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In Reinforced Soil Its Not Only About Tensile Strength Geosynthetic reinforcement is used to provide tensile
strength to a system that otherwise must rely on compressive
shear strength. The reinforcement must sufficiently interact
with the surrounding materials in order to impart its stabilizing
tensile strength to the system. Pullout
resistance, interface friction, and connection strength characteristics
of each unique material combination (i.e. soil-geosynthetic,
block-geosynthetic, etc.) must be measured to assure that the
chosen geosynthetic doesn’t cause a “slip-up” on the job. Interacting
with Surrounding Materials Geosynthetic
reinforcements (geotextiles and geogrids) must interact with soil
and other materials in fills to prevent slippage. This interaction
can be in the form of surface friction or a combination of surface
friction and bearing resistance. Slippage can cause failure of a reinforced
soil system (walls, slopes or embankments), if it results in a
plane of reduced strength. Interaction may be resistance to pullout,
interface friction, or the connection of a facing material. Pullout
Resistance A “block” of reinforced soil is anchored
to a stable soil mass beyond the most likely plane of failure
by extending the geosynthetic reinforcement into the stable soil
mass. The reinforcement is extended as far as is
required to provide sufficient pullout resistance to prevent
movement at the likely failure plane. Pullout
resistance is of most concern for the uppermost reinforcement
layers. Pullout resistance can be derived from interface friction
tests or measured directly from tests that pull the geosynthetic
out of representative soil. Interface Friction
One
form of failure within a reinforced soil zone is horizontal sliding
of a block of reinforced soil. Soil-to-soil friction is almost always higher
than soil-to-geosynthetic interaction and the critical failure
conditions that must be investigated will commonly involve soil-geosynthetic
interface friction, also known as direct shear. Direct shear relates
to the type of test that is employed to accurately determine the
interaction for a given soil and geosynthetic combination. Connection
Strength Reinforced soils are a cost-effective technique for
creating steep to near-vertical structures. As
the structure becomes steeper, its surface becomes more problematic
since it becomes more visually important and more difficult to
maintain. Facing systems have emerged to provide good
aesthetics with a minimum amount of maintenance. Masonry blocks, concrete panels, gabions, and other materials for
the external face of the reinforced zone have been developed. In these structures, the geosynthetic extends
from the reinforced soil zone to these facing systems, connecting
to them and holding them close to the soil surface. Groundwater seeping through the reinforced soil zone or localized
block sliding may exert pressure on the back of the facing units,
stressing the connections. Each
facing material has a unique interaction, or connection strength,
with each type of geosynthetic. This
connection strength has two parts. First,
connection pullout is determined by measuring the force required
to pull the geosynthetic out from between representative facing
units. Additionally, connection shear measures the force required
to cause one facing unit to slide over the top of another both
with and without a geosynthetic “connected”. Interface Friction / Direct
Shear. Soil/geosynthetic
direct shear resistance is determined by laboratory testing in
accordance with ASTM D5321. TRI
has five direct shear apparatus and performs hundreds of soil/geosynthetic
and geosynthetic interface tests annually. Pullout Resistance
The pullout resistance and associated interaction
coefficient (Ci) are used in stability analyses to compute the
mobilized tensile force at the front and tail of each reinforcement
layer in reinforced soil systems. TRI
is installing a state-of-the-art pullout box to fully characterize
pullout performance. Pullout testing is expected to be fully operational
by the end of the 1st Quarter
of 2000. Additionally,
as pullout resistance can also be calculated from direct shear
data, we will continue to estimate direct shear-based characterization
of pullout. Connection Strength Coincident
with the new pullout apparatus, TRI is installing the most advanced
connection strength testing apparatus available. Computerized controls will assure flexible
load application, uniform normal stress distribution, and highly
accurate displacement measurements. Connection
testing is also expected to be fully operational by the end of
the 1st Quarter of 2000. Scope of Tests: Pullout Testing: Each test will consist
of 1 geosynthetic and 1 soil at 1 normal load. Connection Pullout Testing: Each test will consist
of 1 geosynthetic, 1 infill soil, 1 block type, and 1 normal
load range (3 load levels). Connection
Shear Testing: Each
test will consist of 1 geosynthetic, 1 infill soil, 1 block type,
and 1 normal load range (3 load levels).
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