Geoscience Reference
In-Depth Information
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. The
external 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.
3.2 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.
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