TIRUCHIRAPALLI

Transcription

1 MAHALAKHMI ENGINEERINGCOLLEGE TIRUCHIRAPALLI QUESTION WITH ANSWERS DEPARTMENT: CIVIL IV SUB.CODE/ NAME: CE 2033 / Ground Improvement Techniques SEMESTER:VII UNIT IV EARTH REINFORCEMENT PART - A (2 marks) Unit IV - EARTH REINFORCEMENT Concept of reinforcement - Types of reinforcement material - Applications of reinforced earth use of Geotextiles for filtration, drainage and separation in road and other works. Part A 1. Define geosynthetics. (AUC NOV /DEC 2012) Geosynthetics are artificial fibres used in conjunction with soil or rock as an integral part of a man made project They are mainly grouped into two categories a. Geotextiles permeable b. Geomembrane impermeable 2. What are the various types of geosynthetics? Geonets Geomats Geosynthetic clay liners Geofoam Geocells Geocomposites Geotextiles Geogrids Geomembranes 3. How does the use of a geosynthetic as a filter differ from that of Drainage? (AUC NOV /DEC 2010) o Filtration applications are highway underdrain systems, retaining wall drainage, landfill leachate collection systems, as silt fences and curtains, and as flexible forms for bags, tubes and container o Drainage applications for these different geosynthetics are retaining walls, sport fields, dams, canals, reservoirs, and capillary breaks

2 4. Write a brief note on geosynthetics as reinforcement. Containment involves geomembranes, geosynthetic clay liners, or some geocomposites which function as liquid or gas barriers Landfill liners and covers make critical use of these geosynthetics All hydraulic applications (tunnels, dams, canals, reservoir liners, and floating covers) use these geosynthetics as well 5. Define soil nailing. Soil nails are more or less rigid bars driven into soil or pushed into boreholes which are filled with grout.together with the insitu soil, they are intended to form a coherent structural entity supporting on excavation or arresting the movement of on unstable slope. 6. Define Geotextiles. Continuous sheets of woven, nonwoven, knitted or stitch bonded fibres or yarns Flexible and permeable Appearance of a fabric Usually PET or PP 7. Define Geogrids. Open grid like appearance Principally for reinforcement of soil PP, PET, or PE 8. Define Geonets Open grid like material formed by two (or three) sets of coarse, parallel, extruded polymeric strands intersecting at an constant acute angle HDPE 9.Define Geopipes Perforated or solid wall pipes May be wrapped with a geotextile HDPE 10.Define Geomembranes Continuous flexible sheets from one or more synthetic materials Relatively impermeable 1.5mm to 3mm thick, HDPE, LLDPE 11.What is Geosynthetic Clay Liners Bentonite clay layer between two or more geotextiles, in some cases with a geomembrane Relatively impermeable 12.What is mean by Geocells Three dimensional honeycomb like structure formed by strips of joined polymeric sheets, usually HDPE or LDPE

3 13. Define Geofoam Lightweight blocks or slabs formed by expanded polystyrene (EPS) Used for thermal insulation, as lightweight fill or compressible vertical layer against rigid walls 14. Define Geocomposites Combination of two of more geosynthetic types Geotextile geogrid Geopipe - geotextile Geotextile geonet Geonet geomembrane Non-woven geotextile woven geotextile 15. Define ground stabilization Increase bearing capacity over weak subgrades Reduce fill thickness More favourable stress distribution Reduce lateral fill movement Increase lifetime Pavements and railways Working platforms / Crane pads Strength in multiple directions, most biaxial 16. What is mean by reinforced soil? (AUC MAY/JUNE 2013) Theoretically any soil can be used as a fill material Conventionally well graded cohesionless soils are used as fill material but are costly Cohesive soils are cheap and easily available but have long term durability problems A convenient compromise is a fill material that has both cohesiveand Frictional properties. 17. list of functions geotextiles of filters (AUC NOV /DEC 2012) Geosynthetics can allow water to pass across the plane while prevent or retain the soil particles Act similar to sand filter Allow water to move through soil while retaining upstream soil particles Prevent migration through drainage aggregate and pipes Part B 1. With neat sketches explain in detail the various applications of reinforced earth for ground improvement. Theory of Reinforcement Reinforced earth has been in use by man since ancient times with the fundamentals of the techniques being mentioned in the Bible The earliest remaining examples of soil reinforcement are the Agar-Quf Ziggurat and the Great wall of China The Romans, Gauls, Dutch and British have been documented using reinforced

4 soil for various applications The modern concept of earth reinforcement was proposed by Casagrande He idealized the problems in the form of weak soil reinforced by high strength membranes laid horizontally in layers The modern form of earth reinforcement was introduced by Henry Vidal in the 1960s Vidals concept was for a composite material formed from flat reinforcing strips laid horizontally in a frictional soil The interaction between the soil and the reinforcing members was solely by friction generated by gravity This he described as Reinforced Earth, a term now generally being used to refer to all reinforced works Principle of Reinforced Earth It is analogous to reinforced concrete but direct comparison is not completely valid The mode of action of reinforcement in soil is to carry tensile loads or anisotropic reduction of normal strain rate Introduction of reinforcement into soil results in interaction between the two The interaction between the soil and reinforcement can be in the form of either adhesion or friction Failure can occur only if the adhesion or frictiona force is overcome or the reinforcement itself ruptures The reinforcement disrupts the uniform pattern of strain that would have developed if it did not exist Components of Reinforced Earth A reinforced earth structure consists of Soil fill or matrix Reinforcement or anchor system Facing (if necessary) Soil Fill Theoretically any soil can be used as a fill material Conventionally well graded cohesionless soils are used as fill material but are costly Cohesive soils are cheap and easily available but have long term durability problems A convenient compromise is a fill material that has both cohesiveand Frictional properties. o Sometimes waste materials as fill materials for reinforced soil structures is an

5 attractive option from the point of view of environment as well as economy. Reinforcement They can be of a variety of materials and in various shapes The principal requirements of reinforcing materials are Strength Stability Durability Ease of handling High coefficient of friction Adherence with the soil Low cost Ready availability Facing For vertical structures a facing is required The function of facing is to stop erosion of the fill and to provide a suitable architectural treatment to the structure Various materials can be adopted to form the facing and will have its own merits and demerits depending upon scale of structure, shape and material. Some materials used for facing are aluminum, brick or masonry, precast concrete slabs, pressed concrete slabs, geotextiles, plastics, GRC, GRP, Steel, timber etc. 2. Explain with the help of a flow chart the various classifications of geosynthetics in detail. (AUC NOV /DEC 2010)

6 Woven Geotextiles Load carrying filaments, fibres and yarns in woven geotextiles are aligned in specific directions This is usually along the longitudinal direction or warp direction and transverse direction or weft direction They provide separation as well as filtration functions Non Woven Geotextiles The filaments are entangles and bonded together There is no specific direction to the fibres and hence they elongate longer when compared to woven geotextiles They are mostly used as separators Knitted Geotextiles Knitted geotextiles are similar to non-wovens but bundles of load carrying Filaments can be made to be aligned in a given direction and this type of product is called as directionally structured fabric or DSF Geotextiles: They provide an alternative to woven geotextiles One of the two largest groups in geosynthetics They are textiles in the traditional sense, but they consist of synthetic fibers rather than natural ones such as cotton, wool, or silk These synthetic fibers are made into flexible, porous fabrics by standard Weaving machinery or are matted together in a random nonwoven manner or knitted The major point is that geotextiles are porous to liquid flow across their manufactured plane and also within their thickness, but to a widely varying degree The fabric always performs at least one of four discrete functions:

7 separation, reinforcement, filtration, and/or drainage. Geogrids They represent a rapidly growing segment in geosynthetics Geogrids are polymers formed into a very open, gridlike configuration, i.e., they have large apertures between individual ribs in the transverse and longitudinal directions They are made by either Stretching in one or two directions On weaving or knitting machinery By bonding straps or rods There are many specific application areas, however, they function almost exclusively as reinforcement materials Geomembranes (AUC NOV /DEC 2010) They represent the other largest group in geosynthetics They are relatively thin, impervious sheets of polymeric material used primarily for linings and covers of liquids or solid-storage facilities This includes all types of landfills, reservoirs, canals, and other containment facilities Thus the primary function is always containment as a liquid or vapor barrier or both Geonets Geonets, also called geospacers, constitute another specialized segment within the geosynthetics area They are formed by a continuous extrusion of parallel sets of polymeric ribs at acute angles to one another When the ribs are opened, relatively large apertures are formed into a netlike configuration Two types are most common, either biplanar or triplanar Their design function is completely within the drainage area where they are used to convey liquids of all types Geomats A three-dimensional water permeable mat made from extruded and bi-oriented polyethylene grids The underside of the mat is made flat to provide even contact with the prepared soil surface The upper surface is made cuspated to provide excellent soil retention

8 Geosynthetic Clay Liners Geomats are applied to create stable vegetation along river, pond banks and slopes to prevent erosion processes of surfaces Geomats are used in combination with geotextiles to reinforce foundations and increase bearing resistance They are rolls of factory fabricated thin layers of bentonite clay sandwiched between two geotextiles or bonded to a geomembrane Structural integrity of the subsequent composite is obtained by needle-punching, stitching or physical bonding GCLs are used as a composite component beneath a geomembrane or by themselves in geoenvironmental and containment applications. Geofoams Geofoam is a product created by a polymeric expansion process resulting in a foam consisting of many closed, but gas-filled, cells The resulting product is generally in the form of large, but extremely light, blocks which are stacked side-by-side providing lightweight fill in numerous applications The primary function is dictated by the application; however separation is always a consideration Geocells Geocells ( also known as Cellular Confinement Systems) are three- Dimensional honeycombed Cellular structures that form a confinement system when in-filled with compacted soil The cellular confinement reduces the lateral movement of soil particles, thereby maintaining Compaction and forms a stiffened mattress that distributes loads over a wider area Geocomposites Traditionally used in slope protection and earth retention applications, geocells made from Advanced polymers are being increasingly adopted for long-term road and rail load support Much larger geocells are also made from stiff geotextiles sewn into similar, but larger, unit cells that are used for protection bunkers and walls A geocomposite consists of a combination of geotextiles, geogrids, geonets and/or geomembranes in a factory fabricated unit Also, any one of these four materials can be combined with another synthetic material

9 A geonet with geotextiles on both surfaces and a GCL consisting of a geotextile/bentonite/geotextile sandwich are both geocomposites The major functions encompass the entire range of functions listed for geosynthetics Discussed previously: separation, reinforcement, filtration, drainage and containment Geomats: Applications of Geosynthetics The main functions of geosynthetics are Separation Filtration Reinforcement Drainage Containment / Barrier 3. with the help of neat sketches, explain in detail the application of geosynthetics as separation. (AUC NOV /DEC 2010) Geosynthetic can separate two layers of soil and thereby prevent intermixing. Separate two layers of soils with different particle size distributions Prevent road base materials from penetrating soft underlying soils Prevent pumping of fines from subgrade Encourage lateral drainage Usually nonwoven geotextiles Separation The geosynthetic acts to separate two layers of soil that have different particle size distributions. For example, geotextiles are used to prevent road base materials from penetrating into soft underlying soft subgrade soils, thus maintaining design thickness and roadway integrity. Separators also help to prevent fine-grained subgrade soils from being pumped into permeable granular road bases. Separation is the placement of a flexible geosynthetic material, like a porous geotextile, between dissimilar materials so that the integrity and functioning of both materials can remain intact or even be improved Paved roads, unpaved roads, and railroad bases are common applications Also, the use of thick nonwoven geotextiles for cushioning and protection of geomembranes is a separation technique Nonwoven geo-textiles prevent aggregate and ballast from punching into the subgrade and intermixing, reducing maintenance costs and ensuring long-term durability and drainability.

10 Geosynthetic placed between ballasts and sub-grade soil in a rail road Geosynthetic placed between aggregate and foundation soil in a paved road

11 4. Geosynthetics can be used as soil reinforcement Justify in detail with supporting sketches.. REINFORCED EMBANKMENTS ON SOFT FOUNDATIONS Concept The design and construction of embankments on soft foundation soils is a very challenging geotechnical problem. As noted by Leroueil and Rowe (2001), successful projects require a thorough subsurface investigation, properties determination, and settlement and stability analyses. If the settlements are too large or instability is likely, then some type of foundation soil improvement is warranted. Traditional soil improvement methods include preloading/surcharging with drains; lightweight fill; excavation and replacement; deep soil mixing, embankment piles, etc., as discussed by Holtz (1989) and Holtz et al. (2001a). Today, geosynthetic reinforcement must also be considered as a feasible treatment alternative. In some situations, the most economical final design may be some combination of a traditional foundation treatment alternative together with geosynthetic reinforcement. Figure 2a shows the basic concept for using geosynthetic reinforcement. Note that the reinforcement will not reduce the magnitude of long-term consolidation or secondary settlement of the embankment. Design Considerations

12 As with ordinary embankments on soft soils, the basic design approach for reinforced embankments is to design against failure. The ways in which embankments constructed on soft foundations can fail have been described by Terzaghi et al. (1996),

13 Fig. 2. Reinforced embankments: a) concept; b) bearing failure; c) rotational failure; and d) lateral spreading. among others. Figure 2 b-d shows unsatisfactory behavior that can occur in reinforced embankments. The three possible modes of failure indicate the types of stability analyses that are required for design. Overall bearing capacity of the embankment must be adequate, and the reinforcement should be strong enough to prevent rotational failures at the edge of the embankment. Lateral spreading failures can be prevented by the development of adequate shearing resistance between the base of the embankment and the reinforcement. In addition, an analysis to limit geosynthetic deformations must be performed. Finally, the geosynthetic strength requirements in the longitudinal direction, typically the transverse seam strength, must be determined. Discussion of these design concepts as well as detailed design procedures are given

14 by Christopher and Holtz (1985), Bonaparte et al. (1987), Holtz (1989 and 1990), Humphrey and Rowe (1991), Holtz et al. (1997), and Leroueil and Rowe (2001). The calculations required for stability and settlement utilize conventional geotechnical design procedures modified only for the presence of the reinforcement. Because the most critical condition for embankment stability is at the end of construction, the total stress method of analysis is usually performed, which is conservative since the analysis generally assumes that no strength gain occurs in the foundation soil. It is always possible of course to calculate stability in terms of effective stresses provided that effective stress shear strength parameters are available and an accurate estimate of the field pore pressures can be made during the project design phase. Because the prediction of in situ pore pressures in advance of construction is not easy, it is essential that the foundation be instrumented with high quality piezometers during construction to control the rate of embankment filling. Preloading and staged embankment construction are discussed in detail by Ladd (1991) and summarized by Leroueil and Rowe (2001). Material Properties Based on the stability calculations, the minimum geosynthetic strengths required for stability at an appropriate factor of safety can be determined. In addition to its tensile and frictional properties, drainage requirements, construction conditions, and environmental factors must also be considered. Geosynthetic properties required for reinforcement applications are given in Table 1.

15 Table 1. Geosynthetic properties required for reinforcement applications. CRITERIA AND PARAMETER Design requirements: a.mechanical Tensile strength and modulus Seam strength Tension creep Soil-geosynthetic friction b. Hydraulic Piping resistance Permeability Constructability requirements: Tensile strength Puncture resistance Tear resistance Durability: PROPERTY Wide width strength and modulus Wide width strength Tension creep Soilgeosynthetic friction angle Apparent opening size Permeability Grab strength Puncture resistance Trapezoidal tear strength UV stability (if exposed) UV resistance Chemical and biological (if required) Chemical and biological resistance When properly designed and selected, high-strength geotextiles or geogrids can provide adequate embankment reinforcement. Both materials can be used equally well, provided they have the requisite design properties. There are some differences in how they are installed, especially with respect to seaming and field workability. Also, at some very soft sites, especially where there is no root mat or vegetative layer, geogrids may require a lightweight geotextile separator to provide filtration and prevent contamination of the embankment fill. However, a geotextile separator is not required if the fill can adequately filter the foundation soil. A detailed discussion of geosynthetic properties and specifications is given by Holtz et al. (1997) and Koerner and Hsuan (2001), so only a few additional comments are given below. The selection of appropriate fill materials is also an important aspect of the design. When possible, granular fill is preferred, especially for the first few lifts above the geosynthetic.

16 Environmental Considerations For most embankment reinforcement situations, geosynthetics have a high resistance to chemical and biological attack; therefore, chemical and biological compatibility is usually not a concern. However, in unusual situations such as very low (i.e., < 3) or very high (i.e., > 9) ph soils, or other unusual chemical environments (for example, in industrial areas or near mine or other waste dumps), chemical compatibility with the polymer(s) in the geosynthetic should be checked. It is important to assure it will retain the design strength at least until the underlying subsoil is strong enough to support the structure without reinforcement. Constructability (Survivability) Requirements In addition to the design strength requirements, the geotextile or geogrid must also have sufficient strength to survive construction. If the geosynthetic is ripped, punctured, torn or otherwise damaged during construction, its strength will be reduced and failure could result. Constructability property requirements are listed in Table 1. (These are also called survivability requirements.) See Christopher and Holtz (1985) and Holtz et al. (1997) for specific property requirements for reinforced embankment construction with varying subgrade conditions, construction equipment, and lift thicknesses. For all critical applications, high to very high survivability geotextiles and geogrids are recommended. Stiffness and Workability For extremely soft soil conditions, geosynthetic stiffness or workability may be an important consideration. The workability of a geosynthetic is its ability to support workpersons during initial placement and seaming operations and to support construction equipment during the first lift placement. Workability is generally related to geosynthetic stiffness; however, stiffness evaluation techniques and correlations with field workability are very poor (Tan, 1990). See Holtz et al. (1997) for recommendations on stiffness. Construction The importance of proper construction procedures for geosynthetic reinforced embankments cannot be overemphasized. A specific construction sequence is usually required in order to avoid failures during construction. Appropriate site preparation, low ground pressure equipment, small initial lift thicknesses, and partially loaded hauling vehicles may be required. Clean granular fill is recommended especially for the first few construction lifts, and proper fill placement, spreading, and compaction procedures are very important. A detailed discussion of construction procedures for reinforced embankments on very soft foundations is given by Christopher and Holtz (1985) and Holtz et al. (1997). It should be noted that all geosynthetic seams must be positively joined. For geotextiles, this means sewing; for geogrids, some type of positive clamping arrangement must be used. Careful inspection is essential, as the seams are the weak link in the system, and seam failures are common in improperly constructed embankments. Finally, soft ground construction projects usually require geotechnical instrumentation for proper control of construction and fill placement; see Holtz (1989) and Holtz et al. (2001a) for recommendations.

17 REINFORCED STEEP SLOPES Concept The first use of geosynthetics for the stabilization of steep slopes was for the reinstatement of failed slopes. Cost savings resulted because the slide debris could be reused in the repaired slope (together with geosynthetic reinforcement), rather than importing select materials to reconstruct the slope. Even if foundation conditions are satisfactory, costs of fill and right-of-way plus other considerations may require a steeper slope than is stable in compacted embankment soils without reinforcement. As shown in Fig.3, multiple layers of geogrids or geotextiles may be placed in a fill slope during construction or reconstruction to reinforce the soil and provide increased slope stability. Most steep slope reinforcement projects are for the construction of new embankments, alternatives to retaining walls, widening of existing embankments, and repair of failed slopes. Another use of geosynthetics in slopes is for compaction aids (Fig. 3). In this application, narrow geosynthetic strips, 1 to 2 m wide, are placed at the edge of the fill slope to provide increased lateral confinement at the slope face, and therefore increased compacted density over that normally achieved. Even modest amounts of reinforcement in compacted slopes have been found to prevent sloughing and reduce slope erosion. In some cases, thick nonwoven geotextiles with in-plane drainage capabilities allow for rapid pore pressure dissipation in compacted cohesive fill soils.

18 Fig. 3. Examples of multilayer geosynthetic slope reinforcement. Design Considerations The overall design requirements for reinforced slopes are similar to those for unreinforced slopes--the factor of safety must be adequate for both the short- and longterm conditions and for all possible modes of failure. These include: (1) internal--where the failure plane passes through the reinforcing elements; (2) external--where the failure surface passes behind and underneath the reinforced mass; and (3) compound--where the failure surface passes behind and through the reinforced soil mass. Reinforced slopes are analyzed using modified versions of classical limit equilibrium slope stability methods (e.g., Terzaghi et al., 1996). Potential circular or wedge-type failure surfaces are assumed, and the relationship between driving and resisting forces or moments determines the factor of safety. Based on their tensile capacity and orientation, reinforcement layers intersecting the potential failure surface increase the resisting moment or force. The tensile capacity of a reinforcement layer is the minimum of its allowable pullout resistance behind, or in front of, the potential failure surface and/or its long-term design tensile strength, whichever is smaller. A variety of potential failure surfaces must be considered, including deep-seated surfaces through or behind the reinforced zone, and the critical surface requiring the maximum amount reinforcement determines the slope factor of safety. The reinforcement layout and spacing may be varied to achieve an optimum design. Computer programs are available for reinforced slope design which include searching routines to help locate critical surfaces and appropriate consideration of reinforcement strength and pullout capacity. Additional information on reinforced slope design is available in Christopher et al. (1990), Christopher and Leshchinsky (1991), Berg (1993), Holtz et al.(1997), and Bathurst and Jones (2001). For slide repair applications, it is very important that the cause of original failure is addressed in order to insure that the new reinforced soil slope will not have the same problems. Particular attention must be paid to drainage. In natural soil slopes, it is also necessary to identify any weak seams that could affect stability. Material Properties Geosynthetic properties required for reinforced slopes are similar to those listed in Table 1, Section 8.3. Properties are required for design (stability), constructability, and durability. Allowable tensile strength and soil-geosynthetic friction are most important for stability design. Because of uncertainties in creep strength, chemical and biological degradation effects, installation damage, and joints and connections, a partial factor or reduction factor concept is recommended. The ultimate wide width strength is reduced for these various factors, and the reduction depends on how much information is available about the geosynthetics at the time of design and selection. Berg (1993), Holtz et al. (1997), and Koerner and Hsuan (2001) give details about the determination of the allowable geosynthetic tensile strength. They also describe how soil-geosynthetic friction is measured or estimated. An inherent advantage of geosynthetic reinforcement is their longevity, especially in normal soil environments. Recent studies have indicated that the anticipated half-life of reinforfcement geosynthetics in between 500 and 5000 years, although strength

19 characteristics may have to be adjusted to account for potential degradation in the specific environmental conditions. Any soil suitable for embankment construction can be used in a reinforced slope system. From a reinforcement point of view alone, even lower-quality soil than conventionally used in unreinforced slope construction may be used. However, higherquality materials offer less durability concerns, are easier to place and compact, which tends to speed up construction, and they have fewer problems with drainage. See Berg (1993) and Holtz et al. (1997) for discussion of soil gradation, compaction, unit weight, shear strength, and chemical composition. Construction Similarly to reinforced embankments, proper construction is very important to insure adequate performance of a reinforced slope. Considerations of site preparation, reinforcement and fill placement, compaction control, face construction, and field inspection are given by Berg (1993) and Holtz et al. (1997). REINFORCED RETAINING WALLS AND ABUTMENTS Concept Retaining walls are required where a soil slope is uneconomical or not technically feasible. When compared with conventional retaining structures, walls with reinforced backfills offer significant advantages. They are very cost effective, especially for higher walls. Furthermore, these systems are more flexible than conventional earth retaining walls such as reinforced concrete cantilever or gravity walls. Therefore, they are very suitable for sites with poor foundations and for seismically active areas. Modern reinforced soil technology was developed in France by H. Vidal in the mid 1960s. His system is called Reinforced Earth and is shown in Fig. 4. Steel strips are used to reduce the earth pressure against the wall face. The design and construction of Vidaltype reinforced earth walls are now well established, and many thousands have been Fig. 4. Component parts of a Reinforced Earth wall.

20 successfully built throughout the world in the last 25 years. Other similar proprietary reinforcing systems have also been developed using steel bar mats, grids, and gabions. The use of geotextiles as reinforcing elements started in the early 1970 s because of concern over possible corrosion of metallic reinforcement. Systems using sheets of geosynthetics rather than steel strips are shown in Fig 5.. Fig. 5. Reinforced retaining wall systems using geosynthetics: (a) with wrap-around geosynthetic facing, (b) with segmental or modular concrete block, and (c) with full-height (propped) precast panels. The maximum heights of geosynthetic reinforced walls constructed to date are less than 20 m, whereas steel reinforced walls over 40 m high have been built. A significant benefit of using geosynthetics is the wide variety of wall facings available, resulting in greater aesthetic and economic options. Metallic reinforcement is typically used with articulated precast concrete panels or gabion-type facing systems. Design Considerations Reinforced wall design is very similar to conventional retaining wall design, but with the added consideration of internal stability of the reinforced section. External stability is calculated in the conventional way--the bearing capacity must be adequate, the reinforced section may not slide or overturn, and overall slope stability must be adequate. Surcharges (live and dead loads; distributed and point loads) are considered in the conventional manner. Settlement of the reinforced section also should be checked if the foundation is compressible. A number of different approaches to internal design of geotextile reinforced retaining walls have been proposed (Christopher et al., 1990; Allen and Holtz; 1991; Holtz, 1995), but the oldest and most common--and most conservative--method is the tieback wedge analysis. It utilizes classical earth pressure theory combined with tensile resisting tiebacks that extend back of the assumed failure plane (Fig. 6). The K A (or K o) is assumed, depending on the stiffness of the facing and the amount of yielding likely to occur during construction, and the earth pressure at each vertical section of the wall is calculated. This earth pressure must be resisted by the geosynthetic reinforcement at that section.

21 Fig 6. Actual geosynthetic-reinforced wall compared to its analytical model. To design against failure of the reinforcement, there are two possible limiting or failure conditions: rupture of the geosynthetic and pullout of the geosynthetic. The corresponding reinforcement properties are the tensile strength of the geosynthetic and its pullout resistance. In the latter case, the geosynthetic reinforcement must extend some distance behind the assumed failure wedge so that it will not pull out of the backfill. Typically, sliding of the entire reinforced mass controls the length of the reinforcing elements. For a detailed description of the tieback wedge method, see Christopher and Holtz (1985), Bonaparte et al. (1987), Allen and Holtz (1991), and Holtz et al. (1997). Recent research (e.g., Lee et al., 1999; Lee, 2000; Bathurst et al.,2000) has indicated that the tieback wedge approach is overly conservative and uneconomical, and modifications and deformation-based designs are rapidly being developed. Other important design considerations include drainage and potential seismic loading. Material Properties Geosynthetic properties required for reinforced walls are similar to those listed in Table 1, Section 8.3 and discussed in Section 9.3 for reinforced slopes. Properties are required for design (stability), constructability, and durability. Allowable tensile strength and soil-geosynthetic friction are required for stability design, and similar to reinforced slopes, a partial factor or reduction factor approach is common. The ultimate wide width strength is reduced to account for uncertainties in creep strength, chemical and biological degradation effects, installation damage, and joints and connections. Berg (1993), Holtz et al.(1997), and Koerner and Hsuan (2001) give details about the determination of the allowable geosynthetic tensile strength. They also describe how soil-geosynthetic friction is measured or estimated. The discussion on durability and longevity of geosynthetic reinforcement given in Section 9.3 is pertinent here. Backfill for geosynthetic reinforced walls should be free draining if at all possible. If not, then adequate drainage of infiltrating surface or groundwater must be provided. This is important for stability considerations because drainage outward through the wall face may not be adequate. Soil properties required include gradation, percent fines, chemical composition, compaction, unit weight, and shear strength. To insure stability, appropriate consideration of the foundation and overall slope stability at the site is also important (Holtz et al., 2001b).

22 Wall Facing Considerations A significant advantage of geosynthetic reinforced walls over conventional retaining structures is the variety of facings that can be used and the resulting aesthetic options that can be provided. Aesthetic requirements often determine the type of facing systems. Anticipated deflection of the wall face, both laterally and downward, may place further limitations on the type of facing system selected. Tight construction specifications and quality inspection are necessary to insure that the wall face is constructed properly; otherwise an unattractive wall face, or a wall face failure, could result. Facing systems can be installed (1) as the wall is constructed or (2) after the wall is built. Facings installed as the wall is constructed include segmental and full height precast concrete panels, interlocking precast concrete blocks, welded wire panels, gabion baskets, treated timber facings, and geosynthetic face wraps. In these cases, the geosynthetic reinforcement is attached directly to the facing element. Systems installed after construction include shotcrete, cast-in-place concrete facia, and precast concrete or timber panels; the panels are attached to brackets placed between the layers of the geosynthetic wrapped wall face at the end of wall construction or after wall movements are complete. Facings constructed as the wall is constructed must either allow the geosynthetic to deform freely during construction without any buildup of stress on the face, or the facing connection must be designed to take the stress. Although most wall design methods assume that the stress at the face is equal to the maximum horizontal stress in the reinforced backfill, measurements show that considerable stress reduction occurs near the face, depending on the flexibility of the face. See Allen and Holtz (1991) and Holtz et al. (1997) for a detailed discussion of wall facing systems. Constuction Construction procedures for geosynthetic reinforced walls and abutments are given by Christopher and Holtz (1985) and Holtz et al. (1997). Procedures are relatively simple and straightforward, but failures are surprisingly common, especially with proprietary precast segmental concrete block-faced wall systems. It appears that most of these failures are due to (1) inadequate design, particularly of the foundation and back slope of the wall, and/or (2) problems in construction. The latter include poor inspection and quality control, poor compaction, use of inappropriate backfill materials, lack of attention to facing connections, and lack of clear lines of responsibility between designers, material suppliers, and contractors. 5. How do geosynthetics function as a filter? How does it differ in its function for drainage? Explain in detail with sketches Geosynthetics can allow water to pass across the plane while prevent or retain the soil particles Act similar to sand filter Allow water to move through soil while retaining upstream soil particles Prevent migration through drainage aggregate and pipes Prevent soil erosion below rip rap and armour materials The geosynthetic acts similar to a sand filter by allowing water to move through the soil while retaining all upstream soil particles. For example, geotextiles are used to prevent soils from migrating into

23 drainage aggregate or pipes while maintaining flow through the system. Geotextiles are also used below rip rap and other armour materials in coastal and river bank protection systems to prevent soil erosion. Filtration and Drainage Filtration is the equilibrium soil-to-geotextile interaction that allows for adequate liquid flow without soil loss, across the plane of the geotextile over a service lifetime compatible with the application under consideration Filtration applications are highway underdrain systems, retaining wall drainage, landfill leachate collection systems, as silt fences and curtains, and as flexible forms for bags, tubes and container Drainage is the equilibrium soil-to- geosynthetic system that allows for adequate liquid flow without soil loss, within the plane of the geosynthetic over a service lifetime Drainage applications for these different geosynthetics are retaining walls, sport fields, dams, canals, reservoirs, and capillary breaks Geosynthetic placed between earth and gabion for filtration

24 . 6. What is the role of geosynthetics in protecting soil from contamination? Descibe in detail. Some geosynthetics can be used as relatively impermeable barrier to prevent liquids or gases. It can also be used as noise barrier o Containment o Containment involves geomembranes, geosynthetic clay liners, or some geocomposites which function as liquid or gas barriers o Landfill liners and covers make critical use of these geosynthetics o All hydraulic applications (tunnels, dams, canals, reservoir liners, and floating covers) use these geosynthetics as well Applications of Reinforced Earth o Bridgeworks o Bridge Abutment o Economical, high speed of erection, may be used in poor subsoils o Bridge Abutment with Piled Bankseat o Economical, reduced settlement of deck support o Sloping Bridge Abutment o Strong embankment abutment interaction

25 Geosynthetics (geomembrane or geosynthetic clay liners) can act as relatively impermeable barrier to impede flow of fluids or gases in landfills, waste containment, encapsulation of swelling soils, asphalt pavement overlays and reflection cracking. Geosynthetic placed between an existing crack pavement and an asphalt overlay to minimize the reflection cracking or delay the propagation of cracks

26 7. Describe in detail about soil nailing and when is it adopted? Soil nailing Soil nails are more or less rigid bars driven into soil or pushed into boreholes which are filled with grout. Together with the insitu soil, they are intended to form a coherent structural entity supporting on excavation or arresting the movement of on unstable slope. The size of nails varies from then steel bars to light concrete piles. Most nailed supported structures classed as temporary. This is partly due to corrosion and uncertainties with regard to the design assumptions. Analysis of nailed soil: Geometry of failure surface Definition of safety factor Overturning moment Shear strength Friction angle Cohesion Reinforcement properties Direction of tensile stabilizing force provided by nails.

27 Soil nail Design For external stability of the soil nailed structures the following failure modes are generally considered for reinforced soil mass.

28 Overturning Sliding Bearing Overall stability of slope structures Design method (a) German method (Gassler & Gudehus 1981) Assumptions: Bilinear failure surface passing the toe of slopes/cuttings. Soil mass divided into two wedges, 1. Soil wedge at slope/cutting face reinforced with soil nails 2. Soil wedge at back has no reinforcement and considered as a wedge exerting active earth pressure force on reinforced soil wedge in front Only tensile resistance provided by nail reinforcements embedded in passive zone. (b)french method Circular failure surface passing the toe of slopes Failure of nail reinforcements including tensile failure, pullout failure, grout reinforcement failures, bending/shear failure. Is checked in limit equilibrium analysis Modified Davis method: Parabolic failure surface passing through the toe of slopes. Only tensile resistance of nail reinforcement embedded in passive zone is considered. FHWA design method: Bilinear and circular failure surface passing through the toe of slopes. Each nail reinforcement extend beyond critical failure surface. Only tensile resistance provided by nail reinforcement is considered.

29 UK design method: Bilinear failure surface Inter wedge friction is neglected Only tensile resistance provided by nail reinforcement is considered.