Dissertations / Theses on the topic 'Cofferdams'

Cellular cofferdams have primarily been used as temporary systems which serve to allow construction of facilities in open bodies of water. Applications for these structures have been increasing and today they may serve as permanent retaining walls or as navigation or waterfront structures. Conventional design methods for cellular cofferdams are based on semi-empirical approaches largely developed in the 1940s and 1950s. None of the available traditional procedures are capable of predicting cofferdam deformations, a parameter of key importance to the cofferdam performance, and which is often observed during construction for purposes of safety monitoring. Also, there is evidence that much of the conventional design technology is conservative, in some cases predicting loading by more than twice that which actually occurs. Recently, the finite element method has shown promise as a tool which can be used to help resolve some of the outstanding problems with cofferdam design. There are three primary objectives of this work: (1) enhance existing finite element program to allow for more accurate and refined analysis of cellular cofferdams, (2) use the enhanced finite element programs to assess the degree of conservatism in conventional design methods for cofferdams founded on sandy soils, and (3) use the results of parametric studies of cofferdams founded on sandy soils to develop a simplified procedure to predict cofferdam movements and determine potential for internal failure. The first of the objectives involves adding better bending elements to the program SOILSTRUCT to represent the sheet pile system In axisymmetric and plane strain analyses. Also, in the case of the plane strain program, a new method is developed to allow shear transfer through the sheet pile system. Through case history and theoretical analyses, the enhanced programs are demonstrated to yield accurate and realistic results. Parametric studies using the axisymmetric program show that conventional design methods overpredict, in some areas strongly, the interlock forces which develop during filling of the cofferdam. Parametric studies using the plane strain program suggest that there is also considerable conservatism in design methods to predict internal stability of the cofferdam. A new, simplified method is proposed for this type of analysis. In addition, it is shown that the deformations of cofferdams on sand follow consistent trends and can be set into a nondimensionalized context which can be used to predict future cofferdam movements.
Ph. D.
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The construction of water management and navigation structures often requires temporary works to exclude water to facilitate construction in the dry or at least under a lower water level within the construction area. The use of cellular cofferdams for both temporary and permanent earth/water retaining works is very common. A number of theories are presented that describe the failure mechanisms involved. These failure mechanisms were identified from model studies on circular type cofferdams are applied independent of cofferdam geometry. It is common to use diaphragm type cofferdams. To asses the validity of common failure mechanisms associated with cellular cofferdams when applied to diaphragm type cofferdams, a series of numerical modelling analyses were conducted. These were validated using field monitoring results of a large scale diaphragm type cofferdam constructed at St. Germans, Norfolk, UK. A series of plane strain analyses of a diaphragm type cofferdam were conducted using the geometry from a critical section of cofferdam. These analyses identified the lowest factor of safety based on the drained strength of the clay on which the cofferdam was constructed. The water level within the river was increased to accommodate flooding and soil strength was reduced to identify the general failure mechanism. The structural forces were calculated using both 2D and 3D models for a larger width section (13m wide) to allow comparison with wall bending moments and displacements measured in field. The 3D analyses used actual tie spacing and membrane effects whilst reducing the lateral stiffness of the wall to accommodate the stiffness reduction due to variation in the interlock forces. To measure the bending moment in sheet piles, resistance type strain gauges were installed on a 13m wide section of cofferdam. The cell deflection, river and cell water levels were also monitored to identify tidal river effect and the influence of cell water level on performance. Comparison of field and numerical results highlighted a number of important design and construction detail related to diaphragm type cellular cofferdams.

A cellular cofferdam represents a challenging soil-structure interaction problem. The cellular system consists of a combination of a flexible structure formed from interlocking sheet piles that is filled with soil. In the past, the cellular cofferdam has been viewed as a temporary structure, and the design procedures have been based on empirical concepts. Basic to these approaches are assumptions of soil and structural behavior that have, at best, only a rough accounting for soil-structure interaction. In the last decade, work on cofferdams has improved our understanding of the behavior of these systems. Documentation of performance has increased, and in a few cases major instrumentation efforts have been undertaken. Concurrently, finite element methods have been introduced for the analysis of cofferdams. Where the finite element models have been properly calibrated by field performance, they have reasonably predicted the principal aspects of cofferdam behavior. Results of the finite element models have also served to help explain some aspects of the soil-structure interaction process in the cofferdam system. Two finite element programs are used in this research, AXISHL and GPS. The first of these is an axisymmetric analysis tool which is applicable to the case of filling of a main cell. The second program provides a simplified means of analyzing the main/arc cell and common wall system. Both programs are used in a series of parameter studies with the objective to provide information that will allow improvement of the state-of-the-art of design for cofferdams. An analytical solution is proposed which allows an insight to be developed as to how the clamping effect at the dredge line affects the behavior of the system. A simplified calculation procedure which has some of the characteristics of the finite element analysis is developed to supplement the need for a finite element analysis.
Ph. D.

The conventional design methods for sheet-pile cellular cofferdams were developed in the 1940's and 1950's based on field and limited experimental observations. The analytical techniques of the day were unable to account for the complexities involved. The procedures used only rudimentary concepts of soil-structure interaction which do not exhibit the true response of the cofferdam for most circumstances. During the past decade it has been demonstrated that with proper consideration of the soil-structure interaction effects, the two-dimensional finite element models can be powerful tools in the investigation of cellular cofferdam behavior. However, universal implementation of the findings of these analyses was difficult to justify, since uncertainties remain about the assumptions made in arriving at the two-dimensional models. The only way to address these uncertainties was to perform a three-dimensional analysis. This investigation has focused on the study of the three-dimensional behavior of Lock and Dam No. 26 (R) sheet—pile cellular cofferdam. The work involved the development of a new three-dimensional soil-structure interaction finite element code for cellular cofferdam modeling, and the application of the new code to the study of the behavior of the first- and second-stage cofferdam at Lock and Dam No. 26 (R). The new code was used to study the cell filling process where the main cell is filled first with the subsequent filling of the arc cell. The finite element results show that interlock forces in the common wall were 29 to 35 percent higher than those in the main cell which are less than those calculated by conventional methods and compare well with the observed values. After cell filling, the new code was used to model the cofferdam under differential loading due to initial dewatering of the interior of the cofferdam and changes in river levels. The finite element analysis results show that increasing differential water loads cause the confining stresses in the cell fill to increase which results in a decrease in the level of mobilized shear strength in the cell fill. This explains why the cellular cofferdam can withstand extremely high lateral loads and lateral deformations without collapsing.
Ph. D.

Cofferdams are often employed as temporary watertight structures made of sheet piles and internally braced with steel or reinforced concrete ring beams to retain surrounding soil. For urban cofferdam excavations, soil removal is performed following a bottom-up performance and concrete shear walls and foundations are installed in rock or competent soil. The main goal of this study is to compare the observed performance of two cofferdams projects and conduct a series parametric analysis to study the effects of installation activities of steel ring beams. The first case history is the One Museum Park West (OMPW) and the second is the construction of a cofferdam of a structure projected to be the tallest building in America and the deepest basement built in the city. These two cofferdams evidence the need of a strict deformation control plan applicable to every construction stage, including those considered as ancillary.

A finite element program for analyzing the nonlinear behavior of moving contact problems has been developed and used for the analysis of a single sheet pile interlock (Chan and Barker (1985), Wu and Barker (1986). In this study, the program is modified to simulate a sheet pile pull-out test with a new finite element mesh. The improved mesh contains a full-length sheet pile with both of its interlocks connected to two half-length sheet piles. The results are presented and compared with the results of the pull-out tests which were conducted by O'Neil and McDonald at WES (1985). The comparison is not completely satisfactory because the initial slack between interlocks is not modeled. Nonetheless, the general behavior of sheet piles under tensile load is correctly predicted by the finite element program.

For easier interpretation of the output from the finite element analysis, the computer graphics software AutoCAD (Auto desk, 1986) is adopted to serve as a postprocessor. Several features of AutoCAD such as overlaying, zooming, and macro instructions are utilized to serve this purpose. Some intermediate programs are also developed for the communication between the finite element program and AutoCAD.
Master of Science

Inflatable dams are flexible membrane structures, pressurized with either air, water, or both, which have been used in recent years as a means of temporarily impounding water. A number of procedures have been developed to investigate the static behavior of the dams, but the dynamic behavior has been largely neglected. The few studies that have been done on dynamic behavior have used the simplifying assumption that the weight of the membrane was negligible. In this study, equations of equilibrium and equations of motion were derived for an air inflated dam impounding no water, but loaded with its own membrane weight. It was assumed that the effect of membrane extensibility is negligible in the analysis. Derivatives required in the equations of motion were approximated using finite difference equations. Computer programs were written to find solutions for the eigenvalues and eigenvectors of the equations of motion. The computer program plotted the mode shapes of vibration associated with the four lowest eigenvalues, as well as the static shape of the dam. The eigenvalues obtained were the square of the frequencies of the system, so the effects of a series of membrane weights on the frequencies of dams of various base lengths could be analyzed.
M.S.

The aim of this thesis is to undertake simplified design and assessment of cofferdam under vertical surcharge in the form of virtual project. Cofferdam constructed on the sea shore must be able to withstand all loads to enable construction of cut and cover tunnel. As a part of the design it is required to assess ground conditions and it is necessary to review feasibility of the structure on the sea. The construction will be described in construction phases and graphically demonstrated in construction sequence drawing. Because the structure is designed for life time of five years, temperature load on struts is studied in the thesis and further considered in structural analysis. Furthermore, it is required to compare analytical models from GEO5 and PLAXIS. Horizontal frame is analysed separately in Scia Engineer. Structural members – cofferdam wall, waler beam and strut are checked in this thesis. For the design of the cofferdam wall second order theory is considered. All mentioned requirements were accomplished with help of corresponding Eurocodes, books and technical advice. Results of this work are feasible and it was possible to design all members to enable construction of the cut and cover tunnel. The phasing was designed such that deformation of the cofferdam is minimalized and use of machinery is limited to machines on temporary platforms. Comparison of two different analytical models showed that different soil modelling has great effect on internal forces, even though the shape of the bending curve is very similar, values obtained from PLAXIS software are much higher. Assessment of the structural members is done according to Eurocode 3, and horizontal frame is checked also against disproportionate collapse.

Thesis ((MEng)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: When constructing a submarine pipeline, construction teams must work in the hostile environment in the ocean known as the surf zone. The surf zone is the area along a shoreline stretching between the first evident point of wave breaking and the beach line. In order to ensure that the pipeline is shielded from the imposing forces within the surf zone, engineers use a burial technique which leaves the pipeline length in the surf zone buried underneath the active seabed once construction is finished. Thus a temporary surf zone trench is dredged and protected by an open-ended cofferdam built using iron sheet piles. As a result of the incoming wave climate and the surf zone currents created by this wave climate, sedimentation in and around the trench becomes problematic. In this study alternative geometric layouts for the open-ended cofferdam protecting the surf zone trench are investigated, attempting to minimize the sediment build-up in and around the trench. This was done by using both a 3D qualitative physical model conducted at the CSIR in Stellenbosch, and numerical model using MIKE developed by DHI. However, this study only considers sediment build-up and not structural integrity and constructability of the cofferdam designs. Combining the observations of both the physical- and numerical models, a conclusion was drawn that a structure built perpendicular to the shoreline with a 45oextended arm built from the upstream edge of the cofferdam wall, is the most effective. No dimensions are given as the cofferdam design will change depending on the site specific characteristics. Also an increase in structure length will result in the mouth of the structure being located outside the active sediment zone, which leads to a longer period of time before the pipeline pathway is compromised by sediment.
AFRIKAANSE OPSOMMING: Tydens die konstruksie van 'n onderwaterse pyplyn, moet konstruksie spanne in 'n gevaarlike gedeelte van die see werk naamlik die brandersone. Die brandersone kan gedefinieer word as die area tussen die eerste punt waar branders breek en die strandlyn. Om die pyplyn te beskerm teen die kragte wat branders op dit uitoefen, gebruik ingenieurs 'n installasietegniek waar hul die brandersone seksie van die pyplyn onder die aktiewe seebodem begrawe. Om die tegniek te bewerkstellig, grawe kontrakteurs 'n sloot deur die brandersone en beskerm dit met 'n tydelike struktuur bekend as 'n kofferdam. As gevolg van die inkomende branders en die strome wat deur die branders aangedryf word, kan die opbou van sediment in, en rondom die sloot in die brandersone problematies word. Hierdie studie ondersoek alternatiewe uitlegte vir die tydelike kofferdam struktuur met die oog daarop om die opbou van sediment in, en rondom die struktuur te verminder. Die doel was nagestreef deur gebruik te maak van beide 'n 3-dimensionele fisiese model, gebou en gebruik by die WNNR in Stellenbosch, en 'n numeriese model wat op MIKE, ontwikkel deur DHI gedoen was. Let wel die studie het slegs die sediment beweging in die nabye area van die tydelike kofferdam struktuur in ag geneem en nie die praktiese implimentering en strukturele integriteit van die struktuur nie. Deur die observasies van beide die fisiese- en numeriese modelering in ag te neem, is die volgende gevolgtrekkings gemaak. 'n Struktuur wat loodreg met die strandlyn gebou is en met 'n 45o arm wat na die stroom-op kant toe uitstrek, was die mees effektiewe een. Geen dimensies is deurgegee nie aangesien die ontwerp sal verskil afhangende van die spesifieke area waar die projek aangepak word. Daar is ook gesien dat indien die struktuur langer gemaak word, sal die kontrakteur langer tyd h^e voordat daar sediment probleme in die brander sone sloot ondervind sal word.

Stockholm lider idag av omfattande bostadsbrist och ständigt ökande befolkningsmängd. För att möta detta ökande behov av bostäder i Stockholm finns en nödvändighet i att utforska nya vägar till att åstadkomma detta. Här finns en unik möjlighet till en förtätning av staden genom att utnyttja Stockholms stora vattenarealer för att bygga flytande bostäder. Som alternativ till den traditionella byggkonsten ämnar examensarbetet till att demonstrera de kvaliteter som finns med att bo på vatten genom att utforma ett attraktivt bostadsområde med flytande konstruktion. Projektet illustreras i det tidigare hamnområdet vid Liljeholmskajen i Årstadals hamn, Stockholm. För att åstadkomma detta utförs fallstudier på olika typer av flytande konstruktioner som finns på den svenska marknaden för att fastställa de bästa konstruktionslösningarna som leder till färdiga förslagshandlingar. Resultatet visar att ett flytande bostadsområde är genomförbart med dagens teknik som ett sätt att förtäta staden. Vidare utredningar måste dock göras för att i framtiden legalisera och förenkla lagstiftningen kring denna typ av boende.
Present day, Stockholm suffers from a significant lack of dwellings and the population in the county is ever increasing. To meet this escalating demand for residences it is essential to explore new paths of accomplishing this. Here lies a unique opportunity for densification of the city by taking advantage of Stockholm’s grand water areal to build floating residences. This essay aims to demonstrate an alternative to the traditional arts of building construction by presenting the qualities of living on water in a proposition for a floating residential area. The project is displayed as placed in an old industrial port - Liljeholmskajen - in Årstadal, Stockholm. To achieve this, case studies are made on different types of floating constructions on the Swedish market to establish the best constructional solutions, which have led to complete concept drawings. The result illustrates that a floating residential area is achievable with today’s technology as a method of dealing with the densification of cities. Further investigations are necessary to legalize and simplify the legislation of this type of living

As obras de desvio de rios para a construção de barragens, embora tenham caráter provisório, são de estrema importância, pois definem como o empreendimento será executado, garantindo segurança à sua construção, dentro de riscos calculados, devendo no entanto ser o mais econômica possível. As estruturas de desvio devem se encaixar no arranjo das estruturas permanentes, podendo ser utilizadas como tal. Este trabalho tem o intuito de apresentar as principais estruturas utilizadas em esquemas de desvio de rios e caracterizar sua utilização e aplicabilidade. Sempre que possível foram apresentados esquemas utilizados na prática ou previstos em projeto, para melhor exemplificar e caracterizar as diversas estruturas e as possíveis soluções para desvio de rios.
Diversion works for dam construction are temporary. Nevertheless, they are extremely important, since they define how the construction is going to be made, and assuring its safety, within calculated risks. But, they have to be as economical as possible. The diversion structures must fit the global arrangement of the project, being in some cases be used as permanent structures. This document has the objective of present the main diversion structures used in diversion works schemes, as well as characterize their utilization and applicability. Whenever possible, it was presented examples of schemes either already used or just previewed on design, to better characterize and exemplify the many structures and possible solutions for river diversion.

碩士
國立臺灣科技大學
營建工程系
106
The flow of water through soils in the retaining system may induce some alterations on the magnitude of the working stresses in soils, especially the effective overburden stress. Sand boiling is one of the stability issues related to the change of effective stress due to groundwater flow, but only a few researchers investigate the effect from the effective stress point of view. Accordingly, the primary objective of this study is to investigate the impact of effective stress change inside the excavation zone subject to seepage flow and to evaluate the factors affecting the safety against sand boiling. The study was carried out on 23 experimental failure cases with various excavation geometries and soil densities by utilizing PLAXIS and SEEP/W finite element programs. The results indicate that the failure progression of sand boiling is initiated from the excavation surface, then propagates to the area close to the pile. In addition to the failure propagation, the excavation geometry has a visible effect in establishing the failure mechanism and the governing the computed hydraulic gradient. Concerning the requirement of a reliable safety adequacy, the factor of safety adequacy is proposed in this study by adopting the equivalency theory between reliability and factor of safety. Eventually, two actual failure case histories are introduced to validate the applicability of the proposed factor of safety. The results indicate that the application of the proposed method can describe well the failure condition of an excavation with respect to sand boiling. Note also the application of the proposed factor of safety should obey the applied constraints in this study.

碩士
國立高雄應用科技大學
土木工程與防災科技研究所
96
ABSTRACT Regarding development of underground structure, the need of extremely large and deep excavations was found. In this study, a case history of a large-scale circular cofferdam excavation in Kaohsiung retained by diaphragm wall is explored. Back-analyses based on pumping record in this case are conducted so actual permeability of soils are interpreted. In addition, engineering performances of the excavation are examined in order to indicate influences on ground and structure responses from diaphragm walling. Discussions of the relationship between lateral and vertical movements are included in this paper so it is expected to provide a useful reference to similar excavations in Kaohsiung later on. In this study, back-analyses indicate permeability of the 1st and 3rd layer of sand varies from 5×10-3 to 7×10-3 cm/sec. On contrary, permeability of the 5th layer of sand is much smaller, varies from 1.25×10-3 to 1.75×10-3 cm/sec. Further, observations in this study show the maximum building settlement reaches up to 0.018% of depth of the wall. Considering movements induced by excavation, the maximum lateral wall movement is approximately 0.13% to 0.17% of the excavation depth and the maximum building settlement is 0.15% of the excavation depth. Lastly, the influence zone on surface level behind the wall is 4 times of the excavation depth.

Cofferdams are temporary structures used in construction sites. Long-narrow (double-walled), circular, square and rectangular are the commonly seen cofferdam shapes, and flow rate and maximum exit hydraulic gradient are two of the main design parameters required. Commonly, these are evaluated through the 2D ground water flow model solved using flow nets or numerical methods. However, when the flow pattern is 3D, such as flow into the square or rectangular cofferdams, predictions by the 2D models underestimate the flow rate and maximum exit hydraulic gradient values considerably. Method of fragment (MoF) is an approximate technique which can be used for quick estimates of the flow rate and maximum exit hydraulic gradient values for double-walled cofferdams. The accuracy of the MoF solutions depends on the validity of the assumption that the equipotential line at the tip of the cut-off wall is vertical, dividing the flow domain into two fragments. In this research, validity of this assumption was assessed through the extensive numerical simulations, and it was found that, MoF predictions are within acceptable limits, and the effect of deviating from the assumption is always onto the conservative side. Further, MoF was extended to solve circular cofferdam problems, defining two new axisymmetric fragment types. Through a range of numerical simulations, design charts were developed to obtain the required axisymmetric form factors and normalised exit hydraulic gradient values. These were validated against detailed numerical solutions, analytical solutions, and experimental results reported in the literature. Also, a small-scale laboratory model was developed for analysing the circular cofferdam, and using that, series of tests were carried out. Then, the experimental results were compared against solutions derived using the proposed MoF solutions and showed a good agreement. Further, simple analytical expressions were developed and validated for the form factors and normalised exit gradient estimations of both double-walled and circular cofferdams enabling quicker computations and the MoF be implemented in spreadsheets. In addition, a simple method for evaluating the cofferdam safety against possible piping failure is presented. Through a series of finite element simulations, simple expressions were developed and validated to estimate the maximum exit hydraulic gradient for both double-walled and circular cofferdams considering only the shortest seepage path, known as creep length. The proposed solutions, including mean, lower and upper bound values for the exit hydraulic gradient at a given creep length can be applied in both isotropic and anisotropic soil conditions. Using them, a first-order estimate of the required creep length to limit the exit hydraulic gradient to a specific value can be determined. Alternatively, for a given configuration of the cofferdam, the exit hydraulic gradient can also be estimated. These equations can be valuable tools for back-of-the-envelope calculations in the preliminary analysis while selecting the dimensions in a cofferdam. Furthermore, simple expressions were developed and validated for accurately estimating the flow rate and maximum exit hydraulic gradient values of square and rectangular cofferdams founding on an isotropic and homogeneous soil medium. However, when the soil medium is anisotropic and homogeneous, proposed solutions are still applicable with a reasonable level of accuracy. In the proposed solutions, the 3D flow effects of square and rectangular cofferdams have been incorporated through the correction factors. Suggestions are made to improve the expressions given in the Canadian Foundation Engineering Manual, widely used in practice. The solutions proposed in this research can be very useful as a design tool in providing realistic first estimates of the flow rate and maximum exit hydraulic gradients of cofferdams, especially in preliminary assessments and for carrying out parametric studies, before going for a detailed analysis.