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Measurement of ice pressure on a concrete dam with a prototype ice load panel
Cold Regions Science and Technology 170 (2020) 102923
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Measurement of ice pressure on a concrete dam with a prototype ice load panel
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Rikard Hellgrena, , Richard Malma, Lennart Franssonb, Fredrik Johanssona, Erik Nordströma, Marie Westberg Wildea ⁎
a b
KTH Royal Institute of Technology, Department of Civil and Architectural Engineering, Stockholm, Sweden Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Luleå, Sweden
ARTICLE INFO
ABSTRACT
Keywords: Static ice loads Concrete dams Dam safety
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.
1. Introduction In cold regions, where the surface of a river or lake freezes during the winter, submerged structures (such as piers and bridges) and hydraulic structures (such as concrete dams) may be subjected to pressure from the formation, expansion or movement of ice sheets. This pressure can be significant and may constitute a large fraction of the total horizontal load acting on, for instance, a small concrete dam. From a damsafety perspective, it is important to determine the design value of the ice load. This requires a knowledge of the probability distribution function for the annual maximum ice load. Underestimation of the design load may jeopardize the safety of the dam, whereas an overestimation may lead to expensive and unnecessary strengthening. To define a suitable design value for this load, a knowledge of both the possible maximum ice pressure and of the variation and distribution in the maximum annual values is required. The maximum magnitude, as well as the seasonal variation of the ice load, are however uncertain and current understanding of ice loads is limited (Comfort et al., 2003; Gebre et al., 2013). Previous measurements show ice loads in the range of 100 kN/m to 460 kN/m (Adolfi and Eriksson, 2013; Sæther, 2012), whereas design ice line-loads vary between 50 kN/m and 250 kN/m (Sæther, 2012).
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Early models for ice loads were based on thermal stresses due to the restrained thermal expansion of the ice sheet (Bergdahl, 1978; Fransson, 1988; Carter et al., 1998; Petrich et al., 2015). However, it has been shown that, in addition to thermal expansion, a variation in water level may result in a significant contribution to the ice load, where a daily water level variation of about 0.5 m gives the greatest increase in ice load, (Comfort et al., 2003; Carter et al., 1998; Stander, 2006; Taras et al., 2011). A change in water level greater than the current ice thickness results in an immediate decrease in the ice load as the ice is detached from the dam, and the load is thus lowered during the rest of the ice season (Comfort et al., 2003). Other factors that may affect the size of the ice load are the geometry of the reservoir and its beaches, as well as the formation of cracks in the ice cover (Azarnejad and Hrudey, 1998; Carter et al., 1998), and the expected ice pressure on a dam also depends on its geographical location (Carter et al., 1998). Previous measurements have shown that the stress in the ice and the resulting load acting on the structure have a large spatial variation and can vary significantly over a distance of a few meters along the dam at the same time (Taras et al., 2011). This variation is described as chaotic, and the difference between the pressure measured at the ice-dam interface and the stresses in the measured ice sheet in the reservoir varies even more than the stresses along the dam face (Morse et al., 2011).
Corresponding author. E-mail address: [email protected] (R. Hellgren).
https://doi.org/10.1016/j.coldregions.2019.102923 Received 21 March 2019; Received in revised form 12 July 2019; Accepted 28 October 2019 Available online 07 November 2019 0165-232X/ © 2019 Elsevier B.V. All rights reserved.
Cold Regions Science and Technology 170 (2020) 102923
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There are also indications, that even when large peaks occur locally, the average load over a longer distance such as a dam monolith remains lower (Côté et al., 2012). This applies to piers, ships and offshore structures where a reduction of the ice load for larger structural elements is generally implemented in the design codes; see for instance (ISO-19906:2010, 2010; Swedish Maritime Services, 2014; CAN/CSAS472–92, 1992). The ice load on a dam is usually measured with internal stress sensors (Carter et al., 1998; Stander, 2006; Petrich et al., 2015), where measurements at several depths are required to estimate the resultant force over the cross-section of the ice sheet. Internal ice stresses are measured by installing sensors that record the stress or strain in the ice sheet, either by strain gauges or by flat-jacks. A flat-jack consists of an oil-filled sheet-pad with two plates where the oil pressure is measured continuously. The ice pressure is assumed to correspond to the oil pressure. One disadvantage of this type of measurement is that the gauge is usually installed after the ice formation, and the ice sheet is then inevitable disturbed during installation. Another method of measuring an ice load is the interfacial method, where load panels are attached to a structure. A load panel usually consists of two rigid steel plates placed on each side of a material with a known stiffness. The Carter panel consists of four-flat jack stress sensors mounted on a 0.4 m wide, 1 m high and 1 cm thick stainless steel plate to form a load panel (Côté et al., 2012; Carter et al., 1998). Several factors influence the accuracy of the pressure measurements. The pressure is measured only at discrete localized points, and this means that the results must be extrapolated to cover the entire thickness. As the sensors measure only the compressive stresses, they may overestimate the load for an ice sheet subjected to bending, where part of the ice sheet cross-section is in tension. Other factors such as the stress distributions around the cast-in pressure sensor and the thermal expansion of the sensor may also influence the results. For these reasons, it is unclear whether the ice loads measured using these techniques are representative for the ice load acting on the dam. It is, therefore, necessary to further develop methods to measure the ice loads on dams and to continue to evaluate the ice load acting on dams through measurements. In this paper, we address this issue by presenting a prototype ice load panel and reporting the results of measurements of ice load from 2016 to 2018 at the Rätan hydropower dam in Sweden. The ice load panel is attached at the upstream face of the dam and is large enough so that the whole thickness of the ice remains in contact with the panel as the water level varies. The total pressure exerted by the ice is thus measured without interpolation. The panel can also measure tensile forces and the position of the resultant force to the ice pressure can be determined directly from the measured data.
spherical contact surface against the lid. Load cells and screws are placed so that the lid can be considered as a simply supported beam. The screws were pre-stressed so that all three load cells showed 100 kN when the load panel was placed horizontally without external loads. The pre-stressing ensures water-tightness between the plate and the lid and enables tensile and compressive forces from the ice to be measured. 2.2. The dam To test the ice load panel, a concrete dam was selected where the ice load is reasonably idealized against the dam structure. The following criteria were considered for selection of a suitable dam site:
• Geographic location • Distance to spillways and discharge • Amplitude in the reservoir • Dam type • Vertical upstream dam face • Instrumentation level • Heating of the interior of the dam • Insulation wall on the downstream face of the dam From this, three requirements and some preferences were identified. Sweden is a long country, with over 1500 km from north to south. The climate differs over these regions, and the probability of ice and significant ice pressures also differs. This is reflected in the Swedish guidelines, where the design ice load is four times greater in the northern part of Sweden than in the south (RIDAS, 2017). To ensure the presence of ice loads during the test period, the first requirement was that the dam be located in the northern region of Sweden, where the design ice load is 200 kN/m. A long dam was desired to minimize the risk of localized pressure behavior in the ice sheet due to boundary effects at the beaches or from spillways and intakes. Most of the Swedish dam owners use de-icing either by heat or by air circulator as an integral part of their ice management strategy for the flood gates. Since the ice around the flood gates is removed, the ice pressure acting on dam sections located close to a spillway is reduced. The opening in the ice cover may lead to arching which transfers higher loads to the nearby sections, and to a general reduction in the thickness and strength of the ice sheet, which leads to a lower ice pressure. Consequently, the second requirement was that the chosen dam should have a minimum of 28 m continuous dam line and a low probability of winter discharge via the spillways, to ensure an undisturbed ice sheet at the location of the measurement. Since the load panel is attached at a fixed position, low variations in the upstream water level were required to limit the height of the load panel. Therefore, a criterion that the amplitude between the normal top and the minimum operating level should be less than 1.0 m. Small reservoir variations also reduce the risk that the ice sheet breaks from the dam face and ensure that the horizontal ice pressure is as constant as possible. Besides these requirements, some preferences were identified that would simplify and enhance the interpretation of the measurements. The ice load affects the displacements and stresses in the dam, and a dam with a high degree of instrumentation therefore was desired as this would allow the ice load measurements to be verified with a hindcast calculation. For a concrete dam located in the Swedish climate with small water level variation, the seasonal temperature variation is the dominating cause of displacement. To be able to isolate the effects of ice pressure on the dam, the aim was to find a dam where these movements were limited and where the ice load was expected to be a significant part of the load. For this purpose, a dam with a downstream insulation wall and installed heating was desirable. Based on these criteria, several suitable dams were proposed, and the concrete buttress dam at the Rätan hydropower station was chosen, see Fig. 2a. The dam meets all requirements such as low reservoir
2. Development and installation of the ice load panel 2.1. The ice load panel The design of the load panel was inspired by the panels used for the measurements of ice pressure against the Norströmsgrund lighthouse in the Gulf of Bothnia, which was based on an interfacial method (Fransson and Lundqvist, 2006). Fig. 1a shows a 3D sketch of the new load panel, which consists of a rigid steel plate and a lid. It is 1 × 3 m2 in size, see Fig. 1b. The lid is made of homogeneous steel with a thickness of 16 cm and is placed on three compression-only load cells of type HBM (50 t, C2). Two load cells are placed close to the top, and one is placed near the bottom of the panel. The high stiffness of the solid lid limits elastic deformation and causes the lid to behave as a solid body. The total weight of the panel is 6.5 t. The lid is sealed by an O-ring placed around the bottom plate and a vulcanized rubber cover around the sides of the panel. The use of three load cells instead of four prevents the panel from rocking and ensures that the location of the resultant force is measured correctly. The lid is attached to the steel plate by three screw connections in line with the load cells; formed with a 2
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a
b
Fig. 1. Illustration of the ice load panel, a) 3D sketch, b) dimensions including position on the dam and maximum (MAWL), and minimum water levels (MIWL).
amplitude, long dam and reservoir length, good instrumentation level, and a vertical front plate. Rätan is a 28 m high traditional Swedish buttress dam constructed in 1968 and located in the northern region of Sweden. The dam crest is located at an altitude of +352.5 m above sea level (m.a.s.l). The water level in the reservoir varies between the maximum water level (MAWL), +350.44 m.a.s.l, and the minimum operating level (MIWL), +349.94 m.a.s.l. In 2011, an insulation wall was installed on the downstream side of the dam to limit the displacements caused by seasonal temperature variation. After the insulation wall had been installed, the instrumentation for dam-safety monitoring was updated. The dam-safety monitoring at the monolith where the panel is installed consists of pendulum measurement of crest displacements, strain gauges on the front plates and buttress. Recorded data for several external factors are also available. These data are obtained from measurements at the dam, related to dam safety and water management, as well as from the open database of the Swedish Meteorological and Hydrological Institute (SMHI, 2018). Fig. 3 shows, the daily average values for a selection of external factors related to the ice load. The raw data from these measurements have different sampling frequencies. The water level (a) and discharge through turbines and spillways (b) are sampled every 15 min. The temperature of the water in the reservoir (measured at three meters depth) and the ambient air temperature at the dam are measured every hour (c). Daily recordings of precipitation and snowfall from SMHI are available from the weather station Rätan-Handsjön, which is located approximately 7 km east of the dam (in the downstream direction) at a slightly lower elevation (MAWL +350.44 vs. Rätan-Handsjön station +296.0) (d). The data are available in supplementary material A.
3. Results The measured daily max and median ice load are shown in Fig. 4. It is normal practice to discuss ice loads for dams as line loads (N/m). Hence, the measured ice load is hereafter given as line load where the total load has been divided by the length of the panel, which is 1.0 m. Measurements from 2016 are available from the day of installation in February until July, when the logger was disconnected. The panel was thereafter left on the dam over the summer and was thus already in the water prior to the freezing period in the following winter, i.e. the winter of 2016/17. The data-logger was reconnected on November 8, when an ice sheet had already formed on the reservoir. The pressure against the panel was measured with five minutes intervals until the beginning of May. During the next winter, 2017/2018, the computer logger was started in October 26 when reservoir had not yet begun to freeze. On February 12, 2018, the measuring computer crashed for an unknown reason. Since this was not discovered until May, when the winter measurements were to be collected, the data for the spring 2018 are missing. The sampling rate during the measurements was 5 min intervals. When the measured data was analyzed, it was shown that the variations in ice load occur at a much slower rate. For the ice load data to be consistent with the other data, the measurement data have been downsampled, retaining the data for the point closest to every full hour from the ice load data. The ice load data from the three load cells and the total load are available in supplementary material A. 3.1. Calibration and reference values The load cells were calibrated and validated in the laboratory before the installation of the panel. An external load was applied via a hydraulic jack. Four calibrations were performed with the load placed at the center of the panel and over each load cell. The external load was increased to a maximum value of 800 kN and 700 kN for the centric and eccentric loads, respectively. The results show that the seal between the back plate and lid carries some of the applied force but that there is a linear relationship between the applied load and the measured load with a small hysteresis effect. To account for that, a correction factor of 1.25 was used, where the sum of the load cell data is multiplied by the correction factor. Regardless of position, the sum of the load cell multiplied with the correlation factor equals the applied load. As the load cells are pre-stressed, the measurement data have been adjusted so that the measured load is zero when the reservoir is ice-free. The mean of the MAWL and the MIWL, +350.19 m.a.s.l, was chosen as the baseline water level. The calibration was made using data from each
2.3. Installation of the load panel The panel was installed at the dam on 25 February 2016, and a ice thickness of 80 cm was measured at the time of installation. A 120 × 30 cm2 hole was cut in the ice sheet where the panel was placed. The panel was installed with the bottom level at +348.0 m.a.s.l, see Fig. 1b. After the ice in the hole had been removed, the ice attached to the upstream concrete surface was mechanically removed. However, it was estimated that occasional spots with 3–4 cm thick ice remained between the panel and the concrete at the time of installation. The panel was lifted into the correct place with a crane and was attached to the dam vertically by two chains and horizontally with four M20 bolt fasteners, see Fig. 2b. After the installation, the hole between the panel and the ice sheet was filled with the removed ice debris. 3
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Fig. 2. Rätan hydropower dam, a) installed insulation wall on the downstream side (photo; Martin Rosenqvist), b) ice load panel installed at the upstream water line (Photo; Lennart Fransson).
load cell taken at a time when the reservoir was ice-free, and the water surface was as close to the baseline level as possible. For the two first seasons, measurements taken on 18-May 2016 01:00 were used for the calibration. The following data were measured: WL = +350.19 m.a.s.l, LC1 = 87.25 kN, LC2 = 103.9 kN, LC3 = 117.0 kN, Ltot = 308.5 kN/m, where WL is the water level, LCx is load cell x and Ltot is the total load. For the last season, the calibration data were taken on 26 October 2017 18:00, and the following data were measured: WL = 350.19 m.a.s.l, LC1 = 89.65 kN, LC2 = 106.0 kN, LC3 = 123.6 kN, Ltot = 319.3 kN/m. The difference for each load cell was in the range of 2.1–6.6 kN, and the difference in total load was 11.1 kN/m. Compared to the measured maximum loads, this difference was approximately 7%. No adjustment has been made to take into account the variations in water level. This means that the theoretical difference in hydrostatic pressure is from +5.7 kN/m to −5.1 kN/m for maximum and minimum water levels, respectively. Fig. 5a Shows the expected theoretical hydrostatic load compared to the measured load as a function of the water level for measurements registered on July 2016. During this time, the reservoir was guaranteed ice-free and this figure therefore shows how well the load panel performs when the water level variation is the only external factor affecting the total load on the panel. For a given water level, the
Fig. 3. (a) the water level, (b) discharge through turbines and spillways, (c) temperature of the water in the reservoir measured at three meters depth and the air temperature at the dam, (d) precipitation and snowfall from SMHI:s weather station Rätan-Handsjön.
Fig. 4. Max and median ice load from Rätan dam measured during three winters. 4
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together with the level of the resultant load and the water level in February 2017. These three coincide with the daily pattern that follows the operation of the power station. In the evening, the turbine is stopped, and the load against the panel increases as a result of the rising water level in the reservoir. The ice load increases with a short delay, starting just before midnight and reaches its maximum value the following morning. The discharge is then restarted and the water level and ice load decrease. The ice load reaches a minimum value before lunch and is thereafter stable at a low value during the day. The water level continues to drop until the discharge is again stopped the following evening. The position of the resultant force is strongly correlated with the size of the ice load. For the lower residual ice-load, the resultant is located below the water level. But the resultant of the peak loads is located above the water level. For some of the peaks, the resultant force is even located above the retention water level, with a maximum level of +350.62 m.a.s.l, 18 cm over MAWL. This is negative for dam stability, since it means a higher lever arm for the overturning stability than that in the Swedish guidelines where the ice load is assumed to occur 33 cm below the water level if the ice thickness is 1 m. Fig. 8a shows a contour plot of the average total load related to the position of the resultant force. This is relevant from a dam stability perspective where the position of the resultant must be determined for the stability of the overturning failure mode to be calculated. For a resultant force located within the area of the reservoir minimum and maximum operating level, the average load is under 50 kN/m, and the resultant force is located over the top operating water level when the peak loads occur. The greater ice loads thus coincide with an increase in the result position. Fig. 8b shows a contour plot of the number of times the resultant force was located in a specific area, and it can be seen that the normal position of the ice load resultant force is under the operation level. The ice pressure is relatively symmetric in the transverse direction, as the position of the resultant force does not vary by more than ± 20 cm in the transverse direction. Fig. 9 shows the ice load, air temperature, water level and discharge on five days in February 2017 when ice loads peaks occurred three days in succession. Here the relation between the measured ice load, the temperature and the daily operation of the reservoir can be studied in more detail. For the peak occurring on 12 February, the inverse of the temperature and the ice load covary. However, this relationship is not present in the case of other peaks and therefore seems more spurious than causal. The water level and the ice load covary all days when ice load events occur, but the water level varies in the same way on the first day without causing an ice load event. On the last day, a more significant drop in water level occurs and the peak in ice load on that day is significantly reduced. Notable is that the drop in ice load is immediate whereas it takes longer time for the water level to decrease. The drop in ice load seems to be related more to the start of turbines than to the decrease in water level. When the discharge was temporarily lowered at mid-day on 11 February, a small peak in the ice load occurred with only a small change in water level.
Fig. 5. (a) Theoretical, expected, hydrostatic pressure compared with measured loads during July 2016, (b) Distribution of errors between the expected and measured hydrostatic loads.
measured load can vary as much as the expected difference in hydrostatic load between the maximum and minimum levels. Fig. 5b Shows the distribution of the errors between the theoretical and the measured loads. The mean error was 0.49 kN/m and the standard deviation was 1.95 kN/m. Whether the size of this error is absolute or relative, cannot be determined from these data. The errors show no clear relation to the variation in water level or the air temperature. 3.2. Force and resultant force Fig. 4 shows registered daily maximum and median values from the load panel from the three winters. During the first season, in the spring of 2016, peak values of 161 kN/m were registered. These peaks occurred during the first two weeks after installation and thereafter the measured pressure was constantly low with a few individual short peaks. The highest measured load after the first two weeks was 30 kN/ m. From the reconnection of the logger in November 2016 for the second winter until the middle of January 2017, the registered load acting on the panel was relatively constant between 20 and 40 kN/m. Thereafter, the daily maximum load started to increase, while the median value remained at about 50 kN/m. During this period, daily peaks with a magnitude of 70 kN/m occurred. This behavior continued with high residual loads during February, March and early April. The peak values in mid-February were approximately 120 kN/m and a single peak at the end of march reached 164 kN/m. In the 2017/2018 season, the pattern for the initial part of the winter was basically the same as that of the previous year except for some individual smaller peaks. Large daily peaks in January and early February were absent and peak value being 61 kN/m. The 2017/2018 winter was cold, with unusually large amounts of snow, see Fig. 3. It is possible that the part of the winter with a high ice load was delayed and occurred after the measurement recording had crashed. Fig. 6 shows the daily ice load plotted together with the outdoor temperature measured at the dam. The ice-load events with the large peaks appear to be inversely related to the mean air temperature, i.e. most of the load peaks are associated with a drop in temperature. This inverse relationship was not expected, as ice loads are expected to be due to expansion of the ice caused by a temperature increase. The size of the drop in temperature does not correlate to the size of the increase in ice load. For example, from the 3 March to 6 March, the temperature dropped from 0 degrees to −10 °C and the ice load increase from 38 to 84 kN/m, which can be compared to the period 28–30 March, when the temperature dropped from 3 to −1 °C while the ice load reached the seasons peak value of 164 kN/m. Since the temperature in the ice was not measured, no data regarding the actual temperature field in the ice is available. From the three load cells, it is possible to calculate the position of the resultant of the ice load. Fig. 7 shows the measured ice load
3.3. Observations In addition to the measurements, several photographs have also been taken during the winter of 2016/2017. Photos taken immediately after the recording of the greatest values in mid-February 2017 shown in Fig. 10, show a clear crack in the ice sheet along the upstream surface as well as a probable crack located a few meters out from the dam where the slope of the ice surface changes. The position of this fracture is consistent with previous observations and is believed to be the primary mechanism causing ice loads due to variations in water level (Stander, 2006; Comfort et al., 2003). The two fractures act as hinges as the ice sheet between them rotates as the water level varies. When the reservoir is at the top or bottom level, the cracks open and are then filled with water which freezes. When the reservoir level returns to a 5
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Fig. 6. Daily measured ice load and temperature during spring 2017.
level where the ice sheet is completely horizontal, the extra ice in the crack must be compressed for the ice sheet to fit into a horizontal position. It can also be seen in the photographs that the crack in the ice sheet has a curved shape around the panel and that there is a collection of ice and snow in the direct proximity of the panel. These observations make it clear that the panel is disturbing the ice and that the ice sheet at the panel differs from the other ice along the dam. It is not possible from the measurements made, to determine the implications of these observations.
the variation in hydrostatic pressure from water level fluctuations for an ice-free reservoir was nevertheless poor. The errors absolute standard deviation was 1.95 kN. If this is an absolute error it has a negligible impact on the peak load. How this measurement error relates to other measurement methods is unknown, as no similar presentation was found for other measurements. The greatest ice load peaks were due to a variation in water level, which is in line with previous reports (Comfort et al., 2003; Carter et al., 1998; Stander, 2006; Taras et al., 2011). The ice load, the water level and the position of the resultant force of the ice load all follow a daily pattern related to the operation of the power station. However, this relationship is not absolute and it is therefore difficult to treat in an analytical perspective. The water level varies every day, while ice load peaks occur only approximately 40 days a year. During these 40 days, the increase in the ice load correlates with an increase in water level. The water level change thus comes into play first when a certain number of other criteria are met, but it is useless as a sole explanatory mechanism. Based on the findings by Côté et al. (2012), that the ice pressure against the dam varies greatly along the dam face at the same time, it may be difficult to derive a relationship between external factors and measured ice load without extensive further measurements.
4. Discussion The maximum recorded ice load was 161 kN/m, 164 kN/m and 61/ m kN for the winters of 2015/2016, 2016/2017 and 2017/2018, respectively, but the results for the first season when the panel was installed in February are unreliable due to the major disruption of the ice when the load panel was installed. The last season contains measurements only until February. The function of the load panel was validated in the laboratory before transportation and installation, but the panels ability to measure
Fig. 7. Hourly ice load, water level and level of force resultant durring February 2017. 6
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Fig. 8. (a) Meassuered mean total ice load by resultant location and (b) number of regristerd load per resultant location. The horizontal ax load panel.
For further measurements with the load panel, several load panels or “dummies” (without measuring equipment) should be placed next to each other to give a plane surface adjacent to the load panel. The measurements should also be completed with more panels along the dam line. It is nevertheless possible that the panel will create a local effect on the ice sheet since both its thermal properties and surface properties differ from the properties of the dam. Since it is not possible to design a load panel with thermal properties similar to concrete, this effect will always be present. As the next step, it is necessary to compare the results obtained with the load panel with those obtained by other methods. The three different types of sensors, Carter panels, BP gauges and Biaxial gauges, yields similar time history of ice load when used at the same dam (Taras et al., 2011). If the load panel also yields similar results as the other sensors, the belief in the correctness of both the load panel and previous measurements is increased. However, if the results differ, the concerns regarding previous measurements raised in the introduction remain, and the effort to validate the load panel must be continued. The dam on which the panel was installed was chosen to provide large ice loads, with a location in northern Sweden, a water level variation profile that was expected to cause significant loads and long distances to spillways and the water intake for the turbine to represent an undisturbed part of the ice sheet. These criteria must be considered
when evaluating the measurement results. The large values measured at Rätan are probably not representative of dams where these conditions are not meet, and a clear conclusion from this study is that more measurements are needed at other dams. After the load panel is validated, a more extensive measurement campaign at several dams can be designed to find the effect of variation in the external factors. 5. Conclusion In this paper, we present the design and installation of a prototype ice load panel with direct measurement of the ice pressure acting on the dam. The ice load panel is attached at the upstream face of the dam and is large enough to ensure that the whole cross-section of the ice remains in contact with the panel as the water level varies. The total pressure from the ice is thus measured without interpolation. The design of the panel was based on previous experience from similar measurements with sea ice in the Gulf of Bothnia. The panel was installed at the Rätan hydropower dam located in the northern region of Sweden in February 2016, and the location was chosen to provide high ice loads. The maximum recorded loads were 161 kN/m, 164 kN/m and 61 kN/m for the winters of 2015/2016, 2016/2017 and 2017/2018, respectively. A variation in water level is the main mechanism causing the peak
Fig. 9. Ice load, air temperature, water level and discharge.
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Fig. 10. Photos taken of the ice load panel 2017-02-17.
ice loads. During periods when daily ice load peaks occurred, both the ice load and the water level followed a daily pattern related to the operation of the power station. From observations of the ice it is clear that the panel disturbs the naturally formed ice sheet and it cannot be excluded that it may affect the measurement results. The next step is to add dummy panels next to the panel to obtain a plane surface adjacent to the load panel, and to compare the load panel with traditional ice load measurements and models.
Can. J. Civ. Eng. 25 (3), 557–568. Bergdahl, L., 1978. Thermal Ice Pressure in Lake Ice Covers. Chalmers University of Technology, Göteborg, Sweden. CAN/CSA-S472–-92, 1992. General Requirements, Design Criteria, the Environment, and Loads. (A National, s.l.: CAN/CSA-S472–92). Carter, D., et al., 1998. Ice thrust in reservoirs. J. Cold Reg. Eng. 4 (12), 169–183. Comfort, G., Gong, Y., Singh, S., Abdelnour, R., 2003. Static ice loads on dams. Can. J. Civ. Eng. 42–68. Côté, A., Taras, A., Comfort, G., Morse, B., 2012. Hydro Québec ice Load Measurement Program. Saskatoon, SK, Canada, CDA 2012 Annual Conference. Fransson, L., 1988. Thermal ice pressure on structures in ice covers. In: Doctoral thesis 88:67D. Luleå University of Technology, Luleå. Fransson, L., Lundqvist, J.-E., 2006. A statistical approach to extreme ice loads on lighthouse Norströmsgrund. Hamburg, Germany, OMAE. Gebre, S., et al., 2013. Review of ice effects on hydropower systems. J. Cold Reg. Eng. 27 (4), 196–222. ISO-19906:2010, 2010. ISO_19,906 Arctic offshore structures. Petroleum and natural gas industrie, Geneva. Morse, B., et al., 2011. Spatial-Temporal Variability of Static ice Forces. Montréal, Canada, 21st International Conference on Port and Ocean Engineering under Arctic Conditions (POAC’11). Petrich, C., et al., 2015. Time-dependent spatial distribution of thermal stresses in the ice cover of a small reservoir. Cold Reg. Sci. Technol. 120, 35–44. RIDAS, 2017. Swedish Hydropower Companies Guidelines for dam Safety, application guideline 7.3 Concrete dams (In Swedish), Stockholm: Swedish Energy. Sæther, I., 2012. Methods for Prediction of Thermally Induced Ice Loads on Dams and Hydro Elictral Structures. Northern research Institute, Trondheim. Services, Swedish Maritime, 2014. Finnish-Swedish Ice Class Rules: The Structural Design and Engine Output Required of Ship. Swedish Maritime services, Norrköping. SMHI, 2018. Swedish Meteorological and Hydrological Institute Open data. ([Online] Available at: opendata-download.smhi.se [Accessed 15 062018]). Stander, E., 2006. Ice Stresses in Reservoirs: Effect of Water Level Fluctuations. J. Cold Reg. Eng. 20 (2), 52–67. Taras, A., et al., 2011. Measurements of Ice thrust at arnprior and barrett chute dams. Winnipeg 317–328.
Acknowledgements This work was carried out as a part of “Swedish Hydropower Centre - SVC VK10817”. SVC has been established by the Swedish Energy Agency, Elforsk and Svenska Kraftnät together with Luleå University of Technology, KTH Royal Institute of Technology, Chalmers University of Technology and Uppsala University. www.svc.nu. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coldregions.2019.102923. References Adolfi, E., Eriksson, J., 2013. The Ice Loads Effect on the Probability of Sliding and Overturn Failure of Concrete dams. KTH, Stockholm. Azarnejad, A., Hrudey, T.M., 1998. A numerical study of thermal ice loads on structures.
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Measurement of ice pressure on a concrete dam with a prototype ice load panel
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Rikard Hellgrena, , Richard Malma, Lennart Franssonb, Fredrik Johanssona, Erik Nordströma, Marie Westberg Wildea ⁎
a b
KTH Royal Institute of Technology, Department of Civil and Architectural Engineering, Stockholm, Sweden Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Luleå, Sweden
ARTICLE INFO
ABSTRACT
Keywords: Static ice loads Concrete dams Dam safety
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.
1. Introduction In cold regions, where the surface of a river or lake freezes during the winter, submerged structures (such as piers and bridges) and hydraulic structures (such as concrete dams) may be subjected to pressure from the formation, expansion or movement of ice sheets. This pressure can be significant and may constitute a large fraction of the total horizontal load acting on, for instance, a small concrete dam. From a damsafety perspective, it is important to determine the design value of the ice load. This requires a knowledge of the probability distribution function for the annual maximum ice load. Underestimation of the design load may jeopardize the safety of the dam, whereas an overestimation may lead to expensive and unnecessary strengthening. To define a suitable design value for this load, a knowledge of both the possible maximum ice pressure and of the variation and distribution in the maximum annual values is required. The maximum magnitude, as well as the seasonal variation of the ice load, are however uncertain and current understanding of ice loads is limited (Comfort et al., 2003; Gebre et al., 2013). Previous measurements show ice loads in the range of 100 kN/m to 460 kN/m (Adolfi and Eriksson, 2013; Sæther, 2012), whereas design ice line-loads vary between 50 kN/m and 250 kN/m (Sæther, 2012).
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Early models for ice loads were based on thermal stresses due to the restrained thermal expansion of the ice sheet (Bergdahl, 1978; Fransson, 1988; Carter et al., 1998; Petrich et al., 2015). However, it has been shown that, in addition to thermal expansion, a variation in water level may result in a significant contribution to the ice load, where a daily water level variation of about 0.5 m gives the greatest increase in ice load, (Comfort et al., 2003; Carter et al., 1998; Stander, 2006; Taras et al., 2011). A change in water level greater than the current ice thickness results in an immediate decrease in the ice load as the ice is detached from the dam, and the load is thus lowered during the rest of the ice season (Comfort et al., 2003). Other factors that may affect the size of the ice load are the geometry of the reservoir and its beaches, as well as the formation of cracks in the ice cover (Azarnejad and Hrudey, 1998; Carter et al., 1998), and the expected ice pressure on a dam also depends on its geographical location (Carter et al., 1998). Previous measurements have shown that the stress in the ice and the resulting load acting on the structure have a large spatial variation and can vary significantly over a distance of a few meters along the dam at the same time (Taras et al., 2011). This variation is described as chaotic, and the difference between the pressure measured at the ice-dam interface and the stresses in the measured ice sheet in the reservoir varies even more than the stresses along the dam face (Morse et al., 2011).
Corresponding author. E-mail address: [email protected] (R. Hellgren).
https://doi.org/10.1016/j.coldregions.2019.102923 Received 21 March 2019; Received in revised form 12 July 2019; Accepted 28 October 2019 Available online 07 November 2019 0165-232X/ © 2019 Elsevier B.V. All rights reserved.
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There are also indications, that even when large peaks occur locally, the average load over a longer distance such as a dam monolith remains lower (Côté et al., 2012). This applies to piers, ships and offshore structures where a reduction of the ice load for larger structural elements is generally implemented in the design codes; see for instance (ISO-19906:2010, 2010; Swedish Maritime Services, 2014; CAN/CSAS472–92, 1992). The ice load on a dam is usually measured with internal stress sensors (Carter et al., 1998; Stander, 2006; Petrich et al., 2015), where measurements at several depths are required to estimate the resultant force over the cross-section of the ice sheet. Internal ice stresses are measured by installing sensors that record the stress or strain in the ice sheet, either by strain gauges or by flat-jacks. A flat-jack consists of an oil-filled sheet-pad with two plates where the oil pressure is measured continuously. The ice pressure is assumed to correspond to the oil pressure. One disadvantage of this type of measurement is that the gauge is usually installed after the ice formation, and the ice sheet is then inevitable disturbed during installation. Another method of measuring an ice load is the interfacial method, where load panels are attached to a structure. A load panel usually consists of two rigid steel plates placed on each side of a material with a known stiffness. The Carter panel consists of four-flat jack stress sensors mounted on a 0.4 m wide, 1 m high and 1 cm thick stainless steel plate to form a load panel (Côté et al., 2012; Carter et al., 1998). Several factors influence the accuracy of the pressure measurements. The pressure is measured only at discrete localized points, and this means that the results must be extrapolated to cover the entire thickness. As the sensors measure only the compressive stresses, they may overestimate the load for an ice sheet subjected to bending, where part of the ice sheet cross-section is in tension. Other factors such as the stress distributions around the cast-in pressure sensor and the thermal expansion of the sensor may also influence the results. For these reasons, it is unclear whether the ice loads measured using these techniques are representative for the ice load acting on the dam. It is, therefore, necessary to further develop methods to measure the ice loads on dams and to continue to evaluate the ice load acting on dams through measurements. In this paper, we address this issue by presenting a prototype ice load panel and reporting the results of measurements of ice load from 2016 to 2018 at the Rätan hydropower dam in Sweden. The ice load panel is attached at the upstream face of the dam and is large enough so that the whole thickness of the ice remains in contact with the panel as the water level varies. The total pressure exerted by the ice is thus measured without interpolation. The panel can also measure tensile forces and the position of the resultant force to the ice pressure can be determined directly from the measured data.
spherical contact surface against the lid. Load cells and screws are placed so that the lid can be considered as a simply supported beam. The screws were pre-stressed so that all three load cells showed 100 kN when the load panel was placed horizontally without external loads. The pre-stressing ensures water-tightness between the plate and the lid and enables tensile and compressive forces from the ice to be measured. 2.2. The dam To test the ice load panel, a concrete dam was selected where the ice load is reasonably idealized against the dam structure. The following criteria were considered for selection of a suitable dam site:
• Geographic location • Distance to spillways and discharge • Amplitude in the reservoir • Dam type • Vertical upstream dam face • Instrumentation level • Heating of the interior of the dam • Insulation wall on the downstream face of the dam From this, three requirements and some preferences were identified. Sweden is a long country, with over 1500 km from north to south. The climate differs over these regions, and the probability of ice and significant ice pressures also differs. This is reflected in the Swedish guidelines, where the design ice load is four times greater in the northern part of Sweden than in the south (RIDAS, 2017). To ensure the presence of ice loads during the test period, the first requirement was that the dam be located in the northern region of Sweden, where the design ice load is 200 kN/m. A long dam was desired to minimize the risk of localized pressure behavior in the ice sheet due to boundary effects at the beaches or from spillways and intakes. Most of the Swedish dam owners use de-icing either by heat or by air circulator as an integral part of their ice management strategy for the flood gates. Since the ice around the flood gates is removed, the ice pressure acting on dam sections located close to a spillway is reduced. The opening in the ice cover may lead to arching which transfers higher loads to the nearby sections, and to a general reduction in the thickness and strength of the ice sheet, which leads to a lower ice pressure. Consequently, the second requirement was that the chosen dam should have a minimum of 28 m continuous dam line and a low probability of winter discharge via the spillways, to ensure an undisturbed ice sheet at the location of the measurement. Since the load panel is attached at a fixed position, low variations in the upstream water level were required to limit the height of the load panel. Therefore, a criterion that the amplitude between the normal top and the minimum operating level should be less than 1.0 m. Small reservoir variations also reduce the risk that the ice sheet breaks from the dam face and ensure that the horizontal ice pressure is as constant as possible. Besides these requirements, some preferences were identified that would simplify and enhance the interpretation of the measurements. The ice load affects the displacements and stresses in the dam, and a dam with a high degree of instrumentation therefore was desired as this would allow the ice load measurements to be verified with a hindcast calculation. For a concrete dam located in the Swedish climate with small water level variation, the seasonal temperature variation is the dominating cause of displacement. To be able to isolate the effects of ice pressure on the dam, the aim was to find a dam where these movements were limited and where the ice load was expected to be a significant part of the load. For this purpose, a dam with a downstream insulation wall and installed heating was desirable. Based on these criteria, several suitable dams were proposed, and the concrete buttress dam at the Rätan hydropower station was chosen, see Fig. 2a. The dam meets all requirements such as low reservoir
2. Development and installation of the ice load panel 2.1. The ice load panel The design of the load panel was inspired by the panels used for the measurements of ice pressure against the Norströmsgrund lighthouse in the Gulf of Bothnia, which was based on an interfacial method (Fransson and Lundqvist, 2006). Fig. 1a shows a 3D sketch of the new load panel, which consists of a rigid steel plate and a lid. It is 1 × 3 m2 in size, see Fig. 1b. The lid is made of homogeneous steel with a thickness of 16 cm and is placed on three compression-only load cells of type HBM (50 t, C2). Two load cells are placed close to the top, and one is placed near the bottom of the panel. The high stiffness of the solid lid limits elastic deformation and causes the lid to behave as a solid body. The total weight of the panel is 6.5 t. The lid is sealed by an O-ring placed around the bottom plate and a vulcanized rubber cover around the sides of the panel. The use of three load cells instead of four prevents the panel from rocking and ensures that the location of the resultant force is measured correctly. The lid is attached to the steel plate by three screw connections in line with the load cells; formed with a 2
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a
b
Fig. 1. Illustration of the ice load panel, a) 3D sketch, b) dimensions including position on the dam and maximum (MAWL), and minimum water levels (MIWL).
amplitude, long dam and reservoir length, good instrumentation level, and a vertical front plate. Rätan is a 28 m high traditional Swedish buttress dam constructed in 1968 and located in the northern region of Sweden. The dam crest is located at an altitude of +352.5 m above sea level (m.a.s.l). The water level in the reservoir varies between the maximum water level (MAWL), +350.44 m.a.s.l, and the minimum operating level (MIWL), +349.94 m.a.s.l. In 2011, an insulation wall was installed on the downstream side of the dam to limit the displacements caused by seasonal temperature variation. After the insulation wall had been installed, the instrumentation for dam-safety monitoring was updated. The dam-safety monitoring at the monolith where the panel is installed consists of pendulum measurement of crest displacements, strain gauges on the front plates and buttress. Recorded data for several external factors are also available. These data are obtained from measurements at the dam, related to dam safety and water management, as well as from the open database of the Swedish Meteorological and Hydrological Institute (SMHI, 2018). Fig. 3 shows, the daily average values for a selection of external factors related to the ice load. The raw data from these measurements have different sampling frequencies. The water level (a) and discharge through turbines and spillways (b) are sampled every 15 min. The temperature of the water in the reservoir (measured at three meters depth) and the ambient air temperature at the dam are measured every hour (c). Daily recordings of precipitation and snowfall from SMHI are available from the weather station Rätan-Handsjön, which is located approximately 7 km east of the dam (in the downstream direction) at a slightly lower elevation (MAWL +350.44 vs. Rätan-Handsjön station +296.0) (d). The data are available in supplementary material A.
3. Results The measured daily max and median ice load are shown in Fig. 4. It is normal practice to discuss ice loads for dams as line loads (N/m). Hence, the measured ice load is hereafter given as line load where the total load has been divided by the length of the panel, which is 1.0 m. Measurements from 2016 are available from the day of installation in February until July, when the logger was disconnected. The panel was thereafter left on the dam over the summer and was thus already in the water prior to the freezing period in the following winter, i.e. the winter of 2016/17. The data-logger was reconnected on November 8, when an ice sheet had already formed on the reservoir. The pressure against the panel was measured with five minutes intervals until the beginning of May. During the next winter, 2017/2018, the computer logger was started in October 26 when reservoir had not yet begun to freeze. On February 12, 2018, the measuring computer crashed for an unknown reason. Since this was not discovered until May, when the winter measurements were to be collected, the data for the spring 2018 are missing. The sampling rate during the measurements was 5 min intervals. When the measured data was analyzed, it was shown that the variations in ice load occur at a much slower rate. For the ice load data to be consistent with the other data, the measurement data have been downsampled, retaining the data for the point closest to every full hour from the ice load data. The ice load data from the three load cells and the total load are available in supplementary material A. 3.1. Calibration and reference values The load cells were calibrated and validated in the laboratory before the installation of the panel. An external load was applied via a hydraulic jack. Four calibrations were performed with the load placed at the center of the panel and over each load cell. The external load was increased to a maximum value of 800 kN and 700 kN for the centric and eccentric loads, respectively. The results show that the seal between the back plate and lid carries some of the applied force but that there is a linear relationship between the applied load and the measured load with a small hysteresis effect. To account for that, a correction factor of 1.25 was used, where the sum of the load cell data is multiplied by the correction factor. Regardless of position, the sum of the load cell multiplied with the correlation factor equals the applied load. As the load cells are pre-stressed, the measurement data have been adjusted so that the measured load is zero when the reservoir is ice-free. The mean of the MAWL and the MIWL, +350.19 m.a.s.l, was chosen as the baseline water level. The calibration was made using data from each
2.3. Installation of the load panel The panel was installed at the dam on 25 February 2016, and a ice thickness of 80 cm was measured at the time of installation. A 120 × 30 cm2 hole was cut in the ice sheet where the panel was placed. The panel was installed with the bottom level at +348.0 m.a.s.l, see Fig. 1b. After the ice in the hole had been removed, the ice attached to the upstream concrete surface was mechanically removed. However, it was estimated that occasional spots with 3–4 cm thick ice remained between the panel and the concrete at the time of installation. The panel was lifted into the correct place with a crane and was attached to the dam vertically by two chains and horizontally with four M20 bolt fasteners, see Fig. 2b. After the installation, the hole between the panel and the ice sheet was filled with the removed ice debris. 3
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Fig. 2. Rätan hydropower dam, a) installed insulation wall on the downstream side (photo; Martin Rosenqvist), b) ice load panel installed at the upstream water line (Photo; Lennart Fransson).
load cell taken at a time when the reservoir was ice-free, and the water surface was as close to the baseline level as possible. For the two first seasons, measurements taken on 18-May 2016 01:00 were used for the calibration. The following data were measured: WL = +350.19 m.a.s.l, LC1 = 87.25 kN, LC2 = 103.9 kN, LC3 = 117.0 kN, Ltot = 308.5 kN/m, where WL is the water level, LCx is load cell x and Ltot is the total load. For the last season, the calibration data were taken on 26 October 2017 18:00, and the following data were measured: WL = 350.19 m.a.s.l, LC1 = 89.65 kN, LC2 = 106.0 kN, LC3 = 123.6 kN, Ltot = 319.3 kN/m. The difference for each load cell was in the range of 2.1–6.6 kN, and the difference in total load was 11.1 kN/m. Compared to the measured maximum loads, this difference was approximately 7%. No adjustment has been made to take into account the variations in water level. This means that the theoretical difference in hydrostatic pressure is from +5.7 kN/m to −5.1 kN/m for maximum and minimum water levels, respectively. Fig. 5a Shows the expected theoretical hydrostatic load compared to the measured load as a function of the water level for measurements registered on July 2016. During this time, the reservoir was guaranteed ice-free and this figure therefore shows how well the load panel performs when the water level variation is the only external factor affecting the total load on the panel. For a given water level, the
Fig. 3. (a) the water level, (b) discharge through turbines and spillways, (c) temperature of the water in the reservoir measured at three meters depth and the air temperature at the dam, (d) precipitation and snowfall from SMHI:s weather station Rätan-Handsjön.
Fig. 4. Max and median ice load from Rätan dam measured during three winters. 4
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together with the level of the resultant load and the water level in February 2017. These three coincide with the daily pattern that follows the operation of the power station. In the evening, the turbine is stopped, and the load against the panel increases as a result of the rising water level in the reservoir. The ice load increases with a short delay, starting just before midnight and reaches its maximum value the following morning. The discharge is then restarted and the water level and ice load decrease. The ice load reaches a minimum value before lunch and is thereafter stable at a low value during the day. The water level continues to drop until the discharge is again stopped the following evening. The position of the resultant force is strongly correlated with the size of the ice load. For the lower residual ice-load, the resultant is located below the water level. But the resultant of the peak loads is located above the water level. For some of the peaks, the resultant force is even located above the retention water level, with a maximum level of +350.62 m.a.s.l, 18 cm over MAWL. This is negative for dam stability, since it means a higher lever arm for the overturning stability than that in the Swedish guidelines where the ice load is assumed to occur 33 cm below the water level if the ice thickness is 1 m. Fig. 8a shows a contour plot of the average total load related to the position of the resultant force. This is relevant from a dam stability perspective where the position of the resultant must be determined for the stability of the overturning failure mode to be calculated. For a resultant force located within the area of the reservoir minimum and maximum operating level, the average load is under 50 kN/m, and the resultant force is located over the top operating water level when the peak loads occur. The greater ice loads thus coincide with an increase in the result position. Fig. 8b shows a contour plot of the number of times the resultant force was located in a specific area, and it can be seen that the normal position of the ice load resultant force is under the operation level. The ice pressure is relatively symmetric in the transverse direction, as the position of the resultant force does not vary by more than ± 20 cm in the transverse direction. Fig. 9 shows the ice load, air temperature, water level and discharge on five days in February 2017 when ice loads peaks occurred three days in succession. Here the relation between the measured ice load, the temperature and the daily operation of the reservoir can be studied in more detail. For the peak occurring on 12 February, the inverse of the temperature and the ice load covary. However, this relationship is not present in the case of other peaks and therefore seems more spurious than causal. The water level and the ice load covary all days when ice load events occur, but the water level varies in the same way on the first day without causing an ice load event. On the last day, a more significant drop in water level occurs and the peak in ice load on that day is significantly reduced. Notable is that the drop in ice load is immediate whereas it takes longer time for the water level to decrease. The drop in ice load seems to be related more to the start of turbines than to the decrease in water level. When the discharge was temporarily lowered at mid-day on 11 February, a small peak in the ice load occurred with only a small change in water level.
Fig. 5. (a) Theoretical, expected, hydrostatic pressure compared with measured loads during July 2016, (b) Distribution of errors between the expected and measured hydrostatic loads.
measured load can vary as much as the expected difference in hydrostatic load between the maximum and minimum levels. Fig. 5b Shows the distribution of the errors between the theoretical and the measured loads. The mean error was 0.49 kN/m and the standard deviation was 1.95 kN/m. Whether the size of this error is absolute or relative, cannot be determined from these data. The errors show no clear relation to the variation in water level or the air temperature. 3.2. Force and resultant force Fig. 4 shows registered daily maximum and median values from the load panel from the three winters. During the first season, in the spring of 2016, peak values of 161 kN/m were registered. These peaks occurred during the first two weeks after installation and thereafter the measured pressure was constantly low with a few individual short peaks. The highest measured load after the first two weeks was 30 kN/ m. From the reconnection of the logger in November 2016 for the second winter until the middle of January 2017, the registered load acting on the panel was relatively constant between 20 and 40 kN/m. Thereafter, the daily maximum load started to increase, while the median value remained at about 50 kN/m. During this period, daily peaks with a magnitude of 70 kN/m occurred. This behavior continued with high residual loads during February, March and early April. The peak values in mid-February were approximately 120 kN/m and a single peak at the end of march reached 164 kN/m. In the 2017/2018 season, the pattern for the initial part of the winter was basically the same as that of the previous year except for some individual smaller peaks. Large daily peaks in January and early February were absent and peak value being 61 kN/m. The 2017/2018 winter was cold, with unusually large amounts of snow, see Fig. 3. It is possible that the part of the winter with a high ice load was delayed and occurred after the measurement recording had crashed. Fig. 6 shows the daily ice load plotted together with the outdoor temperature measured at the dam. The ice-load events with the large peaks appear to be inversely related to the mean air temperature, i.e. most of the load peaks are associated with a drop in temperature. This inverse relationship was not expected, as ice loads are expected to be due to expansion of the ice caused by a temperature increase. The size of the drop in temperature does not correlate to the size of the increase in ice load. For example, from the 3 March to 6 March, the temperature dropped from 0 degrees to −10 °C and the ice load increase from 38 to 84 kN/m, which can be compared to the period 28–30 March, when the temperature dropped from 3 to −1 °C while the ice load reached the seasons peak value of 164 kN/m. Since the temperature in the ice was not measured, no data regarding the actual temperature field in the ice is available. From the three load cells, it is possible to calculate the position of the resultant of the ice load. Fig. 7 shows the measured ice load
3.3. Observations In addition to the measurements, several photographs have also been taken during the winter of 2016/2017. Photos taken immediately after the recording of the greatest values in mid-February 2017 shown in Fig. 10, show a clear crack in the ice sheet along the upstream surface as well as a probable crack located a few meters out from the dam where the slope of the ice surface changes. The position of this fracture is consistent with previous observations and is believed to be the primary mechanism causing ice loads due to variations in water level (Stander, 2006; Comfort et al., 2003). The two fractures act as hinges as the ice sheet between them rotates as the water level varies. When the reservoir is at the top or bottom level, the cracks open and are then filled with water which freezes. When the reservoir level returns to a 5
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Fig. 6. Daily measured ice load and temperature during spring 2017.
level where the ice sheet is completely horizontal, the extra ice in the crack must be compressed for the ice sheet to fit into a horizontal position. It can also be seen in the photographs that the crack in the ice sheet has a curved shape around the panel and that there is a collection of ice and snow in the direct proximity of the panel. These observations make it clear that the panel is disturbing the ice and that the ice sheet at the panel differs from the other ice along the dam. It is not possible from the measurements made, to determine the implications of these observations.
the variation in hydrostatic pressure from water level fluctuations for an ice-free reservoir was nevertheless poor. The errors absolute standard deviation was 1.95 kN. If this is an absolute error it has a negligible impact on the peak load. How this measurement error relates to other measurement methods is unknown, as no similar presentation was found for other measurements. The greatest ice load peaks were due to a variation in water level, which is in line with previous reports (Comfort et al., 2003; Carter et al., 1998; Stander, 2006; Taras et al., 2011). The ice load, the water level and the position of the resultant force of the ice load all follow a daily pattern related to the operation of the power station. However, this relationship is not absolute and it is therefore difficult to treat in an analytical perspective. The water level varies every day, while ice load peaks occur only approximately 40 days a year. During these 40 days, the increase in the ice load correlates with an increase in water level. The water level change thus comes into play first when a certain number of other criteria are met, but it is useless as a sole explanatory mechanism. Based on the findings by Côté et al. (2012), that the ice pressure against the dam varies greatly along the dam face at the same time, it may be difficult to derive a relationship between external factors and measured ice load without extensive further measurements.
4. Discussion The maximum recorded ice load was 161 kN/m, 164 kN/m and 61/ m kN for the winters of 2015/2016, 2016/2017 and 2017/2018, respectively, but the results for the first season when the panel was installed in February are unreliable due to the major disruption of the ice when the load panel was installed. The last season contains measurements only until February. The function of the load panel was validated in the laboratory before transportation and installation, but the panels ability to measure
Fig. 7. Hourly ice load, water level and level of force resultant durring February 2017. 6
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Fig. 8. (a) Meassuered mean total ice load by resultant location and (b) number of regristerd load per resultant location. The horizontal ax load panel.
For further measurements with the load panel, several load panels or “dummies” (without measuring equipment) should be placed next to each other to give a plane surface adjacent to the load panel. The measurements should also be completed with more panels along the dam line. It is nevertheless possible that the panel will create a local effect on the ice sheet since both its thermal properties and surface properties differ from the properties of the dam. Since it is not possible to design a load panel with thermal properties similar to concrete, this effect will always be present. As the next step, it is necessary to compare the results obtained with the load panel with those obtained by other methods. The three different types of sensors, Carter panels, BP gauges and Biaxial gauges, yields similar time history of ice load when used at the same dam (Taras et al., 2011). If the load panel also yields similar results as the other sensors, the belief in the correctness of both the load panel and previous measurements is increased. However, if the results differ, the concerns regarding previous measurements raised in the introduction remain, and the effort to validate the load panel must be continued. The dam on which the panel was installed was chosen to provide large ice loads, with a location in northern Sweden, a water level variation profile that was expected to cause significant loads and long distances to spillways and the water intake for the turbine to represent an undisturbed part of the ice sheet. These criteria must be considered
when evaluating the measurement results. The large values measured at Rätan are probably not representative of dams where these conditions are not meet, and a clear conclusion from this study is that more measurements are needed at other dams. After the load panel is validated, a more extensive measurement campaign at several dams can be designed to find the effect of variation in the external factors. 5. Conclusion In this paper, we present the design and installation of a prototype ice load panel with direct measurement of the ice pressure acting on the dam. The ice load panel is attached at the upstream face of the dam and is large enough to ensure that the whole cross-section of the ice remains in contact with the panel as the water level varies. The total pressure from the ice is thus measured without interpolation. The design of the panel was based on previous experience from similar measurements with sea ice in the Gulf of Bothnia. The panel was installed at the Rätan hydropower dam located in the northern region of Sweden in February 2016, and the location was chosen to provide high ice loads. The maximum recorded loads were 161 kN/m, 164 kN/m and 61 kN/m for the winters of 2015/2016, 2016/2017 and 2017/2018, respectively. A variation in water level is the main mechanism causing the peak
Fig. 9. Ice load, air temperature, water level and discharge.
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Fig. 10. Photos taken of the ice load panel 2017-02-17.
ice loads. During periods when daily ice load peaks occurred, both the ice load and the water level followed a daily pattern related to the operation of the power station. From observations of the ice it is clear that the panel disturbs the naturally formed ice sheet and it cannot be excluded that it may affect the measurement results. The next step is to add dummy panels next to the panel to obtain a plane surface adjacent to the load panel, and to compare the load panel with traditional ice load measurements and models.
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Acknowledgements This work was carried out as a part of “Swedish Hydropower Centre - SVC VK10817”. SVC has been established by the Swedish Energy Agency, Elforsk and Svenska Kraftnät together with Luleå University of Technology, KTH Royal Institute of Technology, Chalmers University of Technology and Uppsala University. www.svc.nu. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.coldregions.2019.102923. References Adolfi, E., Eriksson, J., 2013. The Ice Loads Effect on the Probability of Sliding and Overturn Failure of Concrete dams. KTH, Stockholm. Azarnejad, A., Hrudey, T.M., 1998. A numerical study of thermal ice loads on structures.
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