MSE Backfill MSE BACKFILL
Part 1 — General
1.1 Section Includes
A.Mechanically Stabilized Earth (MSE) backfill used in constructing MSE retaining walls and Reinforced Soil Slopes (RSS) .
1.2 Related Sections
A.Section 02056: Embankment, Borrow, and Backfill
1.3 References
A.AASHTO T 27: Sieve Analysis of Fine and Coarse Aggregates
B.AASHTO T 90: Determining the Plastic Limit and Plasticity Index of Soils
C.AASHTO T 99: Moisture -Density Relations of Soils Using a 2.5- kg (5.5- lb) Rammer and a 305- mm (12 in.) Drop
D.AASHTO T 104: Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate
E.AASHTO T 236: Direct Shear Test of Soils Under Consolidated Drained Conditions
F.AASHTO T 267: Determination of Organic Content in Soils by Loss on Ignition
G.AASHTO T 288: Determining Minimum Laboratory Soil Resistivity
H.AASHTO T 289: Determining pH of Soil for Use in Corrosion Testing
I.AASHTO T 290: Determining Water -Soluble Sulfate Ion Content in Soil
J.AASHTO T 291: Determining Water -Soluble Chloride Ion Content in Soil
K.AASHTO T 310: In -Place Density and Moisture Content of Soil and Soil - Aggregate by Nuclear Methods (Shallow Depth) MSE Backfill
L.ASTM D 4327: Anions in Water by Suppressed ion Chromatography
M.FHWA Publication No. FHWA -NHI-09-087, Corrosion/Degradation of Soil Reinforcements for M echanically Stabilized Earth Walls and Reinforced Soil Slopes
1.4 Definitions
A.Light Equipment Zone – Zone within 3 ft of the back face of the wall facing units, and within 2 ft of obstructions such as driven piles and drilled shafts .
1.5 SUBMITTALS Not Used
1.6 Quality Control
A.Provide electrochemical testing of MSE backfill during MSE wall construction.
1.Frequency: Every 10,000 yd 3 of MSE backfill used
2.Use an AASHTO accredited laboratory.
1.7 Acceptance
A.Allow t he Engineer to obtain on- site samples during construction at the following frequenc y:
1.Gradation: Every 5,000 yd 3 of MSE backfill used
B.Allow t he Engineer to perform at least one in -place moisture/density determination per lift of backfill for each 100 ft of wall length ( at least two tests per lift) tested according to AASHTO T 310.
1.Tests will be conducted at random locations or at specific locations or both as determined by the Engineer.
Part 2 — Products
2.1 Mse Backfill
A.Use backfill that:
1.Is free from frozen, organic, and otherwise deleterious materials.
2.Has an organic content less than one- half of one percent as determined by AASHTO T 267 on the portion of the material finer than the No. 10 sieve.
3.Is substantially free of shale or other soft particles of poor durability. MSE Backfill
4.Has soundness loss meeting one of the following when tested according to AASHTO T 104 :
a.No more than 15 percent after a test duration of five cycles when using a sodium sulfate solution .
b.No more than 30 percent after a test duration of four cycles when using a magnesium sulfate solution.
B.Conform to the gradation limits in Table 1 as determined by AASHTO T 27. Table 1 MSE Backfill Gradation (percent passing) Sieve Size Metal Reinforcement Geogrid Reinforcement 4 inch 100 – ¾ inch – 100 No. 40 0 – 60 0 – 60 No. 200 0 –15 0 –15
1.The MSE backfill gradation for metal reinforcement in Table 1 may be used with geogrid reinforcement under the following conditions:
a.The Wall Company perform s site and material specific installation damage testing using a representative sample of the proposed MSE backfill material . 1) Refer to FHWA -NHI-09-087 for testing protocols .
b.The installation damage reduction factor determined by the test is not less than 1.2. 1) Refer to FHWA -NHI-09-087 for determination of the installation damage reduction factor.
c.The test results are certified by a professional engineer licensed in the State of Utah.
d.The Engineer may request additional material specific installation damage testing when a change in the backfill characteristics from the original tested representative sample is suspected.
C.Use backfill with a Plasticity Index (PI) of 6 or less, as determined by AASHTO T 90. MSE Backfill
D.Use backfill with an internal friction angle of not less than 34 degrees as determined by AASHTO T 236.
1.Use a sample of the material compacted to 95 percent of maximum density at optimum moisture content as determined by AASHTO T 99, Method D.
2.Internal friction angle testing is not required for backfill material that has at least 80 percent of the material retained on the 3/4 inch sieve.
E.Meet the electrochemical properties in Table 2. Table 2 Electrochemical Properties Property Metal Reinforcement Geogrid Reinforcement Test Method Resistivity Minimum 3000 ohm - cm, at 100% saturation * N/A AASHTO T 288 pH 6.0 – 10.0 5.5 – 10.0** AASHTO T 289 Chlorides Maximum 100 ppm N/A AASHTO T 291 or ASTM D 4327 Sulfates Maximum 200 ppm N/A AASHTO T 290 or ASTM D 4327 * Do not follow Note 6 of AASHTO T 288 to find lowest possible resistivity (where soil is in a slurry state). Saturate soil to 100% saturation where a sheen of water is observed at the top surface of the sample in the soil box. ** Maximum pH value for polyester geogrid is 9.0 unless long- term immersion testing performed according to FHWA -NHI-09-087 shows acceptable performance. For pH > 8.0, use a durability reduction factor (RF D) of 1.3 for polyester geogrid.
2.2 Mse Free Draining Backfill
A.Conform to the gradation limits in Table 3 as determined by AASHTO T 27. MSE Backfill Table 3 MSE Free Draining Backfill Gradation (percent passing) Sieve Size Metal Reinforcement Geogrid Reinforcement 2 inch 100 – 1 1/2 inch 90 – 100 – 3/4 inch – 100 3/8 inch 20 – 75 20 – 75 No. 4 0 – 15 0 – 15 No. 200 0 – 5 0 – 5
B.Meet the electrochemical properties in Table 2.
Part 3 — Execution
3.1 Backfill Placement
A.General
1.Closely f ollow the erection of each course of wall facing units (concrete panel , modular block , or welded wire) with backfill placement.
2.Place backfill in a manner to avoid damage or disturbance of the wall materials , misalign ment of facing panels or modular blocks , and damage to soil reinforcement and facing members .
3.Place backfill using methods that verify that no voids exist directly beneath the soil reinforc ing elements.
4.Place MSE backfill in lifts not to exceed 12 inch thickness before compaction.
a.Decrease the lift thickness as necessary to obtain the specified density.
b.Place backfill in uniformly thick layers.
5.Begin MSE backfill placement 3 ft from the wall face and proceed away from the wall when placing backfill over the soil reinforcement to prevent the soil reinforcement from bunching towards the wall face. MSE Backfill
6.Place each layer of MSE backfill in a level manner before placing subsequent backfill layers.
7.Operate placement and compaction equipment parallel to the wall face.
a.Rubber -tired equipment may be used over the reinforcement at speeds less than 5 mph. 1) Avoid sudden braking and sharp turning.
b.Do not use sheeps -foot or other grid- type rollers for compacting backfill within the limits of the soil reinforcement.
8.Do not place or compact backfill against the facing units until the soil reinforcement has been installed and one lift of MSE backfill has been placed and compacted over the reinforcement layer to avoid pushing the facing unit s out of alignment .
9.Place and compact the MSE backfill to an elevation 2 inch above the reinforcement connection from a point approximately 24 inch behind the back face of the wall facing element to the end of the soil reinforcement zone, unless otherwise shown at specific locations in the authorized retaining wall drawings .
10.Do not proceed with the placement of each layer of soil reinforcement and overlying lift of MSE backfill until the Engineer indicates and records that MSE backfill placement and density requirements (including in the light equipment zone ) have been met.
11.Complete backfilling in front of single -stage walls and the first -stage of two -stage walls before backfilling behind the wall more than 6 ft above the bottom of the wall.
a.Wire facing: Place MSE backfill in lifts not to exceed 10 inch thickness before compaction.
b.Compact backfill to at least 95 percent of the maximum dry density, according to AASHTO T 99.
c.Provide positive drainage away from the wall.
12.Slope the top of the MSE backfill along the wall such that the upper soil reinforcement layer is covered with at least 16 inch of MSE backfil l.
B.MSE Free Draining Backfill
1.Place MSE free draining backfill behind the wall facing to a width of at least 18 inch .
2.Place and compact according to the light equipment zone requirements .
C.Compaction
1.Compact MSE backfill to at least 95 percent of the maximum density, according to AASHTO T 99, Method D. MSE Backfill
2.Light equipment zones
a.Use a maximum lift thickness within this zone as warranted by the type of compaction equipment used, but no t greater than 8 inch before compaction .
b.Compact backfill using at least three passes of a suitable lightweight or medium -weight (hand- held or hand- guided) mechanical roller, tamper, or vibratory compactor.
c.Use compaction equipment with a static weight of less than 800 lb within this zone when compacting a thin leveling lift along the reinforcement connections level.
d.Compact to within 3 inch of the facing unit s.
e.Exercise care in the compaction process to avoid misalignment of the facing unit s.
D.Moisture Content
1.Place MSE backfill with the moisture content at optimum moisture, or between optimum moisture and four percent below optimum moisture.
a.Determine the optimum moisture content according to AASHTO T 99, Method D.
b.Remove MSE backfill with placement moisture content in excess of the optimum moisture content. 1) MSE backfill may be reused, provided it is aerated or otherwise reworked until the moisture content is uniform and acceptable throughout the entire lift.
2.Maintain the moisture content of the MSE backfill uniform throughout each layer during placement and compaction.
3.2 Protection of The Work
A.Remove water to a depth of at least 1 ft below the lowest point of the wall excavation at least 24 hours before placing MSE back fill.
1.Continue to remove water until:
a.Backfill placement in front of the wall facing (including for the first stage of two -stage walls) is complete , and
b.MSE backfill height exceeds 4 ft.
B.At the end of each day's operation, slope the backfill away from the wall to direct runoff of rainwater away from the wall face.
C.Do not allow surface runoff from adjacent areas or groundwater to enter the wall construction site, including at the front face of the wall.
Source: Utah Standard Specifications for Road and Bridge Construction, 2026 Edition. Pages 679–685 of 1,331.