Stability of Riverbanks Under Unsteady Flow Conditions
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Abstract:
Stability of riverbanks under three unsteady river flow conditions is simulated. First is the case of constant water level in the river, second is the case of filling the river to the riverbank top and finally the case of drawdown. Two modes of drawdown are analyzed; the rapid drawdown and the slow drawdown. A finite element model of saturated unsaturated seepage flow was de-coupled with a plain strain elastic-plastic finite element model using strength reduction technique to calculate the stability factor of safety (FOS) of bank material. The influences of location of phreatic surface, pore-water pressure, drawdown rate and ratio on the riverbank stability are discussed in details for each case. Results showed that safety factor for saturated riverbank is nearly 83% of its dry value (for the studied case). The case of filling is most likely to stabilize the riverbank. For the case of slow drawdown, the minimum FOS occurs when the water depth is about 0.25~0.3 the bank height. The case of rapid drawdown is the most critical case.Keywords:
Drawdown (hydrology)
Phreatic
Safety factor
Based on the characteristic of well drawdown,a simplified theoretical model,Neuman model is adopted to predict the spatial and temporal distribution of drawdown when using multi-well dewatering in phreatic zones.By synthesized study on a project case use FORTRAN language,it's found that the predict result coincides with the actual measure result basically,that indicates the Neuman model can be used to lay out the dewatering well and control the dewatering time in projects.
Drawdown (hydrology)
Phreatic
Dewatering
Earthworks
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Analysis of Seepage Effect on the Formation of Cover-collapse Sinkhole Caused by Groundwater Changes
Groundwater flow is one of the important factors affecting sinkhole development in karst area. The formation mechanisms of cover-collapse sinkhole caused by the phreatic level arising and piezometric level drawdown were compared and analyzed from the perspectives of 1D groundwater movement and seepage mechanics. And the effects of drawdown rate and drawdown on the cover-collapse sinkholes were mainly discussed. The results show that: the unsteady seepage due to the groundwater changes (phreatic level arising and piezometric level drawdown) go against the stability of the confining layer above the karst opening; the impact of seepage force resulting from piezometric level drawdown is greater than that of the phreatic level arising; Under the same condition of piezometric level drawdown in confined aquifer, the maximum hydraulic gradient had an exponential relationship with the drawdown rates. Thus, controlling the critical operating head (the maximum drawdown) combining the effect of drawdown rate in the management of pumping will reduce or avoid the occurrence of cover-collapse sinkhole.
Sinkhole
Phreatic
Drawdown (hydrology)
Piezometer
Hydraulic head
Cone of depression
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According to the effect of water level drawdown on the landslide of earth rock dams,based on saturatedunsaturated seepage theory,the Xinji reservoir homogeneous earth dam was studied. The unstable seepage field changing rules at different drawdown rate were analyzed,and the safety factor of the dam under the action of seepage was calculated. Meanwhile the phreatic line within the dam and the dam slope stability were analysed when the reservoir water level was decreased from the 35 m-normal water level to the 5 m-lowest water level with the speed of0. 1 m / d,0. 5 m / d,1 m / d,3 m / d,6 m / d,10 m / d,from which the dam phreatic line and slope safety factor variations with the rate of water level drawdown were obtained. All of this will provide references for the stability analysis and safety evaluation of Xinji reservoir dam.
Phreatic
Drawdown (hydrology)
Safety factor
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To determine the stability of an earth embankment following rapid drawdown, it is necessary to locate the phreatic surface. Using a finite difference technique, the partial differential equation governing unsteady flow in porous media has been solved for various conditions. Curves and equations are presented for locating the phreatic surface in the pervious shell of an embankment with a vertical impervious core, resting on an impervious foundation. The embankment may be constructed with any slope angle. The phreatic surface may be located following instantaneous drawdown, or during and after drawdown proceeding at a constant rate. Calculations using these curves and equations are compared with the results of viscous flow-model studies.
Phreatic
Impervious surface
Drawdown (hydrology)
Embankment dam
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Among all factors affecting the stability of soil slopes, the effects of seepage, and especially drawdown, have long been a topic of study. This paper proposed a novel method for predicting the phreatic surface of three-dimensional (3D) slopes. Based on the solved phreatic surface, the normal stress expression of the slip surface under seepage force was deduced. A strict overall analysis method that combines the seepage force to analyze the stability of the bank slopes with reservoir drawdown was proposed. The credibility of the proposed method was verified by calculating different conditions set by a classic slope model. Finally, the change rules of the phreatic surface and the safety evolution with reservoir drawdown of a practical slope in the Three Gorges reservoir area were studied. The results demonstrated that the safety factor of the slope first decreases and then increases with reservoir drawdown and the most unfavorable water level is at the lower 1/3–1/4 of the total height of the slope.
Phreatic
Drawdown (hydrology)
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In the three-dimensional value simulations for pumping and dewatering,the processing for pumping well is a difficulty.To process the pumping well as a series of virtual units could simulate the changes between the drawdown and the time in each point of the calculating region,research the law of water movement,solve the pumping of group wells at constant rate under the condition of the random boundary,forecast the phreatic surface,instruct the construction progress and appraise the rationality of the water lowering plan.
Drawdown (hydrology)
Phreatic
Dewatering
Water pumping
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To solve the phreatic line location of soil slope in the combined effects of drawdown and rainfall,based on the Boussinesq's differential equation of unsteady-seepage,this paper takes Lijiawan landslide for example to simulate the phreatic line location in the different conditions of drawdown velocity,rainfall intensity and rainfall duration.Then the paper discusses the landslide stability in every condition using transfer coefficient method.The results show that when it dose not rain,the stability of landslide becomes worse with the increasing acceleration of the decline of reservoir level;when it is in the state of rainfall,the stability of landslide gets equally worse with the strengthening of intensity and the rainfall duration.The landslide will probably get failure especially when it suffers from heavy rain weather or the rain lasts for 3 days or more.
Phreatic
Drawdown (hydrology)
Intensity
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Drawdown (hydrology)
Phreatic
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The Rapid drawdown condition is one of the most dangerous conditions for earth slopes. The change of water level in reservoir always causes a slope failure. This paper presents an investigation on the slope stability of the Rama 9 Reservoir under different drawdown conditions depending on the drawdown ratio, the drawdown rate and the loading conditions. Finite element analysis program PLAXIS 2D 2012 were applied to define the displacement and the factor of safety for slopes at selected positions. The results shown that, in the rapid drawdown condition for the drawdown ratio of 0.75, the factor of safety is 1.16. In the slow drawdown condition for the drawdown ratio of 1.00, the factor of safety is 1.26. Both are less than the ratio of allowable security value of 1.3. For the Rama 9 Reservoir, the reduction of the water level should be controlled more carefully.
Drawdown (hydrology)
Safety factor
Factor of safety
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Bank slope seepage field is analyzed by saturated-unsaturated seepage theory under the three times cycle conditions of water level rising and drawdown . The pore water pressure distribution laws of the water level rising period, the stabilization period, the period of decline and decline stable period for every cycle are researched. The result shows soil near slope region is saturated easily after many rounds of water level rising and drawdown, and saturation region gradually increases. Soil phreatic line near slope falls fastly in the drawdown period, away from the slope, phreatic line declines slowly.
Drawdown (hydrology)
Phreatic
Saturation (graph theory)
Water saturation
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