The Älvkarleby test dam, constructed in Sweden, was designed with several intentionally created defects to serve as a controlled environment for evaluating various monitoring techniques. Among these, a unique three-dimensional (3D) Electrical Resistivity Tomography (ERT) setup was implemented. The ERT survey design, combined with full 3D calculation of the geometric factor and inversion constrained by the dam's internal zonation, enabled high-resolution mapping of spatial resistivity variations within the core and filter zones. Up to 14,000 ERT data points were collected daily, providing a detailed representation of the dam's internal conditions. Additional monitoring included pore pressure piezometers, Thomson weirs and turbidity sensors. In this study, a seepage model of the Älvkarleby test dam was developed to obtain comprehensive insights into its performance. A novel modelling approach was applied in which material properties for the core and fine filter were derived from a combination of laboratory test results and interpretations of the 3D ERT data. Anomalous zones identified through ERT, together with the known intentional defects, were explicitly incorporated into the model geometry. For other dam zones than the core material properties were based solely on laboratory measurements. A trial-and-error calibration process was performed by comparing calculated pore pressures and leakage rates with the corresponding measurements until optimal agreement was achieved. Two additional seepage models were developed for comparison: one without any defects and another incorporating the intentional defects only. Results demonstrated that only the seepage model guided by ERT data achieved better agreement with measured values. 

Strains at the bottom of a large-scale experimental embankment dam were measured by fibre optics. The strains were measured in four sections, orthogonal to the dam axis. The fibre optic set up was operational after finalising the dam construction. The measurements were conducted continuously during the impoundment and later during the operation with an upstream water level of a maximum of 3.5 m. Based on the measurements, the post-construction behaviour of the dam was observed, indicating settlements. The foundation of the dam is inclined, which was captured by the measurements, with indications of more shear stresses developing at the downstream side. Variations of strains in the different dam zones were observed. The impoundment is causing most of the strain development in the dam. The strains develop further over time after the reservoir is at its maximum level, indicating a transient process is in place.

In 2019, Vattenfall AB together with several researchers, built an experimental embankment dam in Älvkarleby, Sweden. The dam was a four metres high small-scale conventional rockfill dam. The design and construction methods followed the Swedish dam safety guidelines for conventional dams. The dam was zoned, with a core made of glacial till, surrounded by fine and coarse filter zones and shoulder material. The dam was built and finalized in November 2019. Pore pressure, seepage, deformations and strains have been monitored in the dam. The impoundment of the dam was conducted in the first half of 2020, thereafter the water level in the reservoir was kept constant until early 2022. Then five cycles of emptying and filling up of the reservoir were conducted to evaluate the effects on the cyclic conditions of the embankment dam. In this contribution, the effects of varying reservoir levels on the dam body are assessed, based on the results from the monitoring of the pore pressures. Varying reservoir levels in hydropower dams, in a larger extent is usual today due to the new more flexible energy market, and are thus relevant for existing dams in the green transition.

Modelling can be used as a tool for prediction of the behaviour of embankment dams as a part of the dam safety work. It is advantageous to predict the performance and compare to measurements done, to obtain more knowledge about the dam behaviour, as these structures are complex and potential failures are hazardous. The research presented in this thesis covers parameter identification by backanalysis, interpretation of dam measurements and numerical predictions of dam behaviour. The research highlights the role of numerical modelling as a supportive tool in dam engineering, ratherthan a standalone technique. Two embankment dams were analysed in the research: a 45 metres high existing hydropower dam and a four metres high experimental dam built during the project.

The soil materials in an embankment dam vary significantly, as the zones in a dam have different functions. To create reliable numerical models, parameter values defining the stress-strain relationship of the materials are needed. Obtaining such information for existing embankment dams poses challenges, often due to limited available data and the potential risks associated with traditional field sampling methods. In previous research at Luleå University of Technology, inverse analysis was successfully applied to embankment dam calibration of finite element models against field measurements, by utilizing an optimisation code with a genetic algorithm for optimisation. Inverse analysis provides a non-destructive method for obtaining information about the stress-strain relationship of the material in a dam.

First, applications of inverse analysis are exemplified on an existing embankment dam. The study investigates the impact on the inverse analysis methodology when errors occur in the field measurements. The employed genetic algorithm showed its robustness when dealing with errors, this is important since errors are likely to occur in field measurements. Thereafter, the study examined the usage of parameters identified through inverse analysis in predictions of deformations when a stabilising berm was constructed on the downstream shoulder. The predicted deformations were compared to deformations from inclinometer measurements. The trend of the measured deformations was replicated in the numerical model, and the magnitudes were in the right order. The study shows that predicting future dam behaviour based on results from inverse analysis can be done reasonably well in this case.

Second, the mechanical behaviour of an experimental embankment dam is interpreted and modelled. Monitoring of pore pressure was done with transducers that were installed at different levels covering the whole core and parts of the filters. Measurements were performed continuously. The response of pore pressure in the core, during impoundment and operation, are focused on. A significant delay of the saturation front was observed, as the pore pressure in the bottom of the downstream part of the core was not building up as expected during impoundment and operation. Fully-coupled numerical analyses were performed, in order to better understand the conditions of the core in the experimental dam. The core was initially assumed to be homogeneous, but the numerical results showed poor agreement with the observed behaviour from field. By further analysing the measurements and modelling, the experimental dam was found to be non-homogeneous, even though it was built under very controlled conditions. Variations in the hydraulic conductivity in the dam core were therefore introduced in the numerical model. The hydraulic conductivity changed with height in the dam, was different in the vertical and horizontal direction and was also changing with time at specific places in the core. With these numerical adjustments better correlations against measurements were obtained, compared to the homogeneous case, indicating that homogeneous conditions are not suitable for the core. The study also showed that the values of parameters obtained from laboratory testing are not suitable for the whole core, as the conditions assumed in laboratory do not correspond to the prevailing field conditions.

Measurements of the strain development in the bottom of the embankment dam was done by fibre optics. The settlements in the dam body after construction are captured, mainly as areas of higher lateral strain at the toes of the dam. The 1% sloping foundation towards the downstream side is captured. The measurements show that more shear is activated at the downstream side. Impoundment causes the largest strains, it can be observed that the strains vary in the different dam zones the bottom of the dam body.

In summary, the research presented in this thesis has shown how numerical modelling can be used as support in dam engineering, when combined with monitoring data. Values of parameters that would have been difficult to retrieve otherwise are obtained by inverse analysis, making it possible to perform more reliable predictions. The modelling has also helped to explain unexpected behaviour from monitoring of pore pressure. When the experimental dam was built, it was expected that the core would be homogeneous. The monitoring of the dam and the numerical modelling revealed that the core was non-homogeneous. The experimental dam is small, and it was constructed under very controlled forms. Therefore, it is reasonable to assume that it would be difficult to construct a homogeneous core in a real, large embankment dams. This is an important finding in the thesis, which can influence both how dams should be numerically modelled as well as how dam safety assessments during first impoundment and the beginning of the operation phase should be done.

Suffusion is an internal erosion mechanism observed in embankment dams, caused by washing out fine-grained particles within the dam core as a consequence of seepage with various hydraulic gradients. The initiation of internal erosion is conditional upon three primary factors: grain size distribution, stress conditions, and hydraulic gradient. Graded moraines, such as glacial tills, exhibit an increased susceptibility to suffusion when compared to other soil types used in dam construction. Many Swedish embankment dams in mining and hydropower industry were constructed with glacial till cores over 50 years ago, lacking specific guidelines related to grain size boundaries for core and filter materials. This deficiency has given rise to instances of internal erosion, therefore elevating safety concerns.

Previous research aimed to enhance dam safety by exploring the susceptibility of glacial till soils to suffusion. Silva (2022) reviewed existing methods for assessing soil susceptibility, comparing testing conditions, and presents an experimental study on critical hydraulic gradients for suffusion initiation in glacial till soils. Results indicate the critical hydraulic gradient depends upon testing conditions, including axial loading, the rate of gradient increase, and time intervals. Furthermore, it underscores the efficacy of Kenney and Lau (1985, 1986) method for assessing suffusion susceptibility. These insights offer valuable contributions to the assessment and mitigation of internal erosion in embankment dams, thereby addressing a significant safety concern within the Swedish dams infrastructure.

Silva (2022) has offered valuable insights into suffusion phenomena while the intricacies of erosion processes remain unclear, hindering the implementation of rehabilitation measures to ensure the longevity of embankment dams. A follow-up project utilizing the advantages of transparent soil seeks to further comprehend the migration of fines within a soil matrix. Transparent soils, emerging as a viable alternative with likely properties to sand and clays, consist of a two-phase medium by refractive index allow solids to represent the soil skeleton and a fluid solution to mimic pore fluids. Various solids, such as amorphous silica, silica gel, hydrogel, fused quartz, and laponite, have been employed in conjunction with fluid solutions, depending on the solid, as mineral oil and paraffinic solvent, calcium bromide brine, sucrose solution, or water.

Transparent soils offer the potential to replicate the behavior of glacial till cores employed in embankments, particularly those designed for the storage of tailings material in the mining industry, and water retention for hydropower. The applicability of these findings may address and enhance rehabilitation measures in such structures, which are imperative to mitigate potential socio-economic and environmental ramifications in the event of failure. Given the escalating global demand for mining resources and renewable energy, proactive measures are essential to predict long-term issues looking for a more sustainable and efficient construction methodologies to extend infrastructure lifespans.

The core soil of an embankment dam can be exposed to deteriorating processes, i.e., different kinds of internal erosion due to high hydraulic gradients, disadvantageous particle size distribu-tion, too coarse-grained filters or built-in defects. During internal erosion, fines from the core soil are washed out by the seepage, decreasing the impervious properties of the core. If the internal erosion process is discovered in time, drilling and grouting can be performed to stop the erosion. During drilling and grouting, eroded material from the core soil is replaced.

In this paper, the methodology: “Identification – Localization – Characterization – Remediation” has been proposed. The methodology was tested on a large-scale embankment dam in a laboratory environment. The dam had a central core of moraine and was built inside a watertight concrete structure so a reservoir of water could be created upstream the dam. The left abutment of the dam had higher seepage rates than the rest of the dam and therefore had to be remediated.

During the identification and localization phase, a 10 x 10 cm horizontal, high hydraulic conduc-tivity zone through the core soil was identified and localized at the left abutment at 1 m depth. During drilling at the abutment, it was found that the core soil beneath the damage had become more wet compared to when built. The remedial method used was compaction grouting with a new developed type of non-hardening grout material. The grouting pressures equaled the height of the vertical grout material column with an additional pressure of ~50 kPa to compensate for frictional losses during injection. The grout material was delivered via a novel pipe system where water and air were allowed to be drained. The seepage was lowered by 44 % directly after grout-ing and 60% four months after grouting.

Fyllningsdammen i Burvattnet uppfördes på 1940-talet av jordmaterial som fanns på platsen. I samband med ombyggnation av bottenutskovet skulle fyllningsdammen rivas och återuppbyggas, vilket innebar en möjlighet att studera och dokumentera effekter av åldrande av dammen.