Sudden Drawdown Research Articles

Study on the Influence of Water Level on Earth Dam Reinforced by Cut-Off Wall: A Case Study in Wujing Reservoir

The construction of a cut-off wall is a common reinforcement method for earth rock dams. At present, compared with the in-depth study on homogeneous earth dams, more and more attention is being paid to the stability and deformation of earth dams strengthened by a concrete cut-off wall. In this study, aiming at the Wujing project of the earth dam strengthened by cut-off wall, the influence of the water level rise and fall on the stability of the dam slope, the deformation of the dam body, and the crack width on dam crest were analyzed by numerical calculation and in situ measurement. The analysis results show that when the reservoir encounters a sudden drawdown, the safety factor of the dam slope decreases sharply. The faster the sudden drawdown, the faster the safety factor decreases. When the reservoir water level rises, the dam’s horizontal displacement shifts to the upstream direction, and the change of horizontal displacement of the downstream slope is significantly larger than that at the measuring point of the upstream slope. The water level of the reservoir rises, and the surface of the dam body rises, and the fluctuation of settlement deformation shows that the upstream side is larger than the downstream side, especially during the period of abrupt change in the reservoir water level. The longitudinal cracks on the dam crest show a tendency of shrinkage when the reservoir water level rises, and opening decreases with the decrease of deformation gradient increment and increases with the increase of gradient increment.

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.

A numerical investigation of slope stability influenced by the combined effects of reservoir water level fluctuations and precipitation: A case study of the Bianjiazhai landslide in China

Slope failure near a reservoir area is mainly influenced by rainfall and reservoir water fluctuation. This study provides insights into the deformation and failure process of the Bianjiazhai landslide near the Suofengying reservoir located at the Wu River in Guizhou province, China. Firstly, stability analyses were carried out to assess the overall strength of the slope and identify the most vulnerable section of the site. Secondly, transient hydromechanical analyses were performed to analyze the effects of rainfall and the reservoir water level fluctuations on the selected slope. The hydromechanical analysis is divided into three different scenarios to study combined as well as the individual effects of rainfall and reservoir water level fluctuation on slide formation. The results indicate that the sudden reservoir water level fluctuations are critical for the stability of the slope. Strength reduction analyses confirm that the deformation behavior of the slope is dominated by shear slip phenomena and identify relevant shear zones. The numerical results from both hydromechanical and stability analysis indicate that the stability of the slope is greatly reduced during large and sudden drawdown. The results from this study will be useful for preventing landslide disasters and for guiding reservoir operation in a similar context.

Assessment of the upstream slope failure of a dam due to repeated cyclic drawdown

Khlong Pa Bon Dam in Thailand underwent the greatest differential drawdown in impounded water in 2014. Unexpected deformation of the upstream slope of the dam was observed on June 27 in the year, after operation for 10 years. The drawdown was hypothesized as the possible cause of the slope deformation. The results from piezometers showed that the upstream slope remained partially undrained after sudden drawdowns. A rapid drawdown analysis confirmed that the movement did not occur due to the sudden drawdown of water in 2014. Back analysis revealed that the shear strength of the embankment slope was near its residual value at the time of failure, which was validated by the movement of inclinometers. Ring shear tests were used to determine the residual shear strength of the failure zone, and the results were validated by the finite element method in ABAQUS. Furthermore, the results of the numerical analysis demonstrated that strain softening was the major cause of the slope movement.

An Extension of Taylor’s φ‐Circle Method and Some Stability Charts for Submerged Slopes

Taylor’s φ‐circle method is a classical method for slope stability calculation, which has analytical solutions. Taylor derived equations in two cases separately, namely, (i) the outlet of the critical failure surface is at the slope toe and (ii) the outlet of the failure surfaces is not at the slope toe. The method is only appropriate for two conditions (without underground water table in slopes or totally submerged slopes). In this study, a general equation that unifies the equations of the two cases is proposed and partially submerged condition is introduced. The critical failure surfaces corresponding to the minimum factor of safety are determined using the computer program proposed by the authors. The general expression of the safety factor of slopes under the following four conditions is derived, namely, (i) partly submerged, (ii) completely submerged, (iii) water sudden drawdown, and (iv) water slow drawdown. The corresponding charts for practical use are available.

Study on Effects of Water Drop for High Concrete Face Rockfill Dams in Narrow Valley

In order to analyze the influence of sudden drawdown of dam water level of high concrete face rockfill dam (CFRD), the paper took a narrow valley high CFRD project for example and conducted a comparative study of stress and deformation of panel and dam between different drop amplitude of dam water level. An elastoplastic hardening soil (HS) constitutive model was used to predict the behavior of CFRD. Then, through analyzing the calculation result of six calculation Cases, the paper summarized the effect of dam water level drawdown speed and amplitude on deformation and stress respectively of panel and rockfill zones. The results showed that owing to the arching effect of the CFRD in narrow valley, total deformation of concrete face slab occurring near the dam crest. However, the pressure and tensile stress of concrete face slab along the dam axial direction have a significant influence on the separation of the slab pieces, and potential seepage at the joints in the slab. The study indicates that the hardening soil model could better to analysis of the behavior of high CFRDs and to predict the performances of high CFRDs.

The effect of side slope and clay core shape on the stability of embankment dam: Southern Ethiopia

In a dam construction, the primary concern is the economy and stability of the dam construction, in parallel with the benefit gained from it. The Gidabo rock fill dam has nearly flatter (2H:1V) side slope with 22 m dam height from river bed level, which consume high fill material relative to this height and dam type. In this study, the effect of clay core shape and side slopes on the stability of embankment dam is analyzed and the minimum earthwork consumption with stable dam safety is identified. The identified best side slope and shape of clay core has checked for all loading condition with numerical modeling software called Geo-Studio 2012. Based on computation the factor of safety of 1.514 and 1.611 for upstream and downstream at end of construction, 1.504 for steady state condition, 1.316 for sudden drawdown and flux through the dam has been found to be 1.95 × 10−7 m3/s/m. The horizontal and vertical deformation of the dam at normal pool level is 0.023 m and 0.192 m, respectively, which are all within the allowable limit. Hence, the selected clay core shape and side slope dam fulfill all design criterion and reduce the consumption of shell fill material.

A Simplified Solution for Calculating the Phreatic Line and Slope Stability during a Sudden Drawdown of the Reservoir Water Level

On the basis of the Boussinesq unsteady seepage differential equation, a new simplified formula for the phreatic line of slopes under the condition of decreasing reservoir water level is derived by means of the Laplacian matrix and its inverse transform. In this context, the expression of normal stress on the slip surface under seepage forces is deduced, and a procedure for obtaining the safety factors under hydrodynamic forces is proposed. A case study of the Three Gorges Reservoir is used to analyze the influences of the water level, decreasing velocity and the permeability coefficient on slope stability.

Stability Analysis of Clastic Rock Slope with Mudstone Interlayer Under Rainfall Infiltration

The clastic rock slopes with mudstone interlayer are widely distributed in Guangxi, China. The mudstone interlayer is less permeable than the clastic rock, that may cause slope instability during the rainfall condition. In this study, a numerical analysis was conducted to study the effect of mudstone interlayer on clastic rock slope stability. A two-dimensional slope seepage model based on fluid–solid coupling was developed for the stability analysis of clastic rock slope with mudstone interlayer under different rainfall conditions. Rainfall-induced change of soil matrix suction in slope was analyzed by unstable seepage analysis and the effects of matrix suction on the slope stability and slip surface were then considered. Results indicated that the mudstone interlayer in clastic rock slope could develop to be the slip surface easily and made the clastic rock slope easy to fail under rainfall condition. When the soil matrix suction near the mudstone interlayer surface reaches a certain threshold value caused by rainfall intensity and duration, the most dangerous slip surface moves to the superficial layer, and the slope stability has a sudden drawdown.

Analysis of the return period and correlation between the reservoir-induced seismic frequency and the water level based on a copula: A case study of the Three Gorges reservoir in China

Studies have considered the many factors involved in the mechanism of reservoir seismicity. Focusing on the correlation between reservoir-induced seismicity and the water level, this study proposes to utilize copula theory to build a correlation model to analyze their relationships and perform the risk analysis. The sequences of reservoir induced seismicity events from 2003 to 2011 in the Three Gorges reservoir in China are used as a case study to test this new methodology. Next, we construct four correlation models based on the Gumbel, Clayton, Frank copula and M-copula functions and employ four methods to test the goodness of fit: Q-Q plots, the Kolmogorov-Smirnov (K-S) test, the minimum distance (MD) test and the Akaike Information Criterion (AIC) test. Through a comparison of the four models, the M-copula model fits the sample better than the other three models. Based on the M-copula model, we find that, for the case of a sudden drawdown of the water level, the possibility of seismic frequency decreasing obviously increases, whereas for the case of a sudden rising of the water level, the possibility of seismic frequency increasing obviously increases, with the former being greater than the latter. The seismic frequency is mainly distributed in the low-frequency region (Y⩽20) for the low water level and in the middle-frequency region (20<Y≤80) for both the medium and high water levels; the seismic frequency in the high-frequency region (Y>80) is the least likely. For the conditional return period, it can be seen that the period of the high-frequency seismicity is much longer than those of the normal and medium frequency seismicity, and the high water level shortens the periods.

Analysis of Unsteady Seepage Behavior of Rumei High Core Wall Rockfill Dam by FEM

The Rumei core wall rockfill dam hydropower station is in Lancang River in the Mangkang county of the Tibet autonomous region with maximum height of 315m and crest length of 666.2m. Due to the high core wall rockfill dam has been more than 300m high, water level sudden drawdown of the unsteady seepage condition shall be taken into consideration, according to the given water level drop speed (5m/d) calculate condition, analysis of the seepage field distribution of the different water level, free surface of different time, each material partition of the dam and foundation soil seepage gradient, etc are given. The results showed that the free surface of the upstream dam shell rockfill body dropping fast than the water level dropping speed of the reservoir. The seepage gradient of the upstream shell is very small, which will not cause adverse effect on the upstream dam slope stability. The free surface of the downstream dam shell rockfill body changes little, which will not cause adverse effect on the downstream of the dam slope stability. As the water level falls, the free surface of the core wall dropping speed is slow, far less than the water dropping speed of the reservoir. In the process of emptying reservoir, the free surface of the core wall remains in a high position, which should cause enough attention.

Stability Analysis of Dyke Using Limit Equilibrium and Finite Element Methods

The present study illustrates the slope stability analysis of dyke using limit equilibrium and finite element methods. Stability analysis of the dyke has to be carried with the methods practiced for earthen dam. The stability of the dyke is analyzed using limit equilibrium (SLOPE/W) and finite element (PLAXIS 2D) methods under different construction stages and different operating conditions. The three different conditions viz. stability of dyke just after the end of construction, after deposition of the bottom ash and empty on the other side and rapid drawdown of water on the bottom ash side are assessed. The flow in the third case is unconfined due to the unknown phreatic surface and the unknown nature of the seepage. The variation of phreatic line plays a vital role in the safe performance of the dyke, hence it has to be critically evaluated so that it does not cut the downstream toe of the dyke. In the SLOPE/W software, the Morgenstern-Price method is made use of in the evaluation of factor of safety. The factor of safety has also been evaluated using the PLAXIS 2D software making use of the strength-reduction technique. A typical example considered was the earth dam i.e., dyke having dimensions of 14 m height, 6 m crest with slope of 1V:2H on both the upstream and downstream sides. The subsoil was of soft disintegrated rock (SDR) for a depth of 2 m and underlain by hard rock strata for a depth of 8 m. The factors of safety evaluated for different stability conditions of the dyke have highlighted the importance of considering them separately. Based on the results, it is noted that the most critical among all the conditions is the sudden drawdown case followed by the steady seepage which is critical for the downstream slope of the dyke.

Steady and Nonsteady Seepage Flow Analysis for the Yangtze Embankment due to Bridge Construction

Seepage failure is the main failure mode in the embankment of rivers during the flood period. In order to evaluate the safety of embankment, to predict the form and location of the failure and to give referential instruction of reinforcement of the dyke, it is very important in theory and practice to study the seepage field of the embankment during the flood period. The distribution of seepage field in the embankment may be changed as bridge piers are built nearby. To ensure the stability and long-term safety of the embankment, it is essential to estimate the seepage stability before and after the construction of the bridge piers. In this paper, with the steady and unsteady seepage methods, comparative analysis was performed to estimate the influence of bridge piers on the seepage field of the embankment. The computed results show that the construction of the bridge has little influence on the seepage field and the stability of the embankment will not deteriorate after the construction of bridge foundation, even in the extreme condition of a sudden drawdown of water level.

Analysis of cause of Secondary intracranial hemorrhage during craniotomy for intracranial tumor

Objective To explore the reasons and the treatment counter-measures for secondary intracranial hemorrhage happens in the intracranial tumor craniotomy. Methods Retrospectively analyzed the clinical data of 15 patients with intracranial tumor who suffered secondary intracranial hemorrhage intraoperation. Summarized the tumor characteristics and the situation of corresponding vessels confined by second operation. Results In these 15 cases,the rank of course of disease was 6.5 months to 2 years, mean 1.2 years. The size of the tumor was big with diameter 4.62 ～5. 82cm,mean 5. 12cm,and the tumor was deep surrounding by large range edema,which led to intracranial hypertension. The emissary vein,bridging vein and cortical draining vein were considered as the corresponding vessels for ' secondary intracranial hemorrhage during the second operation carried out for all 15 cases. There wsa no death cases in this research and all patients recovered the nomal ability for self-caring after 3 months following up. Conclusion Sudden drawdown of intracranial pressure and perfusion pressure breakthrough of local vessels had relationship with secondary intracranial hemorrhage during craniotomy for intracranial tumor. Accurate judgement for the occurrence of secondary intracranial hemorrhage intra-operation and quickly taking the effective corresponding measures was the important strategy for prognosis improving for these patients. Key words: Intracranial tumor; Intracranial hemorrhage

Estimation of aquifer lower layer hydraulic conductivity values through base flow hydrograph rising limb analysis

The estimation of catchment‐averaged aquifer hydraulic conductivity values is usually performed through a base flow recession analysis. Relationships between the first time derivatives of the base flow and the base flow values themselves, derived for small and large values of time, are used for this purpose. However, in the derivation of the short‐time equations, an initially fully saturated aquifer without recharge with sudden drawdown is assumed, which occurs very rarely in reality. It is demonstrated that this approach leads to a nonnegligible error in the parameter estimates. A new relationship is derived, valid for the rising limb of a base flow hydrograph, succeeding a long rainless period. Application of this equation leads to accurate estimates of the aquifer lower layer saturated hydraulic conductivity. Further, it has been shown analytically that, if base flow is modeled using the linearized Boussinesq equation, the base flow depends on the effective aquifer depth and the ratio of the saturated hydraulic conductivity to the drainable porosity, not on these three parameters separately. The results of the new short‐time expression are consistent with this finding, as opposed to the use of a traditional base flow recession analysis. When base flow is modeled using the nonlinear Boussinesq equation, the new expression can be used, without a second equation for large values of time, to estimate the aquifer lower layer hydraulic conductivity. Overall, the results in this paper suggest that the new methodology outperforms a traditional recession analysis for the estimation of catchment‐averaged aquifer hydraulic conductivities.

A variably saturated numerical model for seepage and stability analyses of an embankment dam with a central core

Seepage and slope stability analyses are prerequisites for the safe design of embankment dams. In the present study, a variably saturated numerical flow models is developed for seepage and stability analyses of an embankment dam with a central core. The numerical model solves the variably saturated unsteady state seepage through a vertical section of the dam by fully implicit finite difference scheme. The pore water pressure obtained from the seepage analysis is employed to compute the downstream slope factor of safety under full reservoir condition and upstream slope factor of safety under sudden drawdown using modified Bishop's method. The seismic effects on the stability of both upstream and downstream slopes is carried out using Pseudo Static approach. It is observed that the saturated hydraulic conductivity has significant effect on the factors of safety of both upstream and downstream slopes. Further, it is found that the seismic effects reduce the factors of safety of both upstream and downstream slopes significantly at higher values of saturated hydraulic conductivity of the core material.

Performance of a large-scale slope model subjected to rising and lowering water levels

Many slope failures have been observed to occur during times of water level fluctuations. This paper presents a large-scale model test simulating the performance of a sandy silty soil slope subjected to water level rise and drawdown. The slope model is 15 m long, 6 m high and 5 m wide, and the sloping part has an inclination of 1:1 and a height of 4 m. A water level control system was developed to allow the simulation of the rise and sudden drawdown of the water level. Pore-water pressures (negative and positive), total earth pressures, the slip surface and the landslide process were recorded during the simulation process. Data gathered under controlled laboratory conditions was useful for the verification of analytical and numerical modelling methods. Pore-water pressure inside the slope showed a significant delay relative to the drawdown of the water level outside the slope. The failure mode that developed during rapid drawdown was of the multiple retrogressive rotational type. The results provide an improved understanding of the physical behaviour and failure mode of a saturated–unsaturated soil slope subjected to water level fluctuations.

Short- and long-time behavior of aquifer drainage after slow and sudden recharge according to the linearized Laplace equation

During the last 25 years there has been a great interest in deriving aquifer characteristics from outflow data. This analysis has been mainly based of the drainage of a horizontal aquifer after sudden drawdown, using the Boussinesq approximation. Following the general approach of Brutsaert and Lopez [Brutsaert W, Lopez, JP. Basin-scale geohydrologic drought flow features of riparian aquifers in the southern Great Plains. Water Resour Res 1998;34(2):233–40], it was determined that for this geometry the aquifer behavior could be characterized by d Q/d t ∝ Q 3 for small t and by d Q/d t ∝ Q 3/2 for large t. It was remarked that d Q/d t ∝ Q for large t is often observed. In practice, it is also difficult to determine if d Q/d t ∝ Q 3 for small t because this behavior can only be observed over a very short period. Here, we present a similar analysis of aquifer behavior based on the more fundamental Laplace solution for penetrated aquifers. It has been shown that also when the drain does not fully penetrate the aquifer, the solution still produces good results [Szilagyi, J. Sensitivity analysis of aquifer parameter estimations based on the Laplace equation with linearized boundary conditions. Water Resour Res 2003;39(6)]. The Laplace solution quickly shows that d Q/d t ∝ Q for t → ∞ and d Q/d t ∝ Q ∞ for t → 0, after sudden drawdown. This analysis reconfirms previous findings concerning long-time behavior. More importantly, the analysis shows that the exponent B in d Q/d t ∝ Q B does not have a fixed limited value for short times for the given geometry. Further analysis, however, shows that under certain conditions the relation d Q/d t ∝ Q 3 is retained for 0 ≪ t < 1. Detailed examination of the Laplace solution also shows under which types of recharge dynamics a well-identifiable transition takes place between short- and long-term behavior. As long as such a clear transition exists, the aquifer characterization method proposed earlier by Brutsaert and Lopez [Brutsaert W, Lopez, JP. Basin-scale geohydrologic drought flow features of riparian aquifers in the southern Great Plains. Water Resour Res 1998;34(2):233–40] can be applied. It is shown that for a sharp pulse input, the Laplace solution gives similar results as presented by Brutsaert and Lopez [Brutsaert W, Lopez, JP. Basin-scale geohydrologic drought flow features of riparian aquifers in the southern Great Plains. Water Resour Res 1998;34(2):233–40]. For a smooth pulse, the transition becomes unclear. What is “smooth” and “sharp” depends on input and aquifer characteristics, whereby shallow aquifers give clearer transitions than deep aquifers for the same input. The analysis shows that when rain ceases suddenly after the aquifer has come into equilibrium with a steady rain input, a usable transition in the relation between d Q/d t and Q can be found as well. Researchers can use the present analysis to assess whether specific aquifers and recharge events can be used for the previously suggested characterization method.

Briefing: Slope stability of earthfill dams: a case study of Kızılca dam

It is well known that the construction of a wider impervious core for earthfill dams provides greater piping resistance and a greater resistance to earthquakes, which can cause internal cracks within the core. Also, designing a wide core can minimise construction defects. However, the core width and its location in earth or rockfill dams are actually determined by consideration of factors such as the type of material, piping resistance, geological features, cracking, earthquake considerations and stability requirements. In this study the influence of thickness of the impervious core and the magnitude of the earthquake coefficient on the stability of both upstream and downstream slopes of the Kızılca dam are presented. Because the clay core will be a compacted fill and the characteristics of the clay core material are very good, with high dry unit weights and permeabilities less than 10−10 m/s, hopefully there will be no problems of piping. There will be no seepage or stability problems with the geology of the Kızılca dam site; therefore the problem is only to determine the ultimate choice of the impervious core slopes from the point of view of maintaining the optimum stability of the Kızılca dam's outer slopes, to construct an economical impervious fill, and to determine whether or not the alluvium under the dam has to be removed. That is why, using conventional sliding surface methods, the stability analysis for sudden drawdown and full impounded water-level cases was performed for different core slopes with IV : IH and IV : 0·5H.

Briefing: Slope stability of earthfill dams: a case study of Kizilca dam

It is well known that the construction of a wider impervious core for earthfill dams provides greater piping resistance and a greater resistance to earthquakes, which can cause internal cracks within the core. Also, designing a wide core can minimise construction defects. However, the core width and its location in earth or rockfill dams are actually determined by consideration of factors such as the type of material, piping resistance, geological features, cracking, earthquake considerations and stability requirements. In this study the influence of thickness of the impervious core and the magnitude of the earthquake coefficient on the stability of both upstream and downstream slopes of the Kızılca dam are presented. Because the clay core will be a compacted fill and the characteristics of the clay core material are very good, with high dry unit weights and permeabilities less than 10−10 m/s, hopefully there will be no problems of piping. There will be no seepage or stability problems with the geology of the Kızılca dam site; therefore the problem is only to determine the ultimate choice of the impervious core slopes from the point of view of maintaining the optimum stability of the Kızılca dam's outer slopes, to construct an economical impervious fill, and to determine whether or not the alluvium under the dam has to be removed. That is why, using conventional sliding surface methods, the stability analysis for sudden drawdown and full impounded water-level cases was performed for different core slopes with IV : IH and IV : 0·5H.