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REVISION REQUEST 4023

751.24.2.1 Design

Designs of Mechanically Stabilized Earth (MSE) walls shall be completed by consultants or contractors in accordance with Section 11.10 of LRFD specifications, FHWA-NHI-10-024 and FHWA-NHI-10-025 for LRFD. Bridge Pre-qualified Products List (BPPL) provided on MoDOT's web page and in Sharepoint contains a listing of facing unit manufacturers, soil reinforcement suppliers, and wall system suppliers which have been approved for use. See Sec 720 and Sec 1010 for additional information. The Geotechnical Section is responsible for checking global stability of permanent MSE wall systems, which should be reported in the Foundation Investigation Geotechnical Report. For MSE wall preliminary information, see EPG 751.1.4.3 MSE Walls. For design requirements of MSE wall systems and temporary shoring (including temporary MSE walls), see EPG 720 Mechanically Stabilized Earth Wall Systems. For staged bridge construction, see EPG 751.1.2.11 Staged Construction.

For seismic design requirements, see Bridge Seismic Design Flowchart. References for consultants and contractors include Section 11.10 of LRFD, FHWA-NHI-10-024 and FHWA-NHI-10-025.

Design Life

• 75 year minimum for permanent walls (if retained foundation require 100 year than consider 100 year minimum design life for wall).

Global stability:

Global stability will be performed by Geotechnical Section or their agent.

MSE wall contractor/designer responsibility:

MSE wall contractor/designer shall perform following analysis in their design for all applicable limit states.

• External Stability

• Limiting Eccentricity
• Sliding
• Factored Bearing Pressure/Stress ≤ Factored Bearing Resistance

• Internal Stability

• Tensile Resistance of Reinforcement
• Pullout Resistance of Reinforcement
• Structural Resistance of Face Elements
• Structural Resistance of Face Element Connections

• Compound Stability

Capacity/Demand ratio (CDR) for bearing capacity shall be ≥ 1.0

$BearingCapacity(CDR)=\frac{FactoredBearingResistance}{MaximumFactoredBearingStress}\ge 1.0$

Strength Limit States:

Factored bearing resistance = Nominal bearing resistance from Geotech report X Minimum Resistance factor (0.65, Geotech report) LRFD Table 11.5.7-1

Extreme Event I Limit State:

Factored bearing resistance = Nominal bearing resistance from Geotech report X Resistance factor

Resistance factor = 0.9 LRFD 11.8.6.1

Factored bearing stress shall be computed using a uniform base pressure distribution over an effective width of footing determined in accordance with the provisions of LRFD 10.6.3.1 and 10.6.3.2, 11.10.5.4 and Figure 11.6.3.2-1 for foundation supported on soil or rock.

B’ = L – 2e

Where,

L = Soil reinforcement length (For modular block use B in lieu of L as per LRFD 11.10.2-1)

B’ = effective width of footing

e = eccentricity

Note: When the value of eccentricity e is negative then B´ = L.

Capacity/Demand ratio (CDR) for overturning shall be ≥ 1.0

$Overtuning(CDR)=\frac{TotalFactoredResistingMoment}{TotalFactoredDrivingMoment}\ge 1.0$

Capacity/Demand ratio (CDR) for eccentricity shall be ≥ 1.0

$Eccentricity(CDR)=\frac{e_{Limit}}{e_{design}}\ge 1.0$

Capacity/Demand ratio (CDR) for sliding shall be ≥ 1.0      LRFD 11.10.5.3 & 10.6.3.4

$Sliding(CDR)=\frac{TotalFactoredSlidingResistance}{TotalFactoredActiveForce}\ge 1.0$

Capacity/Demand ratio (CDR) for internal stability shall be ≥ 1.0

Eccentricity, (e) Limit for Strength Limit State:      LRFD 11.6.3.3 & C11.10.5.4

For foundations supported on soil or rock, the location of the resultant of the reaction forces shall be within the middle two-thirds of the base width, L or (e ≤ 0.33L).

Eccentricity, (e) Limit for Extreme Event I (Seismic):      LRFD 11.6.5.1

For foundations supported on soil or rock, the location of the resultant of the reaction forces shall be within the middle two-thirds of the base width, L or (e ≤ 0.33L) for γ_EQ = 0.0 and middle eight-tenths of the base width, L or (e ≤ 0.40L) for γ_EQ = 1.0. For γ_EQ between 0.0 and 1.0, interpolate e value linearly between 0.33L and 0.40L. For γ_EQ refer to LRFD 3.4.

Note: Seismic design shall be performed for γ_EQ = 0.5

Eccentricity, (e) Limit for Extreme Event II:

For foundations supported on soil or rock, the location of the resultant of the reaction forces shall be within the middle eight-tenths of the base width, L or (e ≤ 0.40L).

General Guidelines

• Drycast modular block wall (DMBW-MSE) systems are limited to a 10 ft. height in one lift.

• Wetcast modular block wall (WMBW-MSE) systems are limited to a 15 ft. height in one lift.

• For Drycast modular block wall (DMBW-MSE) systems and Wetcast modular block wall (WMBW-MSE) systems, top cap units shall be used and shall be permanently attached by means of a resin anchor system.

• For precast modular panel wall (PMPW-MSE) systems, capstone may be substituted for coping and either shall be permanently attached to wall by panel dowels.

• For precast modular panel wall (PMPW-MSE) systems, form liners are required to produce all panels. Using form liner to produce panel facing is more cost effective than producing flat panels. Standard form liners are specified on the MSE Wall Standard Drawings. Be specific regarding names, types and colors of staining, and names and types of form liner.

• MSE walls shall not be used where exposure to acid water may occur such as in areas of coal mining.

• MSE walls shall not be used where scour is a problem.

• MSE walls with metallic soil reinforcement shall not be used where stray electrical ground currents may occur as would be present near electrical substations.

• No utilities shall be allowed in the reinforced earth if future access to the utilities would require that the reinforcement layers be cut, or if there is a potential for material, which can cause degradation of the soil reinforcement, to leak out of the utilities into the wall backfill, with the exception of storm water drainage.

• All vertical objects shall have at least 4’-6” clear space between back of the wall facing and object for select granular backfill compaction and soil reinforcement skew limit requirements. For piles, see pipe pile spacers guidance.

• The interior angle between two MSE walls should be greater than 70°. However, if unavoidable, then place EPG 751.50 J1.41 note on the design plans.

• Drycast modular block wall (DMBW-MSE) systems and Wetcast modular block wall (WMBW-MSE) systems may be battered up to 1.5 in. per foot. Modular blocks are also known as “segmental blocks”.

• The friction angle used for the computation of horizontal forces within the reinforced soil shall be greater than or equal to 34°.

• For epoxy coated reinforcement requirements, see EPG 751.5.9.2.2 Epoxy Coated Reinforcement Requirements.

• All concrete except facing panels or units shall be CLASS B or B-1.

• The friction angle of the soil to be retained by the reinforced earth shall be listed on the plans as well as the friction angle for the foundation material the wall is to rest on.

• The following requirement shall be considered (from 2009_FHWA-NHI-10-024 MSE wall 132042.pdf, page 200-201) when seismic design is required:

• For seismic design category, SDC C or D (Zones 3 or 4), facing connections in modular block faced walls (MBW) shall use shear resisting devices (shear keys, pin, etc.) between the MBW units and soil reinforcement, and shall not be fully dependent on frictional resistance between the soil reinforcement and facing blocks. For connections partially dependent on friction between the facing blocks and the soil reinforcement, the nominal long-term connection strength T_ac, should be reduced to 80 percent of its static value.

• Seismic design category and acceleration coefficients shall be listed on the plans for categories B, C and D. If a seismic analysis is required that shall also be noted on the plans. See EPG 751.50 A1.1 note.

• Plans note (EPG 751.50 J1.1) is required to clearly identify the responsibilities of the wall designer.

• Do not use Drycast modular block wall (DMBW-MSE) systems in the following locations:

• Within the splash zone from snow removal operations (assumed to be 15 feet from the edge of the shoulder).

• Where the blocks will be continuously wetted, such as around sources of water.

• Where blocks will be located behind barrier or other obstacles that will trap salt-laden snow from removal operations.

• Do not use Drycast modular block wall (DMBW-MSE) systems or Wetcast modular block wall (WMBW-MSE) systems in the following locations:

• For structurally critical applications, such as containing necessary fill around structures.

• In tiered wall systems.

• For locations where Drycast modular block wall (DMBW-MSE) systems and Wetcast modular block wall (WMBW-MSE) systems are not desirable, consider coloring agents and/or architectural forms using precast modular panel wall (PMPW-MSE) systems for aesthetic installations.

• For slab drain location near MSE Wall, see EPG 751.10.3.1 Drain Type, Alignment and Spacing and EPG 751.10.3.3 General Requirements for Location of Slab Drains.

• Roadway runoff should be directed away from running along face of MSE walls used as wing walls on bridge structures.

• Drainage:

• Gutter type should be selected at the core team meeting.

• When gutter is required without fencing, use Type A or Type B gutter (for detail, see Std. Plan 609.00).

• When gutter is required with fencing, use Modified Type A or Modified Type B gutter (for detail, see Std. Plan 607.11).

• When fencing is required without gutter, place in tube and grout behind the MSE wall (for detail, see MSE Wall Standard Drawings - MSEW, Fence Post Connection Behind MSE Wall (without gutter).

• Lower backfill longitudinal drainage pipes behind all MSE walls shall be two-6” (Min.) diameter perforated PVC or PE pipe (See Sec 1013) unless larger sizes are required by design which shall be the responsibility of the District Design Division. Show drainage pipe size on plans. Outlet screens and cleanouts should be detailed for any drain pipe (shown on MoDOT MSE wall plans or roadway plans). Lateral non-perforated drain pipes (below leveling pad) are permitted by Standard Specifications and shall be sized by the District Design Division if necessary. Lateral outlet drain pipe sloped at 2% minimum.

• Identify on MSE wall plans or roadway plans drainage pipe point of entry, point of outlet (daylighting), 2% min. drainage slopes in between points to ensure positive flow and additional longitudinal drainage pipes if required to accommodate ground slope changes and lateral drainage pipes if required by design.

• Adjustment in the vertical alignment of the longitudinal drainage pipes from that depicted on the MSE wall standard drawings may be necessary to ensure positive flow out of the drainage system.

• Identify on MSE wall plans or roadway plans the outlet ends of pipes which shall be located to prevent clogging or backflow into the drainage system. Outlet screens and cleanouts should be detailed for any drain pipe.

• For more information on drainage, see Drainage at MSE Walls.

MSE Wall Construction: Pipe Pile Spacers Guidance

For bridges not longer than 200 feet, pipe pile spacers or pile jackets shall be used at pile locations behind mechanically stabilized earth walls at end bents. Corrugated pipe pile spacers are required when the wall is built prior to driving the piles to protect the wall reinforcement when driving pile for the bridge substructure at end bents(s). Pile spacers or pile jackets may be used when the piles are driven before the wall is built. Pipe pile spacers shall have an inside diameter greater than that of the pile and large enough to avoid damage to the pipe when driving the pile. Use EPG 751.50 Standard Detailing Note E1.2a on bridge plans.

For bridges longer than 200 feet, pipe pile spacers are required and the pile spacer shall be oversized to mitigate the effects of bridge thermal movements on the MSE wall. For HP12, HP14, CIP 14” and CIP 16” piles provide 24-inch inside diameter of pile spacer for bridge movement. Minimum pile spacing shall be 5 feet to allow room for compaction of the soil layers. Use EPG 751.50 Standard Detailing Note E1.2b on bridge plans.

The bottom of the pipe pile spacers shall be placed 5 ft. min. below the bottom of the MSE wall leveling pad. The pipe shall be filled with sand or other approved material after the pile is placed and before driving. Pipe pile spacers shall be accurately located and capped for future pile construction.

Alternatively, for bridges shorter than or equal to 200 feet, the contractor shall be given the option of driving the piles before construction of the mechanically stabilized earth wall and placing the soil reinforcement and backfill material around the piling. In lieu of pipe pile spacers contractor may place pile jackets on the portion of the piles that will be in the MSE soil reinforced zone prior to placing the select granular backfill material and soil reinforcement. The contractor shall adequately support the piling to ensure that proper pile alignment is maintained during the wall construction. The contractor’s plan for bracing the pile shall be submitted to the engineer for review.

Piling shall be designed for downdrag (DD) loads due to either method. Oversized pipe pile spacers with sand placed after driving or pile jacket may be considered to mitigate some of the effects of downdrag (DD) loads. Sizing of pipe pile spacers shall account for pile size, thermal movements of the bridge, pile placement plan, and vertical and horizontal placement tolerances.

When rock is anticipated within the 5 feet zone below the MSE wall leveling pad, prebore into rock and prebore holes shall be sufficiently wide to allow for a minimum 10 feet embedment of pile and pipe pile spacer. When top of rock is anticipated within the 5 to 10 feet zone below the MSE wall leveling pad, prebore into rock to achieve a minimum embedment (pile only) of 10 feet below the bottom of leveling pad. Otherwise, the pipe pile spacer requires a minimum 5 feet embedment below the levelling pad. Consideration shall also be given to oversizing the prebore holes in rock to allow for temperature movements at integral end bents.

For bridges not longer than 200 feet, the minimum clearance from the back face of MSE walls to the front face of the end bent beam, also referred to as setback, shall be 4 ft. 6 in. (typ.) unless larger than 18-inch pipe pile spacer required. The 4 ft. 6 in. dimension serves a dual purpose:

1) the setback ensures that soil reinforcement is not skewed more than 15° for nut and bolt reinforcement connections to clear an 18-inch inside diameter pipe pile spacers by 6 inches per FHWA-NHI-10-24, Figure 5-17C, while considering vertical and horizontal pile placement tolerances

2) the setback helps to reduce the forces imparted on the MSE wall from bridge movements that typically are not accounted for in the wall design and cannot be completely isolated using a pipe pile spacer. Increasing the minimum setback shall be considered when larger diameter pile spacers are required or when other types of soil reinforcement connections are anticipated

For bridges longer than 200 feet, the minimum setback shall be 5 ft. 6 in. based on the use of 24-inch inside diameter of pipe pile spacers.

If interference with soil reinforcement is not a concern and the wall is designed for forces from bridge movement, the following guidance for pipe pile spacers clearance shall be used: pipe pile spacers shall be placed 36 in. clear min. from the back face of MSE wall panels to allow for proper compaction; 12 in. minimum clearance is required between pipe pile spacers and leveling pad and 18 in. minimum clearance is required between leveling pad and pile. For isolated pile (e.g, walls skewed from the bent orientation), the pipe pile spacer may be placed 18 in. clear min. from the back face of MSE wall panels.

MSE Wall Plan and Geometrics

• A plan view shall be drawn showing a baseline or centerline, roadway stations and wall offsets. The plan shall contain enough information to properly locate the wall. The ultimate right of way shall also be shown, unless it is of a significant distance from the wall and will have no effect on the wall design or construction.

• Stations and offsets shall be established between one construction baseline or roadway centerline and a wall control line (baseline). Some wall designs may contain a slight batter, while others are vertical. A wall control line shall be set at the front face of the wall, either along the top or at the base of the wall, whichever is critical to the proposed improvements. For battered walls, in order to allow for batter adjustments of the stepped level pad or variation of the top of the wall, the wall control line (baseline) is to be shown at a fixed elevation. For battered walls, the offset location and elevation of control line shall be indicated. All horizontal breaks in the wall shall be given station-offset points, and walls with curvature shall indicate the station-offsets to the PC and PT of the wall, and the radius, on the plans.

• Any obstacles which could possibly interfere with the soil reinforcement shall be shown. Drainage structures, lighting, or truss pedestals and footings, etc. are to be shown, with station offset to centerline of the obstacle, with obstacle size. Skew angles are shown to indicate the angle between a wall and a pipe or box which runs through the wall.

• Elevations at the top and bottom of the wall shall be shown at 25 ft. intervals and at any break points in the wall.

• Curve data and/or offsets shall be shown at all changes in horizontal alignment. If battered wall systems are used on curved structures, show offsets at 10 ft. (max.) intervals from the baseline.

• Details of any architectural finishes (formliners, concrete coloring, etc.).

• Details of threaded rod connecting the top cap block.

• Estimated quantities, total sq. ft. of mechanically stabilized earth systems.

• Proposed grade and theoretical top of leveling pad elevation shall be shown in constant slope. Slope line shall be adjusted per project. Top of wall or coping elevation and stationing shall be shown in the developed elevation per project. If leveling pad is anticipated to encounter rock, then contact the Geotechnical Section for leveling pad minimum embedment requirements.

MSE Wall Cross Sections

• A typical wall section for general information is shown.

• Additional sections are drawn for any special criteria. The front face of the wall is drawn vertical, regardless of the wall type.

• Any fencing and barrier or railing are shown.

• Barrier if needed are shown on the cross section. Barriers are attached to the roadway or shoulder pavement, not to the MSE wall. Standard barriers are placed along wall faces when traffic has access to the front face of the wall over shoulders of paved areas.

Drainage at MSE Walls

• Drainage Before MSE Wall

Drainage is not allowed to be discharged within 10 ft. from front of MSE wall in order to protect wall embedment, prevent erosion and foundation undermining, and maintain soil strength and stability.

• Drainage Behind MSE Wall

Internal (Subsurface) Drainage

Groundwater and infiltrating surface waters are drained from behind the MSE wall through joints between the face panels or blocks (i.e. wall joints) and two-6 in. (min.) diameter pipes located at the base of the wall and at the basal interface between the reinforced backfill and the retained backfill.

Excessive subsurface draining can lead to increased risk of backfill erosion/washout through the wall joints and erosion at the bottom of walls and at wall terminal ends. Excessive water build-up caused by inadequate drainage at the bottom of the wall can lead to decreased soil strength and wall instability. Bridge underdrainage (vertical drains at end bents and at approach slabs) can exacerbate the problem.

Subsurface drainage pipes should be designed and sized appropriately to carry anticipated groundwater, incidental surface run-off that is not collected otherwise including possible effects of drainage created by an unexpected rupture of any roadway drainage conveyance or storage as an example.

External (Surface) Drainage

External drainage considerations deal with collecting water that could flow externally over and/or around the wall surface taxing the internal drainage and/or creating external erosion issues. It can also infiltrate the reinforced and retained backfill areas behind the MSE wall.

Diverting water flow away from the reinforced soil structure is important. Roadway drainage should be collected in accordance with roadway drainage guidelines and bridge deck drainage should be collected similarly.

• Guidance

ALL MSE WALLS

1. Appropriate measures to prevent surface water infiltration into MSE wall backfill should be included in the design and detail layout for all MSE walls and shown on the roadway plans.

2. Gutters behind MSE walls are required for flat or positive sloping backfills to prevent concentrated infiltration behind the wall facing regardless of when top of backfill is paved or unpaved. This avoids pocket erosion behind facing and protection of nearest-surface wall connections which are vulnerable to corrosion and deterioration. Drainage swales lined with concrete, paved or precast gutter can be used to collect and discharge surface water to an eventual point away from the wall. If rock is used, use impermeable geotextile under rock and align top of gutter to bottom of rock to drain. (For negative sloping backfills away from top of wall, use of gutters is not required.)

District Design Division shall verify the size of the two-6 in. (min.) diameter lower perforated MSE wall drain pipes and where piping will daylight at ends of MSE wall or increase the diameters accordingly. This should be part of the preliminary design of the MSE wall. (This shall include when lateral pipes are required and where lateral drain pipes will daylight/discharge).

BRIDGE ABUTMENTS WITH MSE WALLS

Areas of concern: bridge deck drainage, approach slab drainage, approach roadway drainage, bridge underdrainage: vertical drains at end bents and approach slab underdrainage, showing drainage details on the roadway and MSE wall plans

3. Bridge slab drain design shall be in accordance with EPG 751.10.3 Bridge Deck Drainage – Slab Drains unless as modified below.

4. Coordination is required between the Bridge Division and District Design Division on drainage design and details to be shown on the MSE wall and roadway plans.

5. Bridge deck, approach slab and roadway drainage shall not be allowed to be discharged to MSE wall backfill area or within 10 feet from front of MSE wall.

• (Recommended) Use of a major bridge approach slab and approach pavement is ideal because bridge deck, approach slab and roadway drainage are directed using curbs and collected in drain basins for discharge that protect MSE wall backfill. For bridges not on a major roadway, consideration should be given to requiring a concrete bridge approach slab and pavement incorporating these same design elements (asphalt is permeable).

• (Less Recommended) Use of conduit and gutters:

• Conduit: Drain away from bridge and bury conduit daylighting to natural ground or roadway drainage ditch at an eventual point beyond the limits of the wall. Use expansion fittings to allow for bridge movement and consider placing conduit to front of MSE wall and discharging more than 10 feet from front of wall or using lower drain pipes to intercept slab drainage conduit running through backfill.

• Conduit and Gutters: Drain away from bridge using conduit and 90° elbow (or 45° bend) for smoothly directing drainage flow into gutters and that may be attached to inside of gutters to continue along downward sloping gutters along back of MSE wall to discharge to sewer or to natural drainage system, or to eventual point beyond the limits of the wall. Allow for independent bridge and wall movements by using expansion fittings where needed. See EPG 751.10.3.1 Type, Alignment and Spacing and EPG 751.10.3.3 General Requirements for Location of Slab Drains.

6. Vertical drains at end bents and approach slab underdrainage should be intercepted to drain away from bridge end and MSE wall.

7. Discharging deck drainage using many slab drains would seem to reduce the volume of bridge end drainage over MSE walls.

8. Drain flumes at bridge abutments with MSE walls do not reduce infiltration at MSE wall backfill areas and are not recommended.

DISTRICT DESIGN DIVISION MSE WALLS

Areas of concern: roadway or pavement drainage, MSE wall drainage, showing drainage details on the roadway and MSE wall plans.

9. For long MSE walls, where lower perforated drain pipe slope become excessive, non-perforated lateral drain pipes, permitted by Standard Specifications, shall be designed to intercept them and go underneath the concrete leveling pad with a 2% minimum slope. Lateral drain pipes shall daylight/discharge at least 10 ft. from front of MSE wall. Screens should be installed and maintained on drain pipe outlets.

10. Roadway and pavement drainage shall not be allowed to be discharged to MSE wall backfill area or within 10 feet from front of MSE wall.

11. For district design MSE walls, use roadway or pavement drainage collection pipes to transport and discharge to an eventual point outside the limits of the wall.

Example: Showing drain pipe details on the MSE wall plans.

• 
  ELEVATION SHOWING DRAIN PIPE
• 
  Alternate option

• 
  Section A-A

Notes: (1) To be designed by District Design Division. (2) To be designed by District Design Division if needed. Provide non-perforated lateral drain pipe under leveling pad at 2% minimum slope. (Show on plans). (3) Discharge to drainage system or daylight screened outlet at least 10 feet away from end of wall (typ.). (Skew in the direction of flow as appropriate). (4) Discharge to drainage system or daylight screened outlet at least 10 feet away from front face of wall (typ.). (Skew in the direction of flow as appropriate). (5) Minimum backfill cover = Max(15”, 1.5 x diameter of drain pipe).

E1. Excavation and Fill

(E1.1) Use when specified on the Design Layout.

Existing roadway fill under the ends of the bridge shall be removed as shown. Removal of existing roadway fill will be considered completely covered by the contract unit price for roadway excavation.

Use one of the following two notes where MSE walls support abutment fill.

(E1.2a) [MS Cell] Use when pipe pile spacers are shown on plan details and bridge is 200 feet long or shorter. Add “See special provisions” to the pipe pile spacer callout and add table near the callout.

See special provisions.

Pile Encasement	Option Used (√)
Pipe Pile Spacer
Pile Jacket

MoDOT Construction personnel will indicate the pile encasement used.

(E1.2b) Use note when pipe pile spacers are shown on plan details for HP12, HP14, CIP 14” and CIP 16” piles and bridge is longer than 200 feet. For larger CIP pile size modify following note and use minimum 6” larger pipe pile spacer diameter than CIP pile.

The pipe pile spacers shall have an inside diameter equal to 24 inches.

(E1.4) Use for fill at pile cap end bents. Use the first underlined portion when MSE walls are present. Use approach for semi-deep abutments.

Roadway fill, exclusive of Select Granular Backfill for Structural Systems, shall be completed to the final roadway section and up to the elevation of the bottom of the concrete approach beam within the limits of the structure and for not less than 25 feet in back of the fill face of the end bents before any piles are driven for any bents falling within the embankment section.

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