Rapid Drawdown Conditions Research Articles

Stability analysis of Hub dam under rapid drawdown

Abstract The purpose of this study was to calculate the Hub dam’s slope stability analysis in a rapid drawdown scenario. Using the slide software, we compared four distinct analytic techniques – the Bishop, Janbu, ordinary way of slicing, and Morgenstern-Price approach – to determine whether or not a given slope would remain stable at the highest possible water level. According to the simulation outcomes, the portion of the dam upstream directly impacts the safety factor. The upstream safety factors, calculated using the Bishop approach, are 2.122 for steady-state seepage and 2.023 for the fast drawdown scenario. This ensures the Hub dam’s stability even under rapid drawdown conditions.

Determining the Thickness of the Filter Layers to Protect the Upstream Sideslope of the Homogeneous Earth Dam Due to Rapid Drawdown Condition with Experimental and Numerical Models

Determining the Thickness of the Filter Layers to Protect the Upstream Sideslope of the Homogeneous Earth Dam Due to Rapid Drawdown Condition with Experimental and Numerical Models

Effect of Core Geometry on Earth Dam Slope Stability

Embankment dams are widely constructed due to their suitability with different types of foundation, and it is constructed from the available material at the site. A zoned earth dam consists of a clay core at the center to control seepage supported by a shell. This paper investigated the impacts of side slopes, top width, and shape on upstream and downstream slope stability during different cases. Slide 6.0 software was used to evaluate the factor of safety of side slopes of an earth dam, and its result was validated. Different side slopes range from 0H:1V to 2 H:1V, and crest widths from 3m to 10 m were examined. In addition, for the slanting core case, several cases with varying angles of inclination were provided. The results indicated that by increasing the side slopes of the core, the factor of safety was reduced, especially in the steady state and rapid drawdown conditions. Increasing the top width also reduced the safety factor in the steady state condition. This reduction was because the core material had lower shear strength than the shell material. In addition, in a steady state and rapid drawdown conditions, the cohesion of core material sharply reduced. Compared with the vertical case, increasing the slanting core slopes influenced the slope stability insignificantly during the steady state. The slanting core had benefits in the steady state when the reservoir was full since, in this case, increasing core side slopes affected the slope stability insignificantly. The maximum core side slope in the earth dam and maximum top width, which insignificantly affected the dam’s slope stability, were 0.8:1 and 5 m, respectively.

Stability analysis of an old earth Samarkand Dam in Kazakhstan under rapid drawdown conditions

Despite being potential historical sites, old embankment dams are subjected to many stability challenges due to many factors, including a lack of sufficient stability assessment tools by the time the dam was built and changes in embankment material properties induced by natural and human activities. Therefore, with the current advancement in technology is of great importance to investigate the state of old embankment dams under different potential loading conditions. The stability challenges become of more significant concern when the embankment is subjected to a rapid drawdown loading scenario. In this study, the Samarkand dam located in Karaganda province in Kazakhstan which was put into operation in 1941 is investigated in terms of seepage and slope stability with the help of numerical modelling. Both steady and transient (rapid drawdown) flow conditions are taken into consideration. e finite element method-based modelling is achieved using SEEP/W and SLOPE/W of the GeoStudio software. From the analysis results, it was observed that the old dam can be subjected to a potential failure under rapid drawdown conditions as the minimum factor of safety values were decreasing with the increase in the drawdown rates. For instance, the minimum factor of safety from the instantaneous drawdown rate was equivalent to 32.85% less than the factor of safety retrieved from the long-term steady-state conditions. Also, from Analysis of Variance (ANOVA), a p-value of 9.97× 10-29 was obtained after subjecting the factor of safety values from instantaneous, 5 days, 10 days, and 1 m per day drawdown rates to ANOVA, indicating that the factor of safety differences among the analyzed drawdown rates were statistically significant.

Transparent soil model test of a landslide with umbrella-shaped anchors and different slope angles in response to rapid drawdown

Umbrella-shaped anchors, as new stabilizing structures, have been increasingly adopted in engineering practice due to their simple structure, ease of installation, and high load capacity. Reservoir operation, especially during rapid drawdown, affects the long-term stability of reservoir bank slopes as this process affects the stress, seepage and deformation characteristics of the water-level fluctuation (WLF) zone. Thus, the characteristics of the deformation and failure mode of WLF zones reinforced by umbrella-shaped anchors under rapid drawdown conditions are important for the design and construction of anchoring measures. Five slope-anchor scale models based on the Yingpan landslide were established as transparent soil models considering different slope angles and drawdown rates. Displacement, velocity and macroscopic deformation data were obtained using particle image velocimetry (PIV) technology. The results indicated that the deformation process of both reinforced and unreinforced slopes under rapid drawdown conditions could be divided into four phases: initial, shallow sliding, multisliding and stable phases. In the anchor-reinforced models, loss of soil particles occurred when the water level dropped to reach the anchor heads, causing the anchor plates to loosen and slope deformation to accelerate, especially near the lower row of cables. The anchor heads failed first, and progressive slope sliding ensued. Under rapid drawdown conditions, soil liquefaction and subsequent loosening and sliding occurred, generating a far larger zone of accumulation than the zone of collapse. This indicates that soil liquefaction is another important factor of the link between rapid water level drawdown and slope instability causing local collapses. Combining the experimental results and 47 sets of field monitoring data, it could be concluded that the slope stability decreases with increasing slope angle of the WLF zone. This occurs because a higher water level is needed for deformation initiation in the WLF zone in the case of a higher slope angle.

Coupled Numerical and Analytical Stability Analysis Charts for an Earth-Fill Dam under Rapid Drawdown Conditions

The presented article provides a comprehensive study on the stability analysis of earth-fill dams under rapid drawdown and transient flow conditions used to prepare stability analysis charts by conducting coupled finite-element numerical and analytical limit equilibrium procedures. In this regard, the impacts of different rapid drawdown conditions on the safety factor of the Alavian earth-fill dam are determined. The slope stability charts present for both shallow and deep slip surfaces with various permeabilities are verified by ground information obtained with extensive instrumentation on the dam’s site. The results showed that by decreasing the permeability of the core’s material, despite preventing seepage, the instability risk of the upstream slope as a result of rapid drawdown intensifies. Also, as stability charts can be stated, with increasing the slip surface’s depth and decreasing the hydraulic hydration, the reliability decreases, and the sliding surfaces’ sensitivity increases based on the drawdown rates, which have been revealed to be from 0.2 to 0.6, the most critical state for safety factors, showing significant declines.

The Perfomance of Skewers Foundation Combined with Revetment under Rapid Drawdown Conditions on Balikpapan Coastal Area in Soft Soil using Numerical Analysis Undrained B

This research analyses the skewer foundation with Plaxis 2D considering the stability before and after drawdown conditions of the sea level with variations in the point load on the foundation. The study is carried out using the back-analysis method with the assumption that the protective building foundation is in a balanced state with the safety factor (SF) = 1.5. As a result, the stability of the rapid drawdown gave a different value which is the skewer foundation before rapid drawdown showed a better result of the performance of foundation during point load than after drawdown. Before drawdown conditions, the value of the point load (P) is 12 Tons. Meanwhile, after drawdown conditions, the skewer foundation is only able to accept a load of 5 tons. The value of the movement of the skewer foundation before and after the drawdown of sea-level conditions occurred at the same settlement value of about 0.0179 m = 1.18 cm. In this condition, the skewer foundation is considered capable of resisting the settlement issue where the maximum settlement value for shallow foundations for clay soils is not greater than 65 mm.

The influence of material characteristics on dam stability under rapid drawdown conditions

The influence of material characteristics on dam stability under rapid drawdown conditions

The Influence of Shell Permeability on Stability of Upstream Slope during Rapid Drawdown – Khassa Chai Earth Dam as a Case Study

Several factors affect the stability of earth dam during sudden drawdowns such as permeability and mechanical properties of soil, upstream side slope, drawdown ratio, and drawdown rate. This paper investigates the influence of shell permeability on earth dam upstream slope stability and its role in the change of pore water pressure at different locations of the embankment during the sudden drawdown, using different limit equilibrium methods. To accomplish the objective of this study, (Geo Studio 2012 Software) as one of the powerful geotechnical programs was used for the modeling and numerical analysis. The study shows that decreasing in the shell permeability resulted in the reduction of pore water pressure dissipation and variation of shell hydraulic conductivity plays a vital role in the overall stability of the upstream slope under rapid drawdown conditions.

Analysis of Earth Dam Body Behavior under Rapid Drawdown Conditions in Reservoir Water Level by Laboratory and Numerical Methods

Analysis of Earth Dam Body Behavior under Rapid Drawdown Conditions in Reservoir Water Level by Laboratory and Numerical Methods

DYNAMIC RESPONSE AND STABILITY ANALYSES OF AKOSOMBO DAM USING NUMERICAL ANALYSIS

Dams are very important in Ghanas economic development and environmental improvement. Although Ghana dams are seismically far from the active zone, accurately analysed dams should be evaluated since failure could severely impact the people in the flood environment and the regions economy on a large scale. This paper proposes a numerical procedure for the static, slope stability, and dynamic analysis of the Akosombo embankment dam. Nineteen horizontal acceleration time histories recorded data was used based on Maximum Design Earthquake (MDE), Maximum Credible Earthquake (MCE), Design Basis Earthquake (DBE) and Operating Basis Earthquake (OBE) data. The numerical results estimated showed that the Akosombo embankment dam is likely to experience moderate deformations during the different design earthquakes. The result also indicated that non-linear analysis capable of capturing dominant non-linear mechanisms could be used to assess the stability of embankment dams. The factor of safety (FS) calculated was greater than 1.5 for high reservoir, rapid drawdown condition and low reservoir condition whereas, the FS values were found to be 1.42 for slow drawdown condition.

Toe drain size and slope stability of homogeneous embankment dam under rapid drawdown

The slope stability of an embankment dam has been always a serious concern of any design team. Unfortunately, the information on the potential influence of a toe drain size on the slope stability of an embankment dam under rapid drawdown conditions is still scarce. This study investigated the potential effect of a toe drain size on the slope stability of a homogeneous embankment dam under rapid drawdown conditions. Three different sizes (5m, 10m, and 15m) of the toe drain were investigated under instantaneous (worst scenario) and 5 days (more realistic) drawdown rates with the help of numerical modeling in GeoStudio. From the results, it was observed that the pore-water pressures at the upstream face of the embankment decreased with the increase in the toe drain size, while the pore-water pressures at the downstream toe were increasing with the increase in the toe drain size. The factor of safety values were also observed to be affected by the changes in the toe drain size.

Seepage and Stability Analysis of the Eyvashan Earth Dam under Drawdown Conditions

The rapid drawdown condition to control floods and irrigation is one of the things that may occur over the lifetime of the dam. Also, the stability of the dam at the rapid drawdown will be more important due to the faster reduction of the water level of the dam reservoir than the pore water pressure. In this study, the finite element method and software GeoStudio used to study the seepage from the body earth dam. Also, the complete elastic-plastic model of Mohr-Coulomb is considered in the analysis. In this study, the stability analysis of the Eyvshvan earth dam after rapid drawdown due water to release of the dam reservoir to downstream agricultural lands during drought crisis is investigated. For the validation, first, the results of the pore water pressure instrument were compared with the results of numerical analysis. The results of multivariate regression analysis (coefficient of determination) show very good agreement of about R2=0.98. The results show that the phreatic line remains after 29 days from the start of the rapid drawdown of the reservoir, while half of the volume of the drained reservoir remains at 1842 (1/3 of the crest). The analysis of dam stability during rapid drawdown using both Morgenstern-Price and Bishop Methods showed that the most critical situation would occur after 42 days of discharge with a factor of safety (FoS) of 1.71, with no stability hazard and the upstream slope would be safe.

Judgement of rapid drawdown conditions in slope stability analysis

The rapid drawdown of reservoir may have a significant impact on the stability of adjacent slopes. In order to avoid potential risks, it is important to calculate the change of slope safety factor prior to the reservoir operation. The finite element method is used to analyze the transient seepage during drawdown, and then the pore water pressures are introduced into the stability computation based on limit equilibrium to obtain the transient safety factor. Computations show that for slopes with a specific geometry, the safety factor ratio depends on the parameters K/(Syv) (where K is the permeability coefficient, v is the drawdown speed, and Sy is the specific yield) and c′/(γHtan ϕ′) (where c′ and ϕ′ are the effective cohesion and friction angle, γ is the soil unit weight, and H is the slope height). By considering a wide range of K/(Syv) and c′/(γHtan ϕ′) values and different slope geometries, the percent reduction in critical safety factor of slope during drawdown relative to that during steady-state seepage is obtained. The drawdown condition that causes a large percent reduction in safety factor is judged as a rapid drawdown, and the opposite is a slow drawdown, which does not affect the slope design. This paper presents a series of charts for engineers and designers to judge rapid and slow drawdown conditions. Before the reservoir operation, the appropriate drawdown speed is selected according to the charts to ensure a slow drawdown for adjacent slope, while in the slope stabilization design, only the rapid drawdown stability analysis needs to be performed.

Experimental and numerical study of upstream slope stability in an earth dam reservoir under rapid drawdown conditions

Experimental and numerical study of upstream slope stability in an earth dam reservoir under rapid drawdown conditions

Experimental and numerical study of upstream slope stability in an earth dam reservoir under rapid drawdown conditions

The rapid water level drawdown of the dam reservoirs is one of important factors of earth dam stabilities. This phenomenon might occur over the lifetime of the earth dams and levees. In order to investigate the effect of the hydraulic conductivity on the rapid drawdown of water level, an experimental model was constructed in experimental flume. By obtaining the hydraulic parameters of the materials, the flow through this model was modelled by SEEP/W software. After validation of hydraulic conductivity with the numerical model, the results were compared with the observed data. Finally, a good agreement is observed between the experiments and predictions. Furthermore, the saturated and unsaturated simulations indicate that the unsaturated model has a much better agreement with the experimental model. It is found that the unsaturated model is more convenient for the simulation flow through the homogenous earth dam than the saturated model.

Maximum safe freight train speed on railway embankments under rapid drawdown conditions based on coupled stress-seepage slope stability analysis

The rapid drawdown of the water level adjacent to a railway embankment can reduce its stability, which could compromise the safety of train movements. To investigate the effect drawdown on railway embankments as a function of train speed, a coupled finite element-based model was developed accounting for simultaneous deformations, stresses and seepage. To determine maximum safe train speeds, a comprehensive study was carried out that included slope parameters (e.g. railway embankment height, natural slope height, slope ratio), rapid drawdown ratios, and external freight train loads as a function of speed. The models used the shear strength reduction technique, in which the factor of safety for each model was obtained. Generally, steeper natural slopes with lower railway embankments built upon them are more prone to failure, thus severely limiting train movements when a drawdown condition occurs. However, for a large number of cases, mainly for flatter slopes, a safe passage can be achieved by limiting train speed, while tall railway embankments appear to be able to withstand drawdown conditions without a considerable reduction in train operating speeds. The results of findings are summarized in a table and it is hoped that the study will be useful to increase the safety of freight train operations, particularly in cases when there is a sudden change or an unusual event in the waterways adjacent to a railway embankment.

Experimental and numerical study of upstream slope stability in an earth dam reservoir under rapid drawdown conditions

The rapid ‎drawdown of the dam reservoir is one of the most common situations occurring in the lifetime of a dam. For this reason, one of the main factors in the design of the upstream slope is the rapid drainage of the reservoir. In this case, the upstream slope is in a critical condition and the slope may be unstable. When the water surface in the reservoir is drawdown suddenly, the water level in the dam body does not decrease at the same time as the reservoir water level. The analysis of seepage from the earth dam body and calculation of the water loss play an important role in calculating the amount of pore water pressure, and, consequently, the stability analysis of the dam body. In addition, any seepage analysis is dependent on the hydraulic properties of the dam materials. In order to investigate the effect of hydraulic conductivity on the rapid drawdown of water level and the seepage, an experimental model was constructed of an earth dam. By accurate measurement of hydraulic parameters of the materials in saturated and unsaturated media, the flow through this model was modeled using a disk penetrometer by seep/w software. The results were then compared with the observed data.

Modeling the slope of embankment dam during static and dynamic stability analysis: a case study of Koga dam, Ethiopia

Quantitative assessment of the stability of slopes is very important for the evaluation of an earth fill dam in order to perform the intended function throughout the service life. This study presents the slope stability and analysis of the Koga earth fill dam. The analyses were carried out using finite element-based PLAXIS 2D software. The behavior of both the body and the foundation of the dam was described using the Mohr–Coulomb criterion. Based on the result of this study, the resulting factor of safety values during end of construction for both static and dynamic stability analysis was 1.6221 and 1.3592, respectively. For steady-state condition, the water level was fixed at normal pool level (2015.25 m). The factor of safety obtained for static stability analysis was 1.6136 and the dynamic analysis 1.3157. The rapid drawdown condition is analyzed with normal pool level of 2015.25 m lowered to 2008.5 m. The analysis results showed that the factor of safety for the static and dynamic analysis was 1.2199 and 1.0353, respectively. Using recommended design standards: United States Army Corps of Engineers, British Dam Society and the Canadian Dam Association the slope stability analysis of the Koga earth dam at all critical loading conditions are safe. The displacement result shows the maximum total displacements for static and dynamic analysis were 1.033% and 1.628% of the dam height, respectively. The displacement result coincides with Fell et al. (J Geotech Geoenviron Eng 129(4):307–314, 2003) standards.

On the Stability of Moderate Height Berm-Type Earthen Dams: The Hancagiz Dam Example

This study investigates the effects of berm properties on a moderate height earthen dam stability. The impacts of different berm heights, numbers and widths on the safety of the Hancagiz Dam are determined by analyzing the seepage through the dam and the slopes’ stability under steady-state, rapid fill and rapid drawdown conditions with the finite-element method. The results showed that increasing the upstream berm height, number, or width of berms provides higher safety factors for the upstream slope. However, an excessive increase in the berm height is seen to cause a lower safety factor. Increasing the berm number is found to give maximum stability among all options. Any change made on the upstream berm does not affect the safety of the downstream slope.