Numerical Study on Seepage-induced Failure of Caisson Type Breakwaters Using a Stabilized ISPH

Abstract

This paper presents a stabilized incompressible smoothed particle hydrodynamics (ISPH) method to simultaneously simulate surface and seepage flow. The Navier-Stokes type equation, which manipulates the linear and nonlinear resistance effects caused from porous materials, are employed for tracking fluid motions in porous medium. The SPH method is one of the mesh-less particle methods and its Lagrangian nature is considered to be fit to express the large deformation of free surfaces. Some research on fluid flows in porous medium have been done by using the particle methods. In particular, Akbari introduced the apparent density which is numerically calculated by multiplying the original fluid density with the porosity. By means of his technique, fluid volume in porous medium is successfully conserved. However, fluid pressure around solid boundaries can be overestimated because of the density gap between the fluid and fixed solid boundary particles. With the aim of obtaining the smooth and quantitatively accurate pressure distribution, we proposed a new SPH approximation called as the mass and density correction. Through some fundamental numerical tests, the accuracy of the proposed method was sufficiently confirmed.
This paper also presents a hydraulic experiment on caisson type breakwaters solely focusing on seepage flow through the rubble mound without considering any other factors such as wave force and scour. Two cases of the experiments were implemented with different hydraulic models; a breakwater model without reinforcement and that reinforced with sheet piles. As for the latter one, the sheet piles are set to be quite thin against the whole model structure. This issue can lead to considerably-high particle resolution in the particle methods because a certain number of solid particles need to be placed on the thin structure. In order to overcome this problem, a new boundary treatment using the reversible boundary particles is suggested. We conducted a numerical analysis on this experiment and confirmed that the analysis results show good agreement with the experiments.

Popular Abstract

On March 11, 2011, a huge tsunami induced by the Tohoku-Kanto Earthquake devastated many port structures such as breakwaters and seawalls. A deeper understanding on the failure mechanism of caisson-type breakwaters is one of the significant tasks to reduce the damages which may be brought by next millennium tsunamis. Many research have been done to understand this phenomenon further, and then the following three causes have been determined; (i) the horizontal force due to the water-level difference between the front and back side of a caisson, (ii) the scour induced by tsunami overtopping in the rear of a caisson, and (iii) the seepage-induced piping caused by the degradation of bearing capacity of a mound. Development of a numerical analysis tool, which can simulate the above three causes simultaneously, is desired for practical design of breakwaters. However, due to the complexity of the collapse mechanism of a caisson-type breakwater, it is considerably difficult to develop such multi-physics simulation models. An individual study focusing on each cause is required to be done and a robust analysis scheme for each phenomenon need to be established primarily. For the aforementioned reasons, in this study, the third cause; (iii) seepage-induced piping caused by the bearing capacity degradation of a mound is solely taken into consideration. As an analysis scheme, the stabilized ISPH method　reformulated based on the unified governing equations proposed by Akbari, is adopted to continuously handle both surface and seepage flows. The results were compared with two cases of the experiments implemented with different hydraulic models; namely, a breakwater model without reinforcement and the one reinforced with sheet piles. In terms of the piezo water head and hydraulic gradient, good agreement was observed between the numerical and experimental data.

• author: Goda, Tetsuro
• supervisor: Magnus Larson
• organization: Division of Water Resources Engineering
• course: VVRM01 20162
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Tsunami, Breakwater, Particle method, SPH, Seepage flow
• publication/series: TVVR 17/5007
• report number: 17/5007
• ISSN: 1101-9824
• language: English
• additional info: Examiner: Hans Hanson
• id: 8904246
• date added to LUP: 2017-03-16 17:41:33
• date last changed: 2019-04-01 13:40:06