ITEM 515- MECHANICALLY-STABILIZED EARTH (MSE) RETAINING WALLS
515.1 Description
This item shall consist of furnishing materials and placement of
Mechanically-Stabilized Earth (MSE) retaining walls constructed in
accordance with these specifications and in reasonably close conformity to
the lines, grades, design and dimensions shown on the Plans.
The Mechanically-Stabilized Earth (MSE) retaining walls shall consist of a
non-structural leveling pad, facing panels, and soil reinforcement elements
mechanically connected to each facing panel. Soil reinforcement shall have
sufficient length, strength, and frictional resistance as required by the
design.
515.2 Material Requirements
515.2.1 Concrete Leveling Pad
Concrete leveling pad shall be Class A as specified in Item 405, Structural
Concrete, unless otherwise specified or required by the Engineer.
515.2.2 Geotextile
Geotextile shall be specified in Item 715, Geotextiles, unless otherwise
specified or required by the Engineer.
515.2.3 Mechanically-Stabilized Earth (MSE) Retaining Wall Mate
Mechanically-Stabilized Earth (MSE) retaining wall material shall conform
to the following, unless otherwise specified in the Contract.
515.2.3.1 Concrete Face Panels
Concrete shall conform to Item 405, Structural Concrete, unless otherwise
specified or required by the Engineer.
Reinforcing steel bars shall be as specified in Item 404, Reinforcing,
Steel, unless otherwise specified or required by the Engineer.
Concrete shall have a minimum compressive strength of 30 MPa at 28 days.
Concrete panels shall be fully supported until the concrete has reached a
minimum compressive strength of 7 MPa. The concrete panels shall be shipped
and installed after the concrete has reached a minimum compressive strength
of 24 MPa.
Concrete facing panels shall have a minimum thickness of 140 mm and a
minimum concrete cover on reinforcing steel of 37.5 mm.
The panels shall be cast face down in level forms supported on a flat
working surface. Guides shall be used to locate and support attachment
devices set in the back face of the panel. The concrete in each panel unit
all be placed without interruption and shall be consolidated by the use of
an approved vibrator, supplemented by such hand tamping as may be necessary
to force the concrete into the corners of the forms and to prevent the
formation of stone pockets or cleavage planes. Clear form oil or release
agent shall be used throughout the casting operation.
The rear face of the panel shall be screeded to eliminate open pockets of
aggregate and surface distortions in excess of 6 millimeters. The panels
shall be casted on a flat area. Galvanized connecting devices fasteners
shall not be attached to the face panel steel reinforcement.
The production lot number, date of manufacture and the piece mark shall be
clearly inscribed on an unexposed face of each panel.
All units shall be handled, stored, and shipped in such a manner as to
eliminate the dangers of chipping, discoloration, cracks; fractures, and
excessive bending stresses. Panels in storage shall be supported on firm
wooden blocking to protect the panel connection devices and the exposed
exterior finish.
Concrete panels shall be manufactured within the following tolerances:
- Panel dimensions. Position of panel connection devices shall be within 25 millimeters. All other dimensions within 5 millimeters.
- Panel squareness. Squareness, as determined by the difference between the 2 diagonals, shall · not exceed 13 millimeters.
- Panel surface finish. Surface defects on smooth formed surfaces 1.5 meters or more in length .shall not exceed 3 millimeters. Surface defects on textured-finished surfaces 1.5 meters or more in length shall not exceed 8 millimeters.
Concrete face panels having any or all of the following defects shall be
rejected.
- Defects that indicate imperfect molding
- Defects indicating honeycombed or open texture concrete.
- Cracked or severely chipped panels.
- Color variation on front face of panel due to excess form oil or other reasons.
515.2.3.2 Wire Facing
Wire facing shall be fabricated welded wire fabric conforming to AASHTO M
55M, except that Section 7.4 applies for all longitudinal and transverse
wire sizes. After fabrication wire fabric shall be galvanized according to
AASHTO M 111 M.
515.2.3.3 Gabions
Gabions shall conform to Item 511, Gabions and Mattresses, unless otherwise
specified or required by the Engineer.
515.2.3.4 Backing Mat
Backing mat shall be fabricated welded wire fabric conforming to AASHTO M
55M, except that Section 7.4 applies for all longitudinal and transverse
wire sizes. After fabrication wire fabric shall be galvanized according to
AASHTO M 111 M.
515.2.3.5 Clevis Connector
Clevis connector shall be fabricated from cold-drawn steel wire conforming
to AASHTO M 32M, and welded according to AASHTO M 55M. After fabrication
clevis connector shall be galvanized according to AASHTO M 111M.
515.2.3.6 Connector Bars
Connector bars shall be fabricated from cold-drawn steel wire conforming to
AASHTO M 32M and shall be galvanized according to AASHTO M 111M.
515.2.3. 7 Fasteners
Fasteners shall be made of 13-millimeter diameter, heavy head bolts, nuts,
and washers conforming to AASHTO M 164M and shall be galvanized according
to AASHTO M 232.
515.2.3.8 Hardware Cloth
Hardware cloth shall be fabricated with maximum 7-millimeter square mesh
openings from woven or welded galvanized steel wire fabric conforming to
ASTM A 740.
515.2.3.9 Reinforcing mesh
Reinforcing mesh shall be fabricated from cold-drawn steel wire conforming
to AASHTO M 32M. Wire shall be welded into the finished mesh fabric
according to AASHTO M 55M. Wire shall be galvanized according to AASHTO M
111 M after fabrication. Damaged galvanized coating shall be repaired
before installation.
515.2.3.10 Reinforcing strips
Reinforcing strips shall be fabricated from high-strength, low alloy
structural steel conforming to ASTM A 572M, grade 450, type-3. Reinforcing
strips shall be galvanized according to AASHTO M 111 M after fabrication.
515.2.3.11 Tie strip
Tie strip shall be fabricated from hot-rolled steel conforming to ASTM A
570M, grade 50. Tie strip shall be galvanized according to AASHTO M 111M.
515.2.4 Select Granular Backfill
Select Granular Backfill shall consist of sound, durable, granular material
free from organic matter or other deleterious material (such as shale or
other soft particles with poor durability).
The select granular backfill materials shall conform to Table 515.1,
Grading Requirements.
Table 515.1 -Grading Requirements:
Sieve Designation Standard (mm)
|
Percent by Mass Passing Designated Sieve
(AASHTO T/27 and T 11)
|
100
0.0425
0.075
|
100
0 - 60
0 -15
|
The shear angle of internal friction shall not be less than 340.
Compact samples for AASHTO T 236 to 95 percent of the maximum density
determined according to AASHTO T 99 Method C or D and corrected for
oversized material according to ASSHTO T 99, Note 9.
If the select granular backfill materials are subjected to five (5) cycles
of the sodium sulfate soundness test by AASHTO T 104, the weighted loss
shall not exceed 15 mass percent.
The material shall have a Plasticity Index of not more than 6 as determined
by AASHTO T 90.
Electrochemical requirements for Mechanically-Stabilized Earth (MSE)
retaining walls with metallic reinforcements shall comply with the
following requirements:
Test
|
Requirements
|
Resistivity, AASHTO T 288
pH, AASHTO T 289
Sulfate Content, AASHTO T 290 Chloride Content, AASHTO T
291
|
3000 'Ω-cm min.
5.0 to 10.0
200 ppm max.
100 ppm max.
|
Note:
Tests for sulfate and chloride content are not required when resistivity is
greater than 5,000 Ohm centimeters.
Electrochemical requirements for Mechanically-Stabilized Earth (MSE)
retaining walls with Geosynthetic reinforcements, specify that the pH shall
be between 5.0 to 10.0 as determined by AASHTO T 289.
515.2.5 Structural Backfill
Structural backfill shall consist of free draining granular materials free
from excess moisture, muck, frozen lumps, roots, sod, or other deleterious
material.
The structural backfill materials shall conform to Table 515.2 Grading
Requirements.
Table 515.2 - Grading Requirements
Sieve Standard Designation(mm)
|
Percent by Mass Passing (AASHTO Designated T 27 and Sieve T
11)
|
75
0.075
|
100
15 max
|
The material shall have a liquid limit of not more than 30 as determined by
AASHTO T 89.
515.3 Construction Requirements
515.3.2 General
Survey and verify the limits of the wall installation. Detailed working
drawings such as but not limited to wall elevations, soil reinforcement
length for each section of wall, applied bearing pressure, facing panel
layout, and representative typical details and supporting calculations of
the Mechanically-Stabilized Earth (MSE) retaining walls shall be furnished
by the Contractor to the Engineer at least three (3) weeks prior to
construction. Construction shall not start until the Engineer has reviewed
and approved the design/Plans.
Mechanically-Stabilized Earth (MSE) retaining walls design shall be in
accordance with DPWH – Design Guidelines Criteria and Standards (Latest
Edition)
A minimum horizontal bench1.20 meters wide shall be provided in front of
walls founded on slopes.
515.3.2 Excavation
The ground shall be excavated in accordance with the dimensions, lines and
grades shown on the Plans. Excavation shall be I accordance whenever
applicable, with Section 103.2, Excavation.
Level and layout the foundation for a width equal to the length of
reinforcing elements plus 0.5 meter. When a rocky foundation is situated
anywhere along the wall, place 150 millimeters of select granular backfill
materials under the reinforcing mesh or strips.
515.3.3 Foundation Preparation
Prior to wall construction, the foundation, if not on rock surface shall be
compacted as directed by the Engineer. Any unsuitable foundation material,
as determined by the Engineer, shall be excavated and replaced with
granular material, and shall be compacted in accordance with Section
515.3.5, Backfilling.
The construction requirements for concrete leveling pad shall be in
accordance, whenever applicable, with Section 1405.3, Structural Concrete.
Allowable elevation tolerances are + 3.0 mm and - 6.0 mm from the design
elevation.
515.3.4 Wall Erection
Walls shall be erected in accordance with the approved shop drawings and
the manufacturer's recommendations. When required by the Engineer, an
experienced field representative from the wall system manufacturer shall be
present on site during erection.
The starting point for the wall erection shall be as shown on the Plans or
if the new wall matches an existing wall. Otherwise the wall shall begin on
the leveling pad at the lowest elevation.
The concrete leveling pad shall be allowed to cure for at least 12 hours
before the wall erection.
515.3.4.1 Concrete-Faced-Walls
Where walls or wall sections intersect at an angle of 130° or less, a
special vertical corner element panel shall be used. The corner element
panel shall cover the joint of the panels that abut the corner, to allow
for the independent movement of the abutting panels.
Standard facing panels shall have at least two levels of earth
reinforcements to stabilize the panels against rotation. Top and bottom
half panels shall have at least one (1) level of earth reinforcements. The
wall facing shall be designed to accommodate differential settlement of one
(1) meter in one hundred (100) meters. The spacing between adjacent panels
shall be designed to be 19 millimeters. Joints between panels shall have a
ship lap configuration to protect the joint materials from vandalism. There
shall be no openings through the wall facing except for utilities to pass
through the wall. Slip joints to accommodate excessive differential
settlement shall be included if indicated in the Plans.
515.3.4.1.1 Installation
Precast facing panels shall be erected by means of lifting devices
connected to the upper edge of the panel. Precast facing panels shall be
aligned within 19 millimeters vertically and horizontally when measurement
with a 3-meter straightedge.
Joint openings shall be 19±6 millimeters wide. Joint material shall be
installed according to the drawings. All joints at the backside of the
panels shall be covered with a 300-millimeter wide strip of geotextile.
Geotextile splices shall be overlapped with a minimum of 100 millimeters.
The panels in position shall be supported with temporary wedges or bracing
during backfilling operations. The final position of the wall shall have
maximum overall vertical tolerance (top to bottom) of 13 millimeters per 3
meters of wall height.
515.3.4.2 Wire-faced Walls
Place backing mats and 6-millimeter hardware cloth in successive horizontal
lifts as backfill placement proceeds. Connect, tighten, and anchor soil
reinforcement elements to the wall facing units before placing backfill. An
individual lift shall have a maximum vertical tolerance and an overall wall
(top to bottom) vertical tolerance of 25 millimeters per 3 meters of wall
height. Reinforcement elements shall be placed within 25 millimeters
vertically above the corresponding connection elevation at the wall face.
Reinforcing elements shall not be placed below the corresponding connection
elevations. Designed batter of the wall shall not deviate by more than 25
millimeters per 3 meters· of wall height. Deviation of more than 50
millimeters shall not be allowed at any point in the wall from 3-meter
straightedge placed horizontally on the theoretical plane of the design
face.
The minimum wall embedment shall be 300 mm unless otherwise shown on the
Plans or as established by the Engineer.
515.3.4.3 Gabion-Faced Walls
The construction requirements of gabions shall be in accordance whenever
applicable, with Section 511.3, Gabions and Mattresses.
The first lift of backfill shall be placed before, filling the gabion
baskets. Reinforcement mesh shall be laid horizontally on compacted fill
and normal to the face of the wall. The gabion facing unit shall be
connected to the reinforcement mesh with spiral binders or tie wire at
100-millimeter nominal spacing with alternating single and double locked
loops. Reinforcement mesh taut shall be pulled and anchored before placing
additional backfill.
515.3.5 Backfilling
The stabilized volume shall be backfilled with select granular backfill.
The maximum compacted thickness of any layer shall not exceed 150 mm. All
subsequent layers shall be spread and compacted in a similar manner.
The backfill materials shall be compacted until a uniform density of not
less than 95 mass percent of the maximum dry density determined by AASHTO T
99 Method C is attained, at a moisture content determined by the Engineer
to be suitable for such density. Acceptance of compaction may be based on
adherence to an approved roller pattern developed as set forth in Item 106,
Compaction Equipment and Density Control Strips.
During the progress of the Work, the Engineer shall make density tests of
compacted material in accordance with AASHTO T 310 or other approved field
density tests. If, by such tests, the Engineer determines that the
specified density and moisture conditions have not been attained, the
Contractor shall perform additional work as may be necessary to attain the
specified conditions.
At least one group of three (3) in-situ density tests shall be carried out
for each 500 m of each layer of compacted fill.
Backfilling shall be even on all sides of the structure, and each layer
shall be extended to the limits of the excavation or to natural ground.
Backfill shall be placed in such a manner as to avoid any damage or
disturbance to the wall materials or misalignment of the facing panels. Any
wall materials which become damaged or disturbed during backfill placement
shall either be removed and replaced at the Contractor’s expense or
corrected, as directed by the Engineer.
Backfill shall not be placed against concrete less than 7 -days old or
until 80 percent of the design strength is achieved.
Ensure that no voids exist below the reinforcing mesh or strips. An
acceptable lightweight mechanical or vibratory compactor shall be used
within one (1) meter of the wall face. The specified lift thickness shall
be adjusted as warranted by the type of equipment actually used and as
approved by the Engineer, but no soil density tests need be taken within
this area. Where the stabilized volume supports spread footings for bridges
or other structural loads, the top 1.5 meters shall be compacted to at
least 100 percent of the maximum density.
Facing or reinforcing elements shall not be damaged or disturbed during
backfilling. Equipment shall not be operated directly on top of the
reinforcing mesh or strips. The Contractor shall correct damaged
misaligned, or distorted wall elements.
Backfill and compact behind the stabilized volume with structural backfill
in the same methodology done on select granular backfill. At the end of the
day's operation, the last lift of backfill shall be sloped away from the
wall face to direct surface runoff away from the wall. Temporary drainage
shall be constructed to divert any surface runoff from adjacent areas to
enter the wall construction area.
515.3.6 Placement of Reinforcements
Prior to placing the first layer of reinforcements (strips, mats or grids),
backfill shall be placed and compacted in accordance with Section 515.3.5,
Backfilling.
Bending of reinforcements in the horizontal plane that results in a kink in
their alignment shall not be allowed. Gradual bending in the vertical
direction that does not kink the reinforcements is allowable.
Connection of reinforcements to piles or bending of reinforcements around
piles shall not be allowed. Cutting of reinforcement longitudinal bars
shall not be allowed to avoid conflicts with piles or utility obstructions.
A structural connection (yoke) from the wall panel to the reinforcement
shall be used whenever it is necessary to avoid cutting or excessive
skewing of reinforcements due to pile or utility conflicts.
Soil reinforcements shall be placed normal to the face of the wall, unless
otherwise shown on the Plans or as directed by the Engineer. If skewing of
the soil reinforcements is required due to obstructions in the reinforced
fill, rotatable connections shall be used and the maximum skew angle shall
not exceed 15 degrees from the normal position unless specifically
addressed in design calculations that justify that the skewed
reinforcements are adequate.
515.4 Method of Measurement
The area to be paid for under this item shall be the .number of square
meters (m2) of Mechanically-Stabilized Earth (MSE) retaining
walls furnished, placed and accepted. The area shall be measured on the
facial area of the walls installed. The wall surface area shall include the
surface area of nominal panel joint openings and wall penetrations such as
pipes and other utilities. The payment will include Mechanically-stabilized
earth walls, backing mat, clevis connector, connector bars, fasteners,
hardware cloth, reinforcing mesh, tie strips and other such materials
contained within or attached to the mechanically-stabilized earth walls.
515.4.1 Other Items
The quantities of other Contract items which enter into the completed and
accepted structure will be measured for payment in the manner prescribed
for the several items involved.
515.4.2 Select Granular Backfill/ Structural Backfill
Select Granular Backfill/Structural Backfill material, when specified in
the Contract as a Pay Item shall be measured in cubic place by the meter,
completed and accepted. Cross-sectional measurements shall not exceed the
net dimensions shown on the Plans or as directed by the Engineer.
515.4.3 Excavation
Excavation for Mechanically-Stabilized Earth (MSE) retaining walls shall be
measured and paid for as provided in Item 103, Structure Excavation.
515.4.4 Concrete
Concrete leveling pad/ concrete panels for mechanically-stabilized earth
(MSE) retaining walls shall be measured and paid for as provided in Item
405, Structural Concrete.
515.4.5 Gabions
Gabions for Mechanically-Stabilized Earth (MSE) retaining walls shall be
measured and paid for as provided in Item 511, Gabions and Mattresses.
515.5 Basis of Payment
The accepted quantities, measured as prescribed in Section 515.4 shall be
paid for at the contract unit price for each particular item listed below
that is included in the bill of quantities which price and payment shall be
full compensation for furnishing and placing all materials including the
use of equipment and tools, labor and incidentals necessary to complete the
work.
Payment will be made under:
| Pay item Number | Description | Unit of Measurement |
| 515 (1)
515 (2) 515 (3) |
Mechanically-Stabilized Earth (MSE) Retaining Walls
Select Granular Backfill Structural Backfill |
Sq.m.
Cu.m. Cu.m. |
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