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Thousands year ago, Romans designed and constructed the first road system in Europe. They used cobbles, gravels, rubbles and lime mortar to improve soil stable and quality of the roads (Xeidakis and Varagouli, 1997). Natural materials have also been used in roadway construction to stabilize unstable soil and road edges such as natural fibres, fabrics or wood. But the fundamental problem to use natural materials in soil is the biodegradation. Therefore, natural materials will be converted into minerals or disappeared after chemical breakdown in few years later (Stevenson, 2008).
In the 20th Century, polymer materials have been widely developed in civil engineering. Generally, geosynthetics are manufactured from polymeric materials and planar products. Therefore, geosynthetics are well accepted and used successfully in several areas of civil engineering. Their use offers excellent economic alternatives to the conventional solutions of many civil engineering problems.
1.2. Types and manufacture
Geosynthetics is: A planar, polymeric materials, used with soil, rock and other geotechnical material as an integral part of a civil engineering structure or system. Geosynthetics are made from polypropylene, polyester, polyethylene, polyamide (nylon), PVC, etc. These materials are highly resistant to biological and chemical degradation (Stevenson, 2008). Natural fibres (wool, cotton, jute, bamboo, etc.), can be used as stable soil for temporary application but not widely promoted. Such products may be called geonaturals, they have short life span due to their biodegradable characteristics. They can be considered a companion of geosynthetics rather than a replacement. Typical geosynthetic products: geotextile, geogrid, geonet, geomembrane, geocomposites, etc.
A geotextile is a permeable and polymeric textile product which is form as a flexible sheet. It is good in erosion control and stabilization over wet moisture soil. Geotextiles can be classified into the following categories based on the manufacturing process:
Woven geotextile - Used conventional weaving process to make regular textile structure with yarns. Yarns are formed from one or several fibres.
Non-woven geotextile - it made from either oriented or randomly arrangement of fibres. The fibres are arranged into loose web then bonding with partial melting, needle punching or chemical binding agents such as glue, rubber, latex, etc.
Knitted geotextile - it is formed by interlocking one or more yarns to form a planar structure.
Stitch-bonded geotextile - it made by stitching together of fibres or yarns to form a planar structure.
Figure 1.0 Woven geotextile
Figure 1.1 Nonwoven geotextile
Figure 1.2 Knitted geotextile
Geogrid is a polymeric, regular network formed by intersecting elements called ribs which are joined at the junctions. There are different types of geogrid based on the manufacturing process of ribs connecting such as extruded geogrid, bonded geogrid and woven geogrid. Extruded geogrids can be classfied into the following categories based on the direction of stretching during the process:
Uniaxial geogrids - The polymer punched sheets are stretched longitudinal during the manufacture, therefore, they have higher tensile strength in longitudinal direction rather than transverse direction.
Biaxial geogrids - The polymer punched sheets are stretched both longitudinal and transverse direction during the manufacture, therefore, they have equal tensile strength in both directions.
The special feature of geogrids is the opening in between the ribs, called apertures. The dimensions of the apertures are from about 2.5 to 15cm. It should be large enough to interlock the particle soil. There are vary shapes of apertures such as elongated ellipses, square, rectangles, etc. (Shukla, 2002)
Figure 1.3 Uniaxial geogrid (left); biaxial geogrid (right)
Figure 1.4 The interlocking mechanism in geogrid reinforced soil
Figure 1.5 Bonded geogrid (left); woven geogrid (right)
Geomembrane is a impermeble membrane which can used to control fluid migration. The product may be made from asphaltic or polymeric or combination of both. Geocomposites are manufactured in laminated or composite form from two or more geosynthetic products (geotextiles, geogrids, geomembrane, etc.). In combination of the products, they can perform more effectively to their specific functions than when used separately. There are several combination products such as geotextile-geogrid, geotextile-geomembrane, geomembrane-geonet-geomembrane, etc. (Shukla, 2002)
Figure 1.6 Geomembrane (left); geocomposite (right)
1.3. Functions and applications
Geosynthetics have numerous application areas in civil engineering. They usually have one of these basic functions: separation, filtration, drainage, fluid barrier, reinforcement and erosion control. In some cases, they can perform one or more secondary functions that will be expected to serve during its performance life. In the design, the selection of a geosynthetic product for any field should be highly considered both primary and secondary functions to have efficient performance in specific application. (Holtz, 2001)
Separation If geosynthetic can used to separate two different particle size distributions of soil to prevent intermixing, it is said to perform a separation function. For example, geotextile will apply in between granular road base and soft subgrade soils to prevent road base materials penetrating into underlying soft subgrade. It also prevents soft subgrade soil pumped into permeable granular road base (Bathurst, 2007 & Holtz, 2001). Besides separation function, geosynthetics also have to resistant concentrated stress (tear, puncture and burst) and have aperture sizes compatible with the particle sizes of the materials to be retained.
Figure 1.7 Separation function: a) granular fill-soft soil system without geosynthetic separator; b) granular fill-soft soil system with the geosynthetic separator
White (1990) has noted that under applied loads from vehicle wheels, the aggregate layer will deform after a sufficient quantity of load repetitions. The surface of the layer in contact with the subgrade will begin to separate, since the aggregate cannot resist the tensile force. The aggregate continuity, strength and load spreading ability are reduced. As little as 10 to 20 percent of intermixing of subgrade fines can completely destroy the strength of the aggregate layer. If geotextile prevents the bearing failure then the subgrade should able to carry the design load without deterioration to the pavement system.
Figure 1.8 Geotextile in pavement
Filtration If the geosynthetic has same function as sand filtration to allow fluid flow with limited migration of soil particle across the plane, it is said to have filtration function. It usually used to prevent soil particles migrate into drainage or pipes. There are two conflicting requirements to perform the function: the filter's pore size must be small enough to retain fine soil particles and it also need to permit relatively unimpeded flow of water into drainage (Meccai & Hasan, 2004).
It is important to know that filtration function also provide separation function. However, a principle function has to be identifying between filtration function and separation function with respect to the amount of fluid flow through the geosynthetic. They are also used as filter below riprap and other armor materials in coastal and river bank to prevent soil erosion. Examples include retaining wall drainage, landfill, highway pavement edge drain and slope interceptor drainage (Bathurst, 2007 & Holtz, 2001).
Figure 1.9 Fluid flows through soil and geosynthetic
Drainage The geosynthetic material acts as a drain to allow fluid flows through soil. Geopipe is one of the geosynthetic which provides more efficiency on this function. It usually been used in higher flow places, such as the base of roadway embankment, retaining wall drainage and slope interceptor drainage. Prefabricated vertical drain (PVD) has been used at below embankment and preload fill to accelerate consolidation of soft cohesive foundation soil (Bathurst, 2007 & Holtz, 2001).
A geosynthetic drainage system need to have a drainage core to absorb water at right angles to its horizontal plane and transports the fluid within the latter. To prevent clogging of the drainage core, it needs to combine with a filter layer.
S.K. Shukla (2002) has mentioned some advantages of geosynthetic drains compare to sand drains.
Geosynthetic drains are cost effective in some regions where the granular materials is less in supply and it is restrict to exploitation due to environmental constraints.
The geosynthetic materials are easy to be transported to the construction areas.
The geosynthetic drains are easy to install with some light equipment.
The geosynthetic materials can be manufactured to match the specified requirement and quality can be controlled.
Construction time for geosynthetic drains are faster than sand drains, which can lead to cost saving.
Figure 1.10 Basic mechanism involved in the drainage function.
Fluid or gas barrier The geosynthetic material uses to prevent the migration of liquids or gases into soil or earth surface. Geomembrane is thin film which can be used as fluid barrier to impede the flow of liquid or gas from one location to another location. This function is also used in encapsulation waste containment in landfill (Bathurst, 2007 & Holtz, 2001). It also been used as a sealing function in road rehabilitation, as shown in below this figure. The geotextile is placed on old pavement surface following the application of new overlay asphalt tack coat. The geotextile combine with asphalt to become a waterproofing membrane. It can minimize the flow of water directly into the pavement system (Meccai & Hasan, 2004).
Figure 1.11 Sealing Function
Erosion control Erosion is natural process due to wind force and water runoff. Various activities on construction field will also accelerate erosion of soil. Uncontrolled erosion process can cause some serious damages to the existing structure, construction site and to the environment.
Geosynthetics can be used in works such as: slope protection, channels, waterways, shoreline protection, reclamation, re-vegetation, rockfall netting and embankment. The materials can be used to reduce surface erosion of soil caused by rainfall impact, water runoff and wind force. For example slope surface stability may partially cover with cement paste and geotextile bag filled. Vegetation will be planted on slope to protect it from soil losses caused by the forces of water or wind. Geosynthetic blankets or mats are made from natural (straw or coconut fibres) and polymer meshes to accelerate the establishment of vegetative cover on slope.
Reinforcement The material can act as reinforcement with soil or fill material to improve strength and deformation properties on the soft foundation and heavy surface loading. The geosynthetic products have been successfully practiced for more than 30 years in reinforced soil structures. They are usually used in retaining structure or for stabilizing embankment on soft soil with a poor load bearing capacity (F. Saathoff). Geogrid and geotextile enable to construct in soft foundation of embankment and stable the slope of embankment. It also improves tensile strength to the soil mass in order to create vertical force for retaining wall (Bathurst, Holtz).
Figure 1.12 Application and variability of reinforced soil
There are three main applications for soil reinforcement:
To reinforce the base of embankments constructed on soft foundation
To increase the stability and erosion control on steep slope
To reduce the earth pressure behind the retaining wall and abutments
There are also been used in other reinforcement and stabilization applications such as roads and railroads, large area stabilization and natural slope reinforcement. (Holtz, 2001)
Reinforced embankment on soft foundation
To construct and design the embankment on the very soft foundation, it needs to investigate subsurface, determine soil properties, and settlement and stability analyses. As noted by Holtz, traditional soil improvement methods include preloading/surcharging with drains; lightweight fill; excavation and replacement; deep soil mixing, embankment piles, etc. In some cases, the combination of a traditional foundation treatment with geosynthetic reinforcement in the final design will be more economical. The reinforcement will not reduce the magnitude of long term consolidation or secondary settlement of the embankment.
Figure 2.0 Reinforced embankments: a) concept; b) bearing failure; c) rotational failure; d) lateral spreading.
From the above figure showed that there are three possible of failure modes on embankment. Holtz noted that bearing capacity must be adequate and reinforcement of geosynthetic should strong enough to prevent rotational failures at the edge of the embankment. Lateral spreading failure can be prevented due to the friction force between the base of the embankment and the reinforcement.
There are some criteria needed to consider in design requirements: tensile strength, tear resistance, seam strength, tension creep, soil-geosynthetic friction, hydraulic permeability, UV stability, and chemical and biological resistance. Geotextiles and geogrids provide the requisite design properties to adequate embankment reinforcement. In some soft sites, where there is no root mat or vegetative layer, geogrids may require a lightweight geotextile to provide filtration and prevent contamination of the embankment fill. However, if the fill can completely filter the foundation soil, geotextile is not required (Holtz, 2001).
The proper construction sequence is required in order to avoid failure during construction. If the geosynthetic is ripped, torn or damaged during construction, its strength will be reduced and failure in result. Inspection carefully is absolutely necessary, as seam failure is the main criteria in improperly constructed embankment (Holtz, 2001).
2.1. Reinforced Steep Slopes
The main use of geosynthetics for steep slope was stabilized and restored back the failed slopes. Traditional method to repair the slope is importing select materials to rebuild the slope, it is costly and may not stable if the slope is steep. Geogrids or geotextiles may be used and placed it with multiple layers in the fill slope during construction to increase slope stability and to reinforce the soil (Holtz, 2001)
Figure 2.1 Example of multilayer geosynthetic slope reinforcement.
As shown in figure, geosynthetics not only provide slope stability and also provide compaction aids for embankment slope. The short geosynthetic strips are placed in the edge of the fill slope to increase lateral confinement and compacted density over that normally achieved. (Holtz, 2001)
Holtz noted that the overall design requirement for reinforced slopes must completely achieve the factor of safety for both short and long strip and any possible failure modes. These include:
Internal-where the failure plane passes through the reinforcing elements;
External-where the failure surface passes behind and underneath the reinforced mass;
Compound-where the failure surface passes behind and through the reinforced soil mass.
Drainage systems for the embankment slope also need to pay attention as it may cause slope failure in soil erosion. Geosynthetic properties required for reinforced slopes are similar to embankment. The most important for stability design is tensile strength and friction between soil and geosynthetic. The ultimate wide width strength will be reduced and the life time of design will also reduced because of the unpredictability in creep strength, chemical and biological degradation effects, damaged in installation and other part of reduction factor. Same as reinforced embankment, it needs to have a proper preparation, control and inspection to avoid failure during construction (Holtz, 2001).
Reinforced Retaining Walls and Abutments
Traditional earth retaining structures are usually costly, especially for higher walls such as reinforced concrete cantilever or gravity wall. Horizontal layers of geosynthetic can be consisted in retaining structure with backfill. Backfill soil mass can act as a gravity force to resist the earth pressure against the wall face. Some of the retaining structures have also used galvanized reinforcing bar in backfill to reduce the earth pressure (Holtz, 2001).
Figure 2.2 Component parts of a reinforced earth wall.
Geosynthetic reinforced wall design is similar to traditional retaining wall structures, which needs to consider internal stability, external stability and overall stability. Design modes that need to consider for internal stability is the tensile over stress, pullout and internal sliding; external stability is the base sliding, overturning and bearing capacity; overall stability is connection failure, column shear failure and toppling. Other important design includes drainage system and earthquake (Holtz, 2001 & Bathurst, 2007).
Figure 2.3 Design modes for reinforced soil walls
Geosynthetic properties required for design are similar to reinforced slope. Allowable tensile strength and soil geosynthetic friction are required for stability design. Backfill for geosynthetic reinforced walls should be free draining or provide drainage system of filter surface and groundwater. It is because the drainage outward through the wall may not completely in function. For backfill soil that needs to consider include gradation, percent fines, chemical composition, compaction, unit weight, and shear strength. (Holtz, 2001)
Facing systems can be built by full height precast concrete panels, interlocking precast concrete block, geosynthetic face wraps, treated timber facing and wire baskets. During construct the facing wall, it must have a good quality control and careful inspection the wall faces to avoid has an unattractive wall face or wall face failure.
Figure 2.4 Example of reinforced soil wall types.
Analysis and design of geosynthetic reinforced retaining wall
The steps of design for geosynthetic reinforced retaining wall with wrap-around vertical face without any surcharge (Shukla, 2002):
Step 1: Establish the height of the wall, H
Step 2: Determine the properties of the granular backfill soil, such as unit weight (Î³1) and angle of friction ()
Step 3: Determine the properties of the foundation soil, such as unit weight (Î³2) and angle of friction (), and cohesion).
Step 4: Obtain the friction angle of soil-geotextile interface ()
Step 5: Estimate the Rankine earth pressure coefficient
and determine the active pressure distribution on the wall
Step 6: Select a geotextile that has allowable fabric strength of
Step 7: Determine the vertical spacing of the layers at any depth z from:
The magnitude of is generally taken to be 1.3-1.5.
Step 8: Determine the length of geotextile layer,
The magnitude of is generally taken as 1.3-1.5. If experimental value of is not available, it may assumed to be about 2/3
Step 9: Determine the lap length,
The minimum lap length should be 1m.
Step 10: Check for overturning
Calculate the Rankine active force per unit length of the wall as:
The active force acts a distance of measured from the bottom of the wall.
Calculate the overturning moment, , due to the active force as:
Calculate the resisting moment, , due to the weight of the wall as:
The factor of safety against overturning can then be calculated as:
The factor of safety against overturning should be at least 3.
Step 11: Check for sliding
Calculate the horizontal driving force at the bottom of the wall, , as
Calculate the horizontal resisting force along the bottom of the wall as:
Check the factor of safety against sliding as:
Step 12: Check for bearing capacity failure
Calculate the eccentricity due to the forces on the reinforced block,
The eccentricity should be less than
The stress q on the soil below the reinforced block can be given as:
The ultimate bearing capacity of the soil with the eccentric loading can be given as:
Calculate the factor of safety against bearing capacity failure as: