consider an FS of 1.3 against rotational slope

A second bearing-capacity consideration is the

failure, 1.5 against spreading, 2.0 against sliding

chance of soft foundation material squeezing out.

failure, and 1.3 against excessive rotational dis-

Therefore, the lateral stress and corresponding

placement for the geotextile fabric requirements.

shear forces below the embankment, with respect

Determine minimum geotextile requirements.

to resisting passive forces and shear strength of

soil, are determined.

(1) Ultimate bearing capacity qult for strip

a. Plastic flow method for overall squeeze-

footing on clay.

squeeze between two plates.

= (75)(5.14) = 385 pounds per

(eq 4-8)

square foot (with

surface crust)

= (75)(3.5) = 263 pounds per

where

square foot (without

c = cohesion (shear strength) of soil

surface crust)

a = distance between embankment and

next higher strength foundation soil layer

are standard values for φ =

Values shown for

L = width of embankment slope

0. It has been found from experience that excessive

mud wave formation is minimized when a dried

For the conditions in previous example:

crust has formed on the ground surface.

( )

( 7 0 0 ) 14

2

= lOO(7) = 700 pounds per square

140 + 12

foot

= 32.2

not sufficient for an unreinforced embankment,

Cohesion available is 75 pounds per square foot,

but for a geotextile-reinforced embankment, the

which is greater than 32.2 pounds per square foot

lower portion of its base will act like a mat

required and is therefore satisfactory.

foundation, thus distributing the load uniformly

over the entire embankment width. Then, the

common problem that requires investigating.

average vertical applied stress is:

Therefore, the passive resistance for toe squeeze is

as follows:

(eq 4-9)

(eq 4-10)

Then, the difference:

(eq 4-11)

(eq 4-12)

For the example:

where L = width of embankment slope. If a dried

crust is available on the soft foundation surface,

then the FS is about 1. If no surface crust is

is greater than ; therefore, foundation

available, the FS is less than 1.0, and the embank-

squeeze may occur. Solutions would be to either

ment slopes or crest height would have to be

allow squeezing to occur or construct shallow

modified. Since the embankment is very wide and

berms to stabilize the embankment toe or use

the soft clay layer is located at a shallow depth,

plastic strip drains.

failure is not likely because the bearing-capacity

analysis assumes a uniform soil twice the depth of

bility analysis to determine the required geotextile

the embankment width.

tensile strength and modulus to provide an FS of

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