Section 255 — Mechanically-Stabilized Earth Walls
Description
255.01This work consists of constructing MSE walls.
Material
255.02Conform to the following Subsections: Geotextile 714.01 Mechanically -stabilized earth wall material 720.01 Reinforcement geotextile and geogrid 714.04 Select granular backf ill 704.03 Wall facing fill 705.09
Construction Requirements
255.03General. Survey according to Section 152 and verify the limits of wall installation. Sub mit installation drawings according to Subsection 104.06. Conform to Section 257 for Contractor -designed MSE walls. Do not disturb existing ground until limits of the wall installation have been verified and the CO has approved the installation drawings. Excavate and backfill according to Section 209. Grade the foundation for a width equal to the length of reinforcing elements plus the additional width shown in the plans. If required, as determined by the Contractor, design and construct temporary shoring according to Section 562. For concrete panel and SRWU faced walls, provide a precast reinforced or a non -reinforced cast -in-place concrete leveling pad. Cure cast -in-place leveling pads at least 12 hours before placing wall panels. For all other walls, construct a leveling pad from crushed aggregate or the specified material, if shown in the plans. Construct minor concrete according to Section 601. Excavate wall foundations to within 4 inches horizontally a nd vertically from the staked location. Install reinforcement elements to within 2 inches vertically from the staked location. Bench the excavation as necessary to accommodate variable lengths of reinforcement according to the plans and approved drawings. Install underdrain systems and geocomposite sheet drain systems according to Section 605. Repair damage to galvanized coatings before installation according to Subsection 563.12(b) .
255.04Wall E rection. Erect the wall according to the plans and approved drawings. Have a field representative from the wall system manufacturer on site during the startup of wall erection. Construct MSE walls to the tolerance shown in Table 255-1. Connect reinforcement to facing, pull taut, and anchor far end before placing additional backfill. Do not place reinforcing elements below the corresponding connection elevations, unless approved as part of a utility installation detail. Table 255-1
Construction Tolerance
Facing Type Vertical ,
inch(1) (±) Horizontal , inch(2) (±) Horizontal
Straight Edge
Point Check ,
inch(3) (±)
Precast concrete panel,
segmental retaining wall units 0.5 0.5 0.75 Welded wire, gabions 1 1 2
(1)Wall vertical tolerance at top of wall for every 10 feet of wall height. For example, 5 feet wall height multiply 6.5×value.
(2)Wall horizontal tolerance at top of wall for every 10 feet of wall height.
(3)Maximum horizontal deviation at a point in the wall from a 10- foot metal straightedge placed horizontally or vertically on the theoretical plane of the design face.
(a)Precast concrete panel -faced. Construct panels by the following:
(1)Construct according to Section 578. Use concrete class A(AE).
(2)Finish the front face of the panel with a Class 1 finish according to Subsection 552.14 . Screed the rear face of th e panel to eliminate open pockets of aggregate and surface distortions of more than ¼ inch. Cast the panels on a flat area. Do not attach galvanized connecting devices or fasteners to the face panel reinforcement steel.
(3)Clearly write on an unexposed face of each panel the date of manufacture, the production lot number, and the piece mark. (4) Handle, store, and ship units according to Subsection 578.08.
(5)Manufacture units within the following tolerances:
(a)Panel dimensions. Positio n of panel connection devices within 1 inch. All other dimensions within 3⁄16 inch.
(b)Panel squareness. Do not exceed ½ inch as determined by the difference between the two diagonals. (c) Panel surface finish. Do not exceed ⅛ inch for surface defects on smooth f ormed surfaces of 5 feet or more in length. Do not exceed 5⁄16 inch for surface defects on textured -finished surfaces of 5 feet or more in length.
(6)Concrete face panels having the following defects will be rejected:
(a)Defects that indicate imperfect molding;
(b)Defects indicating honeycombed or open texture concrete;
(c)Cracked or severely chipped panels; or
(d)Color variation on front face of panel. Erect panels by lifting devices connected to the upper edge of the panel. Size the joint openings to prevent damage to facing panels during post -construction structure movement and to prevent loss of wall backfill materials. Construct the joint opening and install joint material according to the approved drawings. Cover joints on the backside of the panels with a 12-inch- wide strip of geotextile filter. Overlap geotextile filter splices at least 4 inches. Hold the panels in position with temporary wedges or bracing during backfilling operations.
(b)Wire- faced. Place facing and backing ma ts in successive horizontal rows as backfill placement proceeds. Do not use hardware cloth or geosynthetic material to retain backfill at the face of the wall unless the product is specifically approved for facing applications and part of the approved drawings.
(c)Gabion- faced. Construct gabion structures according to Section 253. Lay reinforcement mesh horizontally on compacted fill and normal to the face of the wall. Connect the gabion facing unit to reinforcement mesh with spiral binders or tie wire at 4 -inch nominal spacing with alternating single and double locked loops, or as detailed in the approved drawings.
(d)SRWU. Place the first course of wall units on top of and in full contact with the leveling pad. Place the units s ide by side for the full length of the wall such that adjoining units are located with minimal gaps. Place units to ensure only the front face of the unit is visible. Check for proper elevation and alignment every two courses. Install connection devices an d alignment devices as required by the approved drawings. Fill voids in and around units with unit fill as recommended by the manufacturer’s installation guidelines to meet the required connection strength. If the unit fill is required to meet connection strength specifications, completely fill each course of block before proceeding to the next course. Place reinforcement at a vertical spacing no more than two times the block depth or 24 inches, whichever is less. Remove excess material from the top of the units before installing each succeeding course.
255.05Backfillin g. Backfill the reinforced zone with the specified material according to Subsection 209.09. Place facing fill or unit fill as shown in the approved drawings. Place select granular backfill from the back of the wall face, facing fill, or unit fill to the end of the reinforcement plus the additional width shown in the plans. Ensure that no voids exist below the reinforcement. Compact each lift according to Subsection 209.10, except use an approved lightweight mechanical or vibratory compactor within 36 inches of the wall face. Consolidate facing fill or unit fill by rodding or other approved means to produce a uniform, tight fill. Where the reinforced zone supports spread footings for bridges or other structural loads, compact the top 5 feet to at least 100 percent of the maximum density. Do not damage or disturb the facing or reinforcing elements. Do not operate equipment directly on top of the reinforcements. Place at least 6 -inch loose lift of fill before operating rubber- tired equipment over the reinforcements. Limit equipment speeds to 5 miles per hour and limit turning maneuvers to a minimum. Install and main tain reinforcement taut, unwrinkled, and in full contact with the underlying surface. Correct damaged, misaligned, or distorted wall elements as directed before constructing subsequent layers. Backfill and compact behind the reinforced zone with the specif ied material according to Subsections 209.09 and 209.10. At the end of the day’s operation, slope the last lift of backfill away from the wall face to direct surface runoff away from the wall. Do not allow surface runoff from adjacent areas to enter the wall construction area.
255.06Acceptance. See Table 255-2 for sampling, testing, and acceptance requirements. Material for MSE walls will be evaluated under Subsections 106.02 and 106.03. Submit a production certification with each shipment of facing units. Construction of MSE walls will be evaluated under Subsections 106.02 and 106.04. Concrete leveling pads will be evaluated under Section 601. Excavation and backfill will be evaluated under Section 209. Gabions will be evaluated under Section 253. Precast concrete elements will be evaluated under Section 578 . Survey will be evaluated under Section 152. Temporary works will be evaluated under Section 562. Underdrain systems and geocomposite sheet drain systems will be evaluated under Section 605.
Measurement
255.07Measure the Section 255 pay items listed in the bid schedule according to Subsection 109.02 and the following as applicable: When measuring MSE walls by the square foot, measure the front face of wall excluding footings and leveling courses. When measuring select granular backfill within the reinforced zone by the cubic yard, measure in place.
Payment
255.08The accepted quantities will be paid at the contract price per unit of measurement f or the Section 255 pay items listed in the bid schedule. Payment will be full compensation for the work prescribed in this Section. See Subsection 109.05 . Table 255-2 Sampling, Testing, and Acceptance Requirements
Material or
Product
(Subsection) Type of
Acceptance
(Subsection) Characteristic Category Test Methods
Specifications Sampling
Frequency Point of
Sampling Split
Sample Reporting
Time Remarks
Source
Select
granular backfill (704.03 ) Measured & tested for conformance (106.04 & 105) Gradation − AASHTO T 27 & T 11 1 per soil type Source of material Yes Before using in work Not required if using
Government-
furnish ed source
Angle of
internal friction − AASHTO T 236 and
Subsection
704.03(a)(2) " " " " "
Soundness
using sodium sulfate − AASHTO T 104 " " " " "
Plasticity
index − AASHTO R 58, T 89, & T 90 " " " " "
Resistivity
(1)− AASHTO T 288 " " " " " pH(1),(2) − AASHTO T 289 " " " " "
Sulfate
content(1),(3) − AASHTO T 290 " " " " "
Chloride
content(1),(3) − AASHTO T 291 " " " " " Table 255- 2 (continued) Sampling, Testing, and Acceptance Requirements
Material or
Product
(Subsection) Type of
Acceptance
(Subsection) Characteristic Category Test Methods
Specifications Sampling
Frequency Point of
Sampling Split
Sample Reporting
Time Remarks
Production
Select
granular backfill (704.03 ) Measured & tested for conformance (106.04 ) Moisture - density − AASHTO T 99, Method C(4) 1 per soil type Source of material Yes Before using in work − Density − AASHTO T 310 or other approved procedures 1 per lift per 75 linear feet of wall but not less than 2 per lift In-place No Before placing next lift −
(1)Required for MSE walls with metallic reinforcements.
(2)Required for MSE walls with geosynthetic reinforcements.
(3)Tests for sulfate and chloride content are not required if resistivity is greater than 5000 ohm centimeters.
(4)At least 5 points per proctor.