Ground Improvement Technique: Earth Reinforcement

Ground Improvement Technique: Earth Reinforcement

1. Earth-Reinforcement Ground Improvement Technique By: Dr. D. N. Mudgal

2. Introduction Ground improvement is the procedure typically defined as using mechanical means to improve poor ground conditions. Ground improvement methods improve the engineering properties of the soil mass which is treated to meet project performance requirements.

3. Objectives The most common traditional objectives include improvement of the soil and ground for use as a foundation or construction material. Increasing shear strength, durability, stiffness and stability Reducing undesirable properties ( eg . Shrink/ swell potential, compressibility, liquefability ) Modifying permeability, the rate of fluid to flow through a medium; and Improving efficiency and productivity by using methods that save time and expense

4. Factors affecting choice of Improvement Method 1. Soil type: This is one of the most important parameters that will control what approach or materials will be applicable to only certain types of soil types and grain sizes 2. Depth and location of treatment required: Many ground improvement methods have depth limitations that render them unsuitable for applications for deeper soil horizons. 3. Desired/required soil properties: obviously, different methods are use to achieve different engineering properties, and certain methods will provide various levels of uniformity to improved sites. 4.Availability of materials: Depending on the location of the project and materials required for each feasible ground improvements approach.

5. To be conti …… 5. Availability of skills, local experience, and local preferences: While the engineer ma possess the knowledge and understanding of a preferred method. 6.Environmental concerns: With a better understanding and a greater awareness of effects on the natural environment, more attention have been placed on methods that assure less environmental impacts. 7.Economics: when all else has been considered, the final decision on choice of improvement method will often come down to the ultimate cost of a proposed method, or cost will be the deciding factor in choosing between two or more otherwise suitable methods.

6. Ground Improvement Techniques Mechanical modification Hydraulic modification Physical and chemical modification Modification by inclusion and Confinement

7. Soil density is increased by the application of short-term external mechanical forces, including Compaction of surface layers by: Static, Vibratory, Impact rollers, Plate vibrators. Deep compaction by heavy tamping at the surface or vibration at depth Mechanical Modification

8. Hydraulic Modification Free –pore water is forced out of the soil via (by means of) drains of wells. – In coarse grained soils, this is achieved by lowering the ground water level through pumping from boreholes or trenches. In fine-grained soils, the long term application of external loads (preloading) or electrical forces (electro kinetic stabilization) is required

9. Improvement of cohesive soils: Cohesive soils such as soft clay with large void ratio and higher water content have a necessity to improve their characteristics. To reduce void ratio and water content, to increase strength for which increases bearing capacity of soil. Generally following methods are in practice: a. Precompression or Preloading b. Sand Drains c. Wick Drains d. Stone columns

10. Physical and Chemical Modification 1. Additives include: - natural soils - industrial by-products or waste materials (fly ash, slag), - Cementations and other chemicals (lime, cement) which react with each other and the ground. 2. When additives are injected via boreholes under pressure into the voids within the ground or between it and a structure, the process is called GROUTING. Rigs with multiple injectors deliver the stabilizing fluid into the soil. The fluid will prefer to travel into cracks and fissures . 3. Soil stabilization by heating the ground and by freezing the ground come under, Thermal Methods of Modification. a. Heating evaporates water and causes permanent changes in the mineral structure of soils. b. Freezing solidifies part or all of the water and bonds individual particles together

11. Modification by Inclusion and Confinement Reinforcement by Fibers, Strips, Bars, Meshes and Fabrics. In-situ reinforcement is achieved by nails and anchors.

12. Principle of Reinforced Earth Soil has an inherently low tensile strength but a high compressive strength. An objective of incorporating soil reinforcement is to absorb tensile loads or shear stresses within the structure. In absence of the reinforcement, structure my fail in shear or by excess of the deformation. When an axial load is applied to the reinforced soil, it generates an axial compressive strain and lateral tensile strain

13. To be continued…. If the reinforcement has an axial tensile stiffness greater than that of the soil, then lateral movements of the soil will only occur if soil can move relative to the reinforcement. Movement of the soil, relative to the reinforcement, will generate shear stresses at the soil/ reinforcement interface, these shear stresses are redistributed back into the soil in the form of internal confining stress. Due to this, the strain within the reinforced soil mass is less than the strain in unreinforced soil for the same amount of stresses

14. TYPES OF FIBRES FOR GEOTEXTILES Natural Fibers Synthetic Fibers

15. Natural fibers Natural fibers in the form of paper strips, jute nets, wood shavings or wool mulch are being used as geotextiles. In certain soil reinforcement applications, geotextiles have to serve for more than 100 years. Bio-degradable natural geotextiles are deliberately manufactured to have relatively short period of life. They are generally used for prevention of soil erosion until vegetation can become properly established on the ground surface. The commonly used natural fibres are – Ramie Jute

16. To be continued…. These are subtropical bast fibres , which are obtained from their plants 5 to 6 times a year. The fibres have silky luster and have white appearance even in the unbleached condition. They constitute of pure cellulose and possess highest tenacity among all plant fibres

17. Jute This is a versatile vegetable fiber which is biodegradable and has the ability to mix with the soil and serve as a nutrient for vegetation. Their quick biodegradability becomes weakness for their use as a geotextile. However, their life span can be extended even up to 20 years through different treatments and blendings . Thus, it is possible to manufacture designed biodegradable jute geotextile, having specific tenacity, porosity, permeability, transmissibility according to need and location specificity. Soil, soil composition, water, water quality, water flow, landscape etc. physical situation determines the application and choice of what kind of jute geotextiles should be used.

18. To be continued…. In contrast to synthetic geotextiles, though jute geotextiles are less durable but they also have some advantages in certain area to be used particularly in agro-mulching and similar area to where quick consolidation are to take place. For erosion control and rural road considerations, soil protection from natural and seasonal degradation caused by rain, water, monsoon, wind and cold weather are very important parameters. Jute geotextiles, as separator, reinforcing and drainage activities, along with topsoil erosion in shoulder and cracking are used quite satisfactorily. Furthermore, after degradation of jute geotextiles, lignomass is formed, which increases the soil organic content, fertility, texture and also enhance vegetative growth with further consolidation and stability of soil.

19. Synthetic Fibers The four main synthetic polymers most widely used as the raw material for geotextiles are –polyester, polyamide, polyethylene and polypropylene. The oldest of these is polyethylene which was discovered in 1931 by ICI. Another group of polymers with a long production history is the polyamide family, the first of which was discovered in 1935. The next oldest of the four main polymer families relevant to geotextile manufacture is polyester, which was announced in 1941. The most recent polymer family relevant to geotextiles to be developed was polypropylene, which was discovered in 1954.

20. Polyesters (PET) Polyester is synthesized by polymerizing ethylene glycol with dimethyl terephthalate or with terephthalic acid. The fiber has high strength modulus, creep resistance and general chemical inertness due too which it is more suitable for geotextiles. It is attacked by polar solvent like benzyl alcohol, phenol, and meta-cresol. At pH range of 7 to 10, its life span is about 50 years. It possesses high resistance to ultraviolet radiations. However, the installation should be undertaken with care to avoid unnecessary exposure to light.

21. Polyamides (PA): There are two most important types of polyamides, namely Nylon 6 and Nylon 66 but they are used very little in geotextiles. The first one an aliphatic polyamide obtained by the polymerization of petroleum derivative caprolactam. The second type is also an aliphatic polyamide obtained by the polymerization of a salt of adipic acid and hexamethylene diamine. These are manufactured in the form of threads which are cut into granules. They have more strength but less moduli than polypropylene and polyester They are also readily prone to hydrolysis

22. Polyethylene (PE): Polyethylene can be produced in a highly crystalline form, which is an extremely important characteristic in fiber forming polymer. Three main groups of polyethylene are – a. Low density polyethylene (LDPE, density 9.2-9.3 g/cc), b. Linear low density polyethylene (LLDPE, density 9.20-9.45 g/cc) c. High density polyethylene (HDPE, density 9.40- 9.6 g/cc)

23. To be continued…. Polypropylene (PP): Polypropylene is a crystalline thermoplastic produced by polymerizing propylene monomers in the presence of stereo-specific ZeiglerNatta catalytic system. Homo-polymers and copolymers are two types of polypropylene . Homopolymers are used for fibre and yarn applications whereas co-polymers are used for varied industria applications . Propylene is mainly available in granularform

24. Forms of Geo-textile Woven geotextiles Knitted geotextiles Nonwoven geotextiles Stitch-bonded geotextiles Geogrids Geonets Geomembranes Geocomposites

25. Woven geotextiles Woven geotextiles are produced with the interlacement of two sets of yarns at right angles in the weaving process. Woven geotextiles have high strengths and modulus in the warp and weft directions and low elongations at rupture.

26. Knitted geotextiles Knitted geotextiles are produced with the interloping of one or more yarns in the knitting process. These geotextiles are highly extensible and have relatively low strength compared to woven geotextiles, which limits its usage.

27. Nonwoven geotextiles Nonwoven geotextiles are thicker than woven and are made either from continuous filaments or from staple fibers. They are produced in the following bonding techniques: Needle punching Thermal bonding Chemical bonding

28. Stitch-bonded geotextiles Stitch-bonded geotextiles are produced by interlocking fibers or yarns or both, bonded by stitching or sewing. Even strong, heavyweight geotextiles can be produced rapidly. Tubular geotextiles are manufac­tured in a tubular or cylindrical fashion without longitudinal seam.

29. Geogrids Geogrids are materials that have an open grid-like appearance. The principal application for Geogrids is the reinforcement of soil.

30. Geonets Geonets are open grid-like materials formed by two sets of coarse, parallel, extruded polymeric strands intersecting at a constant acute angle . The network forms a sheet with in-plane porosity that is used to carry relatively large fluid or gas flows.

31. Geomembranes Geomembranes are continuous flexible sheets manufactured from one or more synthetic materials . They are relatively impermeable and are used as liners for fluid or gas containment and as vapor barriers

32. Geocomposites Geocomposites are made from a combination of two or more geosyn­thetic types. Examples include geotextile- geonet ; geotextile-geogrid; geonet-geomembrane ; or a geosynthetic clay liner (GCL).

33. Functions of Geotextile Separation Filtration Drainage Reinforcement Moisture and liquid barrier

34. Separation Separation is the process of preventing undesirable mix-up of two dis­similar materials. The geotextile acts as a separating layer between fine aggregates and coarse aggregates or soils that have different particle size distributions to avoid undesirable mix-up. Separators also help to prevent fine-grained subgrade soils from being pumped into permeable granular road bases thereby keeping the structural integrity and functioning of both materials intact.

35. Filtration Geotextile is placed in contact with and down gradient of soil to be drained. The plane of the geotextile is positioned normal to the expected direction of water flow. To perform this function the geotextile needs to satisfy two conflicting requirements: the filter’s pore size must be small enough to retain fine soil particles while the geotextile should permit relatively unimpeded flow of water into the drainage media.

36. Drainage The geotextile acts as a drain to carry fluid flows through less permeable soils. The application of geotextiles in drainage applications has improved the economical usage of blanket and trench drains under and adjacent to the pavement structure, respectively.

37. Reinforcement The geotextiles act as a reinforcement element within a soil mass or in combination with the soil to produce a composite that has improved strength and deformation properties over the unrein­forced soil. The geotextile interacts with soil through frictional or adhesion forces to resist tensile or shear forces.

38. Moisture and liquid barrier The protection of civil structures from the effects of seeping water is a common need. The geotextiles acts as a relatively impermeable barrier to prevent the penetration of liquids or moisture over a projected service period.

39. Erosion control Erosion is the process by which soil and rock are removed from the earth’s surface by exogenetic processes such as wind or water flow, and then transported and deposited in other locations. The geotextile anchored in steep slope protects soil surfaces from the trac­tive forces of moving water or wind and rainfall erosion.

40. Applications Road Construction: Geotextiles are used to reinforce the base of roads, preventing soil erosion and improving the stability of the roadbed. Railway Embankments: Similar to road construction, geotextiles are used to stabilize railway embankments, reducing soil erosion and improving load distribution. Landfill Liners and Covers: Geotextiles are used as part of landfill liners and covers to prevent the leakage of contaminants into the surrounding environment. Erosion Control: Geotextiles are used to stabilize slopes and prevent soil erosion in areas prone to erosion, such as riverbanks, shorelines, and steep hillsides. Retaining Walls: Geotextiles are used behind retaining walls to improve drainage and soil stability, reducing the pressure on the wall and increasing its lifespan. Stormwater Management: Geotextiles are used in stormwater management systems to filter pollutants and control the flow of water, reducing runoff and preventing soil erosion.

41. To be continued… Reinforced Earth Structures: Geotextiles are used in reinforced earth structures, such as reinforced soil slopes and walls, to improve stability and reduce construction costs. Pavement Overlay: Geotextiles are used as a separation layer between old and new pavement layers to prevent the intermixing of materials and improve the performance of the pavement. Subsurface Drainage: Geotextiles are used in subsurface drainage systems to filter water and prevent the clogging of drainage pipes, improving the efficiency of the system. Coastal Protection: Geotextile tubes and bags are used for coastal protection and beach nourishment projects, helping to stabilize shorelines and protect against erosion caused by waves and currents. Agricultural Applications: Geotextiles are used in agriculture for weed control, soil stabilization, and erosion prevention in areas such as crop fields, nurseries, and orchards. Geotextile Tubes: Geotextile tubes are used for dewatering sludge, sediment, and other waste materials in various industries, including wastewater treatment, dredging, and mining.