Effect of armour unit layers and placement mode in the determination of stability of geotextile sand container (GSC) breakwaters

Geosynthetic Sand Containers (GSCs) are increasingly harnessed for their coastal protection capabilities. Recent studies point to its efficacy to be used even as armour units of breakwaters. The current investigation aims at understanding the effect of armour unit layers and placement modes in altering the stability of GSC breakwaters. Single-layered and double-layered GSC structures with slope parallel and perpendicular placement are tested for stability against wave conditions of the Mangaluru coast. A 1:30 scaled monochromatic wave flume model study is adopted to detail the damage levels and stability of various GSC breakwaters. It is observed that the stability of structure increased by up to 17% when supplemented with double layers. Structure tends to be stable with increasing armour units size and fill percentage. Larger bags stacked to double layers is found to be the most stable configuration. 80% filled, slope parallel placement exhibited the least stability. The paper dealt with all factors affecting structure stability and deduced stability nomograms helpful for coastal engineers to design GSC breakwaters.     

Hydraulic permeability of structures made of geotextile sand containers: Laboratory tests and conceptual model

The permeability values were obtained for more than 15 different geotextile sand container (GSCs)-structures. The influence of the size of the sand containers (four different sizes were tested), arrangement and geometry was investigated. First results show that the size of the container represents one of the primary factors influencing the permeability but the method of placement of the GSCs also plays a significant role (different methods of placing resulted in a permeability with more than a 50% difference). The results show that the flow through the GSC-structure is solely governed by the gaps between neighbouring containers and that the flow through the sand fill in the containers can be neglected.In addition, several GSC-structures with the same geometry and the same size of containers but with different arrangements (and thus different permeabilities) were tested under wave action in the twin wave flume of LWI. The results show that the mode of placement of elements strongly affects the permeability and the stability of GSC-structures. Placement of the containers in such a way that the contact areas among containers are maximized resulted in the largest overall stability against wave action. A conceptual model for the determination of the permeability of GSC-structures is proposed.     

Geotextile sand container shoreline protection systems: Design and application

The use geotextile sand containers (GSCs) as shoreline protection systems, has grown moderately since the first applications in the 1970s. This slow growth can be attributed to two factors; firstly, the lack of understanding of coastal processes and design fundamentals by the larger geosyntheticcommunity in order to provide coastal engineers with suitable solutions, and secondly; there has been very little rigorous scientific wave flume testing with which to analyse the wave stability of geotextile sand containers.The application of geotextile containers in coastal protection works can be traced back to early works carried out in 1970s. The application of these types of structures was somewhat haphazard as very little was understood about the wave stability and durability of the structures. Early wave stability work was carried out Ray (1977) and Jacobs (1983) with small containers, however, the testing programs were limited and did not provide sufficient confidence in the product to carry out exhaustive engineering design. As a result, the technology until recently has relied on manufacturers’ design suggestions based on monitoring of actual structures. Over the past five years, coastal population pressure, extreme events and concerns over climate change and sea level rise have resulted in more emphasis being placed on shoreline protection systems. Geotextile manufacturers have responded to the challenges put forward by design engineers and intensive research has been carried out in the field.This paper outlines the current “state of the art” in terms of the design and specification of geotextile sand containers (GSC). This paper covers the key issues which will ensure the long term integrity of a geotextile shoreline protection system is maintained, these issues include: • Container stability; • Detailed analysis of recent large scale wave flume testing which assess filling capacity, size of container, structure slope and scour protection etc.; • Container/geotextile durability; • Methods and specifications used to limit the effects of the fundamental factors affecting the life span of geotextile containers such as vandal resistance, UV degradation and abrasion resistance etc.     

Effect of deformations on the hydraulic stability of coastal structures made of geotextile sand containers

The hydraulic stability of geotextile sand containers (GSCs) for dune reinforcement, seawalls, revetments and artificial reefs for shore protection against storm waves has been studied within an ongoing extensive research program at Leichtweiß Institute. Although the effect of the deformation of the sand containers on the hydraulic stability is significant, no stability formula is available to account for this effect and the associated processes which have led to the observed failures. To achieve a better understanding of these processes and to analyze the influence of the deformations on the stability of coastal structures made of GSCs, different types of scale model tests have been performed. Results from a variety of scale model tests on: (i) wave-induced forces on the sand containers, (ii) internal movement of sand in the containers and (iii) underlying processes leading to the deformations and displacement of the containers have clarified many of the hydrodynamic processes involved, showing that indeed the deformations of the geotextile sand containers substantially affect their stability.     

Uniaxial compressive behavior of scrapped tire and sand-filled wire netted geocell with a geotextile envelope

Cellular structures are widely used in civil engineering. Their design is based on the understanding of the mechanical behavior of geocells. This paper investigates the response of a single geocell to a uniaxial compression test. The geocells were cubic, either 500 mm or 300 mm on a side. The fill materials were sand and scrapped tire and sand mixtures in different mass ratios. The envelope of the geocell was made up of a hexagonal wire netting cage and a containment geotextile. The response of the geocell is discussed based on the axial load and displacement measurements as well as the change in geocell volume.The axial load was found to be globally governed by the interaction between the fill material and the envelope, which depends on the shape of the wire mesh and the volumetric behavior of the fill material.     

Coastal scour stabilisation using granular filter in geosynthetic nonwoven containers

Severe scouring occurred at the Eider storm surge barrier, progressing towards the structure. Due to depth, steepness and tidal currents, traditional coastal protection systems were out of question, but further scouring had to be stopped. A system of filter, stone fill and armour layer was chosen. To avoid segregation of the broadly graded granular filter material, it was dumped in nonwoven geotextile containers. Selection of the materials, design of the filters and placement turned out to be rather difficult. The combination of geotextile and granular filter solved the problem.     

Shock wave attenuation by geotextile encapsulated sand barrier systems

This paper presents a laboratory scale experimental technique to study the performance of the encapsulated sand barrier systems in mitigating shock waves. The geotextile encapsulated sand barrier systems are made of cubical wire mesh formwork lined with geotextile and form a thick protective barrier when filled with granular materials. In the present study, dry sand particles of size varying from microns to few millimeters (fine and coarse) are used as infill granular material. Spherical shaped glass beads are also used as the infill material to study the influence of shape of the infill particle on the attenuation behavior. The process of shock wave attenuation by the sand barrier, with and without the geotextile facing formwork is examined. The experiments are performed using a conventional shock tube, where shock waves with incident Mach number in the range of 1.29–1.70 are generated. The experimental results show that the presence of geotextile layer has contributed significantly towards shock wave attenuation. The geotextile also plays an important role as a regulator, which is able to deliver gradual pressure rise at the downstream end of the barrier.     

Strength enhancement of geotextile-reinforced carbonate sand

The mechanical behavior of carbonate sand reinforced with horizontal layers of geotextile is invetigated using a series of drained compression triaxial tests on unreinforced and reinforced samples. The main factors affecting the mechanical behavior such as the number of geotextile layers, their arrangement in specimens, confining pressure, particle size distribution, geotextile type and relative density of samples were examined and discussed in this research. To make a precise comparison between the behavior of reinforced siliceous and carbonate sand, triaxial tests were performed on both types of sands. Results indicate that geotextile inclusion increases the peak strength and strain at failure, and significantly reduces the post-peak strength loss of carbonate specimens. The amount of strength enhancement rises as the number of geotextile layers increases while two other parameters including confining pressure and particle size affect adversely. The strength enhancement of reinforced carbonate sand is greater than the corresponding siliceous sample at high axial strains. Reinforced and unreinforced carbonate specimens exhibit more contractive behavior than their corresponding siliceous samples and tend to dilate at higher axial strains. By increasing the relative density of the samples, the peak strength of reinforced specimens rises due to enhanced interlocking between geotextile layers and sand particles. This process continues as long as the geotextile is not ruptured. The utilization of geotextiles with high mass per unit areas was found to be uneconomical due to slight differences between the strength augmentation of geotextiles with high and low mass per unit areas. It should be noted that geotextile layers limit the lateral expansion of specimens which leads to changing the failure pattern from a shear plane to bulging between the adjacent layers of geotextile.     

筒型基础防波堤土压力性状的有限元分析

筒型基础防波堤是一种新型港口与海岸工程结构,依靠沉入地基土层中的筒型基础维持结构稳定,作用在筒型基础上的土压力是结构稳定性验算的核心内容。结合某实际工程,建立了筒型基础防波堤三维弹塑性有限元模型,分析了不同波浪力加载值下作用在筒型基础上土压力的大小及竖向和环向分布规律,并将有限元计算的极限状态下的土压力与Rankine主动、被动土压力和静止土压力进行了比较分析。根据筒型基础的位移模式,将土压力分为主动、被动和静止3个区域,建议了不同区域土压力的简化计算方法,为筒型基础结构稳定性验算提供了基本依据。     

基于极限平衡法的箱筒型基础防波堤稳定性分析

箱筒型基础防波堤是一种适用于软土地基和深水波浪条件的新型港口与海岸工程结构，依靠沉入地基中的筒型基础维持结构稳定。根据箱筒型基础防波堤极限状态下的结构运动模式，考虑箱筒型基础防波堤的三维空间几何特性、受力特点、极限状态转动点的位置等因素，建立了箱筒型基础防波堤稳定性计算的极限平衡法，通过与有限元结果对比，相互验证了正确性，与有限元法相比，极限平衡法计算速度快，效率高，更方便工程应用。结合天津港防波堤延伸工程，对箱筒型基础防波堤的稳定性进行了分析。结果表明，基础筒高8～10 m时，天津港延伸工程中的箱筒型基础防波堤的抗滑、抗倾和竖向承载力均具有较高的安全度。     

软弱滩涂上堤基细砂垫层的适用性研究

 结合浙江省平湖市白沙湾—水口围涂一期工程实例及相应原位监测成果,从堤坝工程对砂垫层的渗透性能需求着手,探讨土工织物加筋和细砂垫层应用于软弱滩涂海堤工程的可行性。数值分析结果表明,基于对固结、变形和稳定的综合考虑,在实际选择砂源时,作为水平排水体的砂垫层渗透系数最好大于5×10－3 cm/s;土工加筋对堤基的沉降和孔压影响不大,对提高堤基的整体稳定性作用显著。当细砂垫层由于渗透系数不满足稳定要求时可采用土工织物加筋。实践结果表明,对于场地附近区域缺乏碎石料而砂源充足的软弱滩涂上的堤防工程,采用细砂作为水平排水垫层兼以土工织物加筋是可行的,所得结论和规律可为类似工程提供理论支持,并指导设计和施工。     

Mechanical Behavior of Anisotropic Sand Ground Beneath Structures Subjected to Cyclic Loading Such as Wave Loading

An experimental study on the mechanical response of anisotropic seabed to wave force and oscillation of man-made structures such as wave dissipating blocks and breakwaters was performed on a model soil box with two-dimensional plane strain condition. Using model ground comprised of Toyoura sand reconstituted by air-pluviation method, a series of cyclic loading tests on model structures under several foundation conditions was conducted at a loading pattern which is capable of simulating the various stress states induced by both oscillation of structure and wave force. Test results showed that the strength and deformation characteristics of model sand deposits are basically identical to those estimated from the conventional cyclic undrained triaxial test and are different depending strongly on the changes in the depositional condition of the ground. Based on the results of model testing, the applicability of countermeasures constructed by both side wall and sheet pile was investigated to prevent ground failure. Considering the model testing results, it was also found that the installation of side walls beneath structures and sheet piles into the ground are advantageous as countermeasures against progressive failure on sand ground subjected to wave loading.     

大型疏浚土充填袋筑堤技术研究

为缓解砂石资源紧缺、保护生态环境、降低工程造价和高效利用航道疏浚土,以大型疏浚土充填袋筑堤技术应用于连云港港徐圩港区围堤建设为工程实例,通过稳定性分析计算、离心模型试验开展了大型疏浚土充填袋筑堤技术的理论可行性研究,并结合原型试验提出了新型配套施工工艺,形成了相关成套技术。研究表明:袋体土工织物加筋作用良好,堤体及地基稳定性能够满足安全设计;控制疏浚土充填料初始含水率、采用泥浆泵与浓浆泵串联接力的施工工艺,能够提升大型疏浚土充填袋筑堤施工效率,满足正常围堤建设强度需要;低含水率疏浚土充填料排水较快、袋体实际有效厚度较为理想、堤体地基较为稳定,大型疏浚土充填袋筑堤具有其技术可行性。     

Cyclic behaviour of dry silty sand reinforced with a geotextile

Recent studies on construction material technology have indicated that soil reinforcement improves resistance of soil against compression and tension. Due to the wide use of geotextile reinforcement in road construction, the potential benefit of geotextile reinforcement in cyclic loading should be investigated. In this study we performed a series of cyclic triaxial tests to examine dry silty sand reinforced with geotextile when subjected to dynamic loading. These tests were conducted on reinforced and unreinforced dry sand and sand mixed with varying amounts of silt (0–50%). The main factors affecting the cyclic behaviour, such as the arrangement and number of geotextile layers, confined pressure and silt content are examined and discussed in this paper. The results indicate that geotextile inclusion and increased confining pressure increase the axial modulus and decreased cyclic ductility of dry sand for all silt contents examined. Also, it was found that by increasing the silt content by up to about 35 percent the axial modulus in reinforced and unreinforced sand is decreased and cyclic ductility increased. With further increases in silt content, these values are increased for cyclic axial modulus and decreased for cyclic ductility.     

State-space solution of seabed–geotextile systems subjected to cyclic wave loadings

Analytical solutions for dynamic responses of seabed–geotextile systems subjected to cyclic wave loadings are presented in this paper, which contains the solutions of the transient and harmonic responses. The theory is based on the Biot consolidation equations in which the pore fluid as well as the soil skeleton is considered compressible and the flow in the porous seabed is assumed governed by Darcy's law. The present analysis is completely based on the state-space formulations, which is very effective for laminated systems analysis. Together with Laplace–Fourier transform techniques, state-space methods are used to solve the governing equations. Responses of seabed–geotextile systems can be calculated by using the matrix theory, boundary conditions and inverting integral transform. As illustrative examples, laboratory experiments, which conducted at the Oregon State University Wave Research Facility in USA by McDougal [1981. Ocean wave–soil–geotextile interaction. Ph.D. Dissertation, Oregon State University], are analysed. It is shown that the numerical results are in good agreement with those obtained from laboratory experiments, and the distinction between the transient and harmonic response should be taken into account for design of marine geosynthetic systems. Under the transient condition, the seabed is apt to liquefy. Seabed stability may be increased by placing geotextile beneath an armour layer. The numerical evaluations of the solution in the seabed–geotextile systems can be easily achieved with high efficiency and accuracy.     

Modeling of cable-moored floating breakwaters connected with hinges

In the present paper, the overall performance of a cable-moored array of floating breakwaters connected by hinges is investigated under the action of monochromatic linear waves in the frequency domain. The performance is defined here as: (i) demonstration of acceptable levels of both response of the array and its effectiveness and (ii) non-failure of the mooring lines. The numerical analysis of the array is based on a 3D hydrodynamic formulation of the floating body coupled with the static and dynamic analyses of the mooring lines. The motions of the array of floating breakwaters associated with the hinge vertical translations are considered in the hydrodynamic analysis with the implementation of appropriate generalized modes. The stiffness and damping coefficients caused by the mooring lines in both rigid and generalized degrees of freedom are derived here in the general form. A rigorous parametric study is carried out in order to investigate the effect of different configurations (number of hinge joints and number of mooring lines) on the performance of the cable-moored array of floating breakwaters. Moreover, the performance of the various configurations of cable-moored floating breakwaters connected by hinges examined is compared with the performance of a single cable-moored floating breakwater with no hinges. It is found that the number of hinge joints and mooring lines have a direct effect on the performance of the cable-moored array of floating breakwaters.     

土工模袋砂界面摩擦特性试验研究

土工模袋砂界面特性指标对于各类模袋砂工程的设计至关重要。笔者开展了砂与土工模袋、土工模袋之间界面摩擦特性直剪试验;通过自制试验装置进行了模袋砂间界面摩擦试验,探讨了充填度对模袋砂间界面特性的影响;把试验结果用于典型工程实例变形和稳定性分析,并通过数值计算验证。研究结果表明:模袋与砂界面剪应力-位移关系表现出明显的非线性特征,剪应力峰值及其对应的位移随竖向压力的增大而增大,界面关系服从摩尔-库伦强度理论;砂与模袋间的摩擦角为30.3°,摩擦系数为0.58,黏聚力是3.1 kPa。土工模袋间界面关系也服从摩尔-库伦强度理论,摩擦角为21.3°,摩擦系数是0.39,黏聚力为1.29 kPa。模袋砂之间的摩擦特性指标则因充填度而异,受充填度影响显著,表现为充填度增大,摩擦角变大而粘聚力减小,抗剪强度增大,且竖向压力越大影响越明显。基于试验结果明确了洲头咀沉管隧道模袋砂围堰位移突变的原因及其稳定性,提出了设计控制指标;通过数值计算表明,基于界面摩擦试验结果分析所得结论合理。     

有纺土工织物覆土条件下的渗透特性试验研究

分别配制不同孔隙比的粉砂、标准砂和黏土试样,采用自主研制的一套多功能渗透试验装置,开展了一系列纯土和有纺土工织物覆土条件下的渗透试验,对比分析了这两种条件下渗透系数的差异,并探讨了有纺土工织物与土共同作用下的渗透机理。结果表明,有纺土工织物对于土体渗流略有一定的抑制作用,表现为覆土条件下的渗透系数略小于纯土的渗透系数,但是对于粉砂,当其孔隙比比较大、细砂颗粒的含量相对较多时,细砂颗粒则有可能在渗流作用下通过有纺土工织物孔隙而产生流失,使得覆粉砂条件下的渗透系数略大于纯粉砂的渗透系数。     

Slope protection by membrane structures

Left uncontrolled the effects of weathering can lead to severe degradation of slopes. To assess these effects a field trial has been carried out involving both protected and unprotected slopes. In this trial two types of slope protection were investigated. The first was a simple synthetic mat installed directly over the exposed surface of the slope. The second was a more substantial geotextile reinforced fill facing, incorporating fillow bushes which generate root systems to aid long-term stability of the protective facing. Data are presented to show that appropriate protection systems do prevent significant deterioration in the slope. Also the combination of a geotextile and deep rooting vegetation produce robust structures which are able to withstand high loadings with acceptable deformations. The use of a geotextile reinforced protection system is illustrated through two examples. These show that protection of the face of fill slopes can be achieved either by encapsulating the fill in the geotextile reinforcing layers or by the use of a three-dimensional wire netting applied directly to the face.     

Australian and German experiences on the use of geotextile containers

Geotextile containers and tubes are used for flood emergency protection in dams and dikes, and also as construction elements for erosion control, bottom scour protection and scour fill artificial reefs, groynes, seawalls, breakwaters and dune reinforcement. New shore protection structures, especially at sandy coasts, are increasingly needed. However, due to the increasing storminess associated with climate changes some of the existing dunes must be protected/reinforced. Hydraulic model investigations were carried out at Leichtweiss-Institute for Hydromechanics and Coastal Engineering (LWI) of the Technical University of Braunschweig, to establish reliable stability formulas for sand containers applied as dune protection/reinforcement subject to storm waves.