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

be expedited by using geotextile reinforcement.

The limit equilibrium analysis is recommended for

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

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.

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