasonal maximum ice loads of 100 and 200 kN/m for the two winters, respectively. Winter 2019–20, when the panel measured the largest loads, was mild for the location, with great ice thickness near the dam face (1.2 m) and an almost snow-free ice sheet throughout the winter. Two 2.75 × 1.75 m2 dummy panels were installed adjacent to the load panel prior to the winter 2019–20 to minimize the load panel's protruding effect. These panels significantly reduced the local impact, as evident by the crack-pattern of the ice sheet near the load panel. The load panel recorded large ice loads (>75 kN/m) for all combinations with increasing/decreasing air temperature and/or water level. Identification of temperature change events and water level change events during the winters, shows that a change in air temperature, water level, or any combination of these, is not sufficient alone to explain large ice loads at Rätan dam. These findings suggest that other conditions must be satisfied before a water level or temperature change results in large ice loads. In February 2020, three panels consisting of a steel frame with four stress cells on each were placed on the dummy panels’ upstream face, and one single stress cell was placed 6 m out in the reservoir in front of the load panel. The majority of the stress cells recorded ice pressure larger than their measurement range. At the end of the ice season, only two of the panels’ twelve stress cells were still functional, and the ice vastly deformed the steel frames. From the period before the frames were damaged and unrelated to the choice of interpolation method, the recordings by the three stress cell panels at the dam are among the historically largest inferred ice loads on dams.

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  • 26.
    Hellgren, Rikard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Westberg Wilde, Marie
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Utvärdering av modeller för prediktion av islast mot dammar: RAPPORT 2019:6312019Report (Other academic)
    Abstract [en]

    Concrete dams in the cold Swedish climate may be subjected to a pressure load from the ice. The hydropower industries guidelines for dam safety (RIDAS) specify that dams should be designed for an ice load between 50-200 kN/m, varying from south to north. This load can thereby be significant and may constitute a large portion of the total horizontal load, especially for smaller dams in the northern part of Sweden. Despite this, the current understanding of ice loads is limited.In the previous SVC project, "Load conditions for ice pressure," a prototype ice load panel was developed. The panel was installed on Rätan's concrete dam in 2016 and have measured the ice load against the dam since then.In this report, different models intended to predict ice load are presented and these models are applied to predict the measured ice load from Rätan. For this, two models were used:• A semi-empirical model proposed by Comfort et al. (2003). • A mechanical model where ice pressure is modelled based on ice-stresses, where the thermal ice stresses are modelled with Norton creep as proposed by Petrich, et al. (2015).Both models inaccurately predict the total and the relative magnitude of the measured ice load peaks, and underestimate the maximum measured load during the winter. It is from today's knowledge not possible to determine whether the measurements are correct and if the prediction from the models is wrong, vice versa or a combination of both. Especially the part of the model that predicts the ice loads from water level variations shows low explanatory value, and currently no model exists that accurately can predict ice loads caused by water level variations.Two things seem obvious regarding the modelling of ice loads; ice load caused by water level variations is the mechanism that provides the most significant contribution to ice load on the dam, and at the same time almost all prediction models for ice load on dams only consider thermal loads. There is thus a clear gap between the mechanisms found by measurements and the development of models. The model developed by Comfort takes into account the impact of water level variations. Still, the result shows that this particular part of the model gives the least

  • 27.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Fransson, Lennart
    Nordström, Erik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Westberg Wilde, Marie
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Lastförutsättningar avseende istryck2017Report (Other academic)
  • 28.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hassanzadeh, ManouchehrSWECO.Hellgren, RikardKTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Proceedings of the 14th ICOLD International Benchmark Workshop on Numerical Analysis of Dams2017Conference proceedings (editor) (Refereed)
    Abstract [en]

    Every second year, the Technical Committee A (Computational Aspects of Dam Analysis and Design) of ICOLD organizes an international benchmark workshop on numerical analysis of dams. The purpose is to share knowledge and experience regarding numerical modelling within the fields of dam safety, planning, design, construction as well as operation and maintenance of dams. The Terms of Reference for Committee A state; ”BenchmarkWorkshops are organised to compare numerical models between one another and/or with reference solutions, including the dissemination and publication of results”.

     The 14th ICOLD International Benchmark Workshop on Numerical Analysis of Dams was held from the 6th to the 8th September 2017 in Stockholm, the capital of Sweden. It was organized by the Local Organizing Committee, on behalf of the Swedish National Committee of ICOLD (SwedCold) and the ICOLD Technical Committee A. It was hosted by KTH Royal Institute of Technology, Sweden’s largest technical research and learning institution and one of Europe’s leading technical universities. With respect to hydropower and dam engineering, KTH has for many years been active within of the Swedish hydropower Center (SVC), a key centre for research and educational activities in Sweden. SwedCOLD established in 1931, and is one of the oldest members of the International Commission On Large Dams, ICOLD. The development of hydro power for electricity production was initiated in the late 19th century and made the large scale industrialisation in Sweden possible. About 2000 hydropower plants are in operation in Sweden today, producing almost half of the total electricity supply. Considering the many and relatively old dams in Sweden, organizing the benchmark workshop was important to increase the knowledge regarding managing aging hydropower structures.

    The 14th ICOLD International Benchmark Workshop has addressed current challenges regarding design and maintenance of existing dams and improved the understanding of these by exchange of experience on the use of numerical modelling for design, performance evaluation and safety assessment of dams. On behalf of the organizing committee, I would like to express my sincere appreciation to the formulators/moderators of each theme for their outstanding work to prepare the themes, reviewing the papers and evaluating the results. I also want to thank all participants to the benchmark workshop for their contributions and their presence to this benchmark workshop. Finally, the support from the Technical Committee A and the support from the companies and organizations that sponsored this benchmark workshop is gratefully acknowledged.

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  • 29.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Chaoran, Fu
    AFRY.
    Ekström, Tomas
    AFRY.
    Description and synthesis of theme A – Thermal cracking of a concrete arch dam2018In: Proceedings of the 14th International Benchmark Workshop on Numerical Analysis of Dams / [ed] Malm, R., Hassanzadeh, M., Hellgren, R, Stockholm: KTH Royal Institute of Technology, 2018, p. 18-75Conference paper (Refereed)
  • 30.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Enzell, Jonas
    Lessons Learned Regarding Cracking of a Concrete Arch Dam Due to Seasonal Temperature Variations2020In: Infrastructures, ISSN 2412-3811, Vol. 5, no 2, article id 19Article in journal (Refereed)
    Abstract [en]

    Dams located in cold areas are subjected to large seasonal temperature variations and many concrete dams have cracked as a result. In the 14th International Commission on Large Dams (ICOLD) Benchmark Workshop, a case study was presented where contributors should predict the cracking and displacements due to seasonal variations. In this paper, the conclusions from this case study are presented. Overall, the results from the contributors are well in line with the observations that can be made on the dam and the measurements performed. This shows that using non-linear numerical models is a suitable tool to accurately predict cracking and estimate the displacements of cracked dams. This case study also highlighted important aspects that need special consideration in order to obtain realistic results that can be used to predict the crack pattern, these being: (1) the importance of performing transient thermal analyses based on robin boundary conditions; (2) the influence of contact formulation between the concrete dam and the foundation; and (3) the use of realistic non-linear material properties. The results and conclusions presented in this paper constitute one important step in achieving best practices to estimate dam safety and better understand the potential failure modes and ageing of concrete dams.

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  • 31.
    Malm, Richard
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Johansson, Fredrik
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ríos Bayona, Francisco
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
    Load Capacity of Grouted Rock Bolts Due to Degradation: 2017:3742017Report (Other academic)
    Abstract [en]

    In this report, the influence of degradation on the strength of rock bolts has been studied. A literature study has been performed in order to determine the degradation rate and to present observations and conclusions from available assessments of rock bolts. All cases found in the literature have shown that the rock bolts on concrete dams are in good condition with only minor superficial corrosion even after 50 years of service. In the project, a methodology to account for the degradation mechanism in evaluations of dam safety is presented, where all possible failure modes of rock bolts are considered. The contribution of the rock bolts to the dam stability is based on the failure mode with the lowest strength. The degradation has been taken into account based on a German DIN standard based on the chemical content of the water. Most available field tests have only measured the load capacity of bolts, where the deformation is typically not recorded. The relationship between forces and deformations is however important input to verify numerical and analytical analyses. Therefore, field tests have been performed on rock bolts that been in service for 50 years. A previously developed test rig had been modified in this project to register both load and deflection of the pull-out test. The test rig consists of a hydraulic jack that presses a cylinder towards the rock surrounding the bolt. Due to this configuration of the test equipment, a rock cone failure cannot be captured. The results showed that the test rig may influence the obtained load capacity if the failure occurs in the grout. A case study is presented where analytical, probabilistic and finite element analyses were performed to assess the dam safety. Based on the numerical analyses, it was possible to study the development of forces in the rock bolts due to successively increasing loads. The numerical analyses showed (as expected) that the rock bolts are subjected to both shear and tensile forces at the same time. In addition, the shear force was constantly higher than the tensile forces and that the shear forces were about 10% of the total shear resistance for normal loads. This implies that it is important to use a failure criterion for the rock bolts that considers combinations of tensile and shear forces. Besides this, the probabilistic analyses showed that pure overturning failure is extremely unlikely and cannot be considered as a relevant failure mode. The numerical analyses showed that the deformation start as for overturning failure resulting in that parts of the contact surface (on the upstream side) lose its contact. Thereby, the shear forces have to be transmitted over a reduced area which initiates the sliding failure. This implies that it is more suitable to define a criterion that limits the tensile forces in the upstream toe from serviceability loads, rather than having an overturning failure criterion.

  • 32.
    Sjölander, Andreas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Modelling aspects to predict failure of a bolt-anchored fibre reinforced shotcrete lining2018In: 8th International Symposium on Sprayed Concrete: Modern Use of Wet Mix Sprayed Concrete for Undergro­und Support / [ed] Thomas Beck, Synnove A. Myren, Siri Engen, Trondheim, 2018, p. 278-292Conference paper (Refereed)
    Abstract [en]

     Tunnels in hard and jointed rock are normally excavated with an arch shape to enable the rock to carry itself. The arch effect depends on the stability of individual blocks and too high or too low horizontal stresses could cause blocks to be pushed out or to fall down. To prevent this, systematic rock bolting in combination with fibre reinforced sprayed concrete (FRSC) is commonly used to support the rock. To understand the failure mechanism of the lining when subjected to the load from one block is therefore important for the design. In this paper, the three main failure mechanisms for a rock support shotcrete lining has been identified as failure in the fibre reinforced concrete, bond failure between shotcrete and rock or failure of rock bolts. For each of the failure modes, a short review of numerical methods is presented followed by a selection of a preferred modelling approach. The selected methods are then verified against experimental results from the literature. The selected methods all shows good agreements with tests and demonstrates the ability to simulate each failure mode one by one.

  • 33.
    Sjölander, Andreas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Verification of failure mechanisms and design philosophy for a bolt-anchored and fibre-reinforced shotcrete lining2020In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 116Article in journal (Refereed)
    Abstract [en]

    Falling or sliding of loose blocks is one of the most common failure modes in a rock tunnel. For tunnels in hard and jointed rock, fibre-reinforced shotcrete (sprayed concrete) in combination with rock bolts is one of the most commonly used supports to prevent such failures. The structural behaviour, and especially the failure, of this type of rock support, is complex and involves several failure mechanisms; such as cracking of the shotcrete and interface failure along the shotcrete-rock, bolt-grout and rock-grout interface. Therefore, rock supports are normally designed using analytical solutions based on the independent failure modes. However, these failure modes are derived based on experimental testing and the assumption that no interaction between the failure modes occur. This assumption has not been verified. Therefore, this paper presents a numerical model capable of simulating the failure of a bolt-anchored and fibre-reinforced shotcrete lining. The model includes bond failure between shotcrete and rock, cracking of the shotcrete and pull-out failure of rock bolts. The structural behaviour for each failure mode and the complete structure have been verified against experiments from the literature. This shows that the model is capable of simulating the different phases of failure, and show good agreement with results from full-scale experimental tests from the literature. Furthermore, results from the numerical simulation confirms that the design of the shotcrete lining can be based on individual failure mechanisms. Moreover, it was shown that a design based on the residual strength of the fibre-reinforced shotcrete is conservative compared to a design based on the bond strength.

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  • 34.
    Sjölander, Andreas
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Hellgren, Rikard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Malm, Richard
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Ansell, Anders
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.
    Verification of failure mechanisms and design philosophy for a bolt-anchored and fibre-reinforced shotcrete liningIn: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961Article in journal (Refereed)
    Abstract [en]

    Falling or sliding of loose blocks is one of the most common failure modes in a rock tunnel. For tunnels in hard and jointed rock, fibre-reinforced shotcrete (sprayed concrete) in combination with rock bolts is one of the most commonly used supports to prevent such failures. The structural behaviour, and especially the failure, of this type of rock support, is complex and involves several failure mechanisms; such as cracking of the shotcrete and interface failure along the shotcrete-rock, bolt-grout and rock-grout interface. Therefore, rock supports are normally designed using analytical solutions based on the independent failure modes. However, these failure modes are derived based on experimental testing and the assumption that no interaction between the failure modes occur. This assumption has not been verified. Therefore, this paper presents a numerical model capable of simulating the failure of a bolt-anchored and fibre-reinforced shotcrete lining. The model includes bond failure between shotcrete and rock, cracking of the shotcrete and pull-out failure of rock bolts. The structural behaviour for each failure mode and the complete structure have been verified against experiments from the literature. This shows that the model is capable of simulating the different phases of failure, and show good agreement with results from full-scale experimental tests from the literature. Furthermore, results from the numerical simulation confirms that the design of the shotcrete lining can be based on individual failure mechanisms. Moreover, it was shown that a design based on the residual strength of the fibre-reinforced shotcrete is conservative compared to a design based on the bond strength.

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