Cofferdam Stability Research Articles

Effect of 2-D spatial variability of soil properties on cofferdam stability considering degradation of soft soil foundation

Effect of 2-D spatial variability of soil properties on cofferdam stability considering degradation of soft soil foundation

Simulation of Cofferdam Calculations Using Geo-Studio Application in Peusangan Hydropower Dam Construction

Cofferdam is a building in the form of earth, stone, concrete or masonry filling and holds water, prevents flooding, provides irrigation. The building has many functions such as for agriculture, power generation, and tourist attractions. Cofferdam will also be very helpful in flood control which will control flood water so that it will not overflow excessively which endangers the surrounding community. Besides having great benefits. If the cofferdam collapses (broken) it will cause flash floods that result in casualties. Therefore the cofferdam must be designed safely and technically feasible. The research approach used in this study is a quantitative approach using the 2012 GEO-SLOPE program. The conclusions from the analysis of slope seepage and seepage stability are the Critical Safety Factors for cofferdam stability according to the results of the analysis after running SLOPE/W are 1.711 & 1.512 because it is higher than 1.5 (SNI 8460:2017 Geotechnical Requirements Design, This means that the stability of the cofferdam is safe. The modeling results show that the seepage discharge is 1.81x10-7 m3/s = 1.81x10-4 liters/sec = 15.64 liters/day which is smaller than the maximum allowable discharge of 50 liters/day. So it can be concluded that the seepage discharge is safe.

Stability of Nonuniform Large Geotextile-Reinforced Cofferdam under Seepage and Excavation Effects

This paper investigates the failure mechanism of nonuniform large geotextile-reinforced cofferdam under seepage and excavation effects on soft sediments. A numerical model is established and validated with available testing results. Then, parametric studies are carried out to investigate the potential influence factors. This study found that the difference between the upstream and downstream water levels and the excavation depth both have significant influences on the stability of cofferdams, where excavation is the key influence factor affecting the stability of the cofferdam. Numerical results revealed that the safety factor could reduce by about 20% with excavation of about 1.5 m. An empirical equation is proposed for quantitative prediction of the stability of geotextile-reinforced cofferdams. A thorough understanding about the failure mechanisms of cofferdams as well as an empirical formula for predicting the stability is concluded in this study, which could provide engineering guide for flexible foundations reinforced by geotextiles.

Deformation and Force Analysis of Wood-Piled Island Cofferdam Based on Equivalent Bending Stiffness Principle

This paper analyses the deformation and force behavior of a wood-piled island cofferdam based on the principle of equivalent bending stiffness. The horizontal deformation and bending moments in wood piles and the axial stress in tension bars on top of piles were both analyzed by the finite difference (FD) method. Except for the analysis of the cofferdam construction process, the influence of the pile length, the dam width, the tension bar interval, and the pile interval, among the commonly adopted parameters, were detailly examined in numerical simulations. In addition, a reinforced wood-piled cofferdam model by steel pipe piles has been established to quantify the effect of reinforcement. It was found that the dewatering inside the cofferdam was detrimental to cofferdam stability. The pile deformation reached maxima (roughly 0.6% of the pile length) at solidifying stage after dewatering. The changing trend of the cofferdam structure force within a safe district was consistent with the displacement. The dam width had a vital effect on the stability of the cofferdam, especially on the horizontal deformation. The steel pipe pile reinforcement scheme performed better in further deformation control, providing a new idea for island-type cofferdams with rigorous structural deformation control.

Deformation Characteristics and Safety Evaluation of the Throw Filling Soft Clay Cofferdam under Super‐Historical Flood Conditions

Affected by super‐historical flood, the surface deformation of throw filling soft clay cofferdam is obvious and the stability of the cofferdam slope is difficult to ensure. Relying on the cofferdam project of Jiangxi Xinjiang Shuanggang navigation power junction and according to the data of filed investigation and automatic monitoring, the Plaxis infinite element software is used to analyze the deformation characteristics and stabilities of cofferdam during the super‐historical flood period, and the effective emergency reinforcement measures are presented. The results show that (1) the stability of cofferdam is controlled by downstream face slope of cofferdam during the super‐historical flood period, and there is a most dangerous slide surface interpenetrating through the top to the toe of downstream face slope; (2) the stability of upstream face slope is influenced significantly by the water level fluctuation and it is reduced sharply during the water level decline process; and (3) real‐time remote monitoring can effectively reflect the deformation and failure characteristics of cofferdam under the condition of super‐historical flood level and provide early warning and prediction in time. The results and analyzing methods can be applied to analyze the similar projects excellently.

Analysis of new temporary cofferdam stability

As a temporary cofferdam, the wave force is the biggest threaten to the structure stability. This paper analysed the stability of plug-in steel cylinder under the action of wave. The stability analysis was proposed based on an engineering case and made contribution to the guiding of actual construction.

Simulation of the newly-built temporary cofferdam stability under the regular wave condition

The lateral cyclic wave force challenges the newly-built temporary cofferdam seriously. This paper simulated and assessed the movement of the plugged-in steel cylinder affected by the regular wave force with finite element method. The FEM analysis was conducted for an actual case, and the conclusions may show its advantages in the design work.

Finite Element Analysis on Earth-rock Cofferdam Behavior during Pumping and Drainage of Foundation Pit

The drawdown velocity of foundation pit water level in existing projects is mostly determined empirically, and the influence of water level drawdown velocity on the seepage field and stress field in the weir body is not fully considered. The water level change of the foundation pit is an important factor affecting the stability of the cofferdam. Therefore, it is of great practical significance to study the variation law of seepage field and stress-strain field of cofferdam during the pumping process of foundation pit, to accurately judge the stability of cofferdam slope and to formulate reasonable pumping and drainage scheme. Based on the 1th temporary earth-rock cofferdam of Yongning Water Conservancy Project, the nonlinear finite element method is used to calculate the spatial-temporal evolution of seepage field and stress-strain field of earth-rock cofferdam under different drawdown velocity of water level. The overall deformation of the cofferdam, the shear failure and instability characteristics of the slope, and the influence of water level changes on the stability of the cofferdam slope are analysed. The results show that the distribution of unsaturated zone and negative pore pressure zone is basically the same. The excessive water level drawdown velocity is easy to break the balance between the seepage field and the water level fall time, resulting in the rise of saturation line height and the increase of the upper bending rate. The effect of seepage control is weakened, and the stability of the slope is poor. The shear deformation of the cofferdam slope develops from the foot of the slope and extends to the crest of the cofferdam until it runs through. At the same time, local plastic deformation occurs at the foot of the outer stone slag berm, and the maximum deformation transits to the foot of the inner slope. Under the action of seepage force, the stability of the inner slope is always smaller than that of the outer one; the change of water level drawdown velocity is very sensitive to the stability of the slope, when the drawdown velocity is greater than the critical velocity, the stability of the slope decreases sharply. Combined with the characteristics of earth-rock cofferdam, on the basis of satisfying the stability of cofferdam, the drawdown velocity of water level should be controlled within 1.28m/d. The research results of the thesis have reference significance for the selection of pumping speed of foundation pit similar to earth-rock cofferdam.

Study on Stability of Super-high Straight-web Cellular Steel Sheet Pile Cofferdam

In recent years, steel sheet pile cofferdam has been adopted in some hydropower reconstruction and extension projects because of its many advantages. However, due to insufficient understanding of the bearing mechanism of super-high cellular cofferdam and lack of Engineering experience, there is no exact method to accurately predict the deformation and failure forms of the cofferdam, and there is no perfect safety and economic design theory, which restricts its popularization and application. Taking the Tarbela 4th Extension Project as background, the mechanical behavior and stability evaluation of the cofferdam in 30m deep water during construction and operation are studied by FEM, and influence of structural form and parameters on deformation characteristics of key parts is analysed. Results show that the maximum interlock tension occurs near mudline or top of berm during high lateral load. Development distribution of shear deformation are more in line with the Kitajima Shoichi’ method. The overturning deformation of cell can be effectively restrained by embedment, the rotation point changes regularly with the increase of berm and embedment. The sliding surface in cell conforms to Hansen’s calculation theory, the position of rotation and change of base pressure are closer to the Japanese standard. With the stiffness loss, the change of interlock tension is similar to the damage of low carbon steel, the shear sliding surface is related to berm which above the top overturning deformation is the largest. Based on the stability of cofferdam, there are various combinations of berm height and embedded depth, which can reduce the quantity of embankment works and the cost of later maintenance. The research results are of reference significance for the design and construction of similar super-high steel sheet pile cofferdams.

Analysis and Design Implications on Stability of Cofferdam Subjected to Water Wave Action

Based on Power-Law nonlinear failure criterion and the upper bound theorem of limit analysis, a numerical solution was developed to analyze the shape of collapsing block and landslide thrust of cofferdam subjected to residual pore pressure which was caused by longstanding water wave action. In the upper bound analysis, the effect of water pressure assumed to be a work rate of external force was considered. To evaluate the validity of the present method, the results were compared with previous findings. The good agreement between the present results and the previous findings indicated that the proposed method was valid to assess the shape of collapsing block and landslide thrust of cofferdam subjected to residual pore pressure. As for the influence of the factors on the shape of collapsing block and landslide thrust, a parametric study was conducted. This parametric study was carried out to optimize the design of cofferdam supporting structures and improve analysis of the stability of cofferdam subjected to water wave action.

Research of Hydraulic Characteristics of Cofferdam and Leaching System on Joint Way of Nanjing Second Yangtse Bridge

The hydraulic filling is the construction method of joint way in Nanjing Second Yangze Bridge. The leaching facility, drainage time and cofferdam stability in filling are key points in design. By laboratory test, the compaction, direct shear and permeability test of different compaction are investigated. The deformation through field monitor is carried during the cofferdam construction and hydraulic filling. After parameter test and stability analysis according to hydraulic filling, it is shown that the drainage ditch in base of cofferdam can satisfy drainage requirement, the leaching time is about half of one month. The compactness of cofferdam is 0.92. The slope of cofferdam during construction and hydraulic filling is in stability by analysis. And the cofferdam is in stable situation during hydraulic filling according to deformation monitoring result.

Centrifuge model tests on behavior of double sheet pile wall cofferdam on clay

The behavior of double sheet pile wall cofferdam on a thick clay deposit subjected to a flash flood was studied through a series of centrifuge model tests. In a rectangular container a thick layer of kaolin clay was placed on a layer of fine silica sand. The clay layer was prepared with a defined stress history. The model cofferdam consisted of two aluminum sheet pile walls, with tie rods at the top and ground level. After driving the sheet pile walls to the sand layer, the inside of the sheet pile walls was excavated and filled with the same silica sand as used in preparing the bottom sand layer. Under 70 g centrifugal acceleration, the clay deposit was consolidated under the load from the fill to a certain degree and then water was fed upstream of the cofferdam to simulate high floodwater until the water level reached nearly to the top of the cofferdam or large deflection of the cofferdam was observed. Various factors affecting stability of the cofferdam and mechanism of failure were examined. Stability of the double sheet pile wall cofferdam on a thick clay deposit was also compared with a cofferdam on the sand deposit. Test results implied that: (i) shear deformation of the fill dominates the failure mechanism of the cofferdam, (ii) as the width of the cofferdam increases, the water height at failure increases, (iii) the resistance against high water does not always increase with increasing the embedment depth, (iv) the degree of consolidation of the clay foundation affects the stability of the cofferdam, though degree of consolidation was simulated after construction of the cofferdam, (v) performance with respect to the mechanical behavior of a cofferdam in a sand deposit is better than that in a clay deposit, and (vi) both the upstream and downstream sheet pile walls in a clay foundation are subjected to almost the same flexural stress compared to the larger flexural stress in the downstream sheet pile wall in a sand foundation.

Hydro-Mechanical Coupling Analyses of Cofferdam Stability of Taian Pumped Storage Power Station

Three-dimensional hydro-mechanical coupling analyses have been conducted on the water-tight structure of the cofferdam for both intake and outlet of Taian pumped storage power station, located in Shandong Province of China. In addition, the effects of excavation on the cofferdam and foundation slopes were also studied by using the 3D FLAC. The calculation results show that the central core of high-pressure grouting has a prefect anti-seepage effect and therefore is able to strengthen the stability of the cofferdam and foundation slopes. The excavation process has only some local effects on the cofferdam and does not greatly affect the global stability of the cofferdam. Therefore, no failure takes place around slope toes. The results show that the width of platform left on the excavation side is reasonable.

Geotechnical Design and Instrumentation of an Anchored Cofferdam, Point Marion Lock, Pennsylvania

Point Marion Lock was designed to replace an existing navigation lock that has been in service on the Monongahela River near the Pennsylvania-West Virginia border for some 67 years. As a result of the close proximity of the planned excavation for the new lock, the land wall of the existing lock required extensive stabilization to permit it to be utilized as the river arm of the project cofferdam. Installation of 471 prestressed rock anchors was closely linked with each progressive stage of soil and rock excavation required to build the new lock. Adverse geologic conditions affected cofferdam stability in that the land wall of the existing operating lock was founded on weak sedimentary rocks exhibiting very low shear strength. Critical to the successful performance of the cofferdam was the design and installation of an extensive instrumentation program. A vast array of movement detection systems including inclinometers, alignment pins, shear strips, tilt plates and load cells were interfaced with both on-site and off-site computers to provide real-time monitoring of cofferdam performance throughout the excavation process. Data from instrumentation yielded invaluable information needed to make necessary adjustments in on-going construction activities including excavation and rock anchor installation.

Cellular Cofferdam for Trident Drydock: Performance

The construction and performance of a permanent steel sheet pile cellular cofferdam designed to accommodate a maximum water depth of 79 ft (24 m) is described. The cofferdam is among the first to be constructed in the United States which, because of expected interlock tensions, required the use of high strength steel sheet piles and extruded wye connections. Vibratory probe compaction of the cell and connecting are fill was performed to prohibit earthquake liquefaction, to minimize settlements and to obtain necessary weight for cofferdam stability. A pumped cell fill dewatering system was used to limit interlock tensions during and following cofferdam basin unwatering.