National Institute of Technology Hamirpur Department of Civil

National Institute of Technology, Hamirpur Department of Civil Engineering CE-472(c) Geosynthetics Rakesh Kumar Dutta, Ph. D. Associate Professor

Shallow foundations

The geosynthetic-reinforced foundation soils are being used to support footings of many structures including warehouses, oil drilling platforms, platforms of heavy industrial equipments, parking areas, and bridge abutments. In usual construction practice, one or more layers of geosynthetic are placed inside a controlled granular fill beneath the footings. Such reinforced foundation soils provide improved load-bearing capacity and reduced settlements by distributing the imposed loads over a wider area of weak subsoil. In the conventional construction techniques without any use of the reinforcement, a thick granular layer is needed which may be costly or may not be possible, especially in the sites of limited availability of good-quality granular materials.

Geosynthetic products like ‘Paralink’ as shown in Figure (a) can be very effective for use over soft foundation soils as well as over voids and piles (Fig. (b)).

The ideal reinforcing pattern has geosynthetic layers placed horizontally below the footing, which becomes progressively steeper farther from the footing (Fig. (a)). It means that the reinforcement should be placed in the direction of the major principal strain. However, for practical simplicity, geosynthetic sheets are often laid horizontally as shown in Figure (b).

Filters and Drains

• The role of groundwater flow and good drainage in the stability of pavements, foundations, retaining walls, slopes, and wastecontainment systems is gaining attention from engineers, practitioners, and researchers alike. That is why geosynthetics are being increasingly employed either as filters, in the form of geotextiles (nonwovens and lightweight wovens), in conjunction with granular materials and/or pipes (Fig. (a)), or as both filters and drains in the form of geocomposites (Fig. (b)). Filters also form an essential part of many types of hydraulic structures. Thus, there are several application areas for filters and drains including buried drains as pavement edge drains/underdrains, seepage water transmission systems in pavement base course layers and railway tracks, abutments and retaining wall drainage systems, slope drainage, erosion control systems, landfill leachate collection systems, drains to accelerate consolidation of soft foundation soils, drainage blanket to dissipate the excess pore pressure beneath embankments and within the dams and silt fences/barriers.

A filter consists of any porous material that has openings small enough to prevent movement of soil into the drain and that is sufficiently pervious to offer little resistance to seepage. When a geosynthetic is used as a filter in drainage applications, it prevents upstream soils from entering adjacent granular layers or subsurface drains. When properly designed, the geosynthetic filter promotes the unimpeded flow of water by preventing the unacceptable movement of fines into the drain, which can reduce the performance of the drain. Geosynthetic filters are being used successfully to replace conventional graded granular filters in several drainage applications. In fact, filter structures can be realized by using granular materials (i. e. crushed stone) or geotextiles or a combination of these materials (Fig. ). The choice between the graded granular filter or geotextile filter depends on several factors.

When using riprap–geotextile filter, it is recommended that a layer of aggregate be placed between the geotextile and the riprap, for the following reasons (Giroud, 1992): • to prevent damage of the geotextile by the large rocks • to prevent geotextile degradation by light passing between large rocks • to apply a uniform pressure on the geotextile, thereby ensuring close contact between the geotextile filter and the sloping ground, which is necessary to ensure proper filtration • to prevent geotextile movement between the rocks because of wave action, thereby ensuring permanent contact between the geotextile filter and the sloping ground, which is also necessary to ensure proper filtration.

Vertical strip drains (also called prefabricated vertical band drains (PVD) or wick drains) are geocomposites used for land reclamation or for stabilization of soft ground. They accelerate the consolidation process by reducing the time required for the dissipation of excess pore water pressure. The efficiency of the drains is partly controlled by the transmissivity, that is discharge capacity that can be measured, using the drain tester, to check their short-term and long-term performance. The discharge capacity of drains is affected by several factors such as confining pressure, hydraulic gradient, length of specimen, stiffness of filter and the duration of loading. The experimental study, conducted in the laboratory by Broms et al. (1994), suggests that the effect of the length of the drains and the duration of loading on the discharge capacity of the drain is small, whereas the stiffness of the filter of drain can have a considerable effect. The discharge capacity of the drain decreases with decreasing stiffness of the filter.

In filter applications, the design must be prepared so as to avoid, throughout the design life, the following three phenomena causing decrease of the permeability of the geotextile filter in course of time: 1 blocking 2 blinding 3 clogging.

Erosion Control

The problem of soil movement due to erosive forces by moving water and/or wind as well as by seeping water is called soil erosion. Gravity is also one of the prime agents of soil erosion, particularly on steep slopes. Soil erosion is associated with negative economic and environmental consequences in many areas such as agriculture, river and coastal engineering, highway engineering, slope engineering and some more sections of civil engineering.

Construction sites with unvegetated steep slopes are prime targets for soil erosion. Soil erosion by moving water is caused by two mechanisms: (1) detachment of particles due to raindrop impact and (2) movement of particles from surface water flow. The dislodged particles carry with them seeds and soil nutrients. Natural growth of vegetation on the exposed soil slope surface is thus hindered. High velocity runoff can cause not only surface soil movement downslope, but their scouring effects can cause total undermining of slopes. Rain erosion can act upon a land surface of any degree of slope; however, the severity of rain erosion increases with increasing slope steepness and slope length.

The exposed denuded slopes become increasingly vulnerable to erosion agents and are ultimately destabilized. To control erosion is to curb or restrain the gradual or sudden wearing away of soils by wind and moving water. The goal of any erosion control project should be to stabilize soils and manage erosion in an economical manner. Since surface water flow cannot be eliminated, the most feasible solution to erosion problems is slope protection. The slope protection serves two functions: (1) it slows down the surface water flow and (2) it holds soil particles, grass or seedlings in place. If an element is incorporated into the soil to prevent the detachment and transportation of soil particles, then the slope would be able to withstand greater forces.

The solutions of soil erosion problems typically involve the use of basic erosion control techniques such as soil cover and soil retention. The use of revetments is very common in civil engineering practice for erosion control (Fig. ). A cover layer (called armour) of a revetment can be permeable or impermeable. An open cover layer substantially reduces the uplift pressures, which can be induced in the sublayers and provides protection against the external loads. Riprap, blocks and block mats, grouted stones, gabions and mattresses, and concrete and asphalt slabs are most commonly used as revetment armours.

Three-dimensional erosion control geosynthetic mats and geocells that are nowadays commercially available with various dimensions can be used in permanent erosion control systems. Geocells are threedimensional honeycomb structures that have a unique cellular confinement system formed by a series of self-containing cells up to 20 cm deep. They have the ability to physically confine the soil placed inside the cells (Fig. ). They retain soil, moisture and seed, and thus create situations for the growth of vegetative mats on slopes where vegetation may be difficult to establish. The vegetative mats provide reinforcement and the system’s cells increase the natural resistance of these mats to erosive forces and protect the root zone from soil loss. At the same time, the cellular confinement system facilitates slope drainage.

The most common and natural element used for erosion control is vegetation. Roots of the grasses protect the slope surface from erosion. The deeper roots of plants, shrubs and trees tend to reinforce and stabilize the deeper soils. The application of vegetation as bank protection is preferred rather than the application of conventional materials such as riprap, concrete blocks, etc. If necessary, vegetation and appropriate geosynthetics can be applied in combination (Fig. ). The selection of vegetation must be done on the basis of soil and climatic conditions of the specific area of application. The vegetation will on the one hand stabilize the body of the channel, consolidate the soil mass of the slope and bed and reduce erosion. On the other hand, the presence of vegetation will result in extra turbulence and retardation of flow. Geotextiles and other perforated geosynthetics and open blocks provide additional strength to the root mat and can reduce much of the direct mechanical disturbance to plants and soil.

Geotextiles are also used in toe and bed protection, which consists of the armouring of the beach or bottom surface in front of a structure to prevent scouring and undercutting by water waves and currents (Fig. ). The stability of toe is essential, because its failure will generally lead to failure of the entire structure.

In many cases, geotextile is used to wrap a fill material (sand, gravel, asphalt or mortar), creating geobags, geotubes or geomats, known collectively as geocontainers, which are used in hydraulic and coastal engineering (Fig. ).

Ponds, Reservoirs, and Canals

Liquid containment and conveyance facilities, such as ponds, reservoirs and canals, are required in several areas including hydraulic, irrigation and environmental engineering. Unlined ponds, reservoirs, and canals can lose 20– 50% of their water to seepage. Traditionally, soil, cement, concrete, masonry or other stiff materials have been used for lining ponds, reservoirs and canals. The effectiveness and longevity of such materials are generally limited due to cracking, settlement and erosion. Sometimes the traditional materials may be unavailable or unsuitable due to construction site limitations, and they may also be costly.

Flexible geosynthetic lining materials, such as geomembranes, have been gaining popularity as the most cost-effective lining solution alone or in combination with conventional lining material for a number of applications, including irrigation and potable water. Figure below shows typical schematics of liquid containment and conveyance facilities (ponds, reservoirs, and canals) involving application of geosynthetics in addition to conventional materials.

Geosynthetic liner/barrier materials can be classified as GMBs, GCLs, thin-film geotextiles composites or asphalt cement-impregnated geotextiles. The selection of lining material is governed by the location and environmental factors. Placement, handling and soil covering operations can also affect geosynthetic selection. When GMBs are used as lining material, geotextiles can be used with GMBs for their protection against puncture by the granular protective layer, which may also be required to prevent UV- and infrared-induced ageing of geosynthetics, as well as any effects of vandalism and burrowing animals. A geotextile, if used below the GMB liner, can function as a protection layer as well as a drainage medium for the rapid removal of leaked water, if any. For economical reasons, the GMB liner may be left uncovered.

Earth Dams

Earth dams are water impounding massive structures and are normally constructed using locally available soils and rocks. One of the principal advantages of earth dams is that their construction is very economical compared to the construction costs of concrete dams. Apart from the conventional materials used in the earth dam, geosynthetics are being employed in recent times for new dam constructions and for the rehabilitation of the older dams. Properly designed and correctly installed geosynthetics, in an earth dam, contribute to increase in its safety which corresponds to a positive environmental impact on dam structures (Singh and Shukla, 2002).

The reasons for which geosynthetics are used extensively in earth dam construction and rehabilitation are the following: • The use of geosynthetics in earth dams may serve several functions: water barrier, drainage, filtration, protection and reinforcement. • The geosynthetics are soft and flexible – therefore, they can endure some elasto-plastic deformations resulting from the subsidence, expansion, landslide and seepage of soil. • The geosynthetics (geotextiles and geogrids) possess certain mechanical strength, which is favourable as dam-filling materials. • The permeability of geomembranes is much lower than that of clay or concrete.

The long-term performance of various components of an earth dam is critical to the performance of the dam as a whole. If a geotextile is to be used as a filter, careful assessment of the properties, extensive testing and monitoring are required to ensure its suitability. The locations in earth dams where geotextile filters may be used are in the downstream chimney drain and in the downstream drainage blanket (see Fig. ). If the dam is subjected to rapid drawdown, then drainage systems using geosynthetics may also be installed on the upstream side of the core. In the past, geotextile filters, mostly nonwovens, have been used for the construction or the rehabilitation of numerous embankment dams (i. e. earth or earth and rockfill dams) in various parts of the world.

Tunnels

Tunnels are used for various purposes in civil engineering, including traffic movement and fluid flow. Waterproof tunnels are required at some sections of the highway and railway alignments. A crack-free concrete lining is needed for a waterproof tunnel. Geotextiles and geomembranes are commonly used in modern-day tunnel technology to construct waterproof tunnels.

Figure below shows the cross-section of a tunnel vault with the general arrangement of the lining system. The shotcrete lining placed over the excavated surface provides a smooth surface for the geosynthetics. In addition the rock surface is supported by the shotcrete immediately after excavation so that the radially acting forces can be accepted adhesively (Wagner and Hinkel, 1987). The nonwoven geotextile (generally needlepunched) acts as a drainage layer and as protection for a waterproofing geomembrane. It also acts as a cushion (stressrelieving layer) to significantly reduce the formation of cracks in the inner concrete lining by allowing free shrinkage deformation of the concrete during the setting process.

It should be noted that geomembrane sheet sealing with a protective nonwoven geotextiles drainage layer has predominated over the conventional sealing methods such as asphalt membranes or spray applied glass fibrereinforced plastic or bitumen-latex based products. The geosynthetic system not only meets the demands of the rapid tunnelling rates but also the demands for rough construction treatment.

Stabilization

Slopes can be natural or man-made. Several natural and man-made factors, which have been identified as the cause of instability to slopes, are well known to the civil engineering community. Many of the problems of the stability of natural slopes are radically different from those of man-made slopes mainly in terms of 1. The nature of soil materials involved 2. The environmental conditions, location of groundwater level 3. The stress history In man-made slopes, there also essential differences between cuts and embankments. The latter are structures which are built with relatively well-controlled materials.

In cuts, however, this possibility does not exist. The failures of slopes, called landslides, may result in loss of property and lives and create inconvenience in several forms to our normal activities. Several slope stabilization methods are available to improve the stability of unstable slopes. The slope stabilization methods generally reduce driving forces, increase resisting forces, or both. The advent of geosynthetic reinforcement materials has brought a new dimension of efficiency to stabilize the unstable and failed slopes by constructing various forms of structures such as reinforced slopes, retaining walls, etc. mainly due to their corrosive resistance and long-term stability. In recent years geosynthetic-reinforced slopes have provided innovative and cost-effective solutions to slope stabilization problems, particularly after a slope failure has occurred or if a steeper than safe unreinforced slope is desirable. They provide a wide array of design advantages as mentioned below:

• • • Reduce land requirement to facilitate a change in grade Provide additional usable area at toe or crest of slope Use available on-site soil to balance earthwork quantities Eliminate import costs of select fill or export costs of unsuitable fill Meet steep changes in grade, without the expense of retaining walls Eliminate concrete face treatments, when not required for surficial stability or erosion control Provide a natural vegetated face treatment for environmentally sensitive areas Provide noise abatement for high traffic areas and minimize vandalism Offer a design that is easily adjustable for surcharge loadings from buildings and vehicles.

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