TM 5-818-8/AFJMAN 32-1030
Center sag failure may also occur when low-tensile
using assumed logarithmic spiral or circular fail-
strength or low-modulus geotextiles are used, and
ure surfaces. Another bearing capacity failure is
embankment spreading occurs before adequate
the possibility of lateral squeeze (plastic flow) of
geotextile stresses can be developed to carry the
the underlying soils. Therefore, the lateral stress
dike weight and reduce the stresses on the founda-
and corresponding shear forces developed under
tion. If the foundation capacity is exceeded, then
the embankment should be compared with the
the geotextile must elongate to develop the re-
sum of the resisting passive forces and the product
quired geotextile stress to support the dike weight.
of the shear strength of the soil failure plane area.
Foundation bearing-capacity deformation will oc-
If the overall bearing capacity analysis indicates
cur until either the geotextile fails in tension or
an unsafe condition, stability can be improved by
carries the excess load. Low modulus geotextiles
adding berms or by extending the base of the
generally fail because of excessive foundation dis-
embankment to provide a wide mat, thus spread-
placement that causes these low tensile strength
ing the load to a greater area. These berms or
geotextiles to elongate beyond their ultimate
mats may be reinforced by properly designing
strength. High modulus geotextiles may also fail if
geotextiles to maintain continuity within the em-
their strength is insufficient. This type of failure
bankment to reduce the risk of lateral spreading.
may occur where very steep dikes are constructed,
Wick drains may be used in case of low bearing
and where outside edge anchorage is insufficient.
capacity to consolidate the soil rapidly and achieve
the desired strength. The construction time may
4-3. Recommended Criteria
be expedited by using geotextile reinforcement.
The limit equilibrium analysis is recommended for
b. Slope Stability Analysis. If the overall bear-
design of geotextile-reinforced embankments.
ing capacity of the embankment is determined to
These design procedures are quite similar to con-
be satisfactory, then the rotational failure poten-
ventional bearing capacity or slope stability analy-
tial should be evaluated with conventional limit
sis. Even though the rotational stability analysis
equilibrium slope stability analysis or wedge anal-
assumes that ultimate tensile strength will occur
ysis. The potential failure mode for a circular arc
instantly to resist the active moment, some geotex-
analysis is shown in figure 4-2. The circular arc
tile strain, and consequently embankment dis-
method simply adds the strength of the geotextile
placement, will be necessary to develop tensile
layers to the resistance forces opposing rotational
stress in the geotextile. The amount of movement
sliding because the geotextile must be physically
within the embankment may be limited by the use
torn for the embankment to slide. This analysis
of high tensile modulus geotextiles that exhibit
consists of determining the most critical failure
good soil-geotextile frictional properties. Conven-
surfaces, then adding one or more layers of geotex-
tional slope stability analysis assumes that the
tile at the base of the embankment with sufficient
geotextile reinforcement acts as a horizontal force
to increase the resisting moment. The following
strength at acceptable strain levels to provide the
analytical procedures should be conducted for the
necessary resistance to prevent failure at an ac-
design of a geotextile-reinforced embankment: (1)
ceptable factor of safety. Depending on the nature
overall bearing capacity, (2) edge bearing capacity
of the problem, a wedge-type slope stability analy-
or slope stability, (3) sliding wedge analysis for
sis may be more appropriate. The analysis may be
embankment spreading/splitting, (4) analysis to
conducted by accepted wedge stability methods,
limit geotextile deformation, and (5) determine
where the geotextile is assumed to provide hori-
geotextile strength in a direction transverse to the
zontal resistance to outward wedge sliding and
longitudinal axis of the embankment or the longi-
solving for the tensile strength necessary to give
tudinal direction of the geotextile. In addition,
the desired factor of safety. The critical slip circle
embankment settlements and creep must also be
or potential failure surfaces can be determined by
considered in the overall analysis.
conventional geotechnical limited equilibrium
a. Overall Bearing Capacity. The overall bearing
analysis methods. These methods may be simpli-
capacity of an embankment must be determined
fied by the following assumptions:
whether or not geotextile reinforcement is used. If
(1) Soil shear strength and geotextile tensile
the overall stability of the embankment is not
strength are mobilized simultaneously.
satisfied, then there is no point in reinforcing the
(2) Because of possible tensile crack forma-
embankment. Several bearing capacity procedures
tions in a cohesionless embankment along the
are given in standard foundation engineering text-
critical slip surface, any shear strength developed
books. Bearing capacity analyses follow classical
by the embankment (above the geotextile) should
limiting equilibrium analysis for strip footings,
be neglected.
4-3
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