Nội dung

MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUY LOI UNIVERSITY NGUYEN QUANG LUONG STUDY ON THE HYDRAULIC STABILITY AND STRUCTURAL INTEGRITY OF RANDOMLY-PLACED RAKUNA-IV ON RUBBLE MOUND BREAKWATERS Specialization: Coastal Structure Engineering Code No.: 9580203 SUMMARY OF DOCTORAL DISSERTATION HA NOI, 2020 This scientific work has been accomplished at Thuy loi University Scientific supervisor: Prof. Thieu Quang Tuan Reviewer No.1: Assoc. Prof. Phung Dang Hieu – Vietnam Institute of Seas and Islands - Ministry of Natural Resources and Environment Reviewer No.2: Assoc. Prof. Nguyen Ngoc Thang – Thuyloi University Reviewer No.3: Assoc. Prof. Pham Hien Hau – National Universiy of Civil Engineering This Doctoral Dissertation will be defended at the meeting of the University Doctoral Committee in ............................................................................................................................... ............................................................................................................................... at ………………… on …………………………………………………………. The dissertation can be found at: - The National Library of Vietnam; - The Library of Thuy loi University. INTRODUCTION 1. Rationale of the study In recent years, very large concrete armour units have been used for many coastal protection works, especially rubble mound breakwaters in deep water areas. These concrete blocks have been being modified and improved in order to be well adapted to different wave conditions and to meet the increasing and diverse demands of the construction of deep water ports and coastal protection works. Along with hydraulic stability, structural integrity of concrete armour units has also been considered an important issue for rubble mound breakwaters. Recently, there have been various severe failures of many breakwaters covered by the complex form of slender concrete armour units without reinforcement, which caused by the breakage and failure of these blocks when the stresses exceed the strength or mechanical durability of the material, especially when the concrete armour units rock, rotate and collide under wave impacts – which is known as “rocking” mechanism. This is a common mechanism in case of two-layer and randomly placed slender concrete armour units on seaward slope of rubble mound breakwaters. Therefore, it can be seen that the importance of considering the structural integrity of concrete armour units in addition to hydraulic stability in case of random placement on multi-layer slopes of rubble breakwaters in order to avoid breakage and to ensure the overall stability of concrete armour units. As a general development trend in the world, RAKUNA-IV armour unit was invented by Nikken Kogaku company in 2007 and has been applied to a large number of projects in Japan, recently to Nghi Son breakwater in Thanh Hoa in 2006 and Chan May breakwater in Thua Thien Hue in 2019 in Vietnam, and soon to the breakwaters in Van Phong and Vinh Tan ports. There have been a number of studies that have been conducted previously on RAKUNA-IV armour units but have only focused on hydraulic stability in case of regular placement on 2layer slope under breaking wave conditions. However, there have been no studies on the hydraulic stability of RAKUNA-IV armour units in case of random 1 placement under the impacts of non-overtopping and non-breaking waves, especially on the structural integrity on impact due to rocking movements. 2. Research objectives Study on hydraulic stability and structural integrity of RAKUNA-IV armour units when randomly placed on the seaward slope of a rubble mound breakwater by means of experiments on physical models in a wave flume and simulations in mathematical models. The results of the research were later applied to a specific breakwater. Recommendations and solutions were thereby put forward for the design, production and construction of RAKUNA-IV armour units when applied to rubble mound breakwaters in reality. For the above-mentioned reasons, the author has chosen the topic "Study on hydraulic stability and structural integrity of randomly-placed RAKUNA-IV armour units on rubble mound breakwaters" for the research. 3. Subject and scope of the study 3.1 Subject of the study Randomly-placed RAKUNA-IV armour units on the seaward slope of a rubble mound breakwater under the impacts of non-overtopping and non-breaking waves. 3.2 Scope of the study Hydraulic stability and structural integrity of randomly-placed RAKUNA-IV armour units on the seaward slope of a rubble mound breakwater under the condition of non-breaking and non-overtopping waves. 4. Research methodology In order to attain the above-mentioned objectives and tasks, the dissertation has used the following research methodologies: statistical methods; physical model experiments in wave flume; simulations by means of mathematical models, and expert consultancy. 2 5. Scientific and practical meaning 5.1 Scientific meaning The studies were conducted by means of experiments on physical models in order to investigate the hydraulic stability and structural integrity of RAKUNA-IV armour units on the seaward 2-layer slope in case of random placement under the impacts of non-overtopping and non-breaking waves. In addition, the dissertation has also incorporated the finite element mathematical model to study the structural integrity of RAKUNA-IV armour units when rocking on seaward slope of rubble mound breakwater under wave impacts. 5.2 Practical meaning The research results on hydraulic stability and structural integrity under practical construction and working conditions, especially in areas with great depth, may be used as a reference, in combination with other technical standards to be applied in the design and consultation of rubble mound breakwaters using RAKUNA-IV armour units in practice in order to improve economic and technical efficiency, while reducing repair or maintenance costs. 6. Outline of the dissertation In addition to the introduction, conclusions and recommendations, the dissertation consists of 04 chapters as follows: CHAPTER 1: Overview of concrete armour units on rubble mound breakwaters; CHAPTER 2: Scientific bases for the study on hydraulic stability and structural integrity of concrete armour units on rubble mound breakwaters; CHAPTER 3: Results of the study on hydraulic stability and structural integrity of RAKUNA-IV armour units; CHAPTER 4: Application of the research results to the design of armour layer of Chan May breakwater in Thua Thien Hue province. 3 CHAPTER 1 OVERVIEW OF CONCRETE ARMOUR UNITS ON RUBBLE MOUND BREAKWATERS 1.1 Overview of concrete armour units on rubble mound breakwaters In addition to the interlocking, concrete armour units can also dissipate wave energy very well, thus reducing the pressure on the rubble mound breakwaters. Concrete armour units are often used to cover the seaward and inner slopes at the breakwater head. Various types of armour units have been developed and improved for the adaptation to different wave conditions and greater fulfilment of various increasing and diverse practical demands of the construction of deep water ports and coastal protection works. In the general development trend in the world, RAKUNA-IV armour units was invented by Nikken Kogaku company in 2007. RAKUNA-IV armour unit has the same four-legged structure as Tetrapod but is more angular, especially there are four more hollows on the surface. There were a number of previous studies on the hydraulic stability of RAKUNA-IV armour units but only focused on the case of regular placement on seaward slope of rubble mound breakwaters and there have been no studies on the hydraulic stability as well as the structural integrity of randomly-placed RAKUNA-IV armour units. This is a practical and urgent problem where in most cases (especially in deep-water areas), the concrete armour units are commonly placed in random patterns. 1.2 1.2.1 Overview of hydraulic stability of concrete armour units General introduction So far, there have been a large number of empirical formulae derived from many studies on the hydraulic stability of armour units on the slopes of rubble mound breakwater conducted by Tyrel (1949), Mathews (1951), Rodolf (1951), Iribarren and Nogales (1950), Larras (1952), Hedar (1953), etc. Based on the consideration of the balance of forces, many scientists have determined the formulae for hydraulic stability of the armour units on the slope, such as Iribarren (1938), Iribarren and Nogales (1954), Hudson (1958, 1959), Svee formula 4 (1962). Since then, a series of studies on the stability of armour units on seaward slope of rubble mound breakwater have been conducted and developed with various derived formulae such as Tyrel (1949), Mathews (1951), Rodolf (1951), Iribarren and Nogales (1950). ), Larras (1952), Hedar (1953) etc. 1.2.2 Studies on hydraulic stability of concrete armour units Hudson (1959) proposed a formula for the hydraulic stability of a rock under the impacts of incoming waves based on Iribarren's original formula. This formula was generalized later so that it could be applied to various types of armour units (rock and concrete blocks) under random wave conditions, in which the size of armour units is indicated by the nominal diameter Dn. Based on the results from many physical model experiments in Delft Hydraulics, Van der Meer (1988) proposed a more general formula for 2-layer Tetrapods placed on a slope of 1/1.5 under the impacts of non-breaking and nonovertopping waves. Van der Meer, J.W. and Heydra, G. (1991) incorporated the rocking mechanism into the hydraulic stability of Tetrapod armour units, and concluded that the instability due to rocking occurs mostly in the area around the design water level. These test results can be used to calculate the maximum stresses in the concrete armour units and the number of blocks with breakage, including the possible distribution of loading conditions, internal stresses of the concrete armour units and the effect of resulting collision on the tensile strength of concrete. These testing and measurement methods are the complement to the method proposed by Burcharth and Howell (1988) in order to directly measure the stresses in the concrete armour units. For RAKUNA-IV armour units, Mase, H., Yasuda, T., Mori, N., Matsushita, H. and Reis, M.T. (2011) conducted studies on hydraulic stability of horizontally composite breakwaters, taking into consideration the effects of wave steepness and wave breaking, with a foreshore slope of 1/30, 1/15 and a horizontal bottom (with a constant water depth). Based on the analysis of the research results on the application of RAKUNA-IV armour units to a two-layer slope of rubble mound 5 breakwater under non-overtopping wave conditions, which was conducted in the wave flume at Thuyloi University (formerly Water Resource University) in 2010, Tuan et al. showed that in the initial state, RAKUNA-IV armour unit is about 1.6 times more stable than Tetrapod. Tuan et al. (2012) also derived a formula for the hydraulic stability of RAKUNA-IV armour units, with a similar form to that for Tetrapods introduced by Van der Meer (1998) by means of regression analysis based on the experimental data. Suh, Kyung-Duck & Hoon Lee, Tae & Matsushita, Hiroshi & Ki Nam, Hong (2013) conducted model experiments for different wave conditions and seaward slope in order to establish stability formulae for RAKUNA-IV armour units on rubble mound breakwaters. Based on the results of the physical modelling experiments, Giang (2015) investigated the hydraulic stability of RAKUNA-IV armour units under the impacts of overtopping waves and the overtopping reduction feature which varies according to the interactive nature of the waves on the seaward slope of rubble mound breakwaters. The combination of a physical wave flume and a digital gave a significant insight into the physical nature of the water-cushioning effect, which governs the overtopping reduction feature of RAKUNA-IV armour units, and also an empirical formula was derived in order to determine the level of the stabiliy increasing level in case of overtopping waves in the form of increased stability factor and the overtopping reduction coefficient. 1.3 1.3.1 Overview of structural integrity of concrete armour units General introduction Slender, complex types of armour units such as Tetrapods and Dolosse have been widely used for rubble mound breakwaters. The breakage of these armor units has caused many failures to rubble mound breakwaters, therefore the need for studying stresses in concrete armour units under wave impacts has been fully recognized. The stability of the armour layer will decrease if the concrete armour units are broken and as a result will reduce the interlocking efficiency. Moreover, the debris from broken armour units may be thrown due to wave actions and may 6 increase the damage caused by cracks. In order to reduce cracking, it is necessary to ensure the structural integrity of concrete armour units. 1.3.2 Studies on structural integrity of concrete armour units There have been many studies on experimental models built to measure stresses in Tetrapods and Dolosse armour units under wave impacts by means of mounting stress-strain gauges. Typical studies were those of Burcharth (1980, 1981, 1983, 1986, 1988, 1990, 1991, 1993, 1994), Van de Meer (1990, 1991), Angremond (1994), Howell (1988), Ligteringen (1985) ), Nishigori (1986), Terao (1982) and many others. Many of the recent serious failures of rubble mound breakwaters protected by Dolosse and Tetrapods armour units were caused by the cracking. The breakage occurs before the hydraulic stability of the concrete armour units is no longer maintained. Therefore, there exists an imbalance between the strength (structural integrity) of the concrete armour units and the hydraulic stability (resistance to displacement) of the armour layer. H. F. Burcharth, G. L. Howell and Z. Liu (1991) conducted many experiments on the prototype and small-scale physical models, providing the research results for Dolosse armour units. Dolosse armour units were selected for the study due to their high hydraulic stability and the fact that their structural integrity can be adjusted by varying the waist ratio, in other words, the ratio between the diameter of the fluke and the height of the unit. By increasing the waist ratio to achieve greater durability, the hydraulic stability will be reduced to a certain extent, which is a matter of design consideration. H.F.Burcharth, Liu Zhou, Gary L. Howell, W.G.McDougal, (1991) presented the research and analysis results for the experiments on the model of Dolos armour units by means of load-cell techniques. Based on the results of studies on instrumented small scale model of concrete armour units, Burcharth (1993b), Burcharth and Liu (1995); Burcharth et al. (1995b) also derived an empirical formula to estimate the relative cracking 7 level of Dolos and Tetrapod armour units (in proportion to the total number of blocks). The insertion of the load-cell destroys the homogeneity of the material. This means that the impact stresses recorded in the small scale model tests cannot be scaled up to prototypes by the use of conventional formulae, which are valid only for homogeneous material. This is only possible by determining the apparent elasticity for the instrumented small scale models of the concrete armour unit, which is used for the interpretation of the impact signals recorded in the wave flume tests. 1.4 Conclusions for Chapter 1 Nowadays, the applicable conditions of rubble mound breakwaters are increasingly expanded together with the invention and development of various types of modified concrete armour units, with better efficiency in wave attenuation and thus better economic performance. RAKUNA-IV is one of the new types of concrete armour units that has been studied and developed by Nikken Kogaku company since 2007 with many outstanding features and higher economic efficiency than that of traditional concrete armour units such as Tetrapods. There have been a number of previous studies on the hydraulic stability of RAKUNA-IV armour units but only focused on stability in case of reglular placement on seaward slope of rubble mound breakwater, but there have been no studies on the stability of this type of armour unit in case of random placement on the seaward slope, especially the structural integrity under the impacts of incoming waves. This is an urgent practical issue when in most cases (especially at the breakwater head with great water depth) the concrete blocks for breakwaters are usually placed randomly during construction, therefore the armour units are easily subject to rocking movements under the action of waves or currents resulting in the collision and generating stresses that can lead to cracks, breakage and failures of these armour units. 8