Đồ án Application of mechanically stabilized earth wall to construct sai gon river wall

nce (figure 3.3) depending on Geogrid geometry: Friction develops at locations where there is a relative shear displacement and corresponding shear stress between soil and reinforcement surface. Reinforcing elements where friction is important should be aligned with the direction of soil reinforcement relative movement. Figure 3.2 Frictional stress transfer between soil and reinforcement surfaces Passive resistance occurs through the development of bearing type stresses on "transverse" reinforcement surfaces normal to the direction of soil reinforcement relative movement. Passive resistance is generally considered to be the primary interaction for rigid Geogrid. The transverse ridges on "ribbed" strip reinforcement also provide some passive resistance. Figure 3.3 Soil passive (bearing) resistance on reinforcement surfaces 3.1.2 Bi-Axial Geogrid Properties The properties of Bi-Axial Geogrid made from Polypropylene (PP) with their square apertures, high tensile strength and optimized geometry of nodes and ribs make them equal to any other similar material.  The reinforcing action of these Geogrids lies mainly in confining soil and increasing its shearing resistance by a process of interlocking between the square ribs and the soil. The load dispersal effect from the interlocking mechanism is highly effective and can reduce sub-base thickness and construction cost.  Bi-Axial Geogrid can be used with any kind of mechanical fill material.  Two aperture size ranges are available for optimum matching with project fill. Figure 3.4 Technical shape of Bi-Axial Geogrid The performance: Figure 3.5 The interlocking mechanism The interlocking mechanism means Bi-Axial Georid keeps granular materials inside the mesh. They will create a strong structure, eliminate the phenomenon of the shift of the aggregate particles should be able to prevent the phenomenon of differential settlement and improved foundation bearing capacity. Load distribution on wide area, reduced the thickness of the layer of pavement structure and construction costs. Nghe Đọc ngữ âm Từ điển - Xem từ điển chi tiết Figure 3.6 The model of interlocking mechanism (a) (b) Figure 3.7 The use of Bi-Axial Geogrid generate interlock Bi-Axial Geogrid can be used with any kind of engineering fill: Under continuous wheel loading unrestrained stone fill materials can migrate downwards through soft soil sub-bases. The use of Bi-Axial Geogrid generate interlock that confines the stone fill materials to give a stiff structure and eliminate downward migration. Figure 3.8 The improved load distribution The effect on load spread was evaluated and data indicated a mean angle increasing from 380 in the unreinforced case to more 500 with grid. This simple approach indicates that granular layer thickness maybe reduced by around 50% to give a similar stress on the subgrade. Comparison with Geotextiles Both woven and non-woven Geotextiles can improve pavement performance by providing a separation function. They can prevent contamination of the granular fill by intermixing with the subgrade soil. The only mechanism which allows Geotextiles to offer a structural contribution to a road pavement or trafficked area is as a tensioned membrane under the wheel paths. For this mechanism to work effectively, the Geotextiles must be anchored outside the wheel path and then deform sufficiently so that it can carry tension. Figure 3.9 The use of Geotextiles reinforced the pavement Using Geotextiles improve pavement performance. They can occur following phenomena: The meshes of the Geotextiles are so small that granular materials can slide on them. Uncontrolled lateral displacement of these granular materials cause the differential settlement phenomenon. The ruts can act as invisible sumps, providing a water source to soften the subgrade. Figure 3.10 Confinement versus membrane effect Based on these points, the only types of application likely to benefit from the tensioned membrane approach will be roads where fixed wheel paths are followed, and large rut depths are acceptable. As shown on figure 3.10, the interlock mechanism of Bi-Axial Geogrids is distinctly different to the tensioned membrane. By interlocking with the particles, Bi-Axial Geogrids confine the aggregate layer and prevent lateral displacement. Load is distributed from the wheel to the subgrade within the loaded area. The two materials are not directly interchangeable without design review and amendment. 3.1.3 Tri-Axial Geogrid Properties With its unique triangular structure, Tri-Axial Geogrid is invented from the original biaxial form of Geogrid. Its multi-directional properties leverage the triangular geometry, one of construction’s most stable shapes, to provide a new level of in-plane stiffness. The transition from a rectangular to a triangular grid aperture, coupled with an increase in rib thickness and junction efficiency, offers the construction industry a better alternative to conventional materials and practices. Figure 3.11 Technical shape of Tri-Axial Georid Tri-Axial Geogrid deliver performance in three dimensions Multi-directional load distribution Tri-Axial Geogrid has three principal directions of stiffness, they are further enhanced by their rigid triangular geometry. The triangular geometry provides a significantly different structure than other available Geogrid, delivering high radial stiffness throughout the full 360 degrees. Three-dimensional load distribution acts in a radial manner at all levels within the aggregate. This helps to ensure optimum performance of Geogrid reinforcement in a mechanically stabilized layer. Figure 3.12 Load distribution acts radially Triangular aperture geometry Aggregate particles interlock within the Geogrid and confined within the apertures. These interactions create a stiffened composite layer with improved performance characteristics. Figure 3.13 The unique structure of Tri-Axial Geogrid provides a high degree of in-plane. stiffness, improving performance Figure 3.14 Compared with a Bi-Axial Geogrid, Tri-Axial Geogrid has a greater rib depth contributing to improved confinement Junction integrity and efficiency Tri-Axial Geogrids’ aperture geometry forms a hexagonal junction shape with better junction strength and stiffness to mitigate radial stress imparted from a trafficked surface. Figure 3.15 Tri-Axial Geogrid with a hexagonal junction shape Tri-Axial Geogrid is manufactured from an extruded sheet of polypropylene. During the manufacturing process, each sheet is punched with an array of holes and then carefully stretched to create triangular apertures with greater confinement characteristics. This process yields a Geogrid with very high junction efficiency (ratio of junction strength to ultimate tensile strength) to offer optimal rib-to-rib stress transfer. This index characterizes the need to effectively and uniformly distribute loads for both paved and unpaved applications. The similarities and differences of Bi-Axial Geogrid and Tri-Axial Geogrid Similarities Materials: They made from Polypropylene (PP) Performance: The interlocking mechanism. The reduction of differential settlement and improved foundation bearing capacity. Applications: Roads, railways, ports, airport runways, paved and un-paved areas. Table 3.1The differences are between Bi-Axial Geogrid and Tri-Axial Geogrid Bi-Axial Geogrid Tri-Axial Geogrid Geometry The square aperture geometry. The triangular aperture geometry. Directions Bi-Axial Geogrid offers tensile stiffness primarily in two directions. Tri-Axial Geogrid has three principal directions of stiffness. The tensile properties The conventional Bi-Axial Geogrid in TD (Transverse direction) and MD (Machine direction), with the highest strength typically in TD. The radial stiffness describes tensile properties measured in the TD and 450 and 1350 off TD Performance - The evenly distributed load. - Reduction of soil layer thickness. - Greater ability to distribute wheel loads. - Improved aggregate reduction factor. - Improved applications performance. Table 3.2 Advantages and disadvantages of Geogrids Advantages Disadvantages Uni-Axial Geogrid - High quality durable polymers. - Consistent manufacturing process. - Consistent data and information. - Reliable design parameters. - If the soft soil thickness is large and degree of consolidation is slow, they still need additional support methods such as sand drain, wick drain… Bi-Axial Geogrid - High quality durable polymers. - The interlocking mechanism between Geogrid and aggregate. - High angle of load spread through reinforced granular layers. - Improved pavement performance. Tri-Axial Geogrid - Increased in-plane stiffness. - More efficient material usage. - Near isotropic (3600) properties. - Increased aggregate confinement. 3.2 APPLICATIONS OF GEOGRIDS 3.2.1 Uni-Axial Geogrid Applications in Mechanically Stabilized Earth Walls (MSEW) Main characteristics are high strength and low creep. Used primarily for reinforcement of walls, abutments and slopes under long term high loading. Figure 3.16The four main components of a mechanically stabilized earth wall Vertical walls were necessary for B-Avenue Intersection flyover because of space restrictions, with clear heights up to 7.5 m, using 180mm thick pre-cast T shaped panels as facing elements with feature finishes. Figure 3.17 Reinforced Soil structure for B-Avenue Intersection flyover, New Delhi in India. Geogrids are spread horizontally, they attached to the brick block surface against shear force caused the potential sliding mass and can build the retaining wall of 45 meters high with slopes up to 900. It can be seen as potential applications of this measure are very big in Vietnam. Landslide slope is an alarming phenomenon in Ho Chi Minh trail. It not only caused material damage, human beings, but also disrupting the traffic impact on the economy of the Central Region and Central Highlands. Besides, Ho Chi Minh trail, there are many other mountainous areas in Viet Nam, especially in the North. If we want the economy of those areas of development, we must open the arterial road traffic. Retaining wall is one of the mandatory requirements. With the application of the Geogrids for slopes, the mechanically stabilized earth walls will help ensure the safety of works and cost savings. Some additional successful uses of MSE walls include: Temporary structures, which have been especially cost-effective for temporary detours necessary for highway reconstruction projects. Reinforced soil dikes, which have been used for containment structures for water and waste impoundments around oil and liquid natural gas storage tanks. (The use of reinforced soil containment dikes is economical and can also result in savings of land because a vertical face can be used, which reduces construction time). Dams and seawalls, including increasing the height of existing dams. 3.2.2 Uni-Axial Geogrid Applications in Reinforced Soil Slopes Reinforced Steep Slopes (RSS) Figure 3.18 Typical structure for reinforced steep slopes Construction of steep slopes without hard facings such as concrete panels or blocks can be easily achieved using Uni-Axial Geogrid in conjunction with soil filled bags. A simple wrap around technique is used to enclose the bags. And reinforce the soil structure in the embankment. Bi-Axial Geogrid may be used as secondary reinforcement to assist with compaction and face stability if close spacing of primary reinforcement is not required. Figure 3.19 Embankment for access road to Jinping station in South China Figure 3.20 Reinforced steep slope on a railway project in southern China Steep Slopes with Surface Vegetation Figure 3.21 Green Slope There are many examples of reinforced steep slopes where the Uni-Axial Geogrid is wrapped around each rising layer. Natural fibre bags filled with soil and seed mixtures can be incorporated into the construction that will result in rapid development of vegetation to give protection to the grids from UV Light and to further reinforce the face of the embankment with root systems. Combined M.S.E.W and R.S.S The need for a durable structure sometimes calls for a solution that combines the benefits of mechanically stabilized block faced walls with an environmentally friendly reinforced steep slope with a vegetation covering. (a) (b) Figure 3.22 Retaining wall reinforced by Un-Axial Geogrid with modular block surface with vegetation covering Sand-bags filled with soil and seed make the wrap around structure easy to construct. A few weeks later, after some rainfall and warm sunshine, the resulting a very attractive structure that blends with its surroundings. 3.2.3 Bi-Axial Geogrid Applications in Roads, Railways, Ports, Airports Roads, Railways, Ports One layer or multi-layer Biaxial Geogrid construction distributes loading and disperses stress more effectively, leading to the reduction in differential settlement and improved foundation bearing capacity. Figure 3.23 Installation of Geotextile under Bi Axial E’GRID on a road development near the town of Hearst in Ontarion-Canada Figure 3.24 Ground Stabilisation for Railway track in Port of Tianjin-China Airports A tough platform can be established with Bi-Axial Geogrids reinforcement. Resistance to the impact of planes when taking off and landing is improved with more effective and immediate load dispersal. Figure 3.25 Airports runway in Riga-Latvia Temporary roads and other un-paved areas Bi-Axial Geogrid reinforcement gives ease of construction of temporary pavements with reduced construction time and cost. 3.2.4 Bi-Axial Geogrid Applications in reinforced sea embankments-river embankments Gabions and Mattresses are made from Bi-axially orientated polypropylene grid with high strength and toughness, resistance to corrosion in sea water or freshwater, not affected by acids or alkalis and protected from the sun’s rays by the optimum addition of carbon black. Figure 3.26 Mattress making on site Figure 3.27 Mattresses being laid on a river bank to prevent scour and erosion at times of high fast flowing water levels Gabions combined with Bi-Axial Geogrid solution is widely applied in many countries around the world due to environmental sustainability, installation easy and reduce construction costs for investors. Nghe Đọc ngữ âm Từ điển - Xem từ điển chi tiết Figure 3.28 Gabion for bank protection, Liangshui River, Beijing, China 3.3.5 Tri-Axial Geogrid Applications Paved applications Paved systems often fail prematurely because of progressive lateral displacement and weakening of the granular base course. Tri-Axial Geogrids improve the overall stiffness of roadways, parking lots, taxiways, runways, apron and other structures that support vehicular traffic, leading to enhanced performance. Figure 3.29 Pilot project - A66 Melsonby Quarry Unpaved applications Weak subgrades are a common problem during the construction of haul roads, parking lots, working surfaces, staging areas, storage yards and other unpaved structures. Tri-Axial Geogrid provide a simple solution for stiffening the granular platform and reducing subgrade stress. Enhanced constructability greatly improves site access while significantly reducing up-front costs and future maintenance. Figure 3.30 Lorry park, Cumbernauld 3.3 MECHANICALLY STABILIZED EARTH WALL 3.3.1 Historical Development Retaining structures are essential elements of every highway design. Retaining structures are used not only for bridge abutments and wing walls but also for slope stabilization and to minimize right-of-way for embankments. For many years, retaining structures were almost exclusively made of reinforced concrete and were designed as gravity or cantilever walls which are essentially rigid structures and cannot accommodate significant differential settlements unless founded on deep foundations. With increasing height of soil to be retained and poor subsoil conditions, the cost of reinforced concrete retaining walls increases rapidly. Mechanically Stabilized Earth Walls (MSEW) is cost-effective soil-retaining structures that can tolerate much larger settlements than reinforced concrete walls. By placing tensile reinforcing elements (inclusions) in the soil, the strength of the soil can be improved significantly such that the vertical face of the soil/reinforcement system is essentially self supporting. Use of a facing system to prevent soil raveling between the reinforcing elements allows very steep slopes and vertical walls to be constructed safely. In some cases, the inclusions can also withstand bending from shear stresses, providing additional stability to the system. Inclusions have been used since prehistoric times to improve soil. The use of straw to improve the quality of adobe bricks dates back to earliest human history. Many primitive people used sticks and branches to reinforce mud dwellings. During the 17th and 18th centuries, French settlers along the Bay of Fundy in Canada used sticks to reinforce mud dikes. Some other early examples of man-made soil reinforcement include dikes of earth and tree branches, which have been used in China for at least 1,000 years and along the Mississippi River in the 1880s. Other examples include wooden pegs used for erosion and landslide control in England, and bamboo or wire mesh, used universally for revetment erosion control. Soil reinforcing can also be achieved by using plant roots. The modern methods of soil reinforcement for retaining wall construction were pioneered by the French architect and engineer Henri Vidal in the early 1960s. His research led to the invention and development of Reinforced Earth, a system in which steel strip reinforcement is used. The first wall to use this technology in the United States was built in 1972 on California State Highway 39, northeast of Los Angeles. In the last 25 years, more than 23,000 Reinforced Earth structures representing over 70 million m2 (750 million ft2) of wall facing have been completed in 37 countries. More than 8,000 walls have been built in the United States since 1972. The highest wall constructed in the United States was on the order of 30 meters (98 feet). Currently, most process patents covering soil-reinforced system construction or components have expired, leading to a proliferation of available systems or components that can be separately purchased and assembled by the erecting contractor. The remaining patents in force generally cover only the method of connection between the reinforcement and the facing. Geogrids for soil reinforcement were developed around 1980. The first use of Geogrid in earth reinforcement was in 1981. Extensive use of Geogrid products in the United States started in about 1983, and they now comprise a growing portion of the market. 3.3.2 Current Usage of Mechanically Stabilized Earth Walls It is believed that MSE walls have been constructed in every State in the United States. Major users include transportation agencies in Georgia, Florida, Texas, Pennsylvania, New York, and California, which rank among the largest road building States. It is estimated that more than 700,000 m2 (7,500,000 ft2) of MSE retaining walls with precast facing are constructed on average every year in the United States, which may represent more than half of all retaining wall usage for transportation applications. The majority of the MSE walls for permanent applications either constructed to date or presently planned use a segmental precast concrete facing and galvanized steel reinforcements. The use of Geotextile faced MSE walls in permanent construction has been limited to date. They are quite useful for temporary construction, where more extensive use has been made. Recently, modular block dry cast facing units have gained acceptance due to their lower cost and nationwide availability. These small concrete units are generally mated with grid reinforcement, and the wall system is referred to as modular block wall (MBW). It has been reported that more than 200,000 m2 (2,000,000 ft2) of MBW walls have been constructed yearly in the United States when considering all types of transportation related applications. The current yearly usage for transportation-related applications is estimated at about 50 projects per year. In Vietnam, the constructions are reinforced by the traditional materials have been made at Lach Tray, Hai Phong Nga Tu Vong, Hanoi, Hue. Nowadays, the Mechanically Stabilized Earth Walls are developed in our country. So, they may be mentioned the retaining wall works: path to the bridge of Nuoc Man canal, Tan Duc bridge, Hung Vuong bridge in Long An, or retaining wall projects in Da Nang, Binh Duong, Ho Chi Minh City. We can say, with many of their outstanding characteristics, Mechanically Stabilized Earth Walls are innovative solutions for many different requirements in Viet Nam. In this thesis, I would introduce two solutions for retaining wall design works to stabilize Sai Gon River bank –Thanh My Loi residential area project, which will be clearly the advantages of this new material. 3.4 THEORETICAL BASIS FOR MECHANICALLY STABILIZED EARTH WALLS 3.4.1 Analysis theories Figure 3.31 The structure of mechanically stabilized earth wall It includes the four major components: (1) Facing: There are so many choices for the aesthetic and architectural requirements of investors. - Modular block was precast with many different models. - Precast panels of various sizes and different shapes. (2) Drainage: Using of crushed-stone aggregate and drainage-pipes. (3) Reinforcement: Uni-Axial Geogrid used for reinforcing. (4) Soil: Any filling soil at local. Geogrids are responsible for anchoring soil mass easily slip into the land mass itself is stable, wall surface can only beautify and keep the soil from erosion. Principles of calculation: MSEW based on two main factors to determine the type of Geogrids, embedded length and reinforcement spacing. Sizing for external stability. Sizing for internal stability. Currently, there are many standards in the world of design with MSEW. The contents of this thesis used AASHTO 2002/NHI 043-ASD standard of the United States to implement my design. 3.4.2 Determination of Basic Parameters The active coefficient of earth pressure Ka (Coulomb): It is calculated for vertical walls (defined as walls with a face batter of less than 8 degrees) and a horizontal backslope from: Ka= tan2(45-φ2) (3.1) For an inclined front face equal or greater than 8 degrees, the coefficient of earth pressure can be calculated from the general Coulomb case as: Ka=sin2θ+φsin2θsinθ-δ1+sinφ+δsinφ-βsinθ-δsinθ+β2 (3.2) Where φ : Friction angle of the soil. θ : The face inclination from a horizontal. δ : Wall friction angle. β : Surcharge slope angle. Figure 3.32 The active earth pressures (Coulomb analysis) Wall parameters: Vertical force due to earth pressure: V1=γrHL (3.3) Vertical force due to traffic surcharge: V2=qL (3.4) Resultant of vertical forces: R = V1 + V2 (3.5) The lateral force due to earth pressure: F1=12γbH2Ka (3.6) The lateral force due to traffic surcharge: F2=qHKa (3.7) 3.4.3 External Stability Analysis External stability evaluations for MSEW structures treat the reinforced section as a composite homogeneous soil mass and evaluate the stability according to conventional failure modes for gravity type wall systems. Differences in the present practice exist for internal stability evaluations which determines the reinforcement required, principally in the development of the internal lateral stress and the assumption as to the location of the most critical failure surface. As with classical gravity retaining structures, four potential external failure mechanisms are usually considered in sizing MSE walls. They include: Sliding on the base. Limiting the location of the resultant of all forces (overturning). Bearing capacity. Deep seated stability (rotational slip-surface or slip along a plane of weakness). Define wall geometry and soil properties Select performance criteria Preliminary sizing Evaluate static external stability Settlement/ lateral deform Overall slope stability Bearing capacity Overturning Sliding Establish reinforcement length Figure 3.33 External stability computational sequences are schematically illustrated above Direct Sliding: Figure 3.34 Sliding Check the preliminary sizing with respect to sliding at the base layer, which is the most critical depth as follows: FS sliding = Horizontal resisting forcesHorizontal driving forces= PRPd=V1tanφ(F1+F2) ≥1.5 (3.8) Where: FS sliding: Factor of safety for Sliding. Overturning (eccentricity): Figure 3.35 Overturning Overturning factor of safety: FOSov=MROMO ≥1.5 (3.9) Resisting moment: MRO=V1L2 (3.10)