Seepage Stability Research Articles

SEEPAGE AND SLOPE STABILITY ANALYSIS OF LEVEES SUBJECTED TO FLOODING (THEORITICAL STUDY

SEEPAGE AND SLOPE STABILITY ANALYSIS OF LEVEES SUBJECTED TO FLOODING (THEORITICAL STUDY

The Mass Loss Behavior of Fractured Rock in Seepage Process: The Development and Application of a New Seepage Experimental System

In order to study the water‐inrush mechanism in fractured geological structure, seepage instability theory is picked up, which considers that water‐inrush is the embodiment of seepage instability. Seepage tests are the basis for studying seepage instability in the fractured rock system, while the experimental system and equipment are the foundation of seepage tests. In this paper, we introduce a new experimental system for the seepage test on fractured rock accompanying with mass migration and loss and discuss its development and application. It presents the designed and manufactured experimental system, the subsystems, components and their functions, and the experimental scheme and the experimental process. Compared to the previous experimental system, more functions are satisfied, among which, the most important improvements are realizing a long‐term permeate and keeping water pressure stable at a high level. These improvements are verified by a series of tests, and the results show that our experimental system has higher accuracy, stability, and reliability. The distribution of the permeate times and lost mass of different Talbol power exponents are obtained, and the time‐varying rules of water pressure, water flow, the lost mass, and porosity are also revealed through the results. Although the experimental system also has some limitations, for instance, the measuring accuracy of pressure transducers and flow transducers, the provided maximum pressure of the quantitative displacement piston pump, the fine particles collection subsystem, etc., we will continue to improve it in our further research.

Assessment of Real‐Time Compaction Quality Test Indexes for Rockfill Material Based on Roller Vibratory Acceleration Analysis

Compaction quality is directly related to the structure and seepage stability of a rockfill dam. To timely and accurately test the compaction quality of the rockfill material, four real‐time test indexes were chosen to characterize the soil compaction degree based on the analysis of roller vibratory acceleration, including acceleration peak value (ap), acceleration root mean square value (arms), crest factor value (CF), and compaction meter value (CMV). To determine which of these indexes is the most appropriate, a two‐part field compaction experiment was conducted using a vibratory roller in different filling zones of the dam body. Data on rolling parameters, real‐time test indexes, and compaction quality indexes were collected to perform statistical regression analyses. Combined with the spectrum analysis of the acceleration signal, it was found that the CF index best characterizes the compaction degree of the rockfill material among the four indexes. Furthermore, the quantitative relations between the real‐time index and compaction quality index were established to determine the control criterion of CF, which can instruct the site work of compaction quality control in the rockfill rolling process.

Seepage Piping Evolution Characteristics in Bimsoils -An Experimental Study

Piping is a kind of seepage failure mode that commonly occurs under the influence of seepage force in soil materials. Many studies have been done to study the characteristics of piping for soil materials, however, the initiation and development mechanism of piping in bimsoils is poorly understood. In this work, an experimental program was set for investigating the evolution process of piping, in a self-developed servo-controlled flow–erosion–stress coupled testing system. All the studied samples with rock block percentage (RBP) of 30%, 40%, 50%, 60%, and 70%, were produced as a cylindrical shape(50 mm diameter and 100 mm height) by compaction tests with different hammer strike counts to roughly insure the same void ratio. The results show that the amount of rock blocks in bimsoil samples significantly influenced the initiation and development of piping. Furthermore, the stress state has a crucial influence on the critical hydraulic gradient, seepage velocity, permeability, erosion, and migration of soil particles. Moreover, interactions among soil matrix, rock blocks, and rock–soil interfaces control the seepage stability of the bimsoil sample.

Particle Flow Code Method-Based Meso-scale Identification for Seepage Failure of Soil Levee

The long-term action of internal and external factors leads constantly to seepage failures of soil levee engineering such as soil flow failure and piping failure. It is very disadvantageous to the service safety of levee engineering. Most of disastrous accidents are induced directly or indirectly by seepage. It has been known that the water–soil interaction with particle migration determines the occurrence and development of seepage failure. In this paper, a particle flow code (PFC)-based approach is introduced to implement the numerical simulation for seepage behavior in soil levee and investigate the meso-scale seepage failures. Firstly, according to the granular properties and particle migration characteristics of soil levee, an implementation process is presented to construct the meso-scale soil particle model and analyze the levee failure. Then, considering the coupling action of soil and water, a numerical method combining PFC with computation fluid dynamics is developed to simulate the seepage failure of soil levee. The corresponding criteria on seepage failures of soil levee are given. Lastly, one actual levee engineering is taken as an example. The movements of soil and water in the soil flow and piping processes are investigated with meso-scale soil particle model of selected levee. The meso-scale mechanism of seepage failures is analyzed, and the seepage stability is evaluated. It is indicated that the proposed PFC-based approach can fulfill the real-time observation for the movement status of soil particles and water during seepage failure of soil levee. The seepage stability identification with the introduced meso-scale criteria shows good agreement with the experimental result.

Study on Mechanism of Concentrated Leakage Accesses Formed Due to Current Scouring in Weak Structure Plane

Weak structure plane is high in plasticity and low in strength, which provides the necessary condition for deforming rock mass and forming concentrated leakage accesses and seepage failure under the effect of groundwater in the long term. Seepage stability of weak structure plane has always been an important engineering geological and hydrogeological problem of water conservancy and hydropower project. At present, their understanding of the mechanism of seepage failure still exist many unsolved problems. Since they rarely studied the concentrated leakage accesses formed due to the weak structure of bedrock before, this article will discuss seepage and deformation problems in weak structure plane. It mainly discusses the current scouring development mechanism of weak structure plane and critical flow velocity and corresponding fissure width when soil particles start at different positions. Finally, it introduces living examples like concentrated leakage accesses formed due to weak structure plane, which means weak structure planes of bedrock can form concentrated leakage accesses under appropriate geological conditions.

Numerical simulation of backward erosion piping in heterogeneous fields

AbstractBackward erosion piping (BEP) is one of the major causes of seepage failures in levees. Seepage fields dictate the BEP behaviors and are influenced by the heterogeneity of soil properties. To investigate the effects of the heterogeneity on the seepage failures, we develop a numerical algorithm and conduct simulations to study BEP progressions in geologic media with spatially stochastic parameters. Specifically, the void ratio e, the hydraulic conductivity k, and the ratio of the particle contents r of the media are represented as the stochastic variables. They are characterized by means and variances, the spatial correlation structures, and the cross correlation between variables. Results of the simulations reveal that the heterogeneity accelerates the development of preferential flow paths, which profoundly increase the likelihood of seepage failures. To account for unknown heterogeneity, we define the probability of the seepage instability (PI) to evaluate the failure potential of a given site. Using Monte‐Carlo simulation (MCS), we demonstrate that the PI value is significantly influenced by the mean and the variance of ln k and its spatial correlation scales. But the other parameters, such as means and variances of e and r, and their cross correlation, have minor impacts. Based on PI analyses, we introduce a risk rating system to classify the field into different regions according to risk levels. This rating system is useful for seepage failures prevention and assists decision making when BEP occurs.

An Experimental Investigation for Seepage-Induced Instability of Confined Broken Mudstones with Consideration of Mass Loss

To study and prevent water-mud-outburst disasters of tectonic fracture zones in geotechnical engineering, we tested seepage stability of confined broken mudstones with consideration of mass loss using syringe seepage method and a self-designed seepage testing system, obtained the variation laws of seepage instable duration, total mass loss, and mass loss rate of broken mudstones under different pressure gradients and Talbol power exponents (simplified as Talbol hereafter), and explained their instable seepage behaviors. The results showed that the mass loss is the internal cause of seepage-induced instability of broken rocks and pressure gradient is the external cause, and the persistent migration and loss of particles result in progressive failure process, while the large enough pressure gradient causes sudden overall instability. The seepage instable duration shortens with pressure gradient increasing, with the longest and shortest duration at Talbol of 0.5 and 0.1, respectively. In general, mass loss increases with pressure gradient increasing and with Talbol decreasing. Mass loss rate increases with pressure gradient increasing but shows no monotonic changes with Talbol. Their expressions can be used to establish dynamic model in the further seepage stability researches.

Mathematical model of one-dimensional penetration stability failure for gaseous coal

Based on the theory of fluid dynamics in porous media, combined with the gas state equation, Darcy's law and the discriminant equation of one-dimensional seepage instability, a mathematical model of one-dimensional penetration stability failure is established to study the seepage damage law of coal seam. Assuming that the background pressure of the coal wall is attenuated according to the exponential law, the mathematical equation is solved by using the finite difference method. Furthermore, the process of coal bed instability which is supposed as a form of a 'sublayer' pushing forward was analysed. That is, the coal bed loses its stability layer by layer. The calculation results showed that the thickness of the failure sublayer decreases with the reduction of coal permeability and the acceleration of dissipation rate of the background pressure. The model provides a method which can analyse the outburst process and its intensity quantitatively.

COMPREHENSIVE ANALYSIS ON SEEPAGE AND STRUCTURAL STABILITY OF EARTH-ROCK DAM: A CASE STUDY OF XIQUANYAN DAM IN CHINA

Earth-rock dam is commonly used in the high-dam engineering around the world. It has been widely accepted that the analysis on structural and seepage stability plays a very important role, and it is necessary to take into account while designing the earth-rock dam. In performing the analysis of structural and seepage stability, many remarkable methods are available at current stage. However, there are still some important issues remaining unsolved, including: (1) Finite element methods (FEMs) is a means of solutions to analysis seepage process, but it is often a difficult task to determine the so-called seepage coefficient, because the common-used water injection test is limited in the practical work due to the high cost and complex procedure. (2) It has long been discussed that the key parameters for structural stability analysis show a significant spatial and temporal variations. It may be partly explained by the inhomogeneous dam-filling during construction work and the developing seepage process. The consequence is that one constant value of the parameter cannot represent the above variations. In this context, we solve the above issues and introduce the solution with a practical earth-rock dam project. For determining the seepage coefficient, the data from the piezo metric tube is used to calculate the potential value, based on which the seepage coefficient can be back-analysed. Then the seepage field, as well as the seepage stability are numerically analysed using the FEM-based SEEP/W program. As to the structural safety, we take into account the spatial and temporal variations of the key parameters, and incorporate the Monte-Carlo simulation method into the commonly used M-P method to calculate the frequency distribution of the obtained structural safety factor. In this way, the structural and seepage safety can be well analysed. This study is also beneficial to provide a mature method and a theoretical insight into the earth-rock dam design. The earth-rock dam of Xiquanyan Reservoir near Harbin city in China is also selected as case study to illustrate the described method.

Computational simulation of seepage instability problems in fluid-saturated porous rocks: Potential dynamic mechanisms for controlling mineralisation patterns

Pore-fluid flow associated with seepage instabilities can play an important role in controlling large mineralisation patterns within the upper crust of the Earth. To demonstrate this process, two kinds of seepage instability problems in fluid-saturated porous rocks are considered in this paper. The first kind of seepage instability problem is caused by the temperature-induced buoyancy of pore fluid, so that it can be called the buoyancy-driven seepage instability problem, while the second kind of seepage instability problem is caused by chemical dissolution reactions that are commonly encountered in the upper crust of the Earth, so that it can be called the chemical-dissolution-driven seepage instability problem. After the mathematical governing equations of and computational methods for these two kinds of seepage instability problems are introduced, two numerical examples are used to elucidate how and why these two kinds of seepage instabilities can provide favorable places for the formation of large mineralisation patterns within the upper crust of the Earth. The related computational simulation results have demonstrated that: (1) the convective pore-fluid flow caused by the buoyancy-driven seepage instability not only can dissolve minerals at the lower part of the upper crust, but also can transport the dissolved minerals from the lower part to the upper part of the upper crust, resulting in large mineralisation patterns near the surface of the Earth's upper crust. (2) The chemical-dissolution-driven seepage instability in fluid-saturated porous rock can provide some favorable places, such as finger-like channels created by porosity enhancement in the porous rock, for the formation of large mineralisation patterns within the upper crust of the Earth.

Evaluation of storm wave-induced silty seabed instability and geo-hazards: A case study in the Yellow River delta

Models based on the theoretical framework of soil mechanics are presented to evaluate storm wave-induced silty seabed instability and geo-hazards through a case study in the Yellow River delta. First, the transient and residual mechanisms of wave-induced pore pressure are analyzed. Three typical models (i.e., elastic model, pore pressure development mode and elasto-plastic model) are proposed to calculate wave-induced stresses in the seabed. Next, mechanisms and calculation methods of wave-induced seabed instability modes such as scour, liquefaction, seepage instability and shear slide are proposed. Typical results of storm wave-induced excess pore pressure and seabed instability are given and relevant discussions are made. At last, the formation mechanism of geo-hazards in the Yellow River delta is analyzed based on the proposed mechanism and calculated results. Results and analysis indicate that both transient and residual mechanisms are important to storm wave-induced response of silty seabed and hence the elasto-plastic model is more appropriate. Complete liquefaction does not happen, while other types of instability occur mostly within 2–6m under the seabed surface. Wave-induced scour, seepage instability and shear slide are all possible instability modes under the 1-year storm waves, and scour is predominant for the 50-year storm waves. The formation mechanism of geo-hazards such as shallow slide and storm wave reactivation, pockmarks, silt flow and gully, disturbed stratum and hard crust in the Yellow River are well explained based on the proposed mechanisms and calculated results of storm wave-induced silty seabed instability.

The effects of rainfall on the stability of soil slopes in Khanh Vinh district, Khanh Hoa province

This study presents a procedure for calculating the change of the safety factor for unsaturated slopes of homogenous, residual soils suffering from rainfall infiltration within Khanh Vinh district, Khanh Hòa province. Rainfall is supposed as a main trigger caused failure of the potential sliding slopes. Rainwater into the slope due to infiltration caused an increase in moisture content and negative pore water pressure; a decrease in matric suction and in shear strength on the failure surface. Thus, slopes are reduced stability and can be failed. Soil permeability and rainfall intensity were found to be the primary factors controlling the instability of slopes due to rainfall, while the initial water table location and slope geometry only played a secondary role. A numerical model of analysis coupled seepage-stability used to simulate the seepage and slope stability under conditions of specific environment such as soil permeability, rainfall intensity, water table location and slope geometry in the study area. The relationships between safety factor and rainfall intensity, soil permeability, angle slope, high slope were identified to provide a good indication for the management of landslide hazards under the effects of rainfall.

Water-resisting ability of cemented broken rocks

Using the self-designed testing system, the seepage tests for cemented broken rocks were conducted, and the impact of different factors on water-resisting ability was analyzed. The results show that (1) seepage process of the cemented broken rocks can be divided into two categories: in one category, seepage instability occurs after a period of time, in the other, the permeability decreases slowly and tends to be stable, and seepage instability does not occur; (2) cementing performance of cementing agent and grain size distribution are the decisive factors for water-resisting ability, with the increase of cementing performance and the mass percentage of large grains, the water-resisting ability of the specimen strengthens; (3) aggregate type has little effect on seepage stability, for the specimens with different aggregate types, the permeability and the duration of seepage instability have small difference; (4) initial porosity has a certain effect on the water-resisting ability of the specimen, but has no decisive role. With the increase of the initial porosity, the duration of seepage instability decreases.

Effect of mining on shear sidewall groundwater inrush hazard caused by seepage instability of the penetrated karst collapse pillar

Mining-induced groundwater inrush, a type of serious hazard for underground coal extraction, occurs in tunnel or coal face combined with a large volume of groundwater gushing. It has an impartible relationship with geological structures such as karst collapse pillar (KCP), which is widely distributed in North China. To analyze the effect of coal mining on groundwater inrush caused by seepage instability of the penetrated KCP (PKCP), stress and seepage coupling equations are used to model the seepage rule, and a numerical FLAC3D model is conducted to determine the shear stress, damage zone, pore pressure, seepage vectors and effect of damage zone on seepage field development. The results indicate that when PKCP and the surrounding rocks show dislocation, shear failure may occur, which leads to a shear sidewall channel for groundwater inrush. As mining advances, the damage zone in the PKCP and its crushed area develop gradually, and the simulated damage zone development was consistent with observed thicknesses of crushed rocks. Under the effect of the support pressure and unloading of the coal face, an obvious seepage concentrate channel with greater hydraulic head and seepage vectors will form within the edge of the PKCP; then, the shear sidewall channel will gradually move to the other side with the coal face near and away. When the damage zones are crushed through, the connected fractures will become a strong seepage channel in PKCP, named as pipe flow, which may cause groundwater inrush.

Characteristics of groundwater seepage with cut-off wall in gravel aquifer. I: Field observations

This paper presents a case history of the leakage behavior during dewatering tests in the gravel strata of an excavation pit of a metro station in Hangzhou, China. The groundwater system at the test site is composed of a phreatic aquifer underlain by an aquitard and a confined aquifer with coarse sand and gravel. The sandy gravel stratum has very high hydraulic conductivity. The maximum depth of the excavation is 24 m below the ground surface, which reaches the middle of the aquitard strata, where the thickness of the clayey soil is insufficient to maintain the safety of the base of the excavation. To understand the hydrological characteristics of gravel strata, single- and double-well pumping tests were conducted, where a cut-off wall was installed 43 m deep with its base penetrating 2 to 3 m into the aquifer. Test results show that this partial cut-off of the aquifer cannot effectively protect the base of the excavation from the upward seepage force of the groundwater during excavation. Therefore, a new cut-off wall (second phase) was constructed to a depth of 54 m to cut off the confined aquifer. A second pumping test was conducted after the construction of the second phase cut-off wall, and test results show that this full cut-off combined with dewatering can control groundwater effectively during excavation. This finding indicates that when a deep excavation is conducted in a confined aquifer with high hydraulic conductivity, determination of the depth of the retaining wall should be based on three factors: the stability of the base, the upward seepage stability, and settlement control.

A new second-order numerical manifold method model with an efficient scheme for analyzing free surface flow with inner drains

Numerical manifold method (NMM) is a numerical method known for analyzing continuous and discontinuous mechanical processes in a unified mathematical form. In this study we developed a new second-order NMM model to solve the nonlinear problem of water flow with the free surface priori unknown and the difficulty of modeling drains which could dramatically increase the meshing load. Our study consist of: (1) deriving two forms of NMM second-order approximation; (2) constructing the total potential energy for water flow by our energy-work seepage model considering Dirichlet, Neumann and material boundaries uniformly; (3) locating free surface nodes in two forms of second-order approximation; (4) tracking the free surface with an efficient iteration scheme without re-meshing; (5) deriving velocity and tunnel flux by second-order approximation. We developed a new code and demonstrate our model and code with examples including confined drainage tunnel and free surface flow through a dam. We compare the results such as tunnel flux or free surface with linear NMM, analytical or other available numerical solutions. We prove that: the two forms of second-order NMM (1) yield consistent results; (2) for modeling drains involving local intensive change, could achieve accurate result of tunnel flux calculation and dramatically save computation load with linear velocity distribution in coarse mesh; (3) for free surface iteration, are efficient with fast convergence to accurate results and with rather coarse mesh. As a result, our second-order NMM model is applicable to free surface flow with inner drains for free surface locating and flux calculation, and seepage stability analysis, laying a solid foundation for extending to coupled hydro-mechanical analysis.

Numerical analysis on influence of principal parameters of topography on hillslope instability in a small catchment

This study was conducted to identify the influence of three principal parameters constituting topography (slope inclination, soil depth, and slope length) on hillslope instability in a small catchment, known as Higashifukubegawa of Shikoku Island, western Japan. The typhoon rainfall of 19–20 October 2004 was significant in causing a total of seven slope failures in the catchment, though other rainfall events of various intensities in the same year did not cause failure. To understand the influence of the three principal parameters, numerical modeling of seepage and slope stability was performed in slope profiles constructed by varying the three parameters across their permissible range prepared from the seven slope failures of Higashifukubegawa in GeoStudio (GeoStudio Tutorials includes student edition lessons, Geo-Slope International Ltd., Calgary, 2005 v.4). The change in porewater pressure and slope mass weight due to variation in values of principal parameters was used to interpret the change in factor of safety or instability. The results showed that (1) instability increases with increase in the values of all three selected parameters across their range in Higashifukubegawa with remarkable decreasing trend in factors of safety, (2) slope inclination and soil depth were observed to affect instability through change in both unsaturated zone moisture content and mobilizing force of slope mass, (3) but with slope length, the unsaturated zone moisture content was not found to change considerably which implies that the instability due to slope length is mainly governed by change in slope mass weight. Overall, this study has dealt in great detail with how hillslope instability changes with principal parameters of topography under the same simulating conditions of hydrological and geo-mechanical parameters.

Application of AutoBank in the Dike Reinforcement Calculation of Lower Qinhe River

Calculation of seepage stability is important method in selection and justification of reinforcing dike seepage control measures. In the reinforcement design of the Qinhe river dike downstream, the finite element method of AutoBank is used to calculate the seepage stability of the dike reinforced before and after, and an economic and reasonable reinforcement scheme is selected then.

Experimental Research on Fine Sand and Filter Material of Xing-Long Hydroproject Dam Foundation

The Xing-long hydro project is situated on fine sand foundation, whose seepage stability is a vital factor to the safety of the buildings. By means of field sampling, seepage deformation test and filter test to fine sand and filter material, the seepage deformation character of fine sand and filter material are studied. The seepage failure gradient of fine sand is very low under natural condition, its seepage stability should be satisfied under the protection of filter materials, but the protective capability of natural filter material is limited, it is suggested that the filter material be optimized by getting rid of the particles less than 0.5mm.