Soil Testing Equipment

Referent and Standards Product for Soil Testing Equipment

GEOTEXTILES, GEOGRIDS, AND GEONETS/GEOCOMPOSITES

Geotextiles are the earliest type of multifunctional geosynthetic material. Their functions include reinforcement, separation, filtration, and drainage. When impregnated, they are used as containment. However, some newly developed products perform better than geotextiles in certain functions. For example, geogrids are developed specifically to tensile reinforce soil, while geonets are used to convey large-capacity flow. Although geotextile may also be made impermeable and used as containment by spraying bitumen or other polymers on it, geomembranes should be considered for a watertight containment system. The functions of geotextiles, geogrids, and geonets are described collectively in this section, where one material can be referred to the other.
Geotextile sheets are manufactured from fibers or yarns. Polymers are melted and forced through a spinneret to form fibers and yarns. They are subsequently hardened and stretched. The manufacturing process produces
woven or nonwoven geotextiles. In producing woven fabrics, conventional textile-weaving methodologies are used. For the nonwoven fabrics, the filaments are bonded together by thermal, chemical, or mechanical means (i.e., heating,using resin, or needle-punching).
Geogrids are mainly used as tensile reinforcement. Although biaxial geogrids are available, most geogrids are manufactured to function uniaxially. In manufacturing uniaxial geogrids, circular holes are punched on the polymer sheet, which is subsequently drawn to improve the mechanical properties. For biaxial geogrids, square holes are made on the polymer sheet, which is then drawn longitudinally and transversely. For some geogrids, the junctions between the longitudinal and transverse ribs are bonded by heating or knit-stitching.
Geogrid manufactured from yarns are typically coated with a polymer, latex, or bitumen. Geogrids have higher stiffness and strength than most geotextiles. 

The chapter now describes some major applications of geotextiles and related products

Reinforcement of Steep Slopes, Retaining Structures, and Embankments

Geotextiles and geogrids are used to tensile reinforce steep slopes, retaining structures, and embankments constructed over soft foundation (Fig. 1). Sheets of geotextile/geogrid are embedded horizontally in these soil structures. The shear stress developed in the soil mass is transferred to the geotextile sheets as tensile force through friction. The tensile strength of geotextile/geogrid and its frictional resistance with the soil are the primary items required for design.

The tensile strength of geosynthetic is obtained from the wide-width test. The ASTM standard specifies an aspect ratio (width-to-length) of 2 (i.e., 20 cm to10 cm). Soil confinement may increase the stiffness and strength of nonwoven spun-bonded needle-punched geotextile because of the interactions among the fibers, but it has negligible effect on the heat-bonded nonwoven geotextiles and woven geotextiles. Reduction factors (also known as partial factors of safety) are applied considering possible strength reduction of geotextiles by installation damage, creep, chemical and biological actions. Geotextiles may degrade by exposure to ultraviolet rays, high temperature, oxidation, and hydrolysis (when the environment is highly alkaline), but the effect is minimized when buried in soils.

The frictional behavior of a geotextile with site-specific soil must be determined by direct shear tests. Although the ASTM standard specifies a direct shear box with dimensions of 30 cm by 30 cm, the box with a plane area of 10 cm by 10 cm would be adequate for geotextiles. Pullout tests have been proposed in the last few decades for determining the anchorage capacity of geosynthetics; such tests are not relevant in determining the design parameters because they are subject to scale and boundary effects. For embankments and dikes constructed over a soft foundation that lacks bearing capacity and global stability, a layer or more of geotextile is laid at the base of the embankment. Vertical wick drains of geosynthetic composites or sand drains may be used to accelerate consolidation of the soft foundation. 

Geotextiles have also been used in conjunction with the underwater sand capping of contaminated submarine sediments. In these applications, the seam strength may dominate the design.

Both geotextiles and geogrids are used to reinforce steep slopes and retaining walls. For applications where large tensile stiffness and strength of reinforcement are required, geogrids should be used. A large shear box is
required to determine the frictional properties of the geogrid because the aperture size is large relative to the geotextile. Unlike geotextiles, where frictional behavior dominates the interaction with soil, the junction of some geogrids may provide interlocking. As geotextiles are very flexible, they are typically wrapped around the face of the slope or retaining wall and protected by vegetation, gunite, timber face, or concrete panels to prevent degradation by ultraviolet rays and vandalism.
Geogrids are increasingly used with modular blocks to provide an aesthetically pleasant wall appearance. As such, the connection between the blocks and geogrids plays an important role in design. The creep and stress relaxation behavior of geogrids are also studied in conjunction with wall design. In the design of reinforced slopes and walls, a limit equilibrium approach is used. The structure is checked for internal and external stabilities. In the internal stability analysis, a failure wedge is postulated and it is tied back into the stable soil zone. An adequate strength and length of reinforcement are secured. Theexternal stability is evaluated in a manner similar to conventional gravity/cantilever wall design. In the external stability analysis, possible modes of failure, such as direct sliding, overturning, and bearing capacity, are evaluated. The seismic design of reinforced slopes and retaining walls has also received wide attention in recent years.

Filter and Drainage Layer

Geotextiles are used to replace granular soil filters in the underdrain, as well as paved and unpaved roads. They are also used as chimney drain in an earth dam and behind retaining walls (Fig. 2). The hydraulic properties are a major consideration in design. The flow rate obtained from the tests is reduced using reduction factors considering soil clogging and blinding, creep reduction of void space, intrusion of adjacent materials into geotextile voids, chemical clogging, and biological clogging.
When functioning as a filter, the geotextile sheet is required to retain the soil while possessing adequate permeability to allow cross-plane flow to occur. The permittivity or permeability and apparent opening size or equivalent opening size of the geotextile are used in design. Permittivity is the coefficient of hydraulic conductivity normalized by the thickness of the geotextile. The filter is also expected to function without clogging throughout the lifetime of the system. The gradient ratio test and long-term flow tests may be used to investigate the clogging potential.

 

Figure 2 Geotextile as drainage layer or filter: (a) chimney drain in earth dam; (b) drain behind retaining wall; (c) underdrain; (d) drainage layer in tunnel.

If the geotextiles (usually nonwoven needle-punched geotextiles) are used as a drainage layer, the in-plane permeability is considered. Because the thickness decreases with increasing normal stress acting on it, the term “transmissivity” is used, where the coefficient of hydraulic conductivity is normalized by the geotextile thickness

Large-Capacity Flow with Geonets/Geocomposites

For drainage applications (such as landfills and surface impoundments), geonets and geocomposites are preferable to geotextiles. These are specifically manufactured to allow for large-capacity flow. Geonets have a parallel set of ribs overlying similar sets at various angles for drainage of fluids. Most geonets are manufactured from polyethylene. They are laminated with geotextiles on one or both surfaces to form drainage geocomposites (Fig. 3). Geonets are mostly manufactured from polyethylene; thus they have high resistance to leachate.

In geonets/geocomposites, the flow is no longer laminar, and thus Darcy’s law is invalid. The flow rate is used in lieu of transmissivity or coefficient of hydraulic conductivity to account for the turbulent flow. Because of the large normal stress acting on the plane of geonet/geocomposites, the crushing strength of the core has to be assessed. Geocomposites are sometimes tested with site-specific soils and liquid. A reduction in the flow rate is expected because of the intrusion of the geotextiles into the core. It is also important to ensure that geotextile sheets, if installed along the slope, do not delaminate from the geonets due to shear stress, because geocomposites are installed at a gradient to allow for gravity flow. The drainage systems of a geocomposite are usually constructed for allowance of cleaning by flushing because they are normally subject to biological
action.


 
Figure 3 Geocomposite.

Separation and Reinforcement in Roadways

In the unpaved roads and railways, geotextile separates the subgrade and stonebase/ballast (Fig. 4). The California bearing ratio (CBR) of the soil subgrade may be used to determine if an unpaved road should be designed for separation or for separation and reinforcement. The intrusion of stone aggregates into the soil
subgrade is prevented by the geotextile in a roadway. In a railway, the fine soil particles are stopped from pumping into the stone aggregates. In addition to tensile strength, other mechanical properties of geotextiles, such as resistance to burst, tear, impact, and puncture, are used for designing geotextiles as a separator.

However, existing practice does not emphasize design when geotextiles are used as a separator compared to reinforcement and drainage applications. For unpaved roadways, the use of geotextile reinforcement results in cost savings because the thickness of stone aggregates may be reduced. In paved roads, the geotextiles may prevent reflective cracking. The geotextile or biaxial geogrids may be placed above the cracked old pavement followed by the asphalt overlays. The life of the overlay is prolonged in the presence of geosynthetic
materials, or a reduced thickness of overlay may be used while keeping the lifetime equivalent to the case without using the geotextile. In addition to preventing reflective cracking, the geosynthetic reinforces the asphalt pavement.
 
Figure 4 Geotextile as separator in unpaved roadway.

Coastal and Environmental Protection
Geotextiles are placed under erosion control structures, such as rock ripraps and precast concrete blocks (Fig. 5a). They are also used as silt fences at construction sites so that the soil particles are arrested from the runoff water. Geotextiles are also used as geocontainers on land or underwater as storage for slurry and for coastal protection. On land, the dredged materials or sands are pumped under pressure into sewn geotextile sheets. The geotextile inflates to form a tube (Fig. 5b). Geotextile tubes are extremely effective in dewatering the high-water-content slurry/sludge by acting as a filter. The geotextile tube may also be used as an alternative to dike and coastal protection. In such applications, the strength and filter characteristics of the geotextile are important design criteria.

Geocontainers are used for the disposal of potentially hazardous dredged materials and offer a more environmental-friendly means of disposing dredged materials offshore. The geotextile sheets are laid at the bottom of dump barges, filled with dredged sediments, and sewn. The containers are then transported to
the disposal site and dumped via a split hull barge.


5 comments:

Unknown said...

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