Method of fragments (MoF) solutions for double-walled, circular and rectangular cofferdam seepage problems

Madanayaka, Thushara Asela (2018) Method of fragments (MoF) solutions for double-walled, circular and rectangular cofferdam seepage problems. PhD thesis, James Cook University.

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View at Publisher Website: https://doi.org/10.25903/5c99693025545
 
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Abstract

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.

Item ID: 57790
Item Type: Thesis (PhD)
Keywords: axisymmetric, circular cofferdam, double-walled cofferdam, exit gradient flow rate, method of fragments
Copyright Information: Copyright © 2018 Thushara Asela Madanayaka
Date Deposited: 01 Apr 2019 05:15
FoR Codes: 09 ENGINEERING > 0905 Civil Engineering > 090501 Civil Geotechnical Engineering @ 100%
SEO Codes: 87 CONSTRUCTION > 8702 Construction Design > 870201 Civil Construction Design @ 100%
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Last 12 Months: 10
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