\$\$257.00A

######### Section 257.---ALTERNATE RETAINING WALLS

Revised 16 May 2005

*[INCLUDE THE FOLLOWING FOR ALTERNATE RETAINING WALLS]{.mark}*

\$\$257.04A

257.04**(i)**. Add the following:

> **Design Criteria.** Consider the internal and the external stability
> of the wall. For internal stability, the wall system must be coherent
> and self‑supporting mass under the action of its own weight and any
> externally applied forces. For external stability the wall system must
> be stable against sliding, overturning, foundation failure, and
> overall slope failure including failure of temporary construction
> slopes.

The CO will provide the angle of internal friction used for the
computations of earth pressures in the reinforced and retained soil
backfill.

The CO may reject any proposed wall alternate should the alternate
present unreasonable slope stability risks at any individual site.

\$\$257.04B

Add the following after Subsection 257.04:

**257.04A Mechanically Stabilized Embankment Walls.** Construct
mechanically stabilized embankment walls using either or both extensible
and inextensible reinforcements conforming to Section 255 and to the
following design criteria:

> **(a) Structure Dimensions.** Dimension structures to satisfy the
> following minimum Factors of Safety:
>
> **(1)** Sliding \> 1.5
>
> **(2)** Overturning \> 2.0
>
> **(3)** Bearing Capacity \> 2.5
>
> **(4)** Pullout Resistance \> 1.5
>
> **(5)** Overall Stability \> 1.3 (1.2 for temporary construction
> slopes)

Design the minimum reinforcement length to be 70 percent of the wall
height (as measured from the back of the wall) and not less than 8 feet
[(2.4 meters)]{.mark} for wall heights up to 20 feet [(6.1
meters)]{.mark}. For wall heights over 20 feet [(6.1 meters)]{.mark},
design the minimum reinforcement length to be 70 percent of the wall
height (as measured from the back of the wall) and not less than 15 feet
[(4.6 meters)]{.mark}, unless substantiating evidence is presented to
indicate that shorter lengths are satisfactory. Consider rock intrusion
into the design of the embedment length design. Include a minimum
embedment length of 70 percent of the wall height (as measured from the
back of the wall) if the design embedment length cannot be reached
because of bedrock intrusion. In addition, the reinforcement length must
be sufficient to satisfy all external and internal stability
considerations.

> **(b) Wall Embedment Depth.** Design structures for the following
> minimum embedment unless constructed on rock foundations. The minimum
> embedment is determined as a function of the height of structure (H)
> above the base of the wall.

  ------------------------ ------------ ------- -------------------------------
  [Slope in Front of                            [Minimum Embedment]{.underline}
  Structure]{.underline}                        

  Horizontal               walls                H/20

  3H:1V                    walls                H/10

  2H:1V                    walls                H/7

  3H:2V                    walls                H/5
  ------------------------ ------------ ------- -------------------------------

Design the minimum embedment depth to the top of leveling pad to be 2
feet [(0.6 meters)]{.mark} or below the prevailing depth of frost
penetration.

[]{.mark}

*[INSERT THE FIGURE NUMBERS AND EARTH PRESSURE DIAGRAMS FROM
GEOTECH]{.mark}*

> **(c) External Stability Computations.** Assume the stabilized earth
> mass to be a rigid body for stability computations. Compute the
> coefficient of active earth pressure (K~a~), used to compute the
> horizontal force resulting from the random backfill and other loads,
> based on the friction angle of the random backfill.

Compute the active earth pressure coefficients for random backfill as
follows:

$\text{Ka} = \text{cos}B\left\lbrack \frac{\text{cos}B - \sqrt{(\text{cos}^{2}B - \text{cos}^{2}\phi}}{\text{cos}B + \sqrt{(\text{cos}^{2}B - \text{cos}^{2}\phi}} \right\rbrack$

> Where: *B* = Slope angle above wall

= Internal angle of friction of random backfill

> Figures *[[(insert figure number)]{.underline}]{.mark}* and *[[(insert
> figure number)]{.underline}]{.mark}* illustrate external stability
> equations for walls with horizontal backslope and inclined backslope
> respectively. The resultant force acting on the base lies within the
> middle third of the base width.
>
> **(d) Internal Stability.** Perform internal stability analyses to
> verify that the reinforcing material is not over-stressed and that the
> reinforcement embedment length beyond the failure surface is
> sufficient to prevent pullout.
>
> **(1) Reinforcement Tension.** Use the tie-back wedge method of
> analysis for analysis of retaining walls reinforced with extensible
> reinforcement.
>
> The required reinforcement tension to prevent rupture, T~r~,
> calculated using the tie-back wedge method, is less than or equal to
> the allowable reinforcement tension, T~a~, determined as outlined in
> Subsection 257.04A(f).
>
> **(2) Reinforcement Length.** Design the reinforcement length to be
> the longer of the length required for external stability or internal
> stability. Design the length required for external stability according
> to Subsection 257.04A(c). Determine the length required for internal
> stability as follows:
>
> At each reinforcement level, design the reinforcement length for a
> minimum factor of safety of 1.5 against pullout failure (i.e., at each
> reinforcement level the pullout resistance, T~p~, is at least 1.5
> times the required reinforcement tension, T~r~). Use only the
> effective pullout length extending beyond the Rankine failure surface
> into the resistant zone in the computation of the required pullout
> resistance.

The minimum reinforcement length in the resistant zone shall be 3 feet
[(0.9 meters)]{.mark}. The total reinforcement length at any level is
equal to the sum of the lengths in the active and resistant zones.
Provide uniform reinforcement length throughout any section of the
structure.

> **(e) Reinforcement Pullout Resistance.** Evaluate the pullout
> resistance from the results of pullout tests. The use of direct shear
> tests to determine the pullout resistance of reinforcement is not
> acceptable. Base pullout resistance for design on a maximum elongation
> of the embedded reinforcement of ¾ inch [(19 millimeters)]{.mark} as
> measured at the leading edge of the compressive zone within the soil
> mass and not the ultimate pullout capacity.
>
> For geogrids, where insufficient data exists to evaluate pullout
> resistance as a function of soil type, pullout tests will be required
> on a project specific basis until such time that the engineering
> behavior of the soil‑reinforcement system is clearly defined. Perform
> the pullout tests using vertical stress (~v~) variations and
> reinforcement element configurations simulating the actual project
> conditions.
>
> Perform the pullout tests on samples with a minimum embedded length of
> 2 feet [(0.6 meters)]{.mark}. Perform the tests on samples with a
> minimum width of 1 foot [(0.3 meters)]{.mark} or a width equal to a
> 4 longitudinal reinforcement element, whichever is greater. Conduct
> tests at 70±2 ^o^F [(21±1 ^o^C)]{.mark} at constant strain rates of
> 0.02 inches [(0.5 millimeters)]{.mark} per minute.

Evaluate the pullout resistance for geogrids by the following relation:

T~p~ = 2 tan ~v~ A~s~ fd

where:

T~p~ = Ultimate pullout capacity of tensile reinforcement (pounds
[(kilograms)]{.mark})

~v~ = Vertical stress (pounds per square foot [(kilograms per square
meter)]{.mark})

A~s~ = Total plan area of geogrid beyond failure plans (square feet
[(square meters)]{.mark})

= Internal angle of friction of backfill

fd = Equivalent coefficient of direct sliding derived from pullout tests

The equivalent coefficient of direct sliding is a function of the open
area of the grid. For preliminary analysis estimate from the following
in the absence of product specific data tested with site specific
granular backfills:

+-----------+-----------------------+--------------+--------------------+
| [% Open   |                       | Equivalent   |                    |
| Area of   |                       | Coefficient  |                    |
| Grid]{.u  |                       |              |                    |
| nderline} |                       | [of Direct   |                    |
|           |                       | Sliding]     |                    |
|           |                       | {.underline} |                    |
+-----------+-----------------------+--------------+--------------------+
|           | 80 or more            |              | 0.5                |
+-----------+-----------------------+--------------+--------------------+
|           | 51 to 79              |              | 0.7                |
+-----------+-----------------------+--------------+--------------------+
|           | 50 or less            |              | 0.6                |
+-----------+-----------------------+--------------+--------------------+

> **(f) Reinforcement Properties and Allowable Stress.**
>
> **(1) General.** The long‑term tension‑strain characteristics of
> extensible reinforcements are influenced by:
>
> *(a)* Long‑term polymer creep,
>
> *(b)* Long‑term polymer degradation, and
>
> *(c)* Construction induced reinforcement damage as well as variations
> in the manufacturing process.

Account for and verify these factors by testing to determine the
allowable long‑term reinforcement tensions for design.

> **(2) Creep.** Determine the long-term tension-strain-time behavior of
> polymeric reinforcement from results of controlled laboratory creep
> tests conducted for a minimum duration of 10,000 hours for a range of
> load levels on samples of the finished product to be used on the
> project. Test samples in the direction in which the load will be
> applied in use in either a confined or unconfined mode. Extrapolate
> results using a minimum design life of 100 years.

Determine the following from these tests:

> *(a)* The highest tension level at which the accumulative creep
> strain-rate continues to decrease with Log-time within the required
> design lifetime and which would preclude brittle or ductile failures.
> Term this value of load the limit state reinforcement tensile load and
> designate as T~1~.
>
> *(b)* The tension level at which total strain is not expected to
> exceed 5 percent within the design lifetime. Term this value of load
> the serviceability state reinforcement tension, and designate as T~w~.
>
> **(3) Polymer Durability.** Perform product specific studies to assess
> the effects on the reinforcement\'s durability in differing
> environments. Include the effect on short‑term and long‑term
> mechanical properties as well as changes to the reinforcement
> microstructure, dimensional changes, changes in mass, oxidation,
> environmental stress cracking, hydrolysis, temperature, and
> plasticization in the studies. Provide the test results for
> verification of product durability. Investigate the full range of soil
> environments to which the reinforcements may be potentially exposed
> and the site specific environment. As a minimum include the following:

(a) The pH in the range of 3 to 9 - ASTM D 2165;

(b) Diesel oil - ASTM D 975;

(c) Fungi and Bacteria;

(d) U.V. exposure 500 hours. - ASTM D 4355;

(e) Solvents and agents that are site specific.

> Incorporate the results from these studies into durability reduction
> factors, (FD) and apply to the limit state tensile load T~1~.
> Extrapolate results from short-term tests to the required design life
> of 100 years.

In the absence of site specific product durability information, take FD
as 2.0. In no case shall FD be less than 1.10.

> **(4) Construction Induced Damage.** Determine the effect of
> construction damage on the reinforcement from the results of
> full‑scale construction damage tests on representative fill materials
> to be used in the construction of the reinforced backfill and
> according to construction procedures. Incorporate the effect of
> construction damage tests into the construction damage factor (FC)
> that is applied to the limit and serviceability state reinforcement
> tensions. In the absence of product specific construction damage tests
> on site specific materials, take FC as 3.0. Substantiate lower values
> by construction damage tests for the selected product with the
> specific backfill source from the project.
>
> **(5) Allowable Reinforcement Tension.** The allowable reinforcement
> tension T~a~, will be the lesser of the following 2 determinations:
>
> *(a)* Limit state determination. The allowable long-term reinforcement
> tension based on a limit state criterion is given by:

$T_{\text{al}} = \frac{T_{1}}{\text{FD} \times \text{FC} \times \text{FS}}$

where: FS is an overall factor of safety to account for uncertainties in
structure geometry, fill properties, reinforcement properties, and
externally applied loads. For permanent, vertically faced mechanically
stabilized earth walls the minimum FS shall be 1.5 for overall
stability.

> *(b)* Serviceability state determination. The allowable long-term
> reinforcement tension based on a serviceability state criterion is
> given by:

$T_{\text{as}} = \frac{T_{w}}{\text{FC} \times \text{FD}}$

> For the serviceability state determination no overall factor of safety
> is required.
>
> The allowable reinforcement tension, T~a~, determined in this section
> shall be greater than the required reinforcement tension, T~r~,
> calculated in Subsection 257.04A(d).
>
> **(6) Geogrid Junction Strength.** For geogrids the summation of the
> shear strengths of the joints occurring in a 1-foot
> [(0.3-meter)]{.mark} length of the grid sample shall be greater than
> the ultimate tensile strength of the grid element to which they are
> attached. Otherwise, reduce the allowable reinforcement tensions in
> Subsection 257.04A(f)(5) by the ratio of the shear strengths to the
> ultimate strength. Determine the ultimate tensile strength from the
> wide width tensile test, ASTM D 4595, and translate into an ultimate
> strength per element by dividing by the number of elements per foot
> [(meter)]{.mark} of width. Junction strength may be measured in
> accordance with Geosynthetic Research Institute (G.R.I.)
> Method GG-2-87.
>
> **(g) Backfill Requirements.** Furnish backfill for embankments using
> inextensible metallic reinforcements meeting Subsection 255.06.

Furnish backfill for extensible reinforcements that has a nominal
maximum particle size of 4 inches [(10 centimeters)]{.mark}, be
reasonably free from organic or other deleterious materials, and have a
pH between 3 and 9. The shear strength parameters used for the
computations of earth pressures in the reinforced and retained soil
backfill will be provided on the plans.

> **(h) Facing Connection Design.** Base the size of all structural
> facing elements so the design load stresses do not exceed the
> allowable stresses found in the AASHTO Standard Specifications for
> Highway Bridges. Design extensible reinforcement connections to the
> facing to carry 100 percent of the maximum design tensile load at all
> levels. Load test representative section of connection type with
> vertical stresses, ranging from minimum to maximum, to determine the
> allowable connection working load. The reinforcement allowable
> strength may be no greater than the connection strength.

Conduct the load-test on samples at least 8 inches [(20
centimeters)]{.mark} wide and at a rate of elongation not exceeding
0.5 inches [(14 millimeters)]{.mark} per minute.

> **(i) Corrosion Life.** The corrosion life of any metal components
> which are exposed to soil shall be 100 years. Submit calculations
> concerning corrosion life to the CO for approval.
>
> **(j) Miscellaneous.** If the wall designer/supplier needs additional
> information to complete its design, obtain the information. Calculate
> the maximum bearing pressure under the stabilized mass using the
> Meyerhoff formula and clearly indicate on the plans. Assume passive
> pressure in front of the wall mass to be 0 for design purposes.

Account for all appurtenances behind, in front of, under, mounted upon,
or passing through the wall (such as the drainage structures, or other
items shown on the plans) in the stability design of the wall.

\$\$257.04C

**257.04B Permanent Anchored Walls.** Design permanent anchored walls to
conform to Sections 256, 551, 557, 601, and the following design
criteria:

> **(a)** Design rock and soil anchors to conform to Section 256.
>
> **(b)** Design lagging by sound engineering principles.
>
> **(c)** Furnish a 1-foot [(0.3-meter)]{.mark} reinforced concrete
> facing, designed according to AASHTO Standard and Interim
> Specifications for Highway Bridges. Use Class A concrete, tested
> according to Subsection 552.19(a). Extend the facing 2 feet [(0.6
> meters)]{.mark} below the gutter line or, if applicable, the ground
> line adjacent to the wall. Finish concrete according to Subsection
> 552.16(a).
>
> **(d)** Design soldier piles for shear, bending, and axial stresses.
> Design according to the latest AASHTO design criteria. Soldier piles
> may be steel or concrete with a minimum yield strength for steel of
> 50,000 pounds per square inch [(345 MegaPascals)]{.mark} and the
> minimum compressive strength of concrete of 3,000 pounds per square
> inch [(20.7 MegaPascals)]{.mark}, 28-day strength.
>
> **(e)** Design permanent drainage systems behind the wall. Connect
> drainage systems to the nearest drop inlet by pipe or by free
> drainage.

[]{.mark}

*[INSERT THE FIGURE NUMBERS AND EARTH PRESSURE DIAGRAMS FROM
GEOTECH]{.mark}*

> **(f)** Show loading conditions for soldier piles, lagging, and
> anchors at critical stages of construction as indicated on Figures
> *[(insert figure numbers)]{.mark}*.
>
> **(g)** Ensure that the wall is compatible with horizontal and
> vertical alignment criteria indicated on the plans.
>
> **(h)** Size the tendon such that the design load does not exceed 53
> percent of the guaranteed ultimate tensile strength of the tendon.
>
> **(i)** Analyze the anchor structure system to ensure long-term
> stability of both the anchors and the earth mass being retained.
> Extend the free stressing length at least 5 feet [(1.5 meters)]{.mark}
> beyond any critical failure surface but not less than 15 feet [(4.6
> meters)]{.mark} in soil or 10 feet [(3 meters)]{.mark} in rock.
> Account for the type of foundation, nearness, and susceptibility to
> movement of adjacent facilities in the design.